More Important Topics of Cylinder

The troposphere is the lowest portion of Earth's atmosphere. It is the densest layer of the atmosphere and contains approximately 75% of the mass of the atmosphere and almost all the water vapor and aerosol.

The troposphere extends from the Earth's surface up to the tropopause where the stratosphere begins. The depth of the troposphere is greatest along the equator (about 20 km) and smallest at the poles (about 7 km). The lower part,

where friction on the Earth's surface influences with air flow, is the planetary boundary layer or peplosphere which is 2 km deep on average, depending on the landform, and which is separated from the rest of the troposphere by the capping inversion layer. The border of the troposphere and stratosphere, the tropopause, is also an inversion; the equilibrium level (EL), an atmospheric thermodynamics value.

The word troposphere stems from the Greek "tropos" for "turning" or "mixing." The troposphere is the most turbulent part of the atmosphere and is the part of the atmosphere in which most weather phenomena are seen. Generally, jet aircraft fly just above the troposphere to avoid turbulence.

Pressure and temperature structure

Pressure

The pressure of the atmosphere is lowest at the top and decreases with higher altitude. This is because air at the surface is compressed by the weight of all the air above it. At higher altitudes, the weight of the air above is less so the air is compressed less and has a lower pressure. This change in pressure with height can be predicted with the hydrostatic equation:

where:

g is the standard gravity

? is density

h is height

p is pressure

R is gas constant

T is temperature

Pressure decreases exponentially with height as is shown by solving the above equation: Assuming no temperature changes with height, this linear differential equation yields for p(h)

Temperature

In the troposphere, the temperature decreases with height at an average rate of 6.5 °C for every 1 km (1000 meters) increase in height. This decrease in temperature is caused by adiabatic cooling—as air rises the atmospheric pressure falls so the air expands. In order to expand, the air must do work on its surroundings and therefore its temperature decreases (due to Conservation of energy).

Temperatures decrease at middle latitudes from approx. +17°C at sea level to approx. -52°C at the beginning of the tropopause. At the poles, the troposphere is thinner and the temperature only decreases to -45 °C, while at the equator the temperature at the top of the troposphere can reach -75 °C.

Tropopause

The tropopause is the boundary region between the troposphere and the stratosphere.

Measuring the temperature change with height through the troposphere and the stratosphere identifies the location of the tropopause. In the troposphere, temperature decreases with altitude. In the stratosphere, however, the temperature increases with altitude. The region of the atmosphere where the lapse rate changes from positive (in the troposphere) to negative (in the stratosphere), is defined as the tropopause.

Atmospheric circulation

The basic structure of large-scale circulation in the atmosphere remains fairly constant. There are three convection cells in each hemisphere: the Hadley cell, the Ferrel cell, and the Polar cell which guide the prevailing winds and transport heat from the equator to the poles.

Stratosphere:

The stratosphere is the second layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. It is stratified in temperature, with warmer layers higher up and cooler layers farther down. This is in contrast to the troposphere near the Earth's surface, which is cooler higher up and warmer farther down. The border of the troposphere and stratosphere, the tropopause, is marked by where this inversion begins, which in terms of atmospheric thermodynamics is the equilibrium level. The stratosphere is situated between about 10 km (6 miles) and 50 km (31 miles) altitude above the surface at moderate latitudes, while at the poles it starts at about 8 km (5 miles) altitude.

The stratosphere is layered in temperature because it is heated from above by absorption of ultraviolet radiation from the Sun. Within this layer, temperature increases as altitude increases (see temperature inversion); the top of the stratosphere has a temperature of about 270 K (-3°C or 26.6°F), just slightly below the freezing point of water. This top is called the stratopause, above which temperature again decreases with height. The vertical stratification, with warmer layers above and cooler layers below, makes the stratosphere dynamically stable: there is no regular convection and associated turbulence in this part of the atmosphere. The heating is caused by an ozone layer that absorbs solar ultraviolet radiation, heating the upper layers of the stratosphere. The base of the stratosphere occurs where heating by conduction from above and heating by convection from below (through the troposphere) balance out; hence, the stratosphere begins at lower altitudes near the poles due to the lower ground temperature there.

Commercial airliners typically cruise at an altitude near 10 km in temperate latitudes, in the lower reaches of the stratosphere. They do this to stay above any hard weather. This is to avoid atmospheric turbulence from the convection in the troposphere. Turbulence experienced in the cruise phase of flight is often caused by convective overshoot from the troposphere below. Similarly, most gliders soar on thermal plumes that rise through the troposphere above warm patches of ground; these plumes end at the base of the stratosphere, setting a limit to how high gliders can fly in most parts of the world. (Some gliders do fly higher, using ridge lift from mountain ranges to lift them into the stratosphere.)

The stratosphere is a region of intense interactions among radiative, dynamical, and chemical processes, in which horizontal mixing of gaseous components proceeds much more rapidly than vertical mixing. An interesting feature of stratospheric circulation is the quasi-Biennial Oscillation (QBO) in the tropical latitudes, which is driven by gravity waves that are convectively generated in the troposphere. The QBO induces a secondary circulation that is important for the global stratospheric transport of tracers such as ozone or water vapor.

In northern hemispheric winter, sudden stratospheric warmings can often be observed which are caused by the absorption of Rossby waves in the stratosphere.

Ozone Depletion

The reported main cause of ozone depletion is the presence of chlorofluorocarbons,or CFCs, in the Earth's stratosphere. Chloroflorocarbons are compounds of chlorine, fluorine, and carbon. Because CFCs are stable, inexpensive, non-toxic, non-flammable, and not corrosive, they are used as propellants, as refrigerants, as solvents, etc. However, it is this stability that causes these CFCs to persist within the environment. These molecules eventually find their way to the stratosphere, where they undergo a series of chain reactions which ultimately lead to the destruction of the ozone layer.

The US government banned the use of CFCs for aerosol propellants in 1980. Worldwide efforts to reduce the use of CFCs began in September 1987 and by 1996 an international ban was put into effect preventing the industrial production and release of CFCs. These efforts have been drastically thwarted by black market operations in China and Russia where up to $500 million worth of illegal CFCs are manufactured. The amounts of CFCs in the stratosphere rose until early 2000, and are expected to reach acceptable levels by mid-century.

Mesosphere:

The mesosphere (say mee-so-sfeer) (from the Greek words mesos = middle and sphaira = ball) is the layer of the Earth's atmosphere that is directly above the stratosphere and directly below the thermosphere. The mesosphere is located about 50-80/85km above Earth's surface. Within this layer, temperature decreases with increasing altitude.[1] The main dynamical features in this region are atmospheric tides, internal atmospheric gravity waves (usually just called "gravity waves") and planetary waves. Most of these waves and tides are excited in the troposphere and lower stratosphere, from where they propagate upwards to the mesosphere. In the mesosphere, gravity-wave amplitudes can become so large that the waves dissipate, depositing energy and momentum into the mesosphere. The momentum deposited by these dissipating gravity waves largely drives the global circulation of the mesosphere.

Because it lies between the maximum altitude for most aircraft and the minimum altitude for most spacecraft, for a long time this region of the atmosphere has only been accessed through the use of sounding rockets. As a result the region is one of the most poorly understood in the atmosphere. This has led the mesosphere and the lower thermosphere to be jokingly referred to by scientists as the ignorosphere [1] [2].

Temperatures in the upper mesosphere fall as low as -100°C (-146°F or 173 K) [3], varying according to latitude and season. Millions of meteors burn up daily in the mesosphere as a result of collisions with the gas particles contained there, leading to a high concentration of iron and other metal atoms. The collisions almost always create enough heat to burn the falling objects long before they reach the ground.

The stratosphere and mesosphere are referred to as the middle atmosphere. The mesopause, at an altitude of about 80 km, separates the mesosphere from the thermosphere—the second-outermost layer of the Earth's atmosphere. This is also around the same altitude as the turbopause, below which different chemical species are well mixed due to turbulent eddies. Above this level the scale heights of different chemical species will differ. Noctilucent clouds are located in the mesosphere.

Thermosphere:

The thermosphere is the layer of the earth's atmosphere directly above the mesosphere and directly below the exosphere. Within this layer, ultraviolet radiation causes ionization. (see also: ionosphere). It is the fourth atmospheric layer from earth.

The thermosphere, named from the Greek ?e?µ?? (thermos) for heat, begins about 80 km above the earth. At these high altitudes, the residual atmospheric gases sort into strata according to molecular mass (see turbosphere). Thermospheric temperatures increase with altitude due to absorption of highly energetic solar radiation by the small amount of residual oxygen still present. Temperatures are highly dependent on solar activity, and can rise to 2,000°C. Radiation causes the air particles in this layer to become electrically charged (see ionosphere), enabling radio waves to bounce off and be received beyond the horizon. At the exosphere, beginning at 500 to 1,000km above the earth's surface, the atmosphere blends into space. The few particles of gas here can reach 2,500°C (4500°F) during the day. Even though the temperature is so high, one would not feel warm in the thermosphere. A normal thermometer would read significantly below 0°C. This is due to the distance between the few molecules that are present.

The dynamics of the lower thermosphere (below about 120 km) is dominated by atmospheric tide which is driven, in part, by the very significant diurnal heating. The atmospheric tide dissipates above this level since molecular concentrations do not support the coherent motion needed for fluid flow.

The International Space Station has a stable orbit within the upper part of the thermosphere, between 320 and 380 kilometers. The Northern Lights also occur in the upper thermosphere.

Ionosphere:

The ionosphere is the uppermost part of the atmosphere, distinguished because it is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere. It has practical importance because, among other functions, it influences radio propagation to distant places on the Earth. It is located in the Thermosphere.

Geophysics

The lowest part of the Earth's atmosphere is called the troposphere and it extends from the surface up to about 10 km (6 miles). The atmosphere above 10 km is called the stratosphere, followed by the mesosphere. It is in the stratosphere that incoming solar radiation creates the ozone layer. At heights of above 80 km (50 miles), in the thermosphere, the atmosphere is so thin that free electrons can exist for short periods of time before they are captured by a nearby positive ion. The number of these free electrons is sufficient to affect radio propagation. This portion of the atmosphere is ionized and contains a plasma which is referred to as the ionosphere. In a plasma, the negative free electrons and the positive ions are attracted to each other by the electromagnetic force, but they are too energetic to stay fixed together in an electrically neutral molecule.

Solar radiation at ultraviolet (UV) and shorter X-Ray wavelengths is considered to be ionizing since photons at these frequencies are capable of dislodging an electron from a neutral gas atom or molecule during a collision. At the same time, however, an opposing process called recombination begins to take place in which a free electron is "captured" by a positive ion if it moves close enough to it. As the gas density increases at lower altitudes, the recombination process accelerates since the gas molecules and ions are closer together. The point of balance between these two processes determines the degree of ionization present at any given time.

The ionization depends primarily on the Sun and its activity. The amount of ionization in the ionosphere varies greatly with the amount of radiation received from the sun. Thus there is a diurnal (time of day) effect and a seasonal effect. The local winter hemisphere is tipped away from the Sun, thus there is less received solar radiation. The activity of the sun is associated with the sunspot cycle, with more radiation occurring with more sunspots. Radiation received also varies with geographical location (polar, auroral zones, mid-latitudes, and equatorial regions). There are also mechanisms that disturb the ionosphere and decrease the ionization. There are disturbances such as solar flares and the associated release of charged particles into the solar wind which reaches the Earth and interacts with its geomagnetic field.

The Ionospheric Layers

Solar radiation, acting on the different compositions of the atmosphere with height, generates layers of ionization:

D Layer

The D layer is the innermost layer, 50 km to 90 km above the surface of the Earth. Ionization here is due to Lyman series-alpha hydrogen radiation at a wavelength of 121.5 nanometre (nm) ionizing nitric oxide (NO). In addition, when the sun is active with 50 or more sunspots, hard X-rays (wavelength < 1 nm) ionize the air (N2, O2). During the night cosmic rays produce a residual amount of ionization. Recombination is high in this layer, thus the net ionization effect is very low and as a result high-frequency (HF) radio waves aren't reflected by the D layer. The frequency of collision between electrons and other particles in this region during the day is about 10 million collisions per second. The D layer is mainly responsible for absorption of HF radio waves, particularly at 10 MHz and below, with progressively smaller absorption as the frequency gets higher. The absorption is small at night and greatest about midday. The layer reduces greatly after sunset, but remains due to galactic cosmic rays. A common example of the D layer in action is the disappearance of distant AM broadcast band stations in the daytime.

E Layer

The E layer is the middle layer, 90 km to 120 km above the surface of the Earth. Ionization is due to soft X-ray (1-10 nm) and far ultraviolet (UV) solar radiation ionization of molecular oxygen (O2). This layer can only reflect radio waves having frequencies less than about 10 MHz. It has a negative effect on frequencies above 10 MHz due to its partial absorption of these waves. The vertical structure of the E layer is primarily determined by the competing effects of ionization and recombination. At night the E layer begins to disappear because the primary source of ionization is no longer present. This results in an increase in the height where the layer maximizes because recombination is faster in the lower layers. Diurnal changes in the high altitude neutral winds also plays a role. The increase in the height of the E layer maximum increases the range to which radio waves can travel by reflection from the layer.

This region is also known as the Kennelly-Heaviside Layer layer or simply the Heaviside layer. Its existence was predicted in 1902 independently and almost simultaneously by the American electrical engineer Arthur Edwin Kennelly (1861-1939) and the British physicist Oliver Heaviside (1850-1925). However, it was not until 1924 that its existence was detected by Edward V. Appleton.

In 1899, Nikola Tesla, in his Colorado Springs experiments, transmitted extremely low frequencies between the earth and ionosphere, up to the Kennelly-Heaviside layer (Grotz, 1997). Tesla made mathematical calculations and computations based on his experiments. He predicted the resonant frequency of this area within 15% of modern accepted experimental value. (Corum, 1986) In the 1950s, researchers confirmed the resonant frequency was at the low range 6.8 Hz

ES

The Es layer or sporadic E-layer. Sporadic E propagation is characterized by small clouds of intense ionization, which can support radio wave reflections from 25 – 225 MHz. Sporadic-E events may last for just a few minutes to several hours and make radio amateurs very excited, as propagation paths which are generally unreachable, can open up. There are multiple causes of sporadic-E that are still being pursued by researchers. This propagation occurs most frequently during the summer months with major occurrences during the summer, and minor occurrences during the winter. During the summer, this mode is popular due to its high signal levels. The skip distances are generally around 1000km (620 miles).

F Layer

The F layer or region, also known as the Appleton layer, is 120 km to 400 km above the surface of the Earth. It is the top most layer of the ionosphere. Here extreme ultraviolet (UV) (10-100 nm) solar radiation ionizes atomic oxygen (O). The F region is the most important part of the ionosphere in terms of HF communications. The F layer combines into one layer at night, and in the presence of sunlight (during daytime), it divides into two layers, the F1 and F2. The F layers are responsible for most skywave propagation of radio waves, and are thickest and most reflective of radio on the side of the Earth facing the sun.

From 1972-1975 NASA launched AEROS and AEROS B satellites to study the F-region.

Ionospheric model

The atmospheric physics community contributes to the definition and maintenance of an ionospheric model: the International Reference Ionosphere, through a series of academic committees and conferences. As discoveries are made and generally accepted, the model is improved. (IRI85-6)

Anomalies to the Ideal Model

The statements above assumed that each layer was smooth and uniform. In reality the ionosphere is a lumpy, cloudy layer with irregular patches of ionization.

Winter Anomaly

At mid-latitudes, the F2 layer daytime ion production is higher in the summer, as expected, since the sun shines more directly on the earth. However, there are seasonal changes in the molecular-to-atomic ratio of the neutral atmosphere that cause the summer ion loss rate to be even higher. The result is that the increase in the summertime loss overwhelms the increase in summertime production, and total F2 ionization is actually lower, not higher, in the local summer months. This effect is known as the winter anomaly. The anomaly is always present in the northern hemisphere, but is usually absent in the southern hemisphere during periods of low solar activity.

Equatorial Anomaly

Within approximately ± 20 degrees of the magnetic equator, is the Equatorial Anomaly. It is the occurrence of a trough of concentrated ionization in the F2 layer. The Earth's magnetic field lines are horizontal at the magnetic equator. Solar heating and tidal oscillations in the lower ionosphere move plasma up and across the magnetic field lines. This sets up a sheet of electric current in the E region which, with the horizontal magnetic field, forces ionization up into the F layer, concentrating at ± 20 degrees from the magnetic equator. This phenomenon is known as the equatorial fountain.

Ionospheric perturbations

X-rays: sudden ionospheric disturbances (SID)

When the sun is active, strong solar flares can occur that will hit the Earth with hard X-rays on the sunlit side of the Earth. They will penetrate to the D-region, release electrons which will rapidly increase absorption causing a High Frequency (3-30 MHz) radio blackout. During this time Very Low Frequency (3 - 30 kHz) signals will become reflected by the D layer instead of the E layer, where the increased atmospheric density will usually increase the absorption of the wave, and thus dampen it. As soon as the X-rays end, the sudden ionospheric disturbance (SID) or radio black-out ends as the electrons in the D-region recombine rapidly and signal strengths return to normal.

Protons: polar cap absorption (PCA)

Associated with solar flares is a release of high-energy protons. These particles can hit the Earth within 15 minutes to 2 hours of the solar flare. The protons spiral around and down the magnetic field lines of the Earth and penetrate into the atmosphere near the magnetic poles increasing the ionization of the D and E layers. PCA's typically last anywhere from about an hour to several days, with an average of around 24 to 36 hours.

Geomagnetic storms

A geomagnetic storm is a temporary intense disturbance of the Earth's magnetosphere.

During a geomagnetic storm the F2 layer will become unstable, fragment, and may even disappear completely.

In the Northern and Southern pole regions of the Earth aurora will be observable in the sky.

Lightning

Lightning can cause ionospheric perturbations in the D-region one of two ways. The first is through VLF frequency radio waves launched into the magnetosphere. These so-called "whistler" mode waves can interact with radiation belt particles and cause them to precipitate onto the ionosphere, adding ionization to the D-region. These disturbances are called Lightning-induced Electron Precipitation (LEP) events.

Additional ionization can also occur from direct heating/ionization as a result of huge motions of charge in lightning strikes. These events are called Early/Fast.

Radio application

DX communication, popular among amateur radio enthusiasts, is a term given to communication over great distances. When using High-Frequency bands, the ionosphere is utilized to reflect the transmitted radio beam. The beam returns to the Earth's surface, and may then be reflected back into the ionosphere for a second bounce.

Radio waves "hop" from the Earth to the ionosphere and back to the Earth. When a radio wave reaches the ionosphere, the electric field in the wave forces the electrons in the ionosphere into oscillation at the same frequency as the radio wave. Some of the radio wave energy is given up to this mechanical oscillation. The oscillating electron will then either be lost to recombination or will re-radiate the original wave energy back downward again. Total reflection can occur when the collision frequency of the ionosphere is less than the radio frequency, and if the electron density in the ionosphere is great enough.

The critical frequency is the limiting frequency at or below which a radio wave is reflected by an ionospheric layer at vertical incidence. If the transmitted frequency is higher than the plasma frequency of the ionosphere, then the electrons cannot respond fast enough, and they are not able to re-radiate the signal. It is calculated as shown below:

where N = electron density per cm3 and fcritical is in MHz.

The Maximum Usable Frequency (MUF) is defined as the upper frequency limit that can be used for transmission between two points at a specified time.

where a = angle of attack, the angle of the wave relative to the horizon, and sin is the sine function.

The cutoff frequency is the frequency below which a radio wave fails to penetrate a layer of the ionosphere at the incidence angle required for transmission between two specified points by reflection from the layer.

Other applications

The open system space tether, which uses the ionosphere, is being researched. The space tether uses plasma contactors and the ionosphere as parts of a circuit to extract energy from the Earth's magnetic field by electromagnetic induction.

Measurements

Ionograms

Ionograms show the virtual heights and critical frequencies of the ionospheric layers and which are measured by an ionosonde. An ionosonde sweeps a range of frequencies, usually from 0.1 to 30 MHz, transmitting at vertical incidence to the ionosphere. As the frequency increases, each wave is refracted less by the ionization in the layer, and so each penetrates further before it is reflected. Eventually, a frequency is reached that enables the wave to penetrate the layer without being reflected. For ordinary mode waves, this occurs when the transmitted frequency just exceeds the peak plasma, or critical, frequency of the layer. Tracings of the reflected high frequency radio pulses are known as ionograms.

Incoherent scatter radars

Solar flux

Solar flux is a measurement of the intensity of solar radio emissions at a frequency of 2800 MHz made using a radio telescope located in Ottawa, Canada. Known also as the 10.7 cm flux (the wavelength of the radio signals at 2800 MHz), this solar radio emission has been shown to be proportional to sunspot activity. However, the level of the sun's ultraviolet and X-ray emissions is primarily responsible for causing ionization in the earth's upper atmosphere. We now have data from the GOES spacecraft that measures the background X-ray flux from the sun, a parameter more closely related to the ionization levels in the ionosphere.

The A and K indices are a measurement of the behavior of the horizontal component of the geomagnetic field. The K index uses a scale from 0 to 9 to measure the change in the horizontal component of the geomagnetic field. A new K index is determined at the Table Mountain Observatory, north of Boulder, Colorado.

The geomagnetic activity levels of the earth are measured by the fluctuation of the Earth's magnetic field in SI units called tesla (unit)s (or in non-SI gauss, especially in older literature). The Earth's magnetic field is measured around the planet by many observatories. The data retrieved is processed and turned into measurement indices. Daily measurements for the entire planet are made available through an estimate of the ap index, called the planetary A-index (PAI).

Scientific research on ionospheric propagation

Scientists also are exploring the structure of the ionosphere by a wide variety of methods, including passive observations of optical and radio emissions generated in the ionosphere, bouncing radio waves of different frequencies from it, incoherent scatter radars such as the EISCAT, Sondre Stromfjord, Millstone Hill, Arecibo, and Jicamarca radars, coherent scatter radars such as the Super Dual Auroral Radar Network (SuperDARN) radars, and using special receivers to detect how the reflected waves have changed from the transmitted waves.

A variety of experiments, such as HAARP (High Frequency Active Auroral Research Program), involve high power radio transmitters to modify the properties of the ionosphere. These investigations focus on studying the properties and behavior of ionospheric plasma, with particular emphasis on being able to understand and use it to enhance communications and surveillance systems for both civilian and defense purposes. HAARP was started in 1993 for a proposed twenty year experiment.

The SuperDARN radar project researches the high- and mid-latitudes using coherent backscatter of radio waves in the 8 to 20 MHz range. Coherent backscatter is similar to Bragg scattering in crystals and involves the constructive interference of scattering from ionospheric density irregularities. The project involves more than 11 different countries and multiple radars in both hemispheres.

Scientists are also examining the ionosphere by the changes to radio waves from satellites and stars passing through it. The Arecibo radio telescope located in Puerto Rico, was originally intended to study Earth's ionosphere.

History

In 1899, Nikola Tesla researched ways to utilize the ionosphere to transmit energy wirelessly over long distances. In his experiments, he transmitted extremely low frequencies between the earth and ionosphere, up to what is called the Kennelly-Heaviside Layer (Grotz, 1997). Tesla made mathematical calculations and computations based on his experiments. He predicted the resonant frequency of this area within 15% of modern accepted experimental value. (Corum, 1986) In the 1950s, researchers confirmed the resonant frequency was at the low range 6.8 Hz.

Guglielmo Marconi received the first trans-Atlantic radio signal on December 12, 1901, in St. John's, Newfoundland (now in Canada) using a 400-foot kite-supported antenna for reception. The transmitting station in Poldhu, Cornwall used a spark-gap transmitter to produce a signal with a frequency of approximately 500 kHz and a power of 100 times more than any radio signal previously produced. The message received was three dots, the Morse code for the letter S. To reach Newfoundland the signal would have to bounce off the ionosphere twice. Dr. Jack Belrose has recently contested this, however, based on theoretical and experimental work. However, Marconi did achieve transatlantic wireless communications beyond a shadow of doubt in Glace Bay one year later.

In 1902, Oliver Heaviside proposed the existence of the Kennelly-Heaviside Layer of the ionosphere which bears his name. Heaviside's proposal included means by which radio signals are transmitted around the Earth's curvature. Heaviside's proposal, coupled with Planck's law of black body radiation, may have hampered the growth of radio astronomy for the detection of electromagnetic waves from celestial bodies until 1932 (and the development of high frequency radio transceivers). Also in 1902, Arthur Edwin Kennelly discovered some of the ionosphere's radio-electrical properties.

In 1912, the U.S. Congress imposed the Radio Act of 1912 on amateur radio operators, limiting their operations to frequencies above 1.5 MHz (wavelength 200 meters or smaller). The government thought those frequencies were useless. This led to the discovery of HF radio propagation via the ionosphere in 1923.

In 1926, Scottish physicist Robert Watson-Watt introduced the term ionosphere in a letter published only in 1969 in Nature:

We have in quite recent years seen the universal adoption of the term ‘stratosphere’..and..the companion term ‘troposphere’... The term ‘ionosphere’, for the region in which the main characteristic is large scale ionisation with considerable mean free paths, appears appropriate as an addition to this series.

Edward V. Appleton was awarded in 1947 a Nobel Prize for his confirmation of the existence of the ionosphere in 1927. Lloyd Berkner first measured the height and density of the ionosphere. This permitted the first complete theory of short wave radio propagation. Maurice V. Wilkes and J. A. Ratcliffe researched the topic of radio propagation of very long radio waves in the ionosphere. Vitaly Ginzburg has developed a theory of electromagnetic wave propagation in plasmas such as the ionosphere.

In 1962 the Canadian satellite Alouette 1 was launched to study the ionosphere. Following its success were Alouette 2 in 1965 and the two ISIS satellites in 1969 and 1971, all for measuring the ionosphere.

Exosphere:

The exosphere is the uppermost layer of the atmosphere. On Earth, its lower boundary at the edge of the thermosphere is estimated to be 500 km to 1000 km above the Earth's surface, and its upper boundary at about 10,000 km. It is only from the exosphere that atmospheric gases, atoms, and molecules can, to any appreciable extent, escape into space. The main gases within the exosphere are the lightest gases, mainly hydrogen, with some helium, carbon dioxide, and atomic oxygen near the exobase. The exosphere is the last layer before space.

The atmosphere in this layer is sufficiently rarefied for satellites to orbit the Earth, although they still receive some atmospheric drag.

Exobase, also called the critical level, the lowest altitude of the exosphere, is defined in one of two ways:

1) The height above which there are negligible atomic collisions between the particles and

2) The height above which the constituent atoms are on purely ballistic trajectories.

The detailed mechanism by which the polar ozone holes form is different from that for the mid-latitude thinning, but the most important process in both trends is catalytic

destruction of ozone by atomic chlorine and bromine.[1] The main source of these halogen atoms in the stratosphere is photodissociation of chlorofluorocarbon (CFC) compounds, commonly called freons, and of bromofluorocarbon compounds known as halons. These compounds are transported into the stratosphere after being emitted at the surface. Both ozone depletion mechanisms strengthened as emissions of CFCs and halons increased.

CFCs, halons and other contributory substances are commonly referred to as ozone-depleting substances (ODS). Since the ozone layer prevents most harmful UVB wavelengths (270–315 nm) of ultraviolet light (UV light) from passing through the Earth's atmosphere, observed and projected decreases in ozone have generated worldwide concern leading to adoption of the Montreal Protocol banning the production of CFCs and halons as well as related ozone depleting chemicals such as carbon tetrachloride and trichloroethane (also known as methyl chloroform).

It is suspected that a variety of biological consequences such as increases in skin cancer, damage to plants, and reduction of plankton populations in the ocean's photic zone may result from the increased UV exposure due to ozone depletion.

Ozone cycle overview

Three forms (or allotropes) of oxygen are involved in the ozone-oxygen cycle: Oxygen atoms (O or atomic oxygen), oxygen gas (O2 or diatomic oxygen), and ozone gas (O3 or triatomic oxygen). Ozone is formed in the stratosphere when oxygen molecules photodissociate after absorbing an ultraviolet photon whose wavelength is shorter than 240 nm. This produces two oxygen atoms.

The atomic oxygen then combines with O2 to create O3. Ozone molecules absorb UV light between 310 and 200 nm, following which ozone splits into a molecule of O2 and an oxygen atom. The oxygen atom then joins up with an oxygen molecule to regenerate ozone. This is a continuing process which terminates when an oxygen atom "recombines" with an ozone molecule to make two O2 molecules: O + O3 ? 2 O2

The overall amount of ozone in the stratosphere is determined by a balance between photochemical production and recombination.

Ozone can be destroyed by a number of free radical catalysts, the most important of which are the hydroxyl radical (OH·), the nitric oxide radical (NO·) and atomic chlorine (Cl·) and bromine (Br·). All of these have both natural and anthropogenic (manmade) sources; at the present time, most of the OH· and NO· in the stratosphere is of natural origin, but human activity has dramatically increased the chlorine and bromine.

These elements are found in certain stable organic compounds, especially chlorofluorocarbons (CFCs), which may find their way to the stratosphere without being destroyed in the troposphere due to their low reactivity. Once in the stratosphere, the Cl and Br atoms are liberated from the parent compounds by the action of ultraviolet light, e.g. ('h' is Planck's constant, '?' is frequency of electromagnetic radiation)

CFCl3 + h? ? CFCl2 + Cl

The Cl and Br atoms can then destroy ozone molecules through a variety of catalytic cycles. In the simplest example of such a cycle, a chlorine atom reacts with an ozone molecule, taking an oxygen atom with it (forming ClO) and leaving a normal oxygen molecule. A free oxygen atom then takes away the oxygen from the ClO, and the final result is an oxygen molecule and a chlorine atom, which then reinitiates the cycle. The chemical shorthand for these gas-phase reactions is:

Cl + O3 ? ClO + O2

ClO + O ? Cl + O2

The net reaction is: O3 + O ? 2 O2, the "recombination" reaction given above.

The overall effect is to increase the rate of recombination, leading to an overall decrease in the amount of ozone. For this particular mechanism to operate there must be a source of O atoms, which is primarily the photodissociation of O3; thus this mechanism is only important in the upper stratosphere where such atoms are abundant. More complicated mechanisms have been discovered that lead to ozone destruction in the lower stratosphere as well.

A single chlorine atom would keep on destroying ozone for up to two years (the time scale for transport back down to the troposphere) were it not for reactions that remove them from this cycle by forming reservoir species such as hydrogen chloride (HCl) and chlorine nitrate (ClONO2). On a per atom basis, bromine is even more efficient than chlorine at destroying ozone, but there is much less bromine in the atmosphere at present.

As a result, both chlorine and bromine contribute significantly to the overall ozone depletion. Laboratory studies have shown that fluorine and iodine atoms participate in analogous catalytic cycles. However, in the Earth's stratosphere, fluorine atoms react rapidly with water and methane to form strongly-bound HF, while organic molecules which contain iodine react so rapidly in the lower atmosphere that they do not reach the stratosphere in significant quantities.

Observations

The most pronounced decrease in ozone has been in the lower stratosphere. However, the ozone hole is most usually measured not in terms of ozone concentrations at these levels (which are typically of a few parts per million) but by reduction in the total column ozone, above a point on the Earth's surface, which is normally expressed in Dobson units, abbreviated as "DU". Marked decreases in column ozone in the Antarctic spring and early summer compared to the early 1970s and before have been observed using instruments such as the Total Ozone Mapping Spectrometer (TOMS).

Reductions of up to 70% in the ozone column observed in the austral (southern hemispheric) spring over Antarctica and first reported in 1985 (Farman et al 1985) are continuing. Through the 1990s, total column ozone in September and October have continued to be 40–50% lower than pre-ozone-hole values. In the Arctic the amount lost is more variable year-to-year than in the Antarctic. The greatest declines, up to 30%, are in the winter and spring, when the stratosphere is colder.

Reactions that take place on polar stratospheric clouds (PSCs) play an important role in enhancing ozone depletion. PSCs form more readily in the extreme cold of Antarctic stratosphere. This is why ozone holes first formed, and are deeper, over Antarctica. Early models failed to take PSCs into account and predicted a gradual global depletion, which is why the sudden Antarctic ozone hole was such a surprise to many scientists.

In middle latitudes it is preferable to speak of ozone depletion rather than holes. Declines are about 3% below pre-1980 values for 35–60°N and about 6% for 35–60°S. In the tropics, there are no significant trends.

Ozone depletion also explains much of the observed reduction in stratospheric and upper tropospheric temperatures. The source of the warmth of the stratosphere is the absorption of UV radiation by ozone, hence reduced ozone leads to cooling. Some stratospheric cooling is also predicted from increases in greenhouse gases such as CO2; however the ozone-induced cooling appears to be dominant.

Predictions of ozone levels remain difficult. The World Meteorological Organization Global Ozone Research and Monitoring Project - Report No. 44 comes out strongly in favor for the Montreal Protocol, but notes that a UNEP 1994 Assessment overestimated ozone loss for the 1994–1997 period.

Chemicals in the atmosphere

CFCs in the atmosphere

Chlorofluorocarbons (CFCs) were invented by Thomas Midgley in the 1920s. They were used in air conditioning/cooling units, as aerosol spray propellants prior to the 1980s, and in the cleaning processes of delicate electronic equipment. They also occur as by-products of some chemical processes. No significant natural sources have ever been identified for these compounds — their presence in the atmosphere is due almost entirely to human manufacture.

As mentioned in the ozone cycle overview above, when such ozone-depleting chemicals reach the stratosphere, they are dissociated by ultraviolet light to release chlorine atoms. The chlorine atoms act as a catalyst, and each can break down tens of thousands of ozone molecules before being removed from the stratosphere.

Given the longevity of CFC molecules, recovery times are measured in decades. It is calculated that a CFC molecule takes an average of 15 years to go from the ground level up to the upper atmosphere, and it can stay there for about a century, destroying up to one hundred thousand ozone molecules during that time.

Verification of observations

Scientists have been increasingly able to attribute the observed ozone depletion to the increase of anthropogenic halogen compounds from CFCs by the use of complex chemistry transport models and their validation against observational data (e.g. SLIMCAT, CLaMS).

These models work by combining satellite measurements of chemical concentrations and meteorological fields with chemical reaction rate constants obtained in lab experiments. They are able to identify not only the key chemical reactions but also the transport processes which bring CFC photolysis products into contact with ozone.

The ozone hole and its causes

The Antarctic ozone hole is an area of the Antarctic stratosphere in which the recent ozone levels have dropped to as low as 33% of their pre-1975 values. The ozone hole occurs during the Antarctic spring, from September to early December, as strong westerly winds start to circulate around the continent and create an atmospheric container. Within this "polar vortex", over 50% of the lower stratospheric ozone is destroyed during the Antarctic spring.

As explained above, the overall cause of ozone depletion is the presence of chlorine-containing source gases (primarily CFCs and related halocarbons). In the presence of UV light, these gases dissociate, releasing chlorine atoms, which then go on to catalyze ozone destruction. The Cl-catalyzed ozone depletion can take place in the gas phase, but it is dramatically enhanced in the presence of polar stratospheric clouds (PSCs).

These polar stratospheric clouds form during winter, in the extreme cold. Polar winters are dark, consisting of 3 months without solar radiation (sunlight). Not only lack of sunlight contributes to a decrease in temperature but also the “polar vortex” traps and chills air. Temperatures hover around or below -80 °C.

These low temperatures form cloud particles and are composed of either nitric acid (Type I PSC) or ice (Type II PSC). Both types provide surfaces for chemical reactions that lead to ozone destruction.

The photochemical processes involved are complex but well understood. The key observation is that, ordinarily, most of the chlorine in the stratosphere resides in stable "reservoir" compounds, primarily hydrogen chloride (HCl) and chlorine nitrate (ClONO2).

During the Antarctic winter and spring, however, reactions on the surface of the polar stratospheric cloud particles convert these "reservoir" compounds into reactive free radicals (Cl and ClO). The clouds can also remove NO2 from the atmosphere by converting it to nitric acid, which prevents the newly formed ClO from being converted back into ClONO2.

The role of sunlight in ozone depletion is the reason why the Antarctic ozone depletion is greatest during spring. During winter, even though PSCs are at their most abundant, there is no light over the pole to drive the chemical reactions. During the spring, however, the sun comes out, providing energy to drive photochemical reactions, and melt the polar stratospheric clouds, releasing the trapped compounds.

Most of the ozone that is destroyed is in the lower stratosphere, in contrast to the much smaller ozone depletion through homogeneous gas phase reactions, which occurs primarily in the upper stratosphere.

Warming temperatures near the end of spring break up the vortex around mid-December. As warm, ozone-rich air flows in from lower latitudes, the PSCs are destroyed, the ozone depletion process shuts down, and the ozone hole heals.

Interest in ozone depletion

While the effect of the Antarctic ozone hole in decreasing the global ozone is relatively small, estimated at about 4% per decade, the hole has generated a great deal of interest because:

The decrease in the ozone layer was predicted in the early 1980s to be roughly 7% over a sixty-year period.

The sudden recognition in 1985 that there was a substantial "hole" was widely reported in the press. The especially rapid ozone depletion in Antarctica had previously been dismissed as measurement error.

Many were worried that ozone holes might start to appear over other areas of the globe but to date the only other large-scale depletion is a smaller ozone "dimple" observed during the Arctic spring over the North Pole. Ozone at middle latitudes has declined, but by a much smaller extent (about 4–5% decrease).

If the conditions became more severe (cooler stratospheric temperatures, more stratospheric clouds, more active chlorine), then global ozone may decrease at a much greater pace. Standard global warming theory predicts that the stratosphere will cool.

When the Antarctic ozone hole breaks up, the ozone-depleted air drifts out into nearby areas. Decreases in the ozone level of up to 10% have been reported in New Zealand in the month following the break-up of the Antarctic ozone hole

Consequences of ozone depletion

Since the ozone layer absorbs UVB ultraviolet light from the Sun, ozone layer depletion is expected to increase surface UVB levels, which could lead to damage, including increases in skin cancer.

This was the reason for the Montreal Protocol. Although decreases in stratospheric ozone are well-tied to CFCs and there are good theoretical reasons to believe that decreases in ozone will lead to increases in surface UVB, there is no direct observational evidence linking ozone depletion to higher incidence of skin cancer in human beings.

This is partly due to the fact that UVA, which has also been implicated in some forms of skin cancer, is not absorbed by ozone, and it is nearly impossible to control statistics for lifestyle changes in the populace.

Increased UV

Ozone, while a minority constituent in the earth's atmosphere, is responsible for most of the absorption of UVB radiation. The amount of UVB radiation that penetrates through the ozone layer decreases exponentially with the slant-path thickness/density of the layer. Correspondingly, a decrease in atmospheric ozone is expected to give rise to significantly increased levels of UVB near the surface.

Increases in surface UVB due to the ozone hole can be partially inferred by radiative transfer model calculations, but cannot be calculated from direct measurements because of the lack of reliable historical (pre-ozone-hole) surface UV data, although more recent surface UV observation measurement programmes exist (e.g. at Lauder, New Zealand).

Because it is this same UV radiation that creates ozone in the ozone layer from O2 (regular oxygen) in the first place, a reduction in stratospheric ozone would actually tend to increase photochemical production of ozone at lower levels (in the troposphere), although the overall observed trends in total column ozone still show a decrease, largely because ozone produced lower down has a naturally shorter photochemical lifetime, so it is destroyed before the concentrations could reach a level which would compensate for the ozone reduction higher up.

Biological effects of increased UV

The main public concern regarding the ozone hole has been the effects of surface UV on human health. So far, ozone depletion in most locations has been typically a few percent. Were the high levels of depletion seen in the ozone hole ever to be common across the globe, the effects could be substantially more dramatic.

As the ozone hole over Antarctica has in some instances grown so large as to reach southern parts of Australia and New Zealand, environmentalists have been concerned that the increase in surface UV could be significant.

Effects on Humans

UVB (the higher energy UV radiation absorbed by ozone) is generally accepted to be a contributory factor to skin cancer. In addition, increased surface UV leads to increased tropospheric ozone, which is a health risk to humans. The increased surface UV also represents an increase in the vitamin D synthetic capacity of the sunlight.

The cancer preventive effects of vitamin D represent a possible beneficial effect of ozone depletion. In terms of health costs, the possible benefits of increased UV irradiance may outweigh the burden.

1. Basal and Squamous Cell Carcinomas -- The most common forms of skin cancer in humans, basal and squamous cell carcinomas, have been strongly linked to UVB exposure. The mechanism by which UVB induces these cancers is well understood — absorption of UVB radiation causes the pyrimidine bases in the DNA molecule to form dimers, resulting in transcription errors when the DNA replicates.

These cancers are relatively mild and rarely fatal, although the treatment of squamous cell carcinoma sometimes requires extensive reconstructive surgery. By combining epidemiological data with results of animal studies, scientists have estimated that a one percent decrease in stratospheric ozone would increase the incidence of these cancers by 2%.

2. Malignant Melanoma -- Another form of skin cancer, malignant melanoma, is much less common but far more dangerous, being lethal in about 15% - 20% of the cases diagnosed. The relationship between malignant melanoma and ultraviolet exposure is not yet well understood, but it appears that both UVB and UVA are involved.

Experiments on fish suggest that 90 to 95% of malignant melanomas may be due to UVA and visible radiation whereas experiments on opossums suggest a larger role for UVB.

Because of this uncertainty, it is difficult to estimate the impact of ozone depletion on melanoma incidence. One study showed that a 10% increase in UVB radiation was associated with a 19% increase in melanomas for men and 16% for women.

A study of people in Punta Arenas, at the southern tip of Chile, showed a 56% increase in melanoma and a 46% increase in nonmelanoma skin cancer over a period of seven years, along with decreased ozone and increased UVB levels.

3. Increased Tropospheric Ozone -- Increased surface UV leads to increased tropospheric ozone. Ground-level ozone is generally recognized to be a health risk, as ozone is toxic due to its strong oxidant properties. At this time, ozone at ground level is produced mainly by the action of UV radiation on combustion gases from vehicle exhausts.

Effects on Crops

An increase of UV radiation would be expected to affect crops. A number of economically important species of plants, such as rice, depend on cyanobacteria residing on their roots for the retention of nitrogen. Cyanobacteria are sensitive to UV light and they would be affected by its increase.

Effects on Plankton

Research has shown a widespread extinction of plankton 2 million years ago that coincided with a nearby supernova. Researchers speculate that the extinction was caused by a significant weakening of the ozone layer at that time when the radiation from the supernova produced nitrogen oxides that catalyzed the destruction of ozone (plankton are particularly susceptible to effects of UV light, and are vitally important to marine food webs).

Public policy in response to the ozone hole

The full extent of the damage that CFCs have caused to the ozone layer is not known and will not be known for decades; however, marked decreases in column ozone have already been observed (as explained above).

After a 1976 report by the U.S. National Academy of Sciences concluded that credible scientific evidence supported the ozone depletion hypothesis, a few countries, including the United States, Canada, Sweden, and Norway, moved to eliminate the use of CFCs in aerosol spray cans.

At the time this was widely regarded as a first step towards a more comprehensive regulation policy, but progress in this direction slowed in subsequent years, due to a combination of political factors (continued resistance from the halocarbon industry and a general change in attitude towards environmental regulation during the first two years of the Reagan administration) and scientific developments (subsequent National Academy assessments which indicated that the first estimates of the magnitude of ozone depletion had been overly large).

The European Community rejected proposals to ban CFCs in aerosol sprays while even in the U.S., CFCs continued to be used as refrigerants and for cleaning circuit boards. Worldwide CFC production fell sharply after the U.S. aerosol ban, but by 1986 had returned nearly to its 1976 level. In 1980, DuPont closed down its research program into halocarbon alternatives.

The US Government's attitude began to change again in 1983, when William Ruckelshaus replaced Anne M. Burford as Administrator of the US Environmental Protection Agency. Under Ruckelshaus and his successor, Lee Thomas, the EPA pushed for an international approach to halocarbon regulations.

In 1985 20 nations, including most of the major CFC producers, signed the Vienna Convention which established a framework for negotiating international regulations on ozone-depleting substances. That same year, the discovery of the Antarctic ozone hole was announced, causing a revival in public attention to the issue. In 1987, representatives from 43 nations signed the Montreal Protocol.

Meanwhile, the halocarbon industry shifted its position and started supporting a protocol to limit CFC production. The reasons for this were in part explained by "Dr. Mostafa Tolba, former head of the UN Environment Programme, who was quoted in the June 30, 1990 edition of The New Scientist, '...the chemical industry supported the Montreal Protocol in 1987 because it set up a worldwide schedule for phasing out CFCs, which [were] no longer protected by patents. This provided companies with an equal opportunity to market new, more profitable compounds.'"

At Montreal, the participants agreed to freeze production of CFCs at 1986 levels and to reduce production by 50% by 1999. After a series of scientific expeditions to the Antarctic produced convincing evidence that the ozone hole was indeed caused by chlorine and bromine from manmade organohalogens, the Montreal Protocol was strengthened at a 1990 meeting in London.

The participants agreed to phase out CFCs and halons entirely (aside from a very small amount marked for certain "essential" uses, such as asthma inhalers) by 2000. At a 1992 meeting in Copenhagen, the phase out date was moved up to 1996.

To some extent, CFCs have been replaced by the less damaging hydro-chloro-fluoro-carbons (HCFCs), although concerns remain regarding HCFCs also. In some applications, hydro-fluoro-carbons (HFCs) have been used to replace CFCs. HFCs, which contain no chlorine or bromine, do not contribute at all to ozone depletion although they are potent greenhouse gases.

The best known of these compounds is probably HFC-134a (R-134a), which in the United States has largely replaced CFC-12 (R-12) in automobile air conditioners.

Ozone Diplomacy, by Richard Benedick (Harvard University Press, 1991) gives a detailed account of the negotiation process that led to the Montreal Protocol. Pielke and Betsill provide an extensive review of early US government responses to the emerging science of ozone depletion by CFCs.

Current events and future prospects of ozone depletion

Since the adoption and strengthening of the Montreal Protocol has led to reductions in the emissions of CFCs, atmospheric concentrations of the most significant compounds have been declining.

These substances are being gradually removed from the atmosphere. By 2015, the Antarctic ozone hole would have reduced by only 1 million km² out of 25 (Newman et al., 2004); complete recovery of the Antarctic ozone layer will not occur until the year 2050 or later. Work has suggested that a detectable (and statistically significant) recovery will not occur until around 2024, with ozone levels recovering to 1980 levels by around 2068.

There is a slight caveat to this, however. Global warming from CO2 is expected to cool the stratosphere. This, in turn, would lead to a relative increase in ozone depletion and the frequency of ozone holes.

The effect may not be linear; ozone holes form because of polar stratospheric clouds; the formation of polar stratospheric clouds has a temperature threshold above which they will not form; cooling of the Arctic stratosphere might lead to Antarctic-ozone-hole-like conditions. But at the moment this is not clear.

Even though the stratosphere as a whole is cooling, high-latitude areas may become increasingly predisposed to springtime stratospheric warming events as weather patterns change in response to higher greenhouse gas loading. This would cause PSCs to disappear earlier in the season, and may explain why Antarctic ozone hole seasons have tended to end somewhat earlier since 2000 as compared with the most prolonged ozone holes of the 1990s.

The decrease in ozone-depleting chemicals has also been significantly affected by a decrease in bromine-containing chemicals. The data suggest that substantial natural sources exist for atmospheric methyl bromide (CH3Br).

The 2004 ozone hole ended in November 2004, daily minimum stratospheric temperatures in the Antarctic lower stratosphere increased to levels that are too warm for the formation of polar stratospheric clouds (PSCs) about 2 to 3 weeks earlier than in most recent years.

The Arctic winter of 2005 was extremely cold in the stratosphere; PSCs were abundant over many high-latitude areas until dissipated by a big warming event, which started in the upper stratosphere during February and spread throughout the Arctic stratosphere in March. The size of the Arctic area of anomalously low total ozone in 2004-2005 was larger than in any year since 1997.

The predominance of anomalously low total ozone values in the Arctic region in the winter of 2004-2005 is attributed to the very low stratospheric temperatures and meteorological conditions favorable for ozone destruction along with the continued presence of ozone destroying chemicals in the stratosphere.

A 2005 IPCC summary of ozone issues observed that observations and model calculations suggest that the global average amount of ozone depletion has now approximately stabilized.

Although considerable variability in ozone is expected from year to year, including in polar regions where depletion is largest, the ozone layer is expected to begin to recover in coming decades due to declining ozone-depleting substance concentrations, assuming full compliance with the Montreal Protocol.

Temperatures during the Arctic winter of 2006 stayed fairly close to the long-term average until late January, with minimum readings frequently cold enough to produce PSCs. During the last week of January, however, a major warming event sent temperatures well above normal — much too warm to support PSCs. By the time temperatures dropped back to near normal in March, the seasonal norm was well above the PSC threshold.

Preliminary satellite instrument-generated ozone maps show seasonal ozone buildup slightly below the long-term means for the Northern Hemisphere as a whole, although some high ozone events have occurred.

During March 2006, the Arctic stratosphere poleward of 60 degrees North Latitude was free of anomalously low ozone areas except during the three-day period from March 17 to 19 when the total ozone cover fell below 300 DU over part of the North Atlantic region from Greenland to Scandinavia.

The area where total column ozone is less than 220 DU (the accepted definition of the boundary of the ozone hole) was relatively small until around 20 August 2006. Since then the ozone hole area increased rapidly, peaking at 29 million km² September 24.

In October 2006, NASA reported that the year's ozone hole set a new area record with a daily average of 26 million km² between 7 September and 13 October 2006; total ozone thicknesses fell as low as 85 DU on October 8.

The two factors combined, 2006 sees the worst level of depletion in recorded ozone history. The depletion is attributed to the temperatures above the Antarctic reaching the lowest recording since comprehensive records began in 1979.

The Antarctic ozone hole is expected to continue for decades. Ozone concentrations in the lower stratosphere over Antarctica will increase by 5%–10% by 2020 and return to pre-1980 levels by about 2060–2075, 10–25 years later than predicted in earlier assessments.

This is because of revised estimates of atmospheric concentrations of Ozone Depleting Substances — and a larger predicted future usage in developing countries. Another factor which may aggravate ozone depletion is the draw-down of nitrogen oxides from above the stratosphere due to changing wind patterns.

History of the research

The basic physical and chemical processes that lead to the formation of an ozone layer in the earth's stratosphere were discovered by Sydney Chapman in 1930. These are discussed in the article Ozone-oxygen cycle — briefly, short-wavelength UV radiation splits an oxygen (O2) molecule into two oxygen (O) atoms, which then combine with other oxygen molecules to form ozone.

Ozone is removed when an oxygen atom and an ozone molecule "recombine" to form two oxygen molecules, i.e. O + O3 ? 2O2. In the 1950s, David Bates and Marcel Nicolet presented evidence that various free radicals, in particular hydroxyl (OH) and nitric oxide (NO), could catalyze this recombination reaction, reducing the overall amount of ozone.

These free radicals were known to be present in the stratosphere, and so were regarded as part of the natural balance – it was estimated that in their absence, the ozone layer would be about twice as thick as it currently is.

In 1970 Prof. Paul Crutzen pointed out that emissions of nitrous oxide (N2O), a stable, long-lived gas produced by soil bacteria, from the earth's surface could affect the amount of nitric oxide (NO) in the stratosphere.

Crutzen showed that nitrous oxide lives long enough to reach the stratosphere, where it is converted into NO. Crutzen then noted that increasing use of fertilizers might have led to an increase in nitrous oxide emissions over the natural background, which would in turn result in an increase in the amount of NO in the stratosphere.

Thus human activity could have an impact on the stratospheric ozone layer. In the following year, Crutzen and (independently) Harold Johnston suggested that NO emissions from supersonic aircraft, which fly in the lower stratosphere, could also deplete the ozone layer.

The Rowland-Molina hypothesis

In 1974 Frank Sherwood Rowland, a Chemistry Professor at the University of California at Irvine, and his postdoctoral associate Mario J. Molina suggested that long-lived organic halogen compounds, such as CFCs, might behave in a similar fashion as Crutzen had proposed for nitrous oxide.

James Lovelock (most popularly known as the creator of the Gaia hypothesis) had recently discovered, during a cruise in the South Atlantic in 1971, that almost all of the CFC compounds manufactured since their invention in 1930 were still present in the atmosphere.

Molina and Rowland concluded that, like N2O, the CFCs would reach the stratosphere where they would be dissociated by UV light, releasing Cl atoms. (A year earlier, Richard Stolarski and Ralph Cicerone at the University of Michigan had shown that Cl is even more efficient than NO at catalyzing the destruction of ozone. Similar conclusions were reached by Michael McElroy and Steven Wofsy at Harvard University. Neither group, however, had realized that CFC's were a potentially large source of stratospheric chlorine — instead, they had been investigating the possible effects of HCl emissions from the Space Shuttle, which are very much smaller.)

The Rowland-Molina hypothesis was strongly disputed by representatives of the aerosol and halocarbon industries. The Chair of the Board of DuPont was quoted as saying that ozone depletion theory is "a science fiction tale...a load of rubbish...utter nonsense". Robert Abplanalp, the President of Precision Valve Corporation (and inventor of the first practical aerosol spray can valve), wrote to the Chancellor of UC Irvine to complain about Rowland's public statements (Roan, p 56.)

Nevertheless, within three years most of the basic assumptions made by Rowland and Molina were confirmed by laboratory measurements and by direct observation in the stratosphere.

The concentrations of the source gases (CFC's and related compounds) and the chlorine reservoir species (HCl and ClONO2) were measured throughout the stratosphere, and demonstrated that CFCs were indeed the major source of stratospheric chlorine, and that nearly all of the CFCs emitted would eventually reach the stratosphere.

Even more convincing was the measurement, by James G. Anderson and collaborators, of chlorine monoxide (ClO) in the stratosphere. ClO is produced by the reaction of Cl with ozone — its observation thus demonstrated that Cl radicals not only were present in the stratosphere but also were actually involved in destroying ozone.

McElroy and Wofsy extended the work of Rowland and Molina by showing that Bromine atoms were even more effective catalysts for ozone loss than chlorine atoms and argued that the brominated organic compounds known as halons, widely used in fire extinguishers, were a potentially large source of stratospheric bromine.

In 1976 the U.S. National Academy of Sciences released a report which concluded that the ozone depletion hypothesis was strongly supported by the scientific evidence. Scientists calculated that if CFC production continued to increase at the going rate of 10% per year until 1990 and then remain steady, CFCs would cause a global ozone loss of 5 to 7% by 1995, and a 30 to 50% loss by 2050.

In response the United States, Canada, Sweden and Norway banned the use of CFCs in aerosol spray cans in 1978. However, subsequent research, summarized by the National Academy in reports issued between 1979 and 1984, appeared to show that the earlier estimates of global ozone loss had been too large.

The Ozone Hole

The discovery of the Antarctic "ozone hole" by British Antarctic Survey scientists Farman, Gardiner and Shanklin (announced in a paper in Nature in May 1985) came as a shock to the scientific community, because the observed decline in polar ozone was far larger than anyone had anticipated.

Satellite measurements showing massive depletion of ozone around the south pole were becoming available at the same time. However, these were initially rejected as unreasonable by data quality control algorithms (they were filtered out as errors since the values were unexpectedly low); the ozone hole was detected only in satellite data when the raw data was reprocessed following evidence of ozone depletion in in situ observations.

Susan Solomon, an atmospheric chemist at the National Oceanic and Atmospheric Administration (NOAA), proposed that chemical reactions on polar stratospheric clouds (PSCs) in the cold Antarctic stratosphere caused a massive, though localized and seasonal, increase in the amount of chlorine present in active, ozone-destroying forms.

This hypothesis was decisively confirmed, first by laboratory measurements and subsequently by direct measurements, from the ground and from high-altitude airplanes, of very high concentrations of chlorine monoxide (ClO) in the Antarctic stratosphere.

Alternative hypotheses, which had attributed the ozone hole to variations in solar UV radiation or to changes in atmospheric circulation patterns, were also tested and shown to be untenable. Meanwhile, analysis of ozone measurements from the worldwide network of ground-based Dobson spectrophotometers led an international panel to conclude that the ozone layer was in fact being depleted, at all latitudes outside of the tropics. These trends were confirmed by satellite measurements.

As a consequence, the major halocarbon producing nations agreed to phase out production of CFCs, halons, and related compounds, a process that was completed in 1996. Crutzen, Molina, and Rowland were awarded the 1995 Nobel Prize in Chemistry for their work on stratospheric ozone.

Since 1981 the United Nations Environment Programme has sponsored a series of reports on scientific assessment of ozone depletion. The most recent is from 2006.

Controversy regarding ozone science and policy

That ozone depletion takes place is not seriously disputed in the scientific community. There is a consensus among atmospheric physicists and chemists that the scientific understanding has now reached a level where countermeasures to control CFC emissions are justified, although the decision is ultimately one for policy-makers.

Despite this consensus, the science behind ozone depletion remains complex, and some who oppose the enforcement of countermeasures point to some of the uncertainties. For example, although increased UVB has been shown to constitute a melanoma risk, it has been difficult for statistical studies to establish a direct link between ozone depletion and increased rates of melanoma.

Although melanomas did increase significantly during the period 1970–1990, it is difficult to separate reliably the effect of ozone depletion from the effect of changes in lifestyle factors (e.g., increasing rates of air travel).

Ozone depletion and global warming

Although they are often interlinked in the mass media, the connection between global warming and ozone depletion is not strong. There are four areas of linkage:

The same CO2 radiative forcing that produces near-surface global warming is expected (perhaps surprisingly) to cool the stratosphere. This cooling, in turn, is expected to produce a relative increase in ozone (O3) depletion and the frequency of ozone holes.

Conversely, ozone depletion represents a radiative forcing of the climate system. There are two opposing effects: Reduced ozone causes the stratosphere to absorb less solar radiation, thus cooling the stratosphere while warming the troposphere; the resulting colder stratosphere emits less long-wave radiation downward, thus cooling the troposphere.

Overall, the cooling dominates; the IPCC concludes that "observed stratospheric O3 losses over the past two decades have caused a negative forcing of the surface-troposphere system"[32] of about -0.15 ± 0.10 watts per square meter (W/m2).

One of the strongest predictions of the greenhouse effect theory is that the stratosphere will cool. Although this cooling has been observed, it is not trivial to separate the effects of changes in the concentration of greenhouse gases and ozone depletion since both will lead to cooling.

However, this can be done by numerical stratospheric modeling. Results from the National Oceanic and Atmospheric Administration's Geophysical Fluid Dynamics Laboratory show that above 20 km (12.4 miles), the greenhouse gases dominate the cooling.

Ozone depleting chemicals are also greenhouse gases. The increases in concentrations of these chemicals have produced 0.34 ± 0.03 W/m2 of radiative forcing, corresponding to about 14% of the total radiative forcing from increases in the concentrations of well-mixed greenhouse gases.

Misconceptions about ozone depletion

A few of the more common misunderstandings about ozone depletion are addressed briefly here; more detailed discussions can be found in the ozone-depletion FAQ.

CFCs are "too heavy" to reach the stratosphere

It is sometimes stated that since CFC molecules are much heavier than nitrogen or oxygen, they cannot reach the stratosphere in significant quantities. But atmospheric gases are not sorted by weight; the forces of wind (turbulence) are strong enough to fully intermix gases in the atmosphere.

CFCs are heavier than air, but just like argon, krypton and other heavy gases with a long lifetime, they are uniformly distributed throughout the turbosphere and reach the upper atmosphere.

Man-made chlorine is insignificant compared to natural sources

Another objection occasionally voiced is that It is generally agreed that natural sources of tropospheric chlorine (volcanoes, ocean spray, etc.) are four to five orders of magnitude larger than man-made sources. While strictly true, tropospheric chlorine is irrelevant; it is stratospheric chlorine that matters to ozone depletion. Chlorine from ocean spray is soluble and thus is washed out by rainfall before it reaches the stratosphere.

CFCs, in contrast, are insoluble and long-lived, which allows them to reach the stratosphere. Even in the lower atmosphere there is more chlorine present in the form of CFCs and related haloalkanes than there is in HCl from salt spray, and in the stratosphere the halocarbons dominate overwhelmingly.

Only one of these halocarbons, methyl chloride, has a predominantly natural source, and it is responsible for about 20 percent of the chlorine in the stratosphere; the remaining 80 % comes from manmade compounds.

Very large volcanic eruptions can inject HCl directly into the stratosphere, but direct measurements have shown that their contribution is small compared to that of chlorine from CFCs. A similar erroneous assertion is that soluble halogen compounds from the volcanic plume of Mount Erebus on Ross Island, Antarctica are a major contributor to the Antarctic ozone hole.

An ozone hole was first observed in 1956

G.M.B. Dobson (Exploring the Atmosphere, 2nd Edition, Oxford, 1968) mentioned that when springtime ozone levels over Halley Bay were first measured, he was surprised to find that they were ~320 DU, about 150 DU below spring levels, ~450 DU, in the Arctic. These, however, were the pre-ozone hole normal climatological values. What Dobson describes is essentially the baseline from which the ozone hole is measured: actual ozone hole values are in the 150–100 DU range.

The discrepancy between the Arctic and Antarctic noted by Dobson was primarily a matter of timing: during the Arctic spring ozone levels rose smoothly, peaking in April, whereas in the Antarctic they stayed approximately constant during early spring, rising abruptly in November when the polar vortex broke down.

The behavior seen in the Antarctic ozone hole is completely different. Instead of staying constant, early springtime ozone levels suddenly drop from their already low winter values, by as much as 50%, and normal values are not reached again until December.

If the theory were correct, the ozone hole should be above the sources of

CFCs

CFCs are well mixed in the troposphere and the stratosphere. The reason the ozone hole occurs above Antarctica is not because there are more CFCs there but because the low temperatures allow polar stratospheric clouds to form. There have been anomalous discoveries of significant, serious, localized "holes" above other parts of the globe.

The "ozone hole" is a hole in the ozone layer.

When the "ozone hole" forms, essentially all of the ozone in the lower stratosphere is destroyed. The upper stratosphere is much less affected, however, so that the overall amount of ozone over the continent declines by 50 percent or more. The ozone hole does not go all the way through the layer; on the other hand, it is not a uniform 'thinning' of the layer either. It's a "hole" in the sense of "a hole in the ground", a depression, not in the sense of "a hole in the windshield."

World Ozone Day

In 1994, the United Nations General Assembly voted to designate September 16 as "World Ozone Day", to commemorate the signing of the Montreal Protocol on that date in 1987.

The haloalkanes (also known as halogenoalkanes or alkyl halides) are a group of chemical compounds, consisting of alkanes, such as methane or ethane, with one or more halogens linked, such as chlorine or fluorine, making them a type of organic halide. They are known under many chemical and commercial names. As fire extinguishants,

propellants and solvents they have or had wide use. Some haloalkanes (those containing chlorine or bromine) have negative effects on the environment such as ozone depletion. The most widely known family within this group are the chlorofluorocarbons (CFCs).

General

A haloalkane also known as alkyl halogenide, halogenalkane or halogenoalkane, and alkyl halide is a chemical compound derived from an alkane by substituting one or more hydrogen atoms with halogen atoms. Substitution with fluorine, chlorine, bromine and iodine results in fluoroalkanes, chloroalkanes, bromoalkanes and iodoalkanes, respectively. Mixed compounds are also possible, the best-known examples being the chlorofluorocarbons (CFCs) which are mainly responsible for ozone depletion. Haloalkanes are used in semiconductor device fabrication, as refrigerants, foam blowing agents, solvents, aerosol spray propellants, fire extinguishing agents, and chemical reagents.

Freon is a trade name for a group of chlorofluorocarbons used primarily as a refrigerant. The word Freon is a registered trademark belonging to DuPont.

There are 3 types of haloalkane. In primary (1°) haloalkanes the carbon which carries the halogen atom is only attached to one other alkyl group. However CH3Br is also a primary haloalkane, even though there is no alkyl group. In secondary (2°) haloalkanes the carbon that carries the halogen atom is attached to 2 alkyl groups. In tertiary (3°) haloalkanes the carbon that carries the halogen atom is attached to 3 alkyl groups.

Chlorofluoro compounds (CFC, HCFC)

Chlorofluorocarbons (CFC) are haloalkanes with both chlorine and fluorine. They were formerly used widely in industry, for example as refrigerants, propellants, and cleaning solvents. Their use has been regularly prohibited by the Montreal Protocol, because of effects on the ozone layer (see ozone depletion). They also contribute to global warming. They have a global warming potential (GWP), in terms of carbon dioxide equivalence (over a time period of one hundred years) between 6000 and 9800 per kg.

Hydrochlorofluorocarbons (HCFCs) are of a class of haloalkanes where not all hydrogen has been replaced by chlorine or fluorine. They are used primarily as chlorofluorocarbon (CFC) substitutes, as the ozone depleting effects are only about 10% of the CFCs.

Hydrofluoro compounds (HFC)

Hydrofluorocarbons (HFCs), contain no chlorine. They are composed entirely of carbon, hydrogen, and fluorine. They have an even lower global warming potential than HCFCs, and no known effects at all on the ozone layer. Only compounds containing chlorine and bromine are thought to harm the ozone layer. Fluorine itself is not ozone-toxic.

Polymer haloalkanes

Chlorinated or fluorinated alkenes can be used for polymerization, resulting in polymer haloalkanes with notable chemical resistance properties. Important examples include polychloroethene (polyvinyl chloride, PVC), and polytetrafluoroethylene (PTFE, Teflon), but many more halogenated polymers exist.

History

Original development

Carbon tetrachloride was used in fire extinguishers and glass "anti-fire grenades" from the late nineteenth century until around the end of World War II. Experimentation with chloroalkanes for fire suppression on military aircraft began at least as early as the 1920s.

American engineer Thomas Midgley developed chlorofluorocarbons (CFC) in 1928 as a replacement for ammonia (NH3), chloromethane (CH3Cl), and sulfur dioxide (SO2), which are toxic but were in common use at the time as refrigerants. The new compound developed had to have a low boiling point and be non-toxic and generally non-reactive. In a demonstration for the American Chemical Society, Midgley flamboyantly demonstrated all these properties by inhaling a breath of the gas and using it to blow out a candle.

Midgley specifically developed CCl2F2. However, one of the attractive features is that there exists a whole family of the compounds, each having a unique boiling point which can suit different applications. In addition to their original application as refrigerants, chlorofluoroalkanes have been used as propellants in aerosol cans, cleaning solvents for circuit boards, and blowing agents for making expanded plastics (such as the expanded polystyrene used in packaging materials and disposable coffee cups).

Development of alternatives

During World War II, various early chloroalkanes were in standard use in military aircraft by some combatants, but these early halons suffered from excessive toxicity. Nevertheless, after the war they slowly became more common in civil aviation as well.

In the 1960s, fluoroalkanes and bromofluoroalkanes became available and were quickly recognized as being among the most effective fire-fighting materials discovered. Much early research with Halon 1301 was conducted under the auspices of the US Armed Forces, while Halon 1211 was, initially, mainly developed in the UK. By the late 1960s they were standard in many applications where water and dry-powder extinguishers posed a threat of damage to the protected property, including computer rooms, telecommunications switches, laboratories, museums and art collections. Beginning with warships, in the 1970s, bromofluoroalkanes also progressively came to be associated with rapid knockdown of severe fires in confined spaces with minimal risk to personnel.

Work on alternatives for chlorofluorocarbons in refrigerants began in the late 1970s after the first warnings of damage to stratospheric ozone were published in the journal Nature in 1974 by Molina and Rowland (who shared the 1995 Nobel Prize for Chemistry for their work). Adding hydrogen and thus creating hydrochlorofluorocarbons (HCFC), chemists made the compounds less stable in the lower atmosphere, enabling them to break down before reaching the ozone layer. Later alternatives dispense with the chlorine, creating hydrofluorocarbons (HFC) with even shorter lifetimes in the lower atmosphere.

By the early 1980s, bromofluoroalkanes were in common use on aircraft, ships and large vehicles as well as in computer facilities and galleries. However, concern was beginning to be felt about the impact of chloroalkanes and bromoalkanes on the ozone layer. The Vienna Convention on Ozone Layer Protection did not cover bromofluoroalkanes as it was thought, at the time, that emergency discharge of extinguishing systems was too small in volume to produce a significant impact, and too important to human safety for restriction.

However, by the time of the Montreal Protocol it was realised that deliberate and accidental discharges during system tests and maintenance accounted for substantially larger volumes than emergency discharges, and consequently halons were brought into the treaty, albeit with many exceptions.

Phase out

Use of certain chloroalkanes as solvents for large scale application, such as dry cleaning, have been phased out, for example, by the IPPC directive on greenhouse gases in 1994 and by the Volatile Organic Compounds (VOC) directive of the EU in 1997. Permitted chlorofluoroalkane uses are medicinal only.

Finally, bromofluoroalkanes have been largely phased out and the possession of such equipment is prohibited in some countries like the Netherlands and Belgium, from 1 January 2004, based on the Montreal Protocol and guidelines of the European Union.

Production of new stocks ceased in most (probably all) countries as of 1994. However many countries still require aircraft to be fitted with halon fire suppression systems because no safe and completely satisfactory alternative has been discovered for this application. There are also a few other, highly specialised, uses. These programs recycle halon through "halon banks" coordinated by the Halon Recycling Corporation to ensure that discharge to the atmosphere occurs only in a genuine emergency and to conserve remaining stocks.

Nomenclature

IUPAC nomenclature

The formal naming of haloalkanes should follow IUPAC nomenclature, which put the halogen as a prefix to the alkane. For example, ethane with bromine becomes bromoethane, methane with four chlorine groups becomes tetrachloromethane. However, many of these compounds have already an established trivial name, which is endorsed by the IUPAC nomenclature, for example chloroform (trichloromethane) and methylene chloride (dichloromethane). For unambiguity, this article follows the systematic naming scheme throughout.

Alternative nomenclature for refrigerants

The refrigerant naming system is mainly used for fluorinated and chlorinated short alkanes for refrigerant use. In the US the standard is specified in ANSI/ASHRAE Standard 34-1992, with additional annual supplements.[2] The specified ANSI/ASHRAE prefixes were FC (fluorocarbon) or R (refrigerant), but today most are prefixed by a more specific classification:

CFC—list of chlorofluorocarbons

HCFC—list of hydrochlorofluorocarbons

HFC—list of hydrofluorocarbons

FC—list of fluorocarbons

PFC—list of perfluorocarbons (completely fluorinated)

The decoding system for CFC-01234a is:

0 = Number of double bonds (omitted if zero)

1 = Carbon atoms -1 (omitted if zero)

2 = Hydrogen atoms +1

3 = Fluorine atoms

4 = Replaced by Bromine ("B" prefix added)

a = Letter added to identify isomers, the "normal" isomer in any number has the smallest mass difference on each carbon, and a, b, or c are added as the masses diverge from normal.

Other coding systems are in use as well.

Synthesis

Alkyl halides can be synthesized from alkanes, alkenes, or alcohols.

From alkanes

Alkanes react with halogens by free radical halogenation. In this reaction a hydrogen atom is removed from the alkane, then replaced by a halogen atom by reaction with a diatomic halogen molecule. Thus:

Step 1: X2 ? 2 X· (Initiation step)

Step 2: X· + R-H ? R· + HX (1st propagation step)

Step 3: R· + X2 ? R-X + X· (2nd propagation step)

Steps 2 and 3 keep repeating, each providing the reactive intermediate needed for the other step. This is called a radical chain reaction. This reaction continues until the radicals are used up by one of three termination steps.

Step 4: R· + X· ? R-X (1st termination step)

Step 5: 2 X· ? X2 (2nd termination step)

Step 6: 2 R· ? R-R (3rd termination step)

Note that Step 4 results in the same product as Step 3, the desired haloalkane, but through the destruction of two radicals. Step 5 is just the reverse of Step 1 and Step 6 accounts for the small contamination of this reaction by larger alkanes and their subsequent haloalkanes.

From alkenes

Preparation of haloalkane:

An alkene reacts with a dry hydrogen halide (HX) like hydrogen chloride (HCl) or hydrogen bromide (HBr) to form a haloalkane. The double bond of the alkene is replaced by two new bonds, one with the halogen and one with the hydrogen atom of the hydrohalic acid. Markovnikov's rule states that in this reaction, the halogen is more likely to become attached to the more substituted carbon. This is a electrophilic addition reaction. It gives Markwonikoff addition product. For example:

H3C-CH=CH2 + HBr ? H3C-CHBr-CH3 (primary product) + H3C-CH2-CH2Br (secondary product).

Water must be absent otherwise there will be a side product( water). The reaction is necessarily to be carried out in a dry inert solvent such as CCl4 or directly in the gaseous phase.

Alkenes also react with halogens (X2) to form haloalkanes with two neighboring halogen atoms( Dihaloalkane). This is sometimes known as "decolorizing" the halogen, since the reagent X2 is colored and the product is usually colorless. For example:

2H3C-CH=CH2 + Br2 ? 2H3C-CHBr-CH2Br

From alcohols

Tertiary alkanol reacts with hydrochloric acid directly to produce tertiary chloroalkane, but if primary or secondary alkanol is used, an activator such as zinc chloride is needed. Alternatively the conversion may be performed directly using thionyl chloride which is called the Darzen's process.The Darzen's process is one of the most convinient methods known because the bi-products are gaseous;thus escape leaving behind pure alkyl chloride.Alkanol may likewise be converted to bromoalkane using hydrobromic acid or phosphorus tribromide or iodoalkane using red phosphorus and iodine (equivalent to phosphorus triiodide). Two examples:

(H3C)3C-OH + HCl.H2O ? (H3C)3C-Cl + 2 H2O

CH3-(CH2)6-OH + SOCl2 ? CH3-(CH2)6-Cl + SO2 + HCl

By substitution of alkanol in the absence of water

Halogenating agents are:

Phosphorus pentachloride

Thionyl chloride

hydrogen chloride

Phosphorus with Bromine

Phosphorus with Iodine

Hydrogen chloride with zinc chloride

Reactions of haloalkanes

Haloalkanes are reactive towards nucleophiles. They are polar molecules: the carbon to which the halogen is attached is slightly electropositive where the halogen is slightly electronegative. This results in an electron deficient (electrophilic) carbon which, inevitably, attracts nucleophiles.

Substitution reactions

Substitution reactions involve the replacement of the halogen with another molecule - thus leaving saturated hydrocarbons, as well as the halogen product.

Hydrolysis--a reaction in which water breaks a bond--is a good example of the nucleophilic nature of halogenoalkanes. The polar bond attracts a hydroxide ion, OH-. (NaOH(aq) being a common source of this ion). This OH- is a nucleophile with a clearly negative charge, as it has excess electrons it donates them to the carbon, which results in a covalent bond between the two. Thus C-X is broken by heterolytic fission resulting in a halide ion, X-. As can be seen, the OH is now attached to the alkyl group, creating an alcohol. (Hydrolysis of bromoethane, for example, yields ethanol).

One should note that within the halogen series, the C-X bond weakens as one goes to heavier halogens, and this affects the rate of reaction. Thus, the C-I of an iodoalkane generally reacts faster than the C-F of a fluoroalkane.

Apart from hydrolysis, there are a few other isolated examples of nucleophilic substitution:

Ammonia (NH3) and bromoethane yields a mixture of ethylamine, diethylamine, and triethylamine (as their bromide salts), and tetraethylammonium bromide.

Cyanide (CN-) added to bromoethane will form propionitrile (CH3CH2CN), a nitrile, and Br-. Nitriles can be further hydrolyzed into carboxylic acids.

Elimination reactions

Rather than creating a molecule with the halogen substituted with something else, one can completely eliminate both the halogen and a nearby hydrogen, thus forming an alkene. For example, with bromoethane and NaOH in ethanol, the hydroxide ion OH- attracts a hydrogen atom - thus removing a hydrogen and bromine from bromoethane. This results in C2H4 (ethylene), H2O and Br-.

Applications

Propellant

One major use of CFCs has been as propellants in aerosol inhalers for drugs used to treat asthma. The conversion of these devices and treatments from CFC to halocarbons that do not have the same effect on the ozone layer is well under way. The hydrofluoroalkane propellants ability to solubilize medications and excipients is markedly different from CFCs and as a result require a considerable amount of effort to reformulate. (a significant amount of development effort has also been required to develop non-CFC alternatives to CFC-based refrigerants, particularly for applications where the refrigeration mechanism cannot be modified or replaced.) They have now been outlawed in all 50 U.S. states universally.

Fire extinguishing

At high temperatures, halons decompose to release halogen atoms that combine readily with active hydrogen atoms, quenching the flame propagation reaction even when adequate fuel, oxygen and heat remains. The chemical reaction in a flame proceeds as a free radical chain reaction; by sequestering the radicals which propagate the reaction, halons are able to "poison" the fire at much lower concentrations than are required by fire suppressants using the more traditional methods of cooling, oxygen deprivation, or fuel dilution.

For example, Halon 1301 total flooding systems are typically used at concentrations no higher than 7% v/v in air, and can suppress many fires at 2.9% v/v. By contrast, carbon dioxide fire suppression flood systems are operated from 34% concentration by volume (surface-only combustion of liquid fuels) up to 75% (dust traps). Carbon dioxide can cause severe distress at concentrations of 3 to 6%, and has caused death by respiratory paralysis in a few minutes at 10% concentration. Halon 1301 causes only slight giddiness at its effective concentration of 5%, and even at 15% persons remain conscious but impaired and suffer no long term effects. (Experimental animals have also been exposed to 2% concentrations of Halon 1301 for 30 hours per week for 4 months, with no discernible health effects at all.) Halon 1211 also has low toxicity, although it is more toxic than Halon 1301, and thus considered unsuitable for flooding systems.

However, Halon 1301 fire suppression is not completely non-toxic; very high temperature flame, or contact with red-hot metal, can cause decomposition of Halon 1301 to toxic byproducts. The presence of such byproducts is readily detected because they include hydrobromic acid and hydrofluoric acid, which are intensely irritating. Halons are very effective on Class A (organic solids), B (flammable liquids and gases) and C (electrical) fires, but they are totally unsuitable for Class D (metal) fires, as they will not only produce toxic gas and fail to halt the fire, but in some cases pose a risk of explosion. Halons can be used on Class K (kitchen oils and greases) fires, but offer no advantages over specialised foams.

Halon 1211 is typically used in hand-held extinguishers, in which a stream of liquid halon is directed at a smaller fire by a user. The stream evaporates under reduced pressure, producing strong local cooling, as well as a high concentration of halon in the immediate vicinity of the fire. In this mode, extinguishment is achieved by cooling and oxygen deprivation at the core of the fire, as well as radical quenching over a larger area. After fire suppression, the halon moves away with the surrounding air, leaving no residue.

Halon 1301 is more usually employed in total flooding systems. In these systems, banks of halon cylinders are kept pressurised to about 4 MPa (600 PSI) with compressed nitrogen, and a fixed piping network leads to the protected enclosure. On triggering, the entire measured contents of one or more cylinders are discharged into the enclosure in a few seconds, through nozzles designed to ensure uniform mixing throughout the room. The quantity dumped is pre-calculated to achieve the desired concentration, typically 3-7% v/v. This level is maintained for some time, typically with a minimum of ten minutes and sometimes up to a twenty minute 'soak' time, to ensure all items have cooled so reignition is unlikely to occur, then the air in the enclosure is purged, generally via a fixed purge system that is activated by the proper authorities. During this time the enclosure may be entered by persons wearing SCBA. (There exists a common myth that this is because halon is highly toxic; in fact it is because it can cause giddiness and mildly impaired perception, and also due to the risk of combustion byproducts.)

Flooding systems may be manually operated or automatically triggered by a VESDA or other automatic detection system. In the latter case, a warning siren and strobe lamp will first be activated for a few seconds to warn personnel to evacuate the area. The rapid discharge of halon and consequent rapid cooling fills the air with fog, and is accompanied by a loud, disorienting noise.

Due to environmental concerns, alternatives are being deployed.

Halon 1301 is also used in the F-16 fighters to prevent the fuel vapors in the fuel tanks from becoming explosive; when the aircraft enters area with the possibility of unfriendly fire, Halon 1301 is injected into the fuel tanks for one-time use. Due to environmental concerns, trifluoroiodomethane (CF3I) is being considered as an alternative.

Environmental issues

Since the late 1970s the use of CFCs has been heavily regulated because of its destructive effects on the ozone layer. After the development of his atmospheric CFC detector, James Lovelock was the first to detect the presence of CFC's in the air, finding a concentration of 60 parts per trillion of CFC-11 over Ireland. In a self-funded research expedition ending in 1973, Lovelock went on to measure the concentration of CFC-11 in both the arctic and Antarctic, finding the presence of the gas in each of 50 air samples collected, but incorrectly concluding that CFC's are not hazardous to the environment. The experiment did however provide the first useful data on the presence of CFC's in the atmosphere. The damage caused by CFC's discovered by Sherry Rowland and Mario Molina who, after hearing a lecture on the subject of Lovelocks work, embarked on research resulting in the first published paper suggesting the connection in 1974. It turns out that one of CFCs' most attractive features—their unreactivity—has been instrumental in making them one of the most significant pollutants. CFCs' lack of reactivity gives them a lifespan which can exceed 100 years in some cases. This gives them time to diffuse into the upper stratosphere. Here, the sun's ultraviolet radiation is strong enough to break off the chlorine atom, which on its own is a highly reactive free radical. This catalyzes the break up of ozone into oxygen by means of a variety of mechanisms, of which the simplest is:

Cl· + O3 ? ClO· + O2

ClO· + O3 ? Cl· + 2 O2

Since the chlorine is regenerated at the end of these reactions, a single Cl atom can destroy many thousands of ozone molecules. Reaction schemes similar to this one (but more complicated) are believed to be the cause of the ozone hole observed over the poles and upper latitudes of the Earth. Decreases in stratospheric ozone may lead to increases in skin cancer.

In 1975, the US state of Oregon enacted the world's first ban of CFCs (legislation introduced by Walter F. Brown). The United States and several European countries banned the use of CFCs in aerosol spray cans in 1978, but continued to use them in refrigeration, foam blowing, and as solvents for cleaning electronic equipment. By 1985, scientists observed a dramatic seasonal depletion of the ozone layer over Antarctica. International attention to CFCs resulted in a meeting of world diplomats in Montreal in 1987. They forged a treaty, the Montreal Protocol, which called for drastic reductions in the production of CFCs. On March 2, 1989, 12 European Community nations agreed to ban the production of all CFCs by the end of the century. In 1990, diplomats met in London and voted to significantly strengthen the Montreal Protocol by calling for a complete elimination of CFCs by the year 2000. By the year 2010 CFCs should be completely eliminated from developing countries as well.

Because the only available CFC gases in countries adhering to the treaty is from recycling, their prices have gone up considerably. A worldwide end to production should also terminate the smuggling of this material, such as from Mexico to the United States.

A number of substitutes for CFCs have been introduced. Hydrochlorofluorocarbons (HCFCs) are much more reactive than CFCs, so a large fraction of the HCFCs emitted break down in the troposphere, and hence are removed before they have a chance to affect the ozone layer. Nevertheless, a significant fraction of the HCFCs do break down in the stratosphere and they have contributed to more chlorine buildup there than originally predicted. Development of non-chlorine based chemical compounds as a substitute for CFCs and HCFCs continues. One such class are the hydrofluorocarbons (HFCs), which contain only hydrogen and fluorine. One of these compounds, HFC-134a, is now used in place of CFC-12 in automobile air conditioners.

There is concern that halons are being broken down in the atmosphere to bromine, which reacts with ozone, leading to depletion of the ozone layer (this is similar to the case of chlorofluorocarbons such as freon). These issues are complicated: the kinds of fires that require halon extinguishers to be put out will typically cause more damage to the ozone layer than the halon itself, not to mention human and property damage. However, fire extinguisher systems must be tested regularly, and these tests may lead to damage. As a result, some regulatory measures have been taken, and halons are being phased out in most of the world.

In the United States, purchase and use of freon gases is regulated by the Environmental Protection Agency, and substantial fines have been levied for their careless venting. Also, licenses, good for life, are required to buy or use these chemicals. The EPA website discusses these rules in great detail, and also lists numerous private companies that are approved to give examinations for these certificates.

There are two kinds of licenses. Obtaining a "Section 609" license to use CFCs to recharge old (pre-1993 model year) car air conditioners is fairly easy and requires only an online multiple choice test offered by several companies. Companies that use unlicensed technicians for CFC recharge operations are subject to a US$15,000 fine per technician by the EPA.

The "Section 608" license, needed to recharge CFC-using stationary and non-automobile mobile units, is also multiple choice but more difficult. A general knowledge test is required, plus separate exams for small size (such as home refrigerator) units, and for high and low pressure systems. These are respectively called Parts I, II, and III. A person who takes and passes all tests receives a "Universal" license; otherwise, one that is endorsed only for the respectively passed Parts. While the general knowledge and Part I exams can be taken online, taking them before a proctor (which has to be done for Parts II and III) lets the applicant pass these tests with lower scores.

Safety

Haloalkanes in copper tubing open to the environment can turn into phosgene gas after coming in contact with extreme heat, such as while brazing or in a fire situation. Other ways that phosgene can be created is by passing the Haloalkane through an internal combustion engine, or by inhaling it through a lit cigarette, cigar or pipe. Phosgene is a substance that was used as a chemical weapon in World War I. Low exposure can cause irritation, but high levels cause fluid to collect in the lungs, possibly resulting in death.

Ozone (O3):

Is a triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic species O2. Ground-level ozone is an air pollutant with harmful effects on the respiratory systems of animals. On the other hand, ozone in the upper atmosphere protects living organisms by preventing damaging ultraviolet light from reaching the Earth's surface. It is present in low concentrations throughout the Earth's atmosphere. It has many industrial and consumer applications as well as being used in ozone therapy.

Ozone, the first allotrope of a chemical element to be described by science, was discovered by Christian Friedrich Schönbein in 1840, who named it after the Greek word for smell (ozein), from the peculiar odor in lightning storms. The odor from a lightning strike is from ions produced during the rapid chemical changes, not the ozone itself.

Physical properties

Undiluted ozone is a pale blue gas at standard temperature and pressure; it forms a dark blue liquid below -112 °C and a violet-black solid below -193 °C. At concentrations found in the atmosphere it is colorless. The concentration above which it can be smelled (odor threshold) is between 0.0076 and 0.036 ppm.

Structure

The structure of ozone, according to experimental evidence from microwave spectroscopy, is bent, with C2v symmetry (similar to the water molecule), O – O distance of 127.2 pm and O – O – O angle of 116.78°. The central atom forms an sp2 hybridization with one lone pair. Ozone is a polar molecule with a dipole moment of 0.5337 D. The bonding is single bond on one side and double bond on the other side, and these bonds are blended to become known as resonance structures. The bond order is 1.5 for each side.

Chemistry

Ozone is a powerful oxidizing agent. It is also unstable at high concentrations, decaying to ordinary diatomic oxygen:

2 O3 ? 3 O2.

This reaction proceeds more rapidly with increasing temperature and decreasing pressure. Ozone will oxidize metals (except gold, platinum, and iridium) to oxides of the metals in their highest oxidation state:

2 Cu2+(aq) + 2 H3O+(aq) + O3(g) ? 2 Cu3+(aq) + 3 H2O(l) + O2(g)

Ozone converts oxides to peroxides:

SO2 + O3 ? SO3 + O2

It also increases the oxidation number of oxides:

NO + O3 ? NO2 + O2

The above reaction is accompanied by chemiluminescence. The NO2 can be further oxidized:

NO2 + O3 ? NO3 + O2

The NO3 formed can react with NO2 to form N2O5:

NO2 + NO3 ? N2O5

Ozone reacts with carbon to form carbon dioxide, even at room temperature:

C + 2 O3 ? CO2 + 2 O2

Ozone does not react with ammonium salts but it reacts with ammonia to form ammonium nitrate:

2 NH3 + 4 O3 ? NH4NO3 + 4 O2 + H2O

Ozone reacts with sulfides to make sulfates:

PbS + 4 O3 ? PbSO4 + 4 O2

Sulfuric acid can be produced from ozone, either starting from elemental sulfur or from sulfur dioxide:

S + H2O + O3 ? H2SO4

3 SO2 + 3 H2O + O3 ? 3 H2SO4

All three atoms of ozone may also react, as in the reaction with tin(II) chloride and hydrochloric acid:

3 SnCl2 + 6 HCl + O3 ? 3 SnCl4 + 3 H2O

In the gas phase, ozone reacts with hydrogen sulfide to form sulfur dioxide:

H2S + O3 ? SO2 + H2O

In an aqueous solution, however, two competing simultaneous reactions occur, one to produce elemental sulfur, and one to produce sulfuric acid:

H2S + O3 ? S + O2 + H2O

3 H2S + 4 O3 ? 3 H2SO4

Iodine perchlorate can be made by treating iodine dissolved in cold anhydrous perchloric acid with ozone:

I2 + 6 HClO4 + O3 ? 2 I(ClO4)3 + 3 H2O

Solid nitryl perchlorate can be made from NO2, ClO2, and O3 gases:

2 NO2 + 2 ClO2 + 2 O3 ? 2 NO2ClO4 + O2

Ozone can be used for combustion reactions and combusting gases in ozone provides higher temperatures than combusting in dioxygen (O2). Following is a reaction for the combustion of carbon subnitride:

3 C4N2 + 4 O3 ? 12 CO + 3 N2

Ozone can react at cryogenic temperatures. At 77 K (-196 °C), atomic hydrogen reacts with liquid ozone to form a hydrogen superoxide radical, which dimerizes:

H + O3 ? HO2 + O

2 HO2 ? H2O4

Ozonides can be formed, which contain the ozonide anion, O3-. These compounds are explosive and must be stored at cryogenic temperatures. Ozonides for all the alkali metals are known. KO3, RbO3, and CsO3 can be prepared from their respective superoxides:

KO2 + O3 ? KO3 + O2

Although KO3 can be formed as above, it can also be formed from potassium hydroxide and ozone:

2 KOH + 5 O3 ? 2 KO3 + 5 O2 + H2O

NaO3 and LiO3 must be prepared by action of CsO3 in liquid NH3 on an ion exchange resin containing Na+ or Li+ ions:

CsO3 + Na+ ? Cs+ + NaO3

Treatment with ozone of calcium dissolved in ammonia leads to ammonium ozonide and not calcium ozonide:

3 Ca + 10 NH3 + 6 O3 ? Ca•6NH3 + Ca(OH)2 + Ca(NO3)2 + 2 NH4O3 + 2 O2 + H2

Ozone can be used to remove manganese from the water, forming a precipitate which can be filtered:

2 Mn2+ + 2 O3 + 4 H2O ? 2 MnO(OH)2 (s) + 2 O2 + 4 H+

Ozone will also turn cyanides to the one thousand times less toxic cyanates:

CN- + O3 ? CNO- + O2

Finally, ozone will also completely decompose urea:

(NH2)2CO + O3 ? N2 + CO2 + 2 H2O

Ozone in Earth's atmosphere

The standard way to express total ozone levels (the volume of ozone in a vertical column) in the atmosphere is by using Dobson units. Concentrations at a point are measured in parts per billion (ppb) or in µg/m³.

Ozone layer

The highest levels of ozone in the atmosphere are in the stratosphere, in a region also known as the ozone layer between about 10 km and 50 km above the surface (or between 6.21 and 31.1 miles). Here it filters out the shorter wavelengths (less than 320 nm) of ultraviolet light (270 to 400 nm) from the Sun that would be harmful to most forms of life in large doses. These same wavelengths are also among those responsible for the production of vitamin D, which is essential for human health. Ozone in the stratosphere is mostly produced from ultraviolet rays reacting with oxygen:

O2 + (radiation < 240 nm) ? 2 O

O + O2 ? O3

It is destroyed by the reaction with atomic oxygen:

O3 + O ? 2 O2

(See Ozone-oxygen cycle for more detail.)

The latter reaction is catalysed by the presence of certain free radicals, of which the most important are hydroxyl (OH), nitric oxide (NO) and atomic chlorine (Cl) and bromine (Br). In recent decades the amount of ozone in the stratosphere has been declining mostly due to emissions of CFCs and similar chlorinated and brominated organic molecules, which have increased the concentration of ozone-depleting catalysts above the natural background. See ozone depletion for more information. For more information on stratospheric ozone see Seinfeld and Pandis (1999).

Low level ozone

Low level ozone (or tropospheric ozone) is regarded as a pollutant by the World Health Organization. It is not emitted directly by car engines or by industrial operations. It is formed by the reaction of sunlight on air containing hydrocarbons and nitrogen oxides that react to form ozone directly at the source of the pollution or many kilometers down wind. For more details of the complex chemical reactions that produce low level ozone see tropospheric ozone or Seinfled and Pandis (1998).

Ozone reacts directly with some hydrocarbons such as aldehydes and thus begins their removal from the air, but the products are themselves key components of smog. Ozone photolysis by UV light leads to production of the hydroxyl radical and this plays a part in the removal of hydrocarbons from the air, but is also the first step in the creation of components of smog such as peroxyacyl nitrates which can be powerful eye irritants. The atmospheric lifetime of tropospheric ozone is about 22 days and its main removal mechanisms are being deposited to the ground, the above mentioned reaction giving OH, and by reactions with OH and the peroxy radical HO2· (Stevenson et al, 2006).

As well as having an impact on human health (see below) there is also evidence of significant reduction in agricultural yields due to increased ground-level ozone and pollution which interferes with photosynthesis and stunts overall growth of some plant species.

Ozone as a greenhouse gas

Although ozone was present at ground level before the industrial revolution, peak concentrations are far higher than the pre-industrial levels and even background concentrations well away from sources of pollution are substantially higher. This increase in ozone is of further concern as ozone present in the upper troposphere acts as a greenhouse gas, absorbing some of the infrared energy emitted by the earth. Quantifying the greenhouse gas potency of ozone is difficult as it is not present in uniform concentrations across the globe. However, the most recent scientific review on the climate change (the IPCC Third Assessment Report) suggests that the radiative forcing of tropospheric ozone is about 25% that of carbon dioxide.

Ozone and health

Ozone in air pollution

There is a great deal of evidence to show that high concentrations (ppm) of ozone, created by high concentrations of pollution and daylight UV rays at the earth's surface, can harm lung function and irritate the respiratory system. There has also been shown to be a connection between increased ozone caused by thunderstorms and hospital admissions of asthma sufferers. Air quality guidelines such as those from the World Health Organization are based on detailed studies of what levels can cause measurable health effects.

A common British folk myth dating back to the Victorian era holds that the smell of the sea is caused by ozone, and that this smell has "bracing" health-giving effects. Neither of these is true. The characteristic "smell of the sea" is not caused by ozone, but by the presence of dimethyl sulfide generated by phytoplankton, and dimethyl sulfide, like ozone, is toxic in high concentrations.

Physiology of ozone

Ozone, along with reactive forms of oxygen such as superoxide, singlet oxygen (see oxygen), hydrogen peroxide, and hypochlorite ions, is naturally produced by white blood cells and other biological systems (such as the roots of marigolds) as a means of destroying foreign bodies. Ozone reacts directly with organic double bonds. Also, when ozone breaks down to dioxygen it gives rise to oxygen free radicals, which are highly reactive and capable of damaging many organic molecules. Ozone has been found to convert cholesterol in the blood stream to plaque (which causes hardening and narrowing of arteries). Moreover, it is believed that the powerful oxidizing properties of ozone may be a contributing factor of inflammation. The cause-and-effect relationship of how the ozone is created in the body and what it does is still under consideration and still subject to various interpretations, since other body chemical processes can trigger some of the same reactions. A team headed by Dr. Paul Wentworth Jr. of the Department of Chemistry at the Scripps Research Institute has shown evidence linking the antibody-catalyzed water-oxidation pathway of the human immune response to the production of ozone. In this system, ozone is produced by antibody-catalyzed production of trioxidane from water and neutrophil-produced singlet oxygen. See also trioxidane for more on this biological ozone-producing reaction.

Ozone has also been proven to form specific, cholesterol-derived metabolites that are thought to facilitate the build-up and pathogenesis of atherosclerotic plaques (A form of heart disease). These metabolites have been confirmed as naturally occurring in human atherosclerotic arteries and are categorized into a class of secosterols termed “Atheronals”, generated by ozonolysis of cholesterol's double bond to form a 5,6 secosterol as well as a secondary condensation product via aldolization. Volume: Number: Page: 23 DOI:

Artificial production

Ozone may be formed from O2 by electrical discharges and by action of high energy electromagnetic radiation. Certain electrical equipment generate significant levels of ozone. This is especially true of devices using high voltages, such as ionic air purifiers, laser printers, photocopiers, and arc welders. Electric motors using brushes can generate ozone from repeated sparking inside the unit. Large motors that use brushes, such as those used by elevators (most elevator motors don't have brushes) or hydraulic pumps, will generate more ozone than smaller motors.

Industrial production

Ozone used in industry is measured in g/Nm3 or weight percent. The regime of applied concentrations ranges from 1 to 5 weight percent in air and from 6 to 13 weight percent in oxygen.

Formation and enrichment of ozone is obtained by exposure of an oxygen carrying gas to plasma, which is made of so-called silent or dielectric barrier discharges (DBD). Basically, molecular oxygen is dissociated into atomic oxygen, which subsequently recombines to ozone. The discharges manifest as filamentary transfer of electrons (micro discharges) in a gap between two electrodes. In order to evenly distribute the micro discharges, a dielectric insulator must be used to separate the metallic electrodes and to prevent arcing.

Ozone cannot be stored and transported like other industrial gases and must therefore be produced on site. Available ozone generators vary in the arrangement and design of the high-voltage electrodes. At production capacities higher than 20kg per hour, a gas/water tube heat-exchanger is utilized as ground electrode and assembled with tubular high-voltage electrodes on the gas-side. The regime of typical gas pressures is around 2bar absolute in oxygen and 3bar absolute in air. Several megawatt of electrical power may be installed in large facilities, applied as one phase AC current at 600 to 2000 Hz and peak voltages between 3000 and 20000 volts.

The dominating parameter influencing ozone generation efficiency is the gas temperature, which is controlled by the cooling water temperature. The cooler the water, the better the ozone synthesis. At typical industrial conditions, almost 90 percent of the effective power is dissipated as heat and needs to be removed by a sufficient cooling water flow.

Due to the high reactivity of ozone, only few materials may be used like stainless steel (quality 316L), glass, polytetrafluorethylene, or polyvinylidene fluoride. Viton may be used with the restriction of constant mechanical forces and absence of humidity.

Laboratory production

In the laboratory ozone can be produced by electrolysis using a 9 volt battery, a pencil graphite rod cathode, a platinum wire anode and a 3M sulfuric acid electrolyte.[26] The half cell reactions taking place are

3 H2O ? O3 + 6 H+ + 6 e-; ?Eo = -1.53 V;

6 H+ + 6 e- ? 3 H2; ?Eo = 0 V;

2 H2O ? O2 + 4 H+ + 4 e-; ?Eo = -1.23 V;

so that in the net reaction three equivalents of water are converted into one equivalent of ozone and three equivalents of hydrogen. Oxygen formation is a competing reaction.

Industrial applications

Ozone can be used for bleaching substances and for killing bacteria. Many municipal drinking water systems kill bacteria with ozone instead of the more common chlorine. Ozone has a very high oxidation potential. Ozone does not form organochlorine compounds, but it also does not remain in the water after treatment, so some systems introduce a small amount of chlorine to prevent bacterial growth in the pipes, or may use chlorine intermittently, based on results of periodic testing. Where electrical power is abundant, ozone is a cost-effective method of treating water, as it is produced on demand and does not require transportation and storage of hazardous chemicals. Once it has decayed, it leaves no taste or odor in drinking water. Low level of Ozone is helpful to purify air inside the house.

Industrially, ozone or ozonated water is used to

Disinfect laundry in hospitals, food factories, care homes etc;

disinfect water before it is bottled;

deodorize air and objects, such as after a fire;

kill bacteria on food or on contact surfaces;

ozone swimming pool and spa sanitation

scrub yeast and mold spores from the air in food processing plants;

wash fresh fruits and vegetables to kill yeast, mold and bacteria;

chemically attack contaminants in water (iron, arsenic, hydrogen sulfide, nitrites, and complex organics lumped together as "colour");

provide an aid to flocculation (agglomeration of molecules, which aids in filtration, where the iron and arsenic are removed);

manufacture chemical compounds via chemical synthesis

clean and bleach fabrics (the latter use is patented);

assist in processing plastics to allow adhesion of inks;

age rubber samples to determine the useful life of a batch of rubber;

hospital operating rooms where air needs to be sterile;

eradicate water borne parasites such as Giardia and Cryptosporidium in surface water treatment plants. This process is known as ozonation.

Ozone is a reagent in many organic reactions in the laboratory and in industry. Ozonolysis is the cleavage of an alkene to carbonyl compounds.

Many hospitals in the U.S. and around the world use large ozone generators to decontaminate operating rooms between surgeries. The rooms are cleaned and then sealed airtight before being filled with ozone which effectively kills or neutralizes all remaining bacteria.

Consumer applications

Ozone machines, with or without ionisation, are currently used to sanitise (high ozone output) and deodorize non-inhabited rooms, ductwork, vehicles, boats, woodsheds, and buildings.

Some models of air purifiers that also emit low levels of ozone have been sold in the US. These type of air purifiers claim to imitate nature's "filterless" air purifying mechanisms and claim to "sanitise" the air and/or household surfaces. The government successfully sued one company in 1995, ordering them to stop repeating health claims without supporting scientific studies.

Ozonated water is used to launder clothes, sanitise food, drinking water, and surfaces in the home. According to the FDA, it is "amending the food additive regulations to provide for the safe use of ozone in gaseous and aqueous phases as an antimicrobial agent on food, including meat and poultry." Studies at California Polytechnic University, have proven that low levels of ozone dissolved in filtered tapwater can produce more than a four-log (99.99%) reduction in such food-borne microorganisms as salmonella, e. Coli 0157:H7, campylobacter and others Ironically, while ozone is considered an atmospheric pollutant, pollution and smog by the US government, it can actually reduce pollutants like pesticides in fruits and vegetables.

Ozone is used in spas or hot tubs with reduced levels of chlorine or bromine for keeping the water free of bacteria. As it does not remain in the water after treatment, it is ineffective at preventing bather cross-contamination, and must be used in conjunction with another sanitizer. Ozone gas is created by an ultraviolet light bulb or corona discharge chip and injected into the plumbing system.

Ozone is also widely used in treatment of water in aquaria and fish ponds. Its use can minimize bacterial growth control parasites and removes or reduce "yellowing" of the water. As the Ozone rapidly decomposes, at correctly controlled levels the application has no effect on the fish.

Pharmaceutical applications

Ozone has been used in alternative medicine as a medical treatment in a number of different countries. Its use, however, is controversial.

Cyclic ozone

An alternative molecular structure for ozone is the cyclic structure with D3h symmetry, similar to cyclopropane. The cyclic structure has been studied theoretically using ab initio quantum chemistry methods, and most calculations agree that it is less stable than the bent structure by about 30 kcal/mol, with a ring-opening barrier of about 20 kcal/mol.

This structure would look much like an equilateral triangle with all the 3 oxygen atoms bonded to one another. Unfortunately, this structure has never been synthesized, let alone observed in nature.

Currently, research is being done to synthesize cyclic ozone molecules by hitting it with ultrafast lasers. Creation and isolation of such cyclic molecules could allow more energy to be packed into rockets and hence may allow for farther space travel.

numbers of deaths and cases of respiratory disease. Enforced air quality standards, like the Clean Air Act in the United States, have reduced the presence of some pollutants. While major stationary sources are often identified with air pollution, the greatest source of emissions is actually made up by mobile sources, mainly the automobiles.

Gases such as carbon dioxide, which contribute to global warming, have recently gained recognition as pollutants by some scientists. Others recognize the gas as being essential to life, and therefore incapable of being classed as a pollutant.

Pollutants

There are many substances in the air which may impair the health of plants and animals (including humans), or reduce visibility. These arise both from natural processes and human activity. Substances not naturally found in the air or at greater concentrations or in different locations from usual are referred to as 'pollutants'.

Pollutants can be classified as either primary or secondary. Primary pollutants are substances directly produced by a process, such as ash from a volcanic eruption or the carbon monoxide gas from a motor vehicle exhaust.

Secondary pollutants are not emitted. Rather, they form in the air when primary pollutants react or interact. An important example of a secondary pollutant is ground level ozone - one of the many secondary pollutants that make up photochemical smog.

Note that some pollutants may be both primary and secondary: that is, they are both emitted directly and formed from other primary pollutants.

Primary pollutants produced by human activity include:

oxides of sulfur, nitrogen and carbon

organic compounds, such as hydrocarbons (fuel vapours and solvents)

particulate matter, such as smoke and dust

metal oxides, especially those of lead, cadmium, copper and iron

chlorofluorocarbons (CFCs)

hazardous air pollutants (HAP)

persistent organic pollutants (POPs)

odors

Secondary pollutants include some particles formed from gaseous primary pollutants and compounds in photochemical smog, such as nitrogen dioxide, ground level ozone and peroxyacetyl nitrate (PAN).

Sources of Air Pollution

Anthropogenic sources (human activity) related to burning different kinds of fuel

"Stationary Sources" as smoke stacks of power plants, manufacturing facilities, municipal waste incinerators

"Mobile Sources" as motor vehicles, aircraft etc.

Combustion-fired power plants

Controlled burn practices used in agriculture and forestry management

Motor vehicles generating air pollution emissions.

Marine vessels, such as container ships or cruise ships, and related port air pollution.

Burning wood, fireplaces, stoves, furnaces and incinerators

Other anthropogenic sources

Oil refining, power plant operation and industrial activity in general.

Chemicals, dust and crop waste burning in farming, (see Dust Bowl).

Fumes from paint, hair spray, varnish, aerosol sprays and other solvents.

Waste deposition in landfills, which generate methane.

Military uses, such as nuclear weapons, toxic gases, germ warfare and rocketry.

Natural sources

Dust from natural sources, usually large areas of land with little or no vegetation.

Methane, emitted by the digestion of food by animals, for example cattle.

Radon gas from radioactive decay within the Earth's crust.

Smoke and carbon monoxide from wildfires.

Volcanic activity, which produce sulfur, chlorine, and ash particulates.

Indoor Air Pollution, or Indoor air quality (IAQ)

The lack of ventilation indoors concentrates air pollution where people have greatest exposure times. Radon (Rn) gas, a carcinogen, is exuded from the Earth in certain locations and trapped inside houses. Researchers have found that radon gas is responsible for over 1,800 deaths annually in the United Kingdom.[citation needed] Building materials including carpeting and plywood emit formaldehyde (H2CO) gas.

Paint and solvents give off volatile organic compounds (VOCs) as they dry. Lead paint can degenerate into dust and be inhaled. Intentional air pollution is introduced with the use of air fresheners, incense, and other scented items.

Controlled wood fires in stoves and fireplaces can add significant amounts of smoke particulates into the air, inside and out. Indoor air pollution may arise from such mundane sources as shower water mist containing arsenic or manganese, both of which are damaging to inhale. The arsenic (As3+) can be trapped with a shower nozzle filter.

Indoor pollution fatalities may be caused by using pesticides and other chemical sprays indoors without proper ventilation, and many homes have been destroyed by accidental pesticide explosions.

Carbon monoxide (CO) poisoning is a quick and silent killer, often caused by faulty vents and chimneys, or by the burning of charcoal indoors. 56,000 Americans died from CO in the period 1979-1988. Chronic carbon monoxide poisoning can result even from poorly adjusted pilot lights.

Smoke inhalation is a common cause of death in victims of house fires. Traps are built into all domestic plumbing to keep deadly sewer gas, hydrogen sulfide, out of interiors. Clothing emits tetrachloroethylene, or other dry cleaning fluids, for days after dry cleaning.

Though its use has now been banned in many countries, the extensive use of asbestos in industrial and domestic environments in the past has left a potentially very dangerous material in many localities. Asbestosis is a chronic inflammatory medical condition affecting the tissue of the lungs. It occurs after long-term, heavy exposure to asbestos, e.g. in mining or in the installation or removal of asbestos-containing materials from structures.

Sufferers have severe dyspnea (shortness of breath) and are at an increased risk regarding several different types of lung cancer. As clear explanations are not always stressed in non-technical literature, care should be taken to distinguish between several forms of relevant diseases.

According to the World Health Organisation (WHO), these may defined as; asbestosis, lung cancer, and mesothelioma (generally a very rare form of cancer, when more widespread it is almost always associated with prolonged exposure to asbestos).

Biological sources of air pollution are also found indoors, as gases and airborne particulates.

Pets produce dander, people produce dust from minute skin flakes, dust mites in bedding, carpeting and furniture produce enzymes and micron-sized fecal droppings, inhabitants emit methane, mold forms in walls and generates mycotoxins and spores, air conditioning systems can incubate Legionnaires' disease and mold, toilets can emit feces-tainted mists, and houseplants, soil and surrounding gardens can produce pollen, dust, and mold.

Indoors, the lack of air circulation allows these airborne pollutants to accumulate more than they would otherwise occur in nature.

Health effects

The World Health Organization thinks that 4.6 million people die each year from causes directly attributable to air pollution. Many of these mortalities are attributable to indoor air pollution. Worldwide more deaths per year are linked to air pollution than to automobile accidents. Research published in 2005 suggests that 310,000 Europeans die from air pollution annually.

Direct causes of air pollution related deaths include aggravated asthma, bronchitis, emphysema, lung and heart diseases, and respiratory allergies. The US EPA estimates that a proposed set of changes in diesel engine technology (Tier 2) could result in 12,000 fewer premature mortalities, 15,000 fewer heart attacks, 6,000 fewer emergency room visits by children with asthma, and 8,900 fewer respiratory-related hospital admissions each year in the United States.

The worst short term civilian pollution crisis in India was the 1984 Bhopal Disaster. Leaked industrial vapors from the Union Carbide factory, belonging to Union Carbide, Inc., U.S.A., killed more than 2,000 people outright and injured anywhere from 150,000 to 600,000 others, some 6,000 of whom would later die from their injuries. The United Kingdom suffered its worst air pollution event when the December 4th Great Smog of 1952 formed over London.

In six days more than 4,000 died, and 8,000 more died within the following months. An accidental leak of anthrax spores from a biological warfare laboratory in the former USSR in 1979 near Sverdlovsk is believed to have been the cause of hundreds of civilian deaths. The worst single incident of air pollution to occur in the United States of America occurred in Donora, Pennsylvania in late October, 1948, when 20 people died and over 7,000 were injured.

The health effects caused by air pollutants may range from subtle biochemical and physiological changes to difficulty breathing, wheezing, coughing and aggravation of existing respiratory and cardiac conditions. These effects can result in increased medication use, increased doctor or emergency room visits, more hospital admissions and premature death.

The human health effects of poor air quality are far reaching, but principally affect the body's respiratory system and the cardiovascular system. Individual reactions to air pollutants depend on the type of pollutant a person is exposed to, the degree of exposure, the individual's health status and genetics. People who exercise outdoors, for example, on hot, smoggy days increase their exposure to pollutants in the air.

Reduction efforts

There are many air pollution control technologies and urban planning strategies available to reduce air pollution; however, worldwide costs of addressing the issue are high. Enforced air quality standards, like the Clean Air Act in the United States, have reduced the presence of some pollutants.

Many countries have programs to or are debating how to reduce dependence on fossil fuels for energy production and shift toward renewable energy technologies or nuclear power plants.

Efforts to reduce pollution from mobile sources includes primary regulation (many developing countries have permissive regulations), expanding regulation to new sources (such as cruise and transport ships, farm equipment, and small gas-powered equipment such as lawn trimmers, chainsaws, and snowmobiles), increased fuel efficiency (such as through the use of hybrid vehicles), conversion to cleaner fuels (such as bioethanol, biodiesel), or conversion to electric vehicles with renewable energy sources (batteries or clean fuel such as hydrogen being used for transport and storage).

Control devices

The following items are commonly used as pollution control devices by industry or transportation devices. They can either destroy contaminants or remove them from an exhaust stream before it is emitted into the atmosphere.

Particulate control

Mechanical collectors (dust cyclones, multicyclones)

Electrostatic precipitators

Fabric filters (baghouses)

Particulate scrubbers

NOx control

Low NOx burners

Selective catalytic reduction (SCR)

Selective non-catalytic reduction (SNCR)

NOx scrubbers

Exhaust gas recirculation

Catalytic converter (also for VOC control)

VOC abatement

Adsorption systems, such as activated carbon

Flares

Thermal oxidizers

Catalytic oxidizers

Biofilters

Absorption (scrubbing)

Cryogenic condensers

Acid Gas/SO2 control

Wet scrubbers

Dry scrubbers

Flue gas desulfurization

Mercury control

Sorbent Injection Technology

Electro-Catalytic Oxidation (ECO)

K-Fuel

Dioxin and furan control

Ambient cleaning systems

Associated equipment

Source capturing systems

Continuous emissions monitoring systems (CEMS)

Air quality standards

Air quality targets set by DEFRA are mostly aimed at local government representatives responsible for the management of air quality in cities, where air quality management is the most urgent. The UK has established an air quality network where levels of the key air pollutants are published by monitoring centers.

Air quality in Oxford, Bath and London is particularly poor. One controversial study created by Calor Gas and published in the Guardian newspaper compared walking in Oxford on an average day to smoking over sixty light cigarettes.

'Cigarette equivalents' is obviously a headline capturing measure and more reliable and accepted comparison data can be collected from UK Air Quality Archive which allows the user to compare a cities management of pollutants against objectives set by DEFRA in 2000.

It is, however, important to evaluate several aspects of air pollution, and especially to take into consideration overall average values, rather than localized peak values sometimes cited. The UK National Air Quality Information Archive offers almost real-time monitoring "current maximum" air pollution measurements for many UK towns and cities. This source offers a wide range of constantly updated data, including:

Hourly Mean Ozone (µg/m³);

Hourly Mean Nitrogen dioxide (µg/m³);

max 15 min mean Sulphur dioxide (µg/m³);

8 Hourly Mean Carbon monoxide (mg/m³), and

24 Hour mean PM10 Particles (µg/m³ Grav Equiv)

DEFRA acknowledges that air pollution has a significant effect on health and has produced a simple banding system that is used to create a daily warning system that is issued by the BBC Weather service to indicate air pollution levels. DEFRA has published guidelines for people suffering from respiratory and heart diseases.

United States

In the 1960s, 70s, and 90s, the United States Congress enacted a series of Clean Air Acts which significantly strengthened regulation of air pollution. Individual U.S. states, some European nations and eventually the European Union followed these initiatives. The Clean Air Act sets numerical limits on the concentrations of a basic group of air pollutants and provide reporting and enforcement mechanisms.

In 1999, the United States EPA replaced the Pollution Standards Index (PSI) with the Air Quality Index (AQI) to incorporate new PM2.5 and Ozone standards.

The effects of these laws have been very positive. In the United States between 1970 and 2006, citizens enjoyed the following reductions in annual pollution emissions

carbon monoxide emissions fell from 197 million tons to 89 million tons

nitrogen oxide emissions fell from 27 million tons to 19 million tons

sulfur dioxide emissions fell from 31 million tons to 15 million tons

particulate emissions fell by 80%

lead emissions fell by more than 98%

European Union

National Emission Ceilings (NEC) for certain atmospheric pollutants are regulated by Directive 2001/81/EC (NECD)[2]

As part of the preparatory work associated with the revision of the NECD, the European Commission is assisted by the NEC-PI or NECPI working group (National Emission Ceilings – Policy Instruments ).

Atmospheric dispersion modeling

The basic technology for analyzing air pollution is through the use of a variety of mathematical models for predicting the transport of air pollutants in the lower atmosphere. The principal methodologies are:

Point source dispersion, used for simple industrial sources.

Line source dispersion, used for airport and roadway air dispersion modeling

Area source dispersion, used for forest fires or duststorms

Photochemical models, used to analyze reactive pollutants such as form smog

The point source problem is the best understood, since it involves simpler mathematics and has been studied for a long period of time, dating back to about the year 1900. It uses a Gaussian dispersion model to forecast the air pollution isopleths, with consideration given to wind velocity, stack height, emission rate, stability class (a measure of atmospheric turbulence). This model has been extensively validated and calibrated with experimental data for all sorts of atmospheric conditions.

The roadway air dispersion model was developed starting in the late 1950s and early 1960s in response to requirements of the National Environmental Policy Act and the U.S. Department of Transportation (then known as the Federal Highway Administration) to understand impacts of proposed new highways upon air quality, especially in urban areas.

Several research groups were active in this model development, among which were: the Environmental Research and Technology (ERT) group in Lexington, Massachusetts, the ESL Inc. group in Sunnyvale, California and the California Air Resources Board group in Sacramento, California. The research of the ESL group received a boost with a contract award from the U.S. Environmental Protection Agency to validate a line source model using sulfur hexafluoride as a tracer gas.

This program was successful in validating the line source model developed by ESL inc. Some of the earliest uses of the model were in court cases involving highway air pollution, the Arlington, Virginia portion of Interstate 66 and the New Jersey Turnpike widening project through East Brunswick, New Jersey.

Area source models were developed in 1971 through 1974 by the mid 1970s by the ERT and ESL groups, but addressed a smaller fraction of total air pollution emissions, so that their use and need was not as widespread as the line source model, which enjoyed hundreds of different applications as early as the 1970s.

Similarly photochemical models were developed primarily in the 1960s and 1970s, but their use was more specialized and for regional needs, such as understanding smog formation in the Livermore Valley, California.

Greenhouse effect and ocean acidification

The greenhouse effect is a phenomenon whereby carbon dioxide levels are thought to create a condition in the upper atmosphere, causing a trapping of excess heat and leading to increased surface temperatures. It shares this property with many other gases, and water vapour produces a larger effect than carbon dioxide. Other greenhouse gases include methane and NOx.

This effect has been understood by scientists for about a century, and technological advancements during this period have helped increase the breadth and depth of data relating to the phenomenon.

A number of studies have investigated the potential for long-term rising levels of atmospheric carbon dioxide to cause slight increases in the acidity of ocean waters and the possible effects of this on marine ecosystems. However, carbonic acid is a very weak acid, and is utilized by marine organisms during photosynthesis.

Sulfur dioxide (also sulphur dioxide) is the chemical compound with the formula SO2. This important gas is the main product from the combustion of sulfur compounds and is of significant environmental concern. SO2 is often described as the "smell of burning sulfur" but is not responsible for the smell of rotten eggs.

SO2 is produced by volcanoes and in various industrial processes. Since coal and petroleum contain various amounts of sulfur compounds, their combustion generates

sulfur dioxide. Further oxidation of SO2, usually in the presence of a catalyst such as NO2, forms H2SO4, and thus acid rain.

Preparation

Sulfur dioxide can be prepared by burning sulfur:

S(s) + O2(g) ? SO2(g)

The combustion of hydrogen sulfide and organosulfur compounds proceeds similarly.

2H2S(g) + 3O2(g) ? 2H2O(g) + 2SO2(g)

The roasting of sulfide ores such as iron pyrites, sphalerite (zinc blende) and cinnabar (mercury sulfide) also emits SO2:

4FeS2(s) + 11O2(g) ? 2Fe2O3(s) + 8SO2(g)

2ZnS(s) + 3O2(g) ? 2ZnO(s) + 2SO2(g)

HgS(s) + O2(g) ? Hg(g) + SO2(g)

When anhydrous CaSO4 is heated with coke and sand in the manufacture of cement, CaSiO3, sulfur dioxide is a by-product.

2CaSO4(s) + 2SiO2(s) + C(s) ? 2CaSiO3(s) + 2SO2(g) + CO2(g)

Action of hot concentrated sulfuric acid on copper turnings will produce sulfur dioxide.

Cu(s) + 2H2SO4(aq) ? CuSO4(aq) + SO2(g) + 2H2O(l)

Structure and bonding

SO2 is a bent molecule with C2v symmetry point group.

In terms of electron-counting formalisms, the sulfur atom has an oxidation state of +4, a formal charge of 0, and is surrounded by 5 electron pairs. From the perspective of molecular orbital theory, most of these electron pairs are non-bonding in character, as is typical for hypervalent molecules.

One conventional covalent bond is present between each oxygen and the central sulphur atom, with two further electrons delocalised between the oxygens and the sulphur atom.

Uses

Sulfur dioxide is sometimes used as a preservative in alcoholic drinks, or dried apricots and other dried fruits due to its antimicrobial properties. The preservative is used to maintain the appearance of the fruit rather than prevent rotting. This can give fruit a distinctive chemical taste.

Sulfur dioxide is also a good reductant. In the presence of water, sulfur dioxide is able to decolorize substances that can be reduced by it; thus making it a useful reducing bleach for papers and delicate materials such as clothes.

This bleaching effect normally does not last very long. Oxygen in the atmosphere reoxidizes the reduced dyes, restoring the color.

Sulfur dioxide is also used to make sulfuric acid, being converted to sulfur trioxide, and then to oleum, which is made into sulfuric acid. Sulfur dioxide for this purpose is made when sulfur combines with oxygen. This is called the contact process.

According to Claude Ribbe in The Crime of Napoleon, sulfur dioxide gas was used as an execution poison by the French emperor to suppress a slave revolt in Haiti early in the 19th century.

Sulfur dioxide blocks nerve signals from the pulmonary stretch receptors (PSR's) and abolishes the Hering-Breuer inflation reflex.

Prior to the development of freons, sulfur dioxide was used as a refrigerant in home refrigerators.

Sulfur dioxide is the anhydride of sulfurous acid, H2SO3.

Sulfur dioxide is a very important element in winemaking, and is designated as parts per million in wine. It acts as an antibiotic and antioxidant, protecting wine from spoilage organisms, bacteria, and oxidation, and also helps to keep volatile acidity at desirable levels.

Sulfur dioxide is responsible for the words "contains sulfites" found on wine labels. Wines with SO2 concentrations below 10ppm do not require "contains sulfites" on the label by US law. The upper limit of SO2 allowed in wine is 350ppm. In low concentrations SO2 is mostly undetected in wine, but at over 50ppm, SO2 becomes evident in the nose and taste of wine.

SO2 is also a very important element in winery sanitation. Wineries and equipment must be kept very clean, and because bleach cannot be used in a winery, a mixture of SO2, water, and citric acid is commonly used to clean hoses, tanks, and other equipment to keep it clean and free of bacteria.

Emissions

According to the US EPA (as presented by the 2002 World Almanac or in chart form ), the following amount of sulfur dioxide was released in the U.S. per year, measured in thousands of short tons:

*1999 18,867

*1998 19,491

*1997 19,363

*1996 18,859

*1990 23,678

*1980 25,905

*1970 31,161

Due largely to the US EPA’s Acid Rain Program, the U.S. has witnessed a 33 percent decrease in emissions between 1983 and 2002. This improvement resulted from flue gas desulfurization, a technology that enables SO2 to be chemically bound in power plants burning sulfur-containing coal or oil. In particular, calcium oxide (lime) reacts with sulfur dioxide to form calcium sulfite:

CaO + SO2 ? CaSO3

Aerobic oxidation converts this CaSO3 into CaSO4, gypsum. Most gypsum sold in Europe comes from flue gas desulfurization.

New fuel additive catalysts, such as ferox, are being used in gasoline and diesel engines in order to lower the emission of sulfur oxide gases into the atmosphere. This is also done by forcing the sulfur into stable mineral salts and mixed mineral sulfates as opposed to sulfuric acid and sulfur oxides.

As of 2006, China is the world's largest sulfur dioxide polluter, with 2005 emissions estimated to be 25.49 million tons. This amount represents a 27% increase since 2000, and is roughly comparable with U.S. emissions in 1980.

Al-Mishraq, an Iraqi sulfur plant, was the site of a 2004 disaster resulting in the release of massive amounts of sulfur dioxide into the atmosphere.

Temperature dependence of aqueous solubility

22 g/100ml (0 °C) 15 g/100ml (10 °C)

11 g/100ml (20 °C) 9.4 g/100 ml (25 °C)

8 g/100ml (30 °C) 6.5 g/100ml (40 °C)

5 g/100ml (50 °C) 4 g/100ml (60 °C)

3.5 g/100ml (70 °C) 3.4 g/100ml (80 °C)

3.5 g/100ml (90 °C) 3.7 g/100ml (100 °C)

2) NOx:

The term nitrogen oxide is a general term and can be used to refer to any of these oxides (oxygen compounds) of nitrogen, or to a mixture of them:

Nitric oxide (NO), nitrogen(II) oxide

Nitrogen dioxide (NO2)

Dinitrogen monoxide (N2O), nitrous oxide

Dinitrogen trioxide (N2O3), nitrogen (III) oxide

Dinitrogen tetroxide (N2O4)

Dinitrogen pentoxide (N2O5)

A mixture is often formed in chemical reactions that produce nitrogen oxides, with the proportions depending on the specific reaction and the conditions it is performed in. This is one reason why home production of N2O is undesirable; the other two stable oxides — which are extremely toxic — are liable to be produced.

Note that the last three listed above are unstable.

See the articles for these oxides for details on their properties, as well as the section NOx below.

NOx

This section refers to the chemical term for nitrogen oxides produced during combustion. For other definitions see Nox

NOx is a generic term for mono-nitrogen oxides. These oxides are produced during combustion, and are of interest as air pollution. They are believed to aggravate asthmatic conditions, react with the oxygen in the air to produce ozone, which is also an irritant, and eventually form nitric acid when dissolved in water. When dissolved in atmospheric moisture the result can be acid rain which can damage both trees and entire forest ecosystems.

In an internal combustion engine, a mixture of air and fuel is burned. When the mixture is tuned so as to consume every molecule of reactant (in this case fuel and oxygen) it is said to be "running at stoichiometry".

When this burns, combustion temperatures reach a high enough level to drive endothermic reactions between nitrogen and oxygen in the flame, yielding various oxides of nitrogen, the results of which can be seen over major cities such as Los Angeles, California in the summer in the form of brown clouds of smog.

Not to be confused with the term NOS which is used to refer to nitrous oxide in the context of its use as a booster for internal combustion engines.

Definition of NOx and NOy in atmospheric chemistry

In atmospheric chemistry the term NOx is used to mean the total concentration of NO plus NO2. During daylight NO and NO2 are in equilibrium with the ratio NO/NO2 determined by the intensity of sunshine (which converts NO2 to NO) and ozone (which reacts with NO to give back NO2). NO and NO2 are also central to the formation of tropospheric ozone.

This definition excludes other oxides of nitrogen such as Nitrous Oxide. NOy (reactive odd nitrogen) is defined as the sum of NOx plus the compounds produced from the oxidation of NOx which include nitric acid, peroxyacetyl nitrate and other compounds. In this context nitrous oxide and ammonia are not considered as reactive nitrogen compounds.

Sources of NOx

Generally, three primary sources of NOx formation in combustion processes are documented:

thermal NOx

fuel NOx

prompt NOx

Thermal NOx formation, which is highly temperature dependent, is recognized as the most relevant source when combusting natural gas. Fuel NOx tends to dominate during the combustion of fuels, such as coal, which have a significant nitrogen content, particularly when burned in combustors designed to minimise thermal NOx. The contribution of prompt NOx is normally considered negligible.

A fourth source, called feed NOx is associated with the combustion of nitrogen present in the feed material of cement rotary kilns, at between 300° and 800° C, where it is also a minor contributor.

Thermal NOx

Thermal NOx refers to NOx formed through high temperature oxidation of the diatomic nitrogen found in combustion air. The formation rate is primarily a function of temperature and the residence time of nitrogen at that temperature. At high temperatures, usually above 1600°C (2900°F), molecular nitrogen (N2) and oxygen (O2) in the combustion air disassociate into their atomic states and participate in a series of reactions.

The three principal reactions producing thermal NOx are:

(Extended Zeldovich Mechanism)

N2 + O ? NO + N

N+ O2 ? NO + O

N + OH ? NO + H

All 3 reactions are reversible. Zeldovich was the first to suggest the importance of the first two reactions. The last reaction of atomic Nitrogen with Hydroxyl radical, OH, was added by Lavovie, Heywood and Keck to the mechanism and makes a significiant contribution to the formation of thermal NOx.

Fuel NOx

The major source of NOx production from nitrogen-bearing fuels such as certain coals and oil, is the conversion of fuel bound nitrogen to NOx during combustion. During combustion, the nitrogen bound in the fuel is released as a free radical and ultimately forms free N2, or NO. Fuel NOx can contribute as much as 50% of total emissions when combusting oil and as much as 80% when combusting coal.

Although the complete mechanism is not fully understood, there are two primary paths of formation. The first involves the oxidation of volatile nitrogen species during the initial stages of combustion. During the release and prior to the oxidation of the volatiles, nitrogen reacts to form several intermediaries which are then oxidized into NO.

If the volatiles evolve into a reducing atmosphere, the nitrogen evolved can readily be made to form nitrogen gas, rather than NOx. The second path involves the combustion of nitrogen contained in the char matrix during the combustion of the char portion of the fuels. This reaction occurs much more slowly than the volatile phase. Only around 20% of the char nitrogen is ultimately emitted as NOx, since much of the NOx that forms during this process is reduced to nitrogen by the char, which is nearly pure carbon.

Prompt NOx

This third source is attributed to the reaction of atmospheric nitrogen, N2, with radicals such as C, CH, and CH2 fragments derived from fuel, where this cannot be explained by either the aforementioned thermal or fuel processes. Occurring in the earliest stage of combustion, this results in the formation of fixed species of nitrogen such as NH (nitrogen monohydride), HCN (hydrogen cyanide), H2CN (dihydrogen cyanide) and CN- (cyano radical) which can oxidize to NO. In fuels that contain nitrogen, the incidence of prompt NOx is especially minimal and it is generally only of interest for the most exacting emission targets.

Emission control technologies

Technologies such as flameless oxidation (FLOX®) and staged combustion significantly reduce thermal NOx in industrial processes. Bowin low NOx technology is a hybrid of staged-premixed-radiant combustion technology with a major surface combustion preceded by a minor radiant combustion. In the Bowin burner, air and fuel gas are premixed at a ratio greater than or equal to the stoichiometric combustion requirement.

Water Injection technology, wherby water is introduced into the combustion chamber, is also becoming an important means of NOx reduction through increased efficiency in the overall combustion process. Alternatively, the water (e.g. 10 to 50%) is emulsified into the fuel oil prior to the injection and combustion.

This emulsification can either be made in-line (unstabilized) just before the injection or as a drop-in fuel with chemical additives for long term emulsion stability (stabilized). Other technologies, such as selective catalytic reduction (SCR) and selective non catalytic reduction (SNCR) reduce post combustion NOx. Of particular importance is the introduction of catalytic converters which have significantly reduced emissions from motor vehicles.

Regulation

The USA Environmental Protection Agency (EPA) regulates and enforces NOx emission limits in the U.S. in accordance to legislation passed by Congress.

The Kyoto Protocol, ratified by 54 nations in 1997, calls for a substantial world wide reduction of greenhouse gases including Nitrous Oxide.

3) CO:

Carbon monoxide, with the chemical formula CO, is a colorless, odorless, and tasteless gas. It is the product of the incomplete combustion of carbon-containing compounds, notably in internal-combustion engines. It has significant fuel value, burning in air with a characteristic blue flame, producing carbon dioxide. Despite its serious toxicity, CO is extremely useful and underpins much modern technology, being a precursor to a myriad of useful — even life-saving — products. It consists of one carbon atom covalently bonded to one oxygen atom. It is a gas at room temperature.

Production

Carbon monoxide is so fundamentally important that many methods have been developed for its production.

Producer gas is formed by combustion of carbon in oxygen at high temperatures when there is an excess of carbon. In an oven, air is passed through a bed of coke. The initially produced CO2 equilibrates with the remaining hot carbon to give CO. The reaction of CO2 with carbon to give CO is described as the Boudouard equilibrium. Above 800 °C, CO is the predominant product:

O2 + 2 C ? 2 CO ?H = -221 kJ/mol

The downside of this method is if done with air it leaves a mixture that is mostly nitrogen.

Synthesis gas or Water gas is produced via the endothermic reaction of steam and carbon:

H2O + C ? H2 + CO ?H = 131 kJ/mol

CO also is a byproduct of the reduction of metal oxide ores with carbon, shown in a simplified form as follows:

MO + C ? M + CO ?H = 131 kJ/mol

Since CO is a gas, the reduction process can be driven by heating, exploiting the positive (favorable) entropy of reaction. The Ellingham diagram shows that CO formation is favored over CO2 in high temperatures.

CO is the anhydride of formic acid. As such it is conveniently produced by the dehydration of formic acid, for example with sulfuric acid. Another laboratory preparation for carbon monoxide entails heating an intimate mixture of powdered zinc metal and calcium carbonate.

Zn + CaCO3 ? ZnO + CaO + CO

Structure

The CO molecule is characterized by a bond length of 0.1128 nm. Formal charge and electronegativity difference cancel each other out. The result is a small dipole moment with its negative end on the carbon atom. This distance is consistent with a partial triple bond. The molecule has a small dipole moment and can be represented by three resonance structures:

The leftmost resonance form is the most important.

Nitrogen is isoelectronic to carbon monoxide, which means that these molecules have the same number of electrons and similar bonding. The physical properties of N2 and CO are similar, although CO is more reactive because it is polar.

Principal chemical reactions

Industrial uses

Carbon monoxide is a major industrial gas that has many applications in bulk chemicals manufacturing.

High volume aldehydes are produced by the hydroformylation reaction of alkenes, CO, and H2. In one of many applications of this technology, hydroformylation is coupled to the Shell Higher Olefin Process to give precursors to detergents.

Methanol is produced by the hydrogenation of CO. In a related reaction, the hydrogenation of CO is coupled to C-C bond formation, as in the Fischer-Tropsch process where CO is hydrogenated to liquid hydrocarbon fuels. This technology allows coal to be converted to petrol.

In the Monsanto process, carbon monoxide and methanol react in the presence of a homogeneous rhodium catalyst and HI to give acetic acid. This process is responsible for most of the industrial production of acetic acid.

Coordination chemistry

Most metals form coordination complexes containing covalently attached carbon monoxide. Only those in lower oxidation states will complex with carbon monoxide ligands. This is because there must be sufficient electron density to facilitate back donation from the metal dxz-orbital, to the p* molecular orbital from CO. The lone pair on the carbon atom in CO, also donates electron density to the dx²-y² on the metal to form a sigma bond.

In nickel carbonyl, Ni(CO)4 forms by the direct combination of carbon monoxide and nickel metal at room temperature. For this reason, nickel in any tubing or part must not come into prolonged contact with carbon monoxide (corrosion). Nickel carbonyl decomposes readily back to Ni and CO upon contact with hot surfaces, and this method was once used for the industrial purification of nickel in the Mond process.

In nickel carbonyl and other carbonyls, the electron pair on the carbon interacts with the metal; the carbon monoxide donates the electron pair to the metal. In these situations, carbon monoxide is called the carbonyl ligand. One of the most important metal carbonyls is iron pentacarbonyl, Fe(CO)5,

Many metal-CO complexes are prepared by decarbonylation of organic solvents, not from CO. For instance, iridium trichloride and triphenylphosphine react in boiling methoxyethanol or DMF) to afford IrCl(CO)(PPh3)2.

Organic and main group chemistry

In the presence of strong acids and water, carbon monoxide reacts with olefins to form carboxylic acids in a process known as the Koch-Haaf reaction. In the Gattermann-Koch reaction, arenes are converted to benzaldehyde derivatives in the presence of AlCl3 and HCl. Organolithium compounds, e.g. butyl lithium react with CO, but this reaction enjoys little use.

Although CO reacts with carbocations and carbanions, it is relatively unreactive toward organic compounds without the intervention of metal catalysts.

With main group reagents, CO undergoes several noteworthy reactions. Chlorination of CO is the industrial route to the important compound phosgene. With borane CO forms an adduct, H3BCO, which is isoelectronic with the acylium cation [H3CCO]+. CO reacts with sodium to give products resulting from C-C coupling such as Na2C2O2 (sodium acetylenediolate) and Na2C4O4 (sodium squarate).

Carbon monoxide in the atmosphere

Carbon monoxide, though thought of as a pollutant today, has always been present in the atmosphere, chiefly as a product of volcanic activity. It occurs dissolved in molten volcanic rock at high pressures in the earth's mantle. Carbon monoxide contents of volcanic gases vary from less than 0.01% to as much as 2% depending on the volcano. It also occurs naturally in bushfires. Because natural sources of carbon monoxide are so variable from year to year, it is extremely difficult to accurately measure natural emissions of the gas.

Carbon monoxide has an indirect radiative forcing effect by elevating concentrations of methane and tropospheric ozone through chemical reactions with other atmospheric constituents (e.g., the hydroxyl radical, OH.) that would otherwise destroy them.

Carbon monoxide is created when carbon-containing fuels are burned incompletely. Through natural processes in the atmosphere, it is eventually oxidized to carbon dioxide. Carbon monoxide concentrations are both short-lived in the atmosphere and spatially variable.

Anthropogenic CO from automobile and industrial emissions may contribute to the greenhouse effect and global warming. In urban areas carbon monoxide, along with aldehydes, reacts photochemically to produce peroxy radicals. Peroxy radicals react with nitrogen oxide to increase the ratio of NO2 to NO, which reduces the quantity of NO that is available to react with ozone. Carbon monoxide is also a constituent of tobacco smoke.

Role in physiology and food

Carbon monoxide is used in modified atmosphere packaging systems in the US, mainly with fresh meat products such as beef and pork. The CO combines with myoglobin to form carboxymyoglobin, a bright cherry red pigment. Carboxymyoglobin is more stable than the oxygenated form of myoglobin, oxymyoglobin, which can become oxidized to the brown pigment, metmyoglobin.

This stable red colour can persist much longer than in normally packaged meat and thus increases shelf life. Typical levels of CO used are 0.4% to 0.5%.

The technology was first given generally recognized as safe status by the FDA in 2002 for use as a secondary packaging system. In 2004 the FDA approved CO as primary packaging method, declaring that CO does not mask spoilage odour. Despite this ruling, the technology remains controversial in the US for fears that it is deceptive and masks spoilage.

One reaction in the body produces CO. Carbon monoxide is produced naturally as a breakdown of hemoglobin, heme, is a substrate for the enzyme heme oxygenase which produces CO and biliverdin. The biliverdin is converted to bilirubin by biliverdin reductase in macrophages of the reticuloendothelial system.

The lipid soluble unconjugated bilirubin is transported in the blood bound to albumin, taken up by the hepatocytes, conjugated with glucuronic acid and transported into the bile canaliculi for excretion from the body. The endogenously produced CO may have important physiological roles in the body (eg as a neurotransmitter).

CO is a nutrient for methanogenic bacteria, a building block for acetylcoenzyme A. This theme is the subject for the emerging field of bioorganometallic chemistry. In bacteria, CO is produced via the reduction of carbon dioxide via the enzyme carbon monoxide dehydrogenase, an Fe-Ni-S-containing protein.

A haeme-based CO-sensor protein, CooA, is known. The scope of its biological role is still unclear, it is apparently part of a signalling pathway in bacteria and archaea, but its occurrence in mammals is not established.

CO is also currently being studied for its anti-inflammatory and graft protection properties in the field of transplant immunology.

History

Carbon monoxide was first prepared by the French chemist de Lassone in 1776 by heating zinc oxide with coke. He mistakenly concluded that the gaseous product was hydrogen as it burned with a blue flame. The gas was identified as a compound containing carbon and oxygen by the English chemist William Cruikshank in the year 1800.

The toxic properties of CO were first thoroughly investigated by the French physiologist Claude Bernard around 1846. He poisoned dogs with the gas, and noticed that their blood was more rutilant in all the vessels. 'Rutilant' is a French word, but also has an entry in English dictionaries, meaning ruddy, shimmering, or golden. However, it was translated at the time as crimson, scarlet, and now is famously known as 'cherry pink'.

During World War II, carbon monoxide was used to keep motor vehicles running in parts of the world where gasoline was scarce. External charcoal or wood burners were fitted, and the carbon monoxide produced by gasification was piped to the carburetor. The CO in this case is known as "wood gas". Carbon monoxide was also reportedly used on a small scale during the Holocaust at some Nazi extermination camps.

Toxicity

Carbon monoxide is a significantly toxic gas and is the most common type of fatal poisoning in many countries. Exposures can lead to significant toxicity of the central nervous system and heart. Following poisoning, long-term sequelae often occur. Carbon monoxide can also have severe effects on the fetus of a pregnant woman. OSHA limits workplace exposure levels to 35 ppm.

Symptoms of mild poisoning include headaches and dizziness at concentrations less than 100 ppm. Concentrations as low as 667 ppm can cause up to 50% of the body's hemoglobin to be converted to carboxy-hemoglobin (HbCO) and ineffective for delivering oxygen. Exposures of this level can be life-threatening.

The mechanisms by which carbon monoxide produces toxic effects are not yet fully understood, but hemoglobin, myoglobin, and mitochondrial cytochrome oxidase are thought to be compromised. Treatment largely consists of administering 100% oxygen or hyperbaric oxygen therapy, although the optimum treatment remains controversial. Domestic carbon monoxide poisoning can be prevented by the use of household carbon monoxide detectors.

4) CO2:

Carbon dioxide is a chemical compound composed of one carbon and two oxygen atoms. It is often referred to by its formula CO2. It is present in the Earth's atmosphere at a low concentration of approximately 0.038% and is an important greenhouse gas. In its solid state, it is called dry ice. It is a major component of the carbon cycle.

Chemical and physical properties

Carbon dioxide is a colorless gas which, when inhaled at high concentrations (a dangerous activity because of the associated asphyxiation risk), produces a sour taste in the mouth and a stinging sensation in the nose and throat. These effects result from the gas dissolving in the mucous membranes and saliva, forming a weak solution of carbonic acid. One may notice this sensation if one attempts to stifle a burp after drinking a carbonated beverage.

Its density at standard temperature and pressure is around 1.98 kg/m³, about 1.53 times that of air. The carbon dioxide molecule (O=C=O) contains two double bonds and has a linear shape. It has no electrical dipole. As it is fully oxidized, it is not very reactive and is non-flammable.

Under normal atmospheric pressure (1 atm) at -78.5 °C, carbon dioxide changes directly from a solid phase to a gaseous phase through sublimation or gaseous to solid through deposition. The solid form is typically called "dry ice". Liquid carbon dioxide forms only at pressures above 5.1 atm. Its triple point is -56.6 °C at 416.7 kPa and its critical point is 31.1 °C at 7821 kPa.

History of human understanding

Carbon dioxide was one of the first gases to be described as a substance distinct from air. In the seventeenth century, the Flemish chemist Jan Baptist van Helmont observed that when he burned charcoal in a closed vessel, the mass of the resulting ash was much less than that of the original charcoal. His interpretation was that the rest of the charcoal had been transmuted into an invisible substance he termed a "gas" or "wild spirit" (spiritus sylvestre).

The properties of carbon dioxide were studied more thoroughly in the 1750s by the Scottish physician Joseph Black. He found that limestone (calcium carbonate) could be heated or treated with acids to yield a gas he termed "fixed air."

He observed that the fixed air was denser than air and did not support either flame or animal life. He also found that it would, when bubbled through an aqueous solution of lime (calcium hydroxide), precipitate calcium carbonate, and used this phenomenon to illustrate that carbon dioxide is produced by animal respiration and microbial fermentation.

In 1772, English chemist Joseph Priestley published a paper entitled Impregnating Water with Fixed Air in which he described a process of dripping sulfuric acid (or oil of vitriol as Priestley knew it) onto chalk in order to produce carbon dioxide and forcing the gas to dissolve by agitating a bowl of water in contact with the gas.

Carbon dioxide was first liquefied (at elevated pressures) in 1823 by Humphry Davy and Michael Faraday. The earliest description of solid carbon dioxide was given by Charles Thilorier, who in 1834 opened a pressurized container of liquid carbon dioxide, only to find that the cooling produced by the rapid evaporation of the liquid yielded a "snow" of solid CO2.

Isolation

Carbon dioxide may be obtained from air distillation, however this yields only very small quantities of CO2. A large variety of chemical reactions yield carbon dioxide, such as the reaction between most acids and most metal carbonates. As an example, the reaction between sulfuric acid and calcium carbonate (limestone or chalk) is depicted below:

H2SO4 + CaCO3 ? CaSO4 + H2CO3

The H2CO3 then decomposes to water and CO2. Such reactions are accompanied by foaming and/or bubbling. In industry such reactions are widespread because they can be used to neutralize waste acid streams.

The production of quicklime (CaO) a chemical that has widespread use, from limestone by heating at about 850 oC also produces CO2:

CaCO3 ? CaO + CO2

The combustion of all carbon containing fuels, such as methane (natural gas), petroleum distillates (gasoline, diesel, kerosene, propane), but also of coal and wood, will yield carbon dioxide, and, in most cases, water. As an example the chemical reaction between methane and oxygen is given below.

CH4 + 2 O2 ? CO2 + 2 H2O

Iron is reduced from its oxides with coke in blast furnace, producing pig iron and carbon dioxide:

2 Fe2O3 + 3 C ? 4 Fe + 3 CO2

Carbon dioxide can be used in chemistry to create a carboxylic acid from a Grignard reagent.

R-MgX + CO2 ? R-COOH

Yeast produces carbon dioxide and ethanol, also known as alcohol, in the production of wines, beers and other spirits:

Glucose ? 2 CO2 + 2 C2H5OH

All aerobic organisms produce CO2 when they burn carbohydrates, fatty acids and proteins; it is the prime energy source and the main metabolic pathway in heterotroph organisms such as animals, and also a secondary energy source in phototroph organisms such as plants when not enough light is available for photosynthesis.

The large amount of reactions involved are exceedingly complex and not described easily. Photoautotrophs (i.e. plants, cyanobacteria) utilize another modus operandi: They absorb the CO2 from the air, and, together with water, react it to form carbohydrates:

nCO2 + nH2O ? (CH2O)n + nO2

Carbon dioxide is soluble in water, in which it spontaneously interconverts between CO2 and H2CO3 (carbonic acid). The relative concentrations of CO2, H2CO3, and the deprotonated forms HCO3- (bicarbonate) and CO32-(carbonate) depend on the pH. In neutral or slightly alkaline water (pH > 6.5), the bicarbonate form predominates (>50%) becoming the most prevalent (>95%) at the pH of seawater, while in very alkaline water (pH > 10.4) the predominant (>50%) form is carbonate.

The bicarbonate and carbonate forms are very soluble, such that air-equilibrated ocean water (mildly alkaline with typical pH = 8.2 – 8.5) contains about 120 mg of bicarbonate per liter.

Industrial production

Carbon dioxide is manufactured mainly from six processes:

As a byproduct in ammonia and hydrogen plants, where methane is converted to CO2;

From combustion of carbonaceous fuels;

As a byproduct of fermentation;

From thermal decomposition of CaCO3;

As a byproduct of sodium phosphate manufacture;

Directly from natural carbon dioxide gas wells.

Solid carbon dioxide — "dry ice"

Solid carbon dioxide has the generic trademark "dry ice" and is a versatile cooling agent. Unlike when water passes its solid point and becomes molten, dry ice sublimes, changing directly to a gas. Its sublimation/deposition point is -78.5°C (-109.3°F). The low temperature and direct sublimation to a gas makes dry ice a very effective coolant, since it's colder than ice and leaves no moisture as it changes state. Dry ice is also inexpensive; it costs about US$2 per kilogram.

Dry ice was first observed in 1825 by the French chemist Charles Thilorier. Upon opening the lid of a large cylinder containing liquid carbon dioxide he noted much of the carbon dioxide rapidly evaporated leaving solid dry ice in the container. Throughout the next 60 years, dry ice was observed and tested by many scientists.

Production

Dry ice is readily manufactured:

Carbon dioxide is pressurized and refrigerated until it changes into its liquid form.

The pressure is reduced. When this occurs some liquid carbon dioxide vaporises, and this causes a rapid lowering of temperature of the remaining liquid carbon dioxide. The extreme cold makes the liquid solidify into a snow-like consistency.

The snow-like solid carbon dioxide is compressed into either small pellets or larger blocks of dry ice.

Dry ice is typically produced in two standard sizes: solid blocks and cylindrical pellets. A standard block is most common and will normally be about 30 kg. These are largely used in the shipping industry because they sublime slowly due to a relatively small surface area. The pellets are around 1 cm in diameter and can be bagged easily. This form of dry ice is more suited to small scale use, for example at grocery stores and laboratories.

Safety

Dry ice can be a dangerous substance, if used improperly. It must be handled using protective insulated gloves, because direct contact can cause frostbite. It must not be stored in a sealed container, since its sublimation produces large volumes of gaseous carbon dioxide at high pressure — a sealed container containing dry ice can fail explosively, which could cause shrapnel injuries and hearing loss. Dry ice should never be stored in a functioning freezer or refrigerator, because it is cold enough to freeze the thermostat. Also, due to its temperature, thermal contraction from dry ice can cause brittle materials such as glass or plastic to crack.

Applications

Dry ice has many applications:

Transporting items that need to remain cold or frozen, such as food, without needing any cooling source

Blast cleaning

Freezing warts to make removal easier

Keeping broken or powerless refrigerators and freezers cold

Loosening floor tiles by shrinking and cracking them

Carbonating water and other liquids

Repelling mosquitoes and other insects

Creating low-sinking dense clouds of fog for dramatic effects by putting it in water and therefore accelerating sublimation

Freezing water in pipes with no valves that are leaking or being repaired

Making ice cream

Minor dent repairs - dry ice can force a car's sheet metal to contract, thus popping out a dent.

Dry ice blast cleaning

One of the largest alternative uses of dry ice around the world is dry ice blast cleaning. Dry ice pellets are shot out of a jet nozzle with compressed air. This can remove residues from industrial equipment, for example ink, glue, oil, paint, mold and rubber, replacing sandblasting, steam blasting, water blasting or other (potentially environmentally damaging) solvent blasting.

Dry ice blasting involves three factors:

kinetic energy

thermal shock

thermal kinetic energy

The kinetic energy of the dry ice pellets is transferred when it hits the surface, directly dislodging residues, as in other blasting methods. The thermal shock effect occurs when the cold dry ice hits a much warmer surface and rapid sublimation occurs. The thermal kinetic effect is the result of the rapid sublimation of the dry ice hitting the surface. These factors combine cause small "micro-explosions" of gaseous carbon dioxide where each pellet of dry ice impacts, dislodging the residue.

Solid amorphous carbon dioxide

An alternative form of solid carbon dioxide, an amorphous glass-like form, is possible, although not at atmospheric pressure. This form of glass, called carbonia, was produced by supercooling heated CO2 at extreme pressure (40–48 GPa or about 400,000 atmospheres) in a diamond anvil.

This discovery confirmed the theory that carbon dioxide could exist in a glass state similar to other members of its elemental family, like silicon (silica glass) and germanium. Unlike silica and germanium oxide glasses, however, carbonia glass is not stable at normal pressures and reverts back to gas when pressure is released.

Carbon dioxide in the Earth's atmosphere

Carbon dioxide is present in a low concentration in the Earth's atmosphere. It is essential to photosynthesis in plants and other photoautotrophs, and is also a prominent greenhouse gas.

The 20 year smoothed Law Dome DE02 and DE02-2 ice cores show the levels of earth's atmospheric CO2 to have been 284.3 ppmv (0.02843% by volume) in 1832.[8] As of January 2007, the CO2 concentration was about 383 ppmv, which is 0.0582% by weight. This represents about 2.996×1012 tonnes, and is estimated to be 105 ppm (37.77%) above the pre-industrial average.

Despite its small concentration compared to water vapour, CO2 is a very important component of Earth's atmosphere, because it absorbs infrared radiation at wavelengths of 4.26 µm (asymmetric stretching vibrational mode) and 14.99 µm (bending vibrational mode) and enhances the greenhouse effect. See also Carbon dioxide equivalent.

Biological role

Carbon dioxide is an end product in organisms that obtain energy from breaking down sugars, fats and amino acids with oxygen as part of their metabolism, in a process known as cellular respiration. This includes all plants, animals, many fungi and some bacteria. In higher animals, the carbon dioxide travels in the blood from the body's tissues to the lungs where it is exhaled. In plants using photosynthesis, carbon dioxide is absorbed from the atmosphere.

Role in photosynthesis

Plants remove carbon dioxide from the atmosphere by photosynthesis, also called carbon assimilation, which uses light energy to produce organic plant materials by combining carbon dioxide and water. Free oxygen is released as gas from the decomposition of water molecules, while the hydrogen is split into its protons and electrons and used to generate chemical energy via photophosphorylation. This energy is required for the fixation of carbon dioxide in the Calvin cycle to form sugars. These sugars can then be used for growth within the plant through respiration.

Carbon dioxide gas must be introduced into greenhouses to maintain plant growth, as even in vented greenhouses the concentration of carbon dioxide can fall during daylight hours to as low as 200 ppm, at which level photosynthesis is significantly reduced. Venting can help offset the drop in carbon dioxide, but will never raise it back to ambient levels of 340ppm. Carbon dioxide supplementation is the only known method to overcome this deficiency.

Direct introduction of pure carbon dioxide is ideal, but rarely done because of cost constraints. Most greenhouses burn methane or propane to supply the additional CO2, but care must be taken to have a clean burning system as increased levels of NO2 result in reduced plant growth.

Sensors for SO2 and NO2 are expensive and difficult to maintain, accordingly most systems come with a carbon monoxide (CO) sensor under the assumption that high levels of carbon monoxide mean that significant amounts of NO2 are being produced. Plants can potentially grow up to 50 percent faster in concentrations of 1000ppm CO2 when compared with ambient conditions.

Plants also emit CO2 during respiration, so it is only during growth stages that plants are net absorbers. For example a growing forest will absorb many tonnes of CO2 each year, however a mature forest will produce as much CO2 from respiration and decomposition of dead specimens (e.g. fallen branches) as used in biosynthesis in growing plants. Regardless of this, mature forests are still valuable carbon sinks, helping maintain balance in the Earth's atmosphere.

Animal toxicity

Carbon dioxide content in fresh air varies and is between 0.03% (300 ppm) and 0.06% (600 ppm), depending on location (see graphical map of CO2 in real-time) and in exhaled air approximately 4.5%. When inhaled in high concentrations (greater than 5% by volume), it is immediately dangerous to the life and health of humans and other animals.

The current threshold limit value (TLV) or maximum level that is considered safe for healthy adults for an 8-hour work day is 0.5% (5000 ppm). The maximum safe level for infants, children, the elderly and individuals with cardio-pulmonary health issues would be significantly less.

These figures are valid for carbon dioxide supplied in "pure" form. In indoor spaces occupied by humans the carbon dioxide concentration will also reach a level higher than in pure outdoor air. Concentrations higher than 1000 ppm will cause discomfort in more than 20% of occupants, and the discomfort will increase with increasing CO2 concentration.

The discomfort will be caused by various gases coming from human respiration and perspiration, and not by CO2 itself. At 2000 ppm will the majority of occupants feel a significant degree of discomfort, and many will develop nausea and headache. The CO2 concentration between 300 and 2500 ppm is used as an indicator of indoor air quality in spaces polluted by human occupation.

Acute carbon dioxide toxicity is sometimes known as Choke damp, an old mining industry term, and was the cause of death at Lake Nyos in Cameroon, where an upwelling of CO2-laden lake water in 1986 covered a wide area in a blanket of the gas, killing nearly 2000.

The lowering of carbon dioxide in the atmosphere is largely due to absorption by plants, which convert it to sugars through photosynthesis. Phytoplankton photosynthesis absorbs dissolved CO2 in the upper ocean and thereby promotes the absorption of CO2 from the atmosphere.

Carbon dioxide is a surrogate for indoor pollutants that may cause occupants to grow drowsy, get headaches, or function at lower activity levels. To eliminate most Indoor Air Quality complaints, total indoor carbon dioxide must be reduced to below 600 ppm. NIOSH considers that indoor air concentrations of carbon dioxide that exceed 1000 ppm are a marker suggesting inadequate ventilation (1,000 ppm equals 0.1%). ASHRAE recommends that CO2 levels not exceed 1000 ppm inside a space.

OSHA limits carbon dioxide concentration in the workplace to 0.5% for prolonged periods. The U.S. National Institute for Occupational Safety and Health limits brief exposures (up to ten minutes) to 3% and considers concentrations exceeding 4% as "immediately dangerous to life and health."

People who breathe 5% carbon dioxide for more than half an hour show signs of acute hypercapnia, while breathing 7 – 10% carbon dioxide can produce unconsciousness in only a few minutes. Carbon dioxide, either as a gas or as dry ice, should be handled only in well-ventilated areas.

Human physiology

CO2 is carried in blood in three different ways. (The exact percentages vary depending whether it is arterial or venous blood).

Most of it (about 80% – 90%) is converted to bicarbonate ions HCO3- by the enzyme carbonic anhydrase in the red blood cells.

5% – 10% is dissolved in the plasma

5% – 10% is bound to hemoglobin as carbamino compounds.

The CO2 bound to hemoglobin does not bind to the same site as oxygen; rather it combines with the N-terminal groups on the four globin chains. However, because of allosteric effects on the hemoglobin molecule, the binding of CO2 does decrease the amount of oxygen that is bound for a given partial pressure of oxygen.

Hemoglobin, the main oxygen-carrying molecule in red blood cells, can carry both oxygen and carbon dioxide, although in quite different ways. The decreased binding to carbon dioxide in the blood due to increased oxygen levels is known as the Haldane Effect, and is important in the transport of carbon dioxide from the tissues to the lungs. Conversely, a rise in the partial pressure of CO2 or a lower pH will cause offloading of oxygen from hemoglobin. This is known as the Bohr Effect.

Carbon dioxide may be one of the mediators of local autoregulation of blood supply. If it is high, the capillaries expand to allow a greater blood flow to that tissue.

Bicarbonate ions are crucial for regulating blood pH. As breathing rate influences the level of CO2 in blood, too slow or shallow breathing causes respiratory acidosis, while too rapid breathing, hyperventilation, leads to respiratory alkalosis.

It is interesting to note that although it is oxygen that the body requires for metabolism, it is not low oxygen levels that stimulate breathing, but is instead higher carbon dioxide levels. As a result, breathing low-pressure air or a gas mixture with no oxygen at all (e.g., pure nitrogen) leads to loss of consciousness without subjective breathing problems.

This is especially perilous for high-altitude fighter pilots, and is also the reason why the instructions in commercial airplanes for case of loss of cabin pressure stress that one should apply the oxygen mask to oneself before helping others — otherwise one risks going unconscious without being aware of the imminent peril.

According to a study by the USDA, an average person's respiration generates approximately 450 liters (roughly 900 grams) of carbon dioxide per day.

Carbon dioxide sequestering

Methods of CO2 extraction/separation include:

Aqueous solutions

Amine extraction

High pH solutions

For example, Carbon dioxide reacts with dissolved CaO, to form Calcite (CaCO3)[

Adsorption

Molecular Sieves

Activated Carbon

Metal-organic frameworks (MOF's)

Solid reactants

Serpentine, Olivine, Quicklime

Membrane gas separation

Regenerative Carbon Dioxide Removal System (RCRS)

The RCRS on the space shuttle Orbiter uses a two-bed system that provides continuous removal of CO2 without expendable products. Regenerable systems allow a shuttle mission a longer stay in space without having to replenish its sorbent canisters. Older lithium hydroxide (LiOH)-based systems, which are non-regenerable, are being replaced by regenerable metal-oxide-based systems.

A metal-oxide-based system primarily consists of a metal oxide sorbent canister and a regenerator assembly. This system works by removing carbon dioxide using a sorbent material and then regenerating the sorbent material. The metal-oxide sorbent is regenerated by pumping air heated to around 200 °C at 7.5 standard cubic feet per minute through its canister for 10 hours.

Algae Bioreactor Technology

Originally developed at MIT using power plant flue gas to support bio diesel feed stock, they use algae to process out the CO2. Commercial studies have been performed on over 2000 MW of power plants in the United States since 2001. As of March 2007, this is the only commercially installed technology for CO2 mitigation on active power plants.

The largest test site for an Algae bioreactor system is connected directly to smokestack of Arizona Public Service Redhawk 1,040 megawatt power plant, producing renewable biofuels as a process by product. At commercial scale, this organic process holds the potential to "scrub" CO2 without the considerable solid and fluid waste issues associated with other technologies

Underground geological storage.

Deep Ocean storage. At sufficiently high pressure, around 500 m depth, carbon dioxide forms a solid hydrate with water.

Terra preta - Charcoal enhanced soils

Amazon soils that are valued today for their rich agricultural abilities are found to contain charcoal that was put into the soils by Amazonians thousands of years ago. Plant and organic material converted to charcoal can be used to enhance soils and keep CO2 out of the atmosphere for thousands of years.

Oak Ridge National Laboratory has found a way to further enhance charcoal's agricultural benefits and capture more CO2 by combining ammonia and fossil fuel exhaust to form ammonium bicarbonate in the charcoal lattices. The work by Oak Ridge National Laboratory is currently being commercialized by a corporation called EPRIDA, Inc.

Water pollution is a large set of adverse effects upon water bodies such as lakes, rivers, oceans, and groundwater caused by human activities.

Although natural phenomena such as volcanoes, algae blooms, storms, and earthquakes also cause major

changes in water quality and the ecological status of water, these are not deemed to be pollution. Water pollution has many causes and characteristics. Increases in nutrient loading may lead to eutrophication. Organic wastes such as sewage impose high oxygen demands on the receiving water leading to oxygen depletion with potentially severe impacts on the whole eco-system.

Industries discharge a variety of pollutants in their wastewater including heavy metals, organic toxins, oils, nutrients, and solids. Discharges can also have thermal effects, especially those from power stations, and these too reduce the available oxygen.

Silt-bearing runoff from many activities including construction sites, deforestation and agriculture can inhibit the penetration of sunlight through the water column, restricting photosynthesis and causing blanketing of the lake or river bed, in turn damaging ecological systems.

Pollutants in water include a wide spectrum of chemicals, pathogens, and physical chemistry or sensory changes. Many of the chemical substances are toxic. Pathogens can obviously produce waterborne diseases in either human or animal hosts. Alteration of water's physical chemistry include acidity, conductivity, temperature, and eutrophication.

Eutrophication is the fertilisation of surface water by nutrients that were previously scarce. Even many of the municipal water supplies in developed countries can present health risks. Water pollution is a major problem in the global context. It has been suggested that it is the leading worldwide cause of deaths and diseases, and that it accounts for the deaths of more than 14,000 people daily

Sources of water pollution

Some of the principal sources of water pollution are:

geology of aquifers from which groundwater is abstracted

industrial discharge of chemical wastes and byproducts

discharge of poorly-treated or untreated sewage

surface runoff containing pesticides or fertilizers

slash and burn farming practice, which is often an element within shifting cultivation agricultural systems

surface runoff containing spilled petroleum products

surface runoff from construction sites, farms, or paved and other impervious surfaces e.g. silt

discharge of contaminated and/or heated water used for industrial processes

acid rain caused by industrial discharge of sulfur dioxide (by burning high-sulfur fossil fuels)

excess nutrients added (eutrophication) by runoff containing detergents or fertilizers

underground storage tank leakage, leading to soil contamination, thence aquifer contamination

Contaminants

Contaminants may include organic and inorganic substances.

Some organic water pollutants are:

insecticides and herbicides, a huge range of organohalide and other chemicals

bacteria, often is from sewage or livestock operations;

food processing waste, including pathogens

tree and brush debris from logging operations

VOCs (Volatile Organic Compounds, industrial solvents) from improper storage

Some inorganic water pollutants include:

heavy metals including acid mine drainage

acidity caused by industrial discharges (especially sulfur dioxide from power plants)

chemical waste as industrial by products

fertilizers, in runoff from agriculture including nitrates and phosphates

silt in surface runoff from construction sites, logging, slash and burn practices or land clearing sites

Transport and chemical reactions of water pollutants

Most water pollutants are eventually carried by the rivers into the oceans. In some areas of the world the influence can be traced hundred miles from the mouth by studies using hydrology transport models. Advanced computer models such as SWMM or the DSSAM Model have been used in many locations worldwide to examine the fate of pollutants in aquatic systems. Indicator filter feeding species such as copepods have also been used to study pollutant fates in the New York Bight, for example.

The highest toxin loads are not directly at the mouth of the Hudson River, but 100 kilometers south, since several days are required for incorporation into planktonic tissue. The Hudson discharge flows south along the coast due to coriolis force. Further south then are areas of oxygen depletion, caused by chemicals using up oxygen and by algae blooms, caused by excess nutrients from algal cell death and decomposition.

Fish and shellfish kills have been reported, because toxins climb the foodchain after small fish consume copepods, then large fish eat smaller fish, etc. Each successive step up the food chain causes a stepwise concentration of pollutants such as heavy metals (e.g. mercury) and persistent organic pollutants such as DDT.

The big gyres in the oceans trap floating plastic debris. The North Pacific Gyre for example has collected the so-called Great Pacific Garbage Patch that is now about the size of Texas. Many of these long-lasting pieces wind up in the stomachs of marine birds and animals.

Many chemicals undergo reactive decay or change especially over long periods of time in groundwater reservoirs. A noteworthy class of such chemicals are the chlorinated hydrocarbons such as trichloroethylene (used in industrial metal degreasing) and tetrachloroethylene used in the dry cleaning industry. Both of these chemicals, which are carcinogens themselves, undergo partial decomposition reactions, leading to new hazardous chemicals.

Groundwater pollution is much more difficult to abate than surface pollution because groundwater can move great distances through unseen aquifers. Non-porous aquifers such as clays partially purify water of bacteria by simple filtration (adsorption and absorption), dilution, and, in some cases, chemical reactions and biological activity: however, in some cases, the pollutants merely transform to soil contaminants. Groundwater that moves through cracks and caverns is not filtered and can be transported as easily as surface water. In fact, this can be aggravated by the human tendency to use natural sinkholes as dumps in areas of Karst topography.

There are a variety of secondary effects stemming not from the original pollutant, but a derivative condition. Some of these secondary impacts are:

Silt bearing surface runoff from can inhibit the penetration of sunlight through the water column, hampering Photosynthesis in aquatic plants.

Thermal pollution can induce fish kills and invasion by new thermophyllic species freaking

Regulatory framework

In the UK there are common law rights (civil rights) to protect the passage of water across land unfettered in either quality of quantity. Criminal laws dating back to the 16th century exercised some control over water pollution but it was not until the River (Prevention of pollution )Acts 1951 - 1961 were enacted that any systematic control over water pollutuion was established.

These laws were strengthened and extended in the Control of Pollution Act 1984 which has since been updated and modified by a series of further acts. It is a criminal offence to either pollute a lake, river, groundwater or the sea or to discharge any liquid into such water bodies without proper authority. In England and Wales such permission can only be issued by the Environment Agency and in Scotland by SEPA.

In the USA, concern over water pollution resulted in the enactment of state anti-pollution laws in the latter half of the 19th century, and federal legislation enacted in 1899.

The Refuse Act of the federal Rivers and Harbors Act of 1899 prohibits the disposal of any refuse matter from into either the nation's navigable rivers, lakes, streams, and other navigable bodies of water, or any tributary to such waters, unless one has first obtained a permit. The Water Pollution Control Act, passed in 1948, gave authority to the Surgeon General to reduce water pollution.

Growing public awareness and concern for controlling water pollution led to enactment of the Federal Water Pollution Control Act Amendments of 1972. As amended in 1977, this law became commonly known as the Clean Water Act. The Act established the basic mechanisms for regulating contaminant discharge.

It established the authority for EPA to implement wastewater standards for industry. The Clean Water Act also continued requirements to set water quality standards for all contaminants in surface waters. Further amplification of the Act continued including the enactment of the Great Lakes Legacy Act of 2002.

Sewage is correctly the subset of wastewater that is contaminated with feces or urine, but is often used to mean any waste water. "Sewage" includes domestic, municipal, or industrial liquid waste products disposed of, usually via a pipe or sewer or similar structure, sometimes in a cesspool emptier.

The physical infrastructure, including pipes, pumps, screens, channels etc. used to convey sewage from its origin to the point of eventual treatment or disposal is termed sewerage. In the past the word "sewage" also meant what is now called Possibly because of that, the word "sewerage" is often mistakenly used to mean "sewage".

Wastewater origin

Wastewater or sewage can come from (text in brackets indicates likely inclusions or contaminants) :-

Human waste, usually from lavatories: (fæces, used toilet paper, wipes, urine, other bodily fluids) also known as black water

Cesspit leakage

Septic tank discharge

Sewage treatment plant discharge

Washing water (personal, clothes, floors, dishes, etc.) also known as greywater or sullage

Rainfall collected on roofs, yards, hard-standings, etc. (traces of oils and fuel but generally clean)

Groundwater infiltrated into sewerage.

Surplus manufactured liquids from domestic sources (drinks, cooking oil, pesticides, lubricating oil, paint, cleaning liquids, etc.)

Urban rainfall run-off from roads, car-parks, roofs, side-walks or pavements (contains oils, animal faeces, litter, fuel residues, rubber residues, metals from vehicle exhausts etc)

Seawater ingress (salt, micro-biota, high volumes)

Direct ingress of river water (micro-biota, high volumes)

Direct ingress of man-made liquids (illegal disposal of pesticides, used oils, etc.)

Highway drainage (oil, de-icing agents, rubber residues)

Storm drains (almost anything including cars, shopping trolleys, trees, cattle etc.)

Black water - surface water contaminated by sewage

Industrial waste:-

industrial site drainage (silt, sand, alkali, oil, chemical)

Industrial cooling waters (biocides, heat, slimes, silt)

Industrial process waters

Organic - bio-degradable - includes waste from abattoirs and creameries and ice-cream manufacture.

Organic - non bio-degradable or difficult to treat - for example Pharmaceutical or Pesticide manufacturing

Inorganic - for example from the metalworking industry

extreme pH - from acid/alkali manufacturing, metal plating

Toxic - e.g. from metal plating, cyanide production, pesticide manufacturing

Solids and Emulsions - e.g. Paper manufacturing, food stuffs, lubricating and hydraulic oil manufacture

agricultural drainage - direct and diffuse

Wastewater constituents

The composition of wastewater varies widely. This is a partial list of what it may contain:

Water ( > 95%) which is often added during flushing to carry the waste down a drain

Pathogens such as bacteria, viruses, prions and parasitic worms.

Non-pathogenic bacteria (> 100,000 / ml for sewage)

Organic particles such as faeces, hairs, food, vomit, paper fibres, plant material, humus, etc.

Soluble organic material such as urea, fruit sugars, soluble proteins, drugs, pharmaceuticals, etc.

Inorganic particles such as sand, grit, metal particles, ceramics, etc.

Soluble inorganic material such as ammonia, road-salt, sea-salt, cyanide, hydrogen sulphide, thiocyanates, thiosulphates, etc.

Animals such as protozoa, insects, arthropods, small fish, etc.

Macro-solids such as sanitary towels, nappies/ diapers, condoms, needles, children's toys, dead pets, body parts, etc.

Gases such as hydrogen sulphide, carbon dioxide, methane, etc.

Emulsions such as paints, adhesives, mayonnaise, hair colourants, emulsified oils, etc.

Toxins such as pesticides, poisons, herbicides, etc.

Wastewater quality indicators

Any oxidizable material present in a natural waterway or in an industrial wastewater will be oxidized both by biochemical (bacterial) or chemical processes. The result is that the oxygen content of the water will be decreased. Basically, the reaction for biochemical oxidation may be written as:

Oxidizable material + bacteria + nutrient + O2 ? CO2 + H2O + oxidized inorganics such as NO3 or SO4

Oxygen consumption by reducing chemicals such as sulfides and nitrites is typified as follows:

S-- + 2 O2 ? SO4--

NO2- + ½ O2 ? NO3-

Since all natural waterways contain bacteria and nutrient, almost any waste compounds introduced into such waterways will initiate biochemical reactions (such as shown above). Those biochemical reactions create what is measured in the laboratory as the Biochemical oxygen demand (BOD).

Oxidizable chemicals (such as reducing chemicals) introduced into a natural water will similarly initiate chemical reactions (such as shown above). Those chemical reactions create what is measured in the laboratory as the Chemical oxygen demand (COD).

Both the BOD and COD tests are a measure of the relative oxygen-depletion effect of a waste contaminant. Both have been widely adopted as a measure of pollution effect. The BOD test measures the oxygen demand of biodegradable pollutants whereas the COD test measures the oxygen demand of biogradable pollutants plus the oxygen demand of non-biodegradable oxidizable pollutants.

The so-called 5-day BOD measures the amount of oxygen consumed by biochemical oxidation of waste contaminants in a 5-day period. The total amount of oxygen consumed when the biochemical reaction is allowed to proceed to completion is called the Ultimate BOD. The Ultimate BOD is too time consuming, so the 5-day BOD has almost universally been adopted as a measure of relative pollution effect.

There are also many different COD tests. Perhaps, the most common is the 4-hour COD.

It should be emphasized that there is no generalized correlation between the 5-day BOD and the Ultimate BOD. Likewise, there is no generalized correlation between BOD and COD. It is possible to develop such correlations for a specific waste contaminant in a specific wastewater stream ... but such correlations cannot be generalized for use with any other waste contaminants or wastewater streams.

The laboratory test procedures for the determining the above oxygen demands are detailed in the following sections of the "Standard Methods For the Examination Of Water and Wastewater" available at www.standardmethods.org:

5-day BOD and Ultimate BOD: Sections 5210B and 5210C

COD: Section 5220

Sewage disposal

In some urban areas, sewage is carried separately in sanitary sewers and runoff from streets is carried in storm drains. Access to either of these is typically through a manhole. During high precipitation periods a sanitary sewer overflow can occur, causing potential public health and ecological damage.

Sewage may drain directly into major watersheds with minimal or no treatment. When untreated, sewage can have serious impacts on the quality of an environment and on the health of people. Pathogens can cause a variety of illnesses. Some chemicals pose risks even at very low concentrations and can remain a threat for long periods of time because of bioaccumulation in animal or human tissue.

Treatment

For more details on this topic, see Sewage treatment.

There are numerous processes that can be used to clean up waste waters depending on the type and extent of contamination. Most wastewater is treated in industrial-scale wastewater treatment plants (WWTPs) which may include physical, chemical and biological treatment processes.

However, the use of septic tanks and other On-Site Sewage Facilities (OSSF) is widespread in rural areas, serving up to one quarter of the homes in the U.S. The most important aerobic treatment system is the activated sludge process, based on the maintenance and recirculation of a complex biomass composed by micro-organisms able to degrade the organic matter carried in the wastewater.

Anaerobic processes are widely applied in the treatment of industrial wastewaters and biological sludge. Some wastewater may be highly treated and reused as reclaimed water. For some waste waters ecological approaches using reed bed systems such as constructed wetlands may be appropriate. Modern systems include tertiary treatment by micro filtration or synthetic membranes.

After membrane filtration, the treated wastewater is indistinguishable from waters of natural origin of drinking quality. Nitrates can be removed from wastewater by microbial denitrification, for which a small amount of methanol is typically added to provide the bacteria with a source of carbon.

Ozone Waste Water Treatment is also growing in popularity, and requires the use of an ozone generator, which decontaminates the water as Ozone bubbles percolate through the tank.

Disposal of wastewaters from an industrial plant is a difficult and costly problem. Most petroleum refineries, chemical and petrochemical plants have onsite facilities to treat their wastewaters so that the pollutant concentrations in the treated wastewater comply with the local and/or national regulations regarding disposal of wastewaters into community treatment plants or into rivers, lakes or oceans.

Soil contamination is the presence of man-made chemicals or other alteration of the natural soil environment.

This type of contamination typically arises from the rupture of underground storage tanks, application of pesticides, percolation of contaminated surface water to subsurface strata, leaching of wastes from landfills or direct discharge of industrial wastes to the soil. The most common chemicals involved are petroleum hydrocarbons, solvents,

pesticides, lead and other heavy metals. This occurrence of this phenomenon is correlated with the degree of industrialization and intensity of chemical usage.

The concern over soil contamination stems primarily from health risks, both of direct contact and from secondary contamination of water supplies. Mapping of contaminated soil sites and the resulting cleanup are time consuming and expensive tasks, requiring extensive amounts of geology, hydrology, chemistry and computer modeling skills.

It is in North America and Western Europe that the extent of contaminated land is most well known, with many of countries in these areas having a legal framework to identify and deal with this environmental problem; this however may well be just the tip of the iceberg with developing countries very likely to be the next generation of new soil contamination cases.

The immense and sustained growth of the People's Republic of China since the 1970s has exacted a price from the land in increased soil pollution. The State Environmental Protection Administration believes it to be a threat to the environment, to food safety and to sustainable agriculture.

According to a scientific sampling, 150 million mi (100,000 square kilometres) of China’s cultivated land have been polluted, with contaminated water being used to irrigate a further 32.5 million mi (21,670 square kilometres) and another 2 million mi (1,300 square kilometres) covered or destroyed by solid waste.

In total, the area accounts for one-tenth of China’s cultivatable land, and is mostly in economically developed areas. An estimated 12 million tonnes of grain are contaminated by heavy metals every year, causing direct losses of 20 billion yuan (US$2.57 billion).

The United States, while having some of the most widespread soil contamination, has actually been a leader in defining and implementing standards for cleanup. Other industrialized countries have a large number of contaminated sites, but lag the U.S. in executing remediation. Developing countries may be leading in the next generation of new soil contamination cases.

Each year in the U.S., thousands of sites complete soil contamination cleanup, some by using microbes that “eat up” toxic chemicals in soil[4], many others by simple excavation and others by more expensive high-tech soil vapor extraction or air stripping.

At the same time, efforts proceed worldwide in creating and identifying new sites of soil contamination, particularly in industrial countries other than the U.S., and in developing countries which lack the money and the technology to adequately protect soil resources.

Microanalysis of soil contamination

To understand the fundamental nature of soil contamination, it is necessary to envision the variety of mechanisms for pollutants to become entrained in soil. Soil particulates may be composed of a gamut of organic and inorganic chemicals with variations in cation exchange capacity, buffering capacity, and redox poise.

For example, at the extremes, one has a sand component, a coarse grained, inert, and totally inorganic substance; whereas peat soils are dominated by a fine organic material, made of decomposing organic material and highly active. Most soils are mixtures of soil subtypes and thus have quite complex characteristics.

There is also a great diversity of soil porosity, ranging from gravels to sands to silt to clay (in increasing order of porosity), pore size, and pore tortuosity (both in decreasing order). Finally there is a wide spectrum of chemical bonding or adhesion characteristics: each contaminant has a different interaction or bonding mechanism with a given soil type.

On balance, some contaminants may literally drain through soils such as sand and gravel and move to other soils or deeper aquifers, while polar or organic chemicals discharged into a clay soil will have a very high adsorption.

Thus most soil contamination is the result of pollutants adhering to the soil particle surface, or lodging in interstices of a soil matrix. Clearly the equilibrium reached is a dynamic one, where new pollutants may lodge on new soil particles and the action of groundwater movement may over time transport some of the soil contaminants to other locations or depths.

Soil contamination results when hazardous substances are either spilled or buried directly in the soil or migrate to the soil from a spill that has occurred elsewhere. For example, soil can become contaminated when small particles containing hazardous substances are released from a smokestack and are deposited on the surrounding soil as they fall out of the air.

Another source of soil contamination could be water that washes contamination from an area containing hazardous substances and deposits the contamination in the soil as it flows over or through it.

Health effects

The major concern is that there are many sensitive land uses where people are in direct contact with soils such as residences, parks, schools and playgrounds. Other contact mechanisms include contamination of drinking water or inhalation of soil contaminants which have vaporized.

There is a very large set of health consequences from exposure to soil contamination depending on pollutant type, pathway of attack and vulnerability of the exposed population. Chromium and many of the pesticide and herbicide formulations are carcinogenic to all populations.

Lead is especially hazardous to young children, in which group there is a high risk of developmental damage to the brain and nervous system, while to all populations kidney damage is a risk.

Chronic exposure to benzene at sufficient concentrations is known to be associated with higher incidence of leukemia. Mercury and cyclodienes are known to induce higher incidences of kidney damage, some irreversible. PCBs and cyclodienes are linked to liver toxicity.

Organophosphates and carbamates can induce a chain of responses leading to neuromuscular blockage. Many chlorinated solvents induce liver changes, kidney changes and depression of the central nervous system. There is an entire spectrum of further health effects such as headache, nausea, fatigue, eye irritation and skin rash for the above cited and other chemicals. At sufficient dosages a large number of soil contaminants cause death.

Ecosystem effects

Not unexpectedly, soil contaminants can have significant deleterious consequences for ecosystems. There are radical soil chemistry changes which can arise from the presence of many hazardous chemicals even at low concentration of the contaminant species.

These changes can manifest in the alteration of metabolism of endemic microorganisms and arthropods resident in a given soil environment. The result can be virtual eradication of some of the primary food chain, which in turn have major consequences for predator or consumer species. Even if the chemical effect on lower life forms is small, the lower pyramid levels of the food chain may ingest alien chemicals, which normally become more concentrated for each consuming rung of the food chain.

Many of these effects are now well known, such as the concentration of persistent DDT materials for avian consumers, leading to weakening of egg shells, increased chick mortality and potentially species extinction.

Effects occur to agricultural lands which have certain types of soil contamination. Contaminants typically alter plant metabolism, most commonly to reduce crop yields. This has a secondary effect upon soil conservation, since the languishing crops cannot shield the earth's soil mantle from erosion phenomena.

Some of these chemical contaminants have long half-lives and in other cases derivative chemicals are formed from decay of primary soil contaminants.

Regulatory framework

United States of America

Until about 1970 there was little widespread awareness of the worldwide scope of soil contamination or its health risks. In fact, areas of concern such as Love Canal were often viewed as unusual or isolated incidents. In the U.S., passage of the National Environmental Policy Act in 1969 required careful analysis of the consequences of any federally funded project. Passage of The Resource Conservation and Recovery Act (RCRA) by the U.S.

Congress in 1976 established guidelines not only for handling of hazardous materials but transport and hauling, such as required in cleanup of soil contaminants[8]. In 1980 the U.S. Comprehensive Emergency Response Compensation and Liability Act (CERCLA) was passed to establish, for the first time, strict rules on legal liability for soil contamination.

Not only did CERCLA stimulate identification and cleanup of thousands of sites, but it raised awareness of property buyers and sellers to make soil contamination a focal issue of land use and management practices; moreover, preparation of a Phase I Environmental Site Assessment has become standard practice for many parts of the western world and Japan.

While estimates of remaining soil cleanup in the U.S. may exceed 200,000 sites, in other industrialized countries there is a lag of identification and cleanup functions. Lesser developed countries are not without a share of the creation of soil contamination. Even though their use of chemicals is far less than industrialized countries, often their controls and regulatory framework is quite weak.

For example, some persistent pesticides banned in the U.S. for decades are in widespread uncontrolled use in developing countries. It is worth noting that the cost of cleaning up a soil contaminated site can range from as little as about $10,000 for a small spill, which can be simply excavated, to millions of dollars for a widespread event, especially for a chemical that is very mobile such as MTBE or perchloroethylene.

China

China, an economy that regularly records double digit annual economic growth, has little or no legislation to protect the environment. Currently, given the amount of land in question (up to one-tenth of China's cultivatable land may be polluted), the degree of the pollution in specific locations is unclear, making both prevention and remedy difficult.

There are no laws or environmental standards regarding soil. Funding is limited, too, so there is little advanced scientific study of China’s soil taking place. The severity of the pollution is not understood by either the public or business, and the situation is worsening.

United Kingdom

Two sources of published generic guidance are currently commonly used in the UK:

The Contaminated Land Exposure Assessment (CLEA) Guidelines

The Dutch Standards.

Guidance by the Inter Departmental Committee for the Redevelopment of Contaminated Land (ICRCL) has been formally withdrawn by the Department for Environment, Food and Rural Affairs (DEFRA), for use as a prescriptive document to determine the potential need for remediation or further assessment. Therefore, no further reference is made to these former guideline values.

Other generic guidance that may be referred to (to put the concentration of a particular contaminant in context), include the United States EPA Region 9 Preliminary Remediation Goals (US PRGs), the US EPA Region 3 Risk Based Concentrations (US EPA RBCs) and National Environment Protection Council of Australia Guideline on Investigation Levels in Soil and Groundwater.

The CLEA model published by DEFRA and the Environment Agency (EA) in March 2002 sets a framework for the appropriate assessment of risks to human health from contaminated land, as required by Part IIA of the Environmental Protection Act 1990. As part of this framework, generic Soil Guideline Values (SGVs) have currently been derived for ten contaminants to be used as “intervention values”. These values should not be considered as remedial targets but values above which further detailed assessment should be considered.

Three sets of CLEA SGVs have been produced for three different land uses, namely:

residential (with and without plant uptake)

allotments

commercial/industrial

It is intended that the SGVs replace the former ICRCL values. It should be noted that the CLEA SGVs relate to assessing chronic (long term) risks to human health and do not apply to the protection of ground workers during construction, or other potential receptors such as groundwater, buildings, plants or other ecosystems. The CLEA SGVs are not directly applicable to a site completely covered in hardstanding, as there is no direct exposure route to contaminated soils.

To date, the first ten of fifty-five contaminant SGVs have been published, for the following: arsenic, cadmium, chromium, lead, inorganic mercury, nickel, selenium ethyl benzene, phenol and toluene. Draft SGVs for benzene, naphthalene and xylene have been produced but their publication is on hold.

Toxicological data (Tox) has been published for each of these contaminants as well as for benzo[a]pyrene, benzene, dioxins, furans and dioxin-like PCBs, naphthalene, vinyl chloride, 1,1,2,2 tetrachloroethane and 1,1,1,2 tetrachloroethane, 1,1,1 trichloroethane, tetrachloroethene, carbon tetrachloride, 1,2-dichloroethane, trichloroethene and xylene.

The SGVs for ethyl benzene, phenol and toluene are dependent on the soil organic matter (SOM) content (which can be calculated from the total organic carbon (TOC) content). As an initial screen the SGVs for 1% SOM are considered to be appropriate.

Cleanup options

Cleanup or remediation is analyzed by environmental scientists who utilize field measurement of soil chemicals and also apply computer models for analyzing transport and fate of soil chemicals. Thousands of soil contamination cases are currently in active cleanup across the U.S. as of 2006. There are several principal strategies for remediation:

Excavate soil and remove it to a disposal site away from ready pathways for human or sensitive ecosystem contact. This technique also applies to dredging of bay muds containing toxins.

Aeration of soils at the contaminated site (with attendant risk of creating air pollution)

Bioremediation, involving microbial digestion of certain organic chemicals. Techniques used in bioremediation include landfarming, biostimulation and bioaugmentation soil biota with commercially available microflora.

Extraction of groundwater or soil vapor with an active electromechanical system, with subsequent stripping of the contaminants from the extract.

Containment of the soil contaminants (such as by capping or paving over in place).

Landfill

A landfill, also known as a dump (and historically as a midden), is a site for the disposal of waste materials by burial and is the oldest form of waste treatment. Historically, landfills have been the most common methods of organized waste disposal and remain so in many places around the world.

Landfills may include internal waste disposal sites (where a producer of waste carries out their own waste disposal at the place of production) as well as sites used by many producers. Many landfills are also used for other waste management purposes, such as the temporary storage, consolidation and transfer, or processing of waste material (sorting, treatment, or recycling).

A landfill also may refer to ground that has been filled in with soil and rocks instead of waste materials, so that it can be used for a specific purpose, such as for building houses. Unless they are stabilized, these areas may experience severe shaking or liquefaction of the ground in a large earthquake.

Types & construction methods

Most modern landfills are classified according to the type(s) of waste material disposed of into them. Landfills can be engineered to a high standard in order to contain liquid leachate or landfill gas produced by decomposing organic waste. Modern landfills generally require a minimum of one landfill liner, consisting of a layer of compacted clay with a minimum required thickness and a maximum allowable hydraulic conductivity.

Others also require the addition of one or more layers of impermeable membrane, such as high-density polyethylene (HDPE) together with geotextile. Various final cover systems are used to 'cap' landfills (such as clay or topsoil), depending on the type of wastes present within the landfill.

Landfills, based on the waste type that is disposed within them, may be classified as:

Hazardous waste landfill: waste disposal units constructed to specific design criteria and which receive wastes meeting the local definition of hazardous waste. These landfills are generally constructed to be secure repositories for material that presents a serious hazard to human health, such as high-level radioactive waste.

They are restricted, by permit or law, to the types of waste that they may handle (chemical vs. radioactive, liquid vs. dry). Double liner systems are the norm for hazardous waste landfills. Deep geological repository of high level radioactive waste is not generally classified as landfilling.

Sanitary landfills: also called modern, engineered or secure landfills, these usually have physical barriers such as liners and leachate collection systems, and procedures to protect the public from exposure to the disposed wastes. The term sanitary landfill normally refers to those where municipal solid waste is disposed of, as well as other wastes high in organic material. In some countries, all landfills are sanitary landfills.

Inert waste landfill: waste disposal units that receive wastes which are chemically and physically stable and do not undergo decomposition, such as sand, bricks, concrete or gravel.

Dumps: also simply called landfills, dumps are landfills that are not engineered with the special protective measures required by sanitary landfills. They are most common in rural, remote, and developing areas. Many jurisdictions prohibit the use of non-sanitary landfills for the disposal of municipal solid waste.

Other jurisdictions that do allow dumps may require them to be constructed according to some engineering standard to mitigate the risk for environmental contamination, such as by limiting the slope, requiring compaction, or ensuring that the cell is high enough above the groundwater table.

Subsystems

A typical landfill consists of subsystems such as the:

Landfill liner

Leachate collection and management system

Landfill gas management system

Landfill gas monitoring & leachate monitoring systems

Road network

Drainage system

Final landfill cap

Their function is to secure the normal landfill operations and to control the anticipated emissions generated mainly by the decomposition of organic matter, such as leachate and landfill gas.

Site construction requirements

The construction of a landfill requires a staged approach. Landfill designers are primarily concerned with the viability of a site. To be commercially and environmentally viable a landfill must be constructed in accord with specific requirements, which are related to:

Location

Easy access to transport by road

Transfer stations if rail network is preferred

Land value

Cost of meeting government requirements, such as the Environment Agency in England and Wales

Location of community served

Type of construction (more than one may be used at single site)

Pit - filling existing holes in the ground, typically left behind by mining

Canyon - filling in naturally occurring valleys or canyons

Mound - piling the waste up above the ground

Stability

Underlying geology

Nearby earthquake faults

Water table

Location of nearby rivers, streams, and flood plains

Capacity The available voidspace must be calculated by comparison of the landform with a proposed restoration profile.

This calulation of capacity is based on,

Density of the wastes

Amount of intermediate and daily cover

Amount of settlement that the waste will undergo following tipping

Thickness of capping

Construction of lining and drainage layers.

Protection of soil and water through:

Installation of liner and collection systems.

Storm water control

Leachate management.

Landfill gas management.

Nuisances and hazards management.

Costs

Feasiability studies

Site after care

Site investigations (costs involved make small sites uneconomic).

Operations

Typically, in non hazardous waste landfills, in order to meet predefined specifications, techniques are applied by which the wastes are:

1: Confined to as small an area as possible.

2: Compacted to reduce their volume.

3: Covered (usually daily) with layers of soil.

During landfill operations the waste collection vehicles are weighed at a weigh-bridge on arrival and their load is inspected for wastes that do not accord with the landfill’s waste acceptance criteria. Afterwards, the waste collection vehicles use the existing road network on their way to the tipping face or working front where they unload their load. After loads are deposited, compactors or dozers are used to spread and compact the waste on the working face.

Before leaving the landfill boundaries, the waste collection vehicles pass through the wheel cleaning facility. If necessary, they return to weighbridge in order to be weighed without their load. Through the weighing process, the daily incoming waste tonnage can be calculated and listed in databases. In addition to trucks, some landfills may be equipped to handle railroad containers. The use of 'rail-haul' permits landfills to be located at more remote sites, without the problems associated with many truck trips.

Typically, in the working face, the compacted waste is covered with soil daily. Alternative waste cover materials are several sprayed on foam products and temporary blankets. Blankets can be lifted into place with tracked excavators and then removed the following day prior to waste placement.

Chipped wood and chemically 'fixed' bio-solids, may also be used as an alternate daily cover. The space that is occupied daily by the compacted waste and the cover material is called daily cell. Waste compaction is critical to extending the landfill life. Factors such as waste compressibility, waste layer thickness and the number of passes of the compactor over the waste affect the waste densities.

Land reclamation

As human overcrowding of developed areas intensified during the 20th century, it has become important to develop land re-use strategies for completed landfills. Some of the most common usages are for parks, golf courses and other sports fields. Increasingly, however, office buildings and industrial uses are made of a completed landfill. In these latter uses, methane capture is customarily carried out to minimize explosive hazard within the building.

An example of a Class A office building constructed over a landfill is the Dakin Building at Sierra Point, Brisbane, California. The underlying fill was deposited from 1965 to 1985, mostly consisting of construction debris from San Francisco and some municipal wastes. Aerial photographs prior to 1965 show this area to be tidelands of the San Francisco Bay. A clay cap was constructed over the debris prior to building approval.

Another strategy for landfill reclamation is the incineration of landfill trash at high temperature via the plasma-arc gasification process, which is currently used at two facilities in Japan, and will be used at a planned facility in St. Lucie County, Florida.

Impacts

A number of problems can occur from landfill operations. These impacts can vary: fatal accidents (e.g., scavengers buried under waste piles), infrastructure damage (e.g., damage to access roads by heavy vehicles), pollution of the local environment (such as contamination of groundwater and/or aquifers by leakage and residual soil contamination after landfill closure), injuries to wildlif and simple nuisance problems (e.g., dust, odour, vermin, or noise pollution).

Environmental noise and dust are generated from vehicles accessing a landfill as well as from working face operations. These impacts are best to intercept at the planning stage where access routes and landfill geometrics can be used to mitigate such issues. Vector control is also important, but can be managed reasonably well with the daily cover protocols.

Most modern landfills are operated with controls to manage problems such as these. Analysis of common landfill operational problems are available in.

Some local authorities have found it difficult to locate new landfills. These authorities may charge a fee or levy in order to discourage waste and/or recover the costs of site operations. Some landfills are operated for profit as commercial businesses.

The greenhouse effect, discovered by Joseph Fourier in 1829 and first investigated quantitatively by Svante Arrhenius in 1896, is the process in which the emission of infrared radiation by the atmosphere warms a planet's surface.

The name comes from an analogy with the warming of air inside a greenhouse compared to the air outside the

greenhouse. The Earth's average surface temperature is about 20-30°C warmer than it would be without the greenhouse effect . In addition to the Earth, Mars and especially Venus have greenhouse effects.

In common usage, "greenhouse effect" may refer either to the natural greenhouse effect due to naturally occurring greenhouse gases, or to the enhanced (anthropogenic) greenhouse effect which results from gases emitted as a result of human activities (see also global warming, scientific opinion on climate change and attribution of recent climate change).

The basic mechanism

The Earth receives energy from the Sun in the form of radiation. The Earth reflects about 30% of the incident solar flux; the remaining 70% is absorbed, warming the land, atmosphere and oceans.

To the extent that the Earth is in a steady state, the energy stored in the atmosphere and ocean does not change in time, so energy equal to the absorbed solar radiation must be radiated back to space. Earth radiates energy into space as black-body radiation, which maintains a thermal equilibrium.

This thermal, infrared radiation increases with increasing temperature. One can think of the Earth's temperature as being determined by the infrared flux needed to balance the absorbed solar flux.

The visible solar radiation heats the surface, not the atmosphere, whereas most of the infrared radiation escaping to space is emitted from the upper atmosphere, not the surface. The infrared photons emitted by the surface are mostly absorbed by the atmosphere and do not escape directly to space.

The reason this warms the surface is most easily understood by starting with a simplified model of a purely radiative greenhouse effect that ignores energy transfer in the atmosphere by convection (sensible heat transport) and by the evaporation and condensation of water vapor (latent heat transport).

In this purely radiative case, one can think of the atmosphere as emitting infrared radiation both upwards and downwards. The upward infrared flux emitted by the surface must balance not only the absorbed solar flux but also this downward infrared flux emitted by the atmosphere. The surface temperature will rise until it generates thermal radiation equivalent to the sum of these two incident radiation streams.

A more realistic picture taking into account the convective and latent heat fluxes is somewhat more complex. But the following simple model captures the essence. The starting point is to note that the opacity of the atmosphere to infrared radiation determines the height in the atmosphere from which most of the photons emitted to space are emitted.

If the atmosphere is more opaque, the typical photon escaping to space will be emitted from higher in the atmosphere, because one then has to go to higher altitudes to see out to space in the infrared. Since the emission of infrared radiation is a function of temperature, it is the temperature of the atmosphere at this emission level that is effectively determined by the requirement that the emitted flux balance the absorbed solar flux.

But the temperature of the atmosphere generally decreases with height above the surface, at a rate of roughly 6.5 °C per kilometer on average, until one reaches the stratosphere 10-15 km above the surface. (Most infrared photons escaping to space are emitted by the troposphere, the region bounded by the surface and the stratosphere, so we can ignore the stratosphere in this simple picture.)

A very simple model, but one that proves to be remarkably useful, involves the assumption that this temperature profile is simply fixed, by the non-radiative energy fluxes. Given the temperature at the emission level of the infrared flux escaping to space, one then computes the surface temperature by increasing temperature at the rate of 6.5 °C per kilometer, the environmental lapse rate, until one reaches the surface.

The more opaque the atmosphere, and the higher the emission level of the escaping infrared radiation, the warmer the surface, since one then needs to follow this lapse rate over a larger distance in the vertical. While less intuitive than the purely radiative greenhouse effect, this less familiar radiative-convective picture is the starting point for most discussions of the greenhouse effect in the climate modeling literature.

The term "greenhouse effect" is a source of confusion in that actual greenhouses do not warm by this same mechanism (e.g.).

The greenhouse gases

Quantum mechanics provides the basis for computing the interactions between molecules and radiation. Most of this interaction occurs when the frequency of the radiation closely matches that of the spectral lines of the molecule, determined by the quantization of the modes of vibration and rotation of the molecule. (The electronic excitations are generally not relevant for infrared radiation, as they require energy larger than that in an infrared photon.)

The width of a spectral line is an important element in understanding its importance for the absorption of radiation. In the Earth’s atmosphere these spectral widths are primarily determined by “pressure broadening”, which is the distortion of the spectrum due to the collision with another molecule.

Most of the infrared absorption in the atmosphere can be thought of as occurring while two molecules are colliding. The absorption due to a photon interacting with a lone molecule is relatively small. This three-body aspect of the problem, one photon and two molecules, makes direct quantum mechanical computation for molecules of interest more challenging.

Careful laboratory spectroscopic measurements, rather than ab initio quantum mechanical computations, provide the basis for most of the radiative transfer calculations used in studies of the atmosphere.

The molecules/atoms that constitute the bulk of the atmosphere; oxygen (O2), nitrogen (N2) and argon; do not interact with infrared radiation significantly. While the oxygen and nitrogen molecules can vibrate, because of their symmetry these vibrations do not create any transient charge separation that enhances the interaction with radiation.

In the Earth’s atmosphere, the dominant infrared absorbing gases are water vapor, carbon dioxide, and ozone (O3), these molecules being “floppier” so that their rotation/vibration modes are more easily excited.

For example, carbon dioxide is a linear molecule, but it has an important vibrational mode in which the molecule bends with the carbon in the middle moving one way and the oxygens on the ends moving the other way, creating some charge separation, a dipole moment. A substantial part of the greenhouse effect due to carbon dioxide exists because this vibration is easily excited by infrared radiation.

Clouds are also very important infrared absorbers. Therefore, water has multiple effects on infrared radiation, through its vapor phase and through its condensed phases. Other absorbers of significance include methane, nitrous oxide and the chlorofluorocarbons.

Discussion of the relative importance of different infrared absorbers is confused by the overlap between the spectral lines due to different gases, widened by pressure broadening. As a result, the absorption due to one gas cannot be thought of as independent of the presence of other gases.

One convenient approach is to remove the chosen constituent, leaving all other absorbers, and the temperatures, untouched, and monitoring the infrared radiation escaping to space. The reduction in infrared absorbtion is then a measure of the importance of that constituent.

More precisely, define the greenhouse effect (GE) to be the difference between the infrared radiation that the surface would radiate to space if there were no atmosphere and the actual infrared radiation escaping to space.Then compute the percentage reduction in GE when a constituent is removed. The table below is computed by this method, using a particular 1-dimensional model of the atmosphere. More recent 3D computations lead to similar results.

Gas removed

percent reduction in GE

H2O 36%

CO2 12%

O3 3%

(Source: Ramanathan and Coakley, Rev. Geophys and Space Phys., 16 465 (1978)); see also .

By this particular measure, water vapor can be thought of as providing 36% of the greenhouse effect, and carbon dioxide 12%, but the effect of removal of both of these constituents will be greater than 48%. An additional proviso is that these numbers are computed holding the cloud distribution fixed.

But removing water vapor from the atmosphere while holding clouds fixed is not likely to be physically relevant. In addition, the effects of a given gas are typically nonlinear in the amount of that gas, since the absorption by the gas at one level in the atmosphere can remove photons that would otherwise interact with the gas at another altitude.

The kinds of estimates presented in the table, while often encountered in the controversies surrounding global warming, must be treated with caution. Different estimates found in different sources typically result from different definitions and do not reflect uncertainties in the underlying radiative transfer.

Positive feedback and runaway greenhouse effect

When the concentration of a greenhouse gas (A) is itself a function of temperature, there is a positive feedback from the increase in another greenhouse gas (B), whereby increase in B increases the temperature which, in turn, increases the concentration of A, which increases temperatures further, and so on. This feedback is bound to stop, since the overall supply of the gas A must be finite. If this feedback ends after producing a major temperature increase, it is called a runaway greenhouse effect.

According to some climate models (Clathrate gun hypothesis), such a runaway greenhouse effect, involving liberation of methane gas from hydrates by global warming, caused the Permian-Triassic extinction event. It is also thought that large quantities of methane could be released from the Siberian tundra as it begins to thaw, methane being 21-times more potent a greenhouse gas than carbon dioxide.

A runaway greenhouse effect involving CO2 and water vapor may have occurred on Venus. On Venus today there is little water vapor in the atmosphere. If water vapor did contribute to the warmth of Venus at one time, this water is thought to have escaped to space. Venus is sufficiently strongly heated by the Sun that water vapor can rise much higher in the atmosphere and is split into hydrogen and oxygen by ultraviolet light.

The hydrogen can then escape from the atmosphere and the oxygen recombines. Carbon dioxide, the dominant greenhouse gas in the current Venusian atmosphere, likely owes its larger concentration to the weakness of carbon recycling as compared to Earth, where the carbon dioxide emitted from volcanoes is efficiently subducted into the Earth by plate tectonics on geologic time scales.

Anthropogenic greenhouse effect

CO2 production from increased industrial activity (fossil fuel burning) and other human activities such as cement production and tropical deforestation has increased the CO2 concentrations in the atmosphere.

Measurements of carbon dioxide amounts from Mauna Loa observatory show that CO2 has increased from about 313 ppm (parts per million) in 1960 to about 375 ppm in 2005. The current observed amount of CO2 exceeds the geological record of CO2 maxima (~300 ppm) from ice core data (Hansen, J., Climatic Change, 68, 269, 2005 ISSN 0165-0009).

Because it is a greenhouse gas, elevated CO2 levels will increase global mean temperature; based on an extensive review of the scientific literature, the Intergovernmental Panel on Climate Change concludes that "most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations" .

Over the past 800,000 years , ice core data shows unambiguously that carbon dixoide has varied from values as low as 180 parts per million (ppm) to the pre-industrial level of 270ppm . Certain paleoclimatologists consider variations in carbon dioxide to be a fundamental factor in controlling climate variations over this time scale.

Real greenhouses

The term 'greenhouse effect' originally came from the greenhouses used for gardening, but it is a misnomer since greenhouses operate differently. A greenhouse is built of glass; it heats up primarily because the Sun warms the ground inside it, which warms the air near the ground, and this air is prevented from rising and flowing away.

The warming inside a greenhouse thus occurs by suppressing convection and turbulent mixing. This can be demonstrated by opening a small window near the roof of a greenhouse: the temperature will drop considerably. It has also been demonstrated experimentally (Wood, 1909): a "greenhouse" built of rock salt (which is transparent to IR) heats up just as one built of glass does.

Greenhouses thus work primarily by preventing convection; the atmospheric greenhouse effect however reduces radiation loss, not convection. It is quite common, however, to find sources (e.g.,) that make the "greenhouse" analogy. Although the primary mechanism for warming greenhouses is the prevention of mixing with the free atmosphere, the radiative properties of the glazing can still be important to commercial growers.

With the modern development of new plastic surfaces and glazings for greenhouses, this has permitted construction of greenhouses which selectively control radiation transmittance in order to better control the growing environment.

The term acid rain also known as acid precipitation is commonly used to mean the deposition of acidic components in rain, snow, dew, or dry particles. The more accurate term is "acid precipitation."

Acid rain occurs when sulfur dioxide and nitrogen oxides are emitted into the atmosphere, undergo chemical transformations and are absorbed by water droplets in

clouds. The droplets then fall to earth as rain, snow, mist, dry dust, hail, or sleet. This increases the acidity of the soil, and affects the chemical balance of lakes and streams.

The term "acid rain" is sometimes used more generally to include all forms of acid deposition — both wet deposition, where acidic gases and particles are removed by rain or other precipitation, and dry deposition removal of gases and particles to the Earth's surface in the absence of precipitation.

Acid rain is defined as any type of precipitation with a pH that is unusually low. Dissolved carbon dioxide dissociates to form weak carbonic acid giving a pH of approximately 5.6 at typical atmospheric concentrations of CO2. Therefore a pH of less than 5.6 has sometimes been used as a definition of acid rain.

However, natural sources of acidity mean that in remote areas, rain has a pH which is between 4.5 and 5.6 with an average value of 5.0 and so rain with a pH of less than 5 is a more appropriate definition.

The US EPA says, "Acid rain is a serious environmental problem that affects large parts of the US and Canada" Acid rain accelerates weathering in carbonate rocks and accelerates building weathering. It also contributes to acidification of rivers, streams, and forest damage at high elevations. When the acid builds up in rivers and streams it can kill fish.

History and trends

Evidence for an increase in the levels of acid rain comes from analyzing layers of glacial ice. These show a sudden decrease in pH from the start of the Industrial Revolution of 6 to 4.5 or 4. Other information has been gathered from studying organisms known as diatoms which inhabit ponds.

Over the years these die and are deposited in layers of sediment on the bottoms of the ponds. Diatoms thrive in certain pH levels, so the numbers of diatoms found in sediment layers of increasing depth give an indication of the change in pH over the years.

Since the industrial revolution, emissions of sulfur and nitrogen oxides to the atmosphere have increased. Occasional pH readings of well below 2.4 (the acidity of vinegar) have been reported in industrialized areas. Industrial acid rain is a substantial problem in the People's Republic of China, Eastern Europe, Russia and areas down-wind from them. These areas all burn sulfur-containing coal to generate heat and electricity.

The problem of acid rain not only has increased with population and industrial growth, but has become more widespread. The use of tall smokestacks to reduce local pollution has contributed to the spread of acid rain by releasing gases into regional atmospheric circulation.

Often deposition occurs a considerable distance downwind of the emissions, with mountainous regions tending to receive the most (simply because of their higher rainfall). An example of this effect is the low pH of rain (compared to the local emissions) which falls in Scandinavia.

Acid rain was first found in Manchester, England. In 1852, Robert Angus Smith found the relationship between acid rain and atmospheric pollution. Though acid rain was discovered in 1852, it wasn't until the late 1960s that scientists began widely observing and studying the phenomenon. Canadian Harold Harvey was among the first to research a "dead" lake.

Public awareness of acid rain in the U.S increased in the 1990s after the New York Times promulgated reports from the Hubbard Brook Experimental Forest in New Hampshire of the myriad deleterious environmental effects demonstrated to result from it.

Emissions of chemicals leading to acidification

The most important gas which leads to acidification is sulfur dioxide. Emissions of nitrogen oxides which are oxidized to form nitric acid are of increasing importance due to stricter controls on emissions of sulfur containing compounds. 70 Tg(S) per year in the form of SO2 comes from fossil fuel combustion and industry, 2.8 Tg(S) from wildfires and 7-8 Tg(S) per year from volcanoes.

Natural phenomena

The principal natural phenomena that contribute acid-producing gases to the atmosphere are emissions from volcanoes and those from biological processes that occur on the land, in wetlands, and in the oceans. The major biological source of sulfur containing compounds is dimethyl sulfide.

The effects of acidic deposits have been detected in glacial ice thousands of years old in remote parts of the globe.

Human activity

The principal cause of acid rain is sulfuric and nitrogen compounds from human sources, such as electricity generation, factories and motor vehicles. Coal power plants are one of the most polluting. The gases can be carried hundreds of kilometres in the atmosphere before they are converted to acids and deposited.

Factories used to have short funnels to let out smoke, but this caused many problems, so now, factories have longer smoke funnels. The problem with this, is those pollutants get carried far off, where it creates more destruction.

Gas phase chemistry

Chemistry in cloud droplets

When clouds are present the loss rate of SO2 is faster than can be explained by gas phase chemistry alone. This is due to reactions in the liquid water droplets

Hydrolysis

Sulfur dioxide dissolves in water and then, like carbon dioxide, hydrolyses in a series of equilibrium reactions:

SO2 (g)+ H2O ? SO2·H2O

SO2·H2O ? H++HSO3-

HSO3- ? H++SO32-

Oxidation

There are a large number of aqueous reactions that oxidise sulfur from S(IV) to S(VI), leading to the formation of sulfuric acid. The most important oxidation reactions are with ozone, hydrogen peroxide and oxygen (reactions with oxygen are catalysed by iron and manganese in the cloud droplets).

For more information see Seinfeld and Pandis (1998).

Acid deposition

Wet deposition

Wet deposition of acids occurs when any form of precipitation (rain, snow, etc) removes acids from the atmosphere and delivers it to the Earth's surface. This can result from the deposition of acids produced in the raindrops (see aqueous phase chemistry above) or by the precipitation removing the acids either in clouds or below clouds. Wet removal of both gases and aerosol are both of importance for wet deposition.

Dry deposition

Acid deposition also occurs via dry deposition in the absence of precipitation. This can be responsible for as much as 20 to 60% of total acid deposition. This occurs when particles and gases stick to the ground, plants or other surfaces.

Surface waters and aquatic animals

Both the lower pH and higher aluminium concentrations in surface water that occur as a result of acid rain can cause damage to fish and other aquatic animals.

At pHs lower than 5 most fish eggs will not hatch and lower pHs can kill adult fish. As lakes become more acidic biodiversity is reduced.

Acid rain has eliminated insect life and some fish species, including the brook trout in some Appalachian streams and

creeks.'

Soils

Soil biology can be seriously damaged by acid rain. Some tropical microbes can quickly consume acids but other microbes are unable to tolerate low pHs and are killed. The enzymes of these microbes are denatured (changed in shape so they no longer function) by the acid. The hydronium ions of acid rain also mobilize toxins and leach away essential nutrients and minerals

Forests and other vegetation

Acid rain can slow the growth of forests, cause leaves and needles to turn brown and fall off and die. In extreme cases trees or whole areas of forest can die. The death of trees is not usually a direct result of acid rain, often it weakens trees and makes them more susceptible to other threats.

Damage to soils (see above) can also cause problems. High altitude forests are especially vulnerable as they are often surrounded by clouds and fog which are more acidic than rain.

Other plants can also be damaged by acid rain but the effect on food crops is minimized by the application of fertilizers to replace lost nutrients. In cultivated areas, limestone may also be added to increase the ability of the soil to keep the pH stable, but this tactic is largely unusable in the case of wilderness lands. Acid Rain depletes minerals from the soil and then it stunts the growth of the plant.

Human health

Some scientists have suggested direct links to human health, but none have been proven. However, fine particles, a large fraction of which are formed from the same gases as acid rain (sulfur dioxide and nitrogen dioxide), have been shown to cause illness and premature deaths such as cancer and other deadly diseases For more information on the health effects of aerosol see: Particulate#Health effects.

Other adverse effects

Acid rain can also cause damage to certain building materials and historical monuments. Acid rain can cause weathering on ancient and valuable statues and has caused considerable damage. This is because the sulfuric acid in the rain chemically reacts with the calcium compounds in the stones (limestone, sandstone, marble and granite) to create gypsum, which then flakes off.

This is also commonly seen on old gravestones where the acid rain can cause the inscription to become completely illegible. Acid rain also causes an increased rate of oxidation for iron. Visibility is also reduced by sulfate and nitrate in the atmosphere.

Prevention methods

Technical solutions

In the United States, many coal-burning power plants use Flue gas desulfurization (FGD) to remove sulfur-containing gases from their stack gases. An example of FGD is the wet scrubber which is commonly used in the U.S. and many other countries. A wet scrubber is basically a reaction tower equipped with a fan that extracts hot smoke stack gases from a power plant into the tower.

Lime or limestone in slurry form is also injected into the tower to mix with the stack gases and combine with the sulfur dioxide present. The calcium carbonate of the limestone produces pH-neutral calcium sulfate that is physically removed from the scrubber. That is, the scrubber turns sulfur pollution into industrial sulfates.

In some areas the sulfates are sold to chemical companies as gypsum when the purity of calcium sulfate is high. In others, they are placed in landfill. However, the effects of acid rain can last for generations, as the effects of pH level change can stimulate the continued leaching of undesirable chemicals into otherwise pristine water sources, killing off vulnerable insect and fish species and blocking efforts to restore native life.

International treaties

A number of international treaties on the long range transport of atmospheric pollutants have been agreed e.g. Sulphur Emissions Reduction Protocol and Convention on Long-Range Transboundary Air Pollution.

Emissions trading

An even more benign regulatory scheme involves emissions trading. In this scheme, every current polluting facility is given an emissions license that becomes part of capital equipment. Operators can then install pollution control equipment, and sell parts of their emissions licenses. The main effect of this is to give operators real economic incentives to install pollution controls.

Since public interest groups can retire the licenses by purchasing them, the net result is a continuously decreasing and more diffused set of pollution sources. At the same time, no particular operator is ever forced to spend money without a return of value from commercial sale of assets.

The presence of oil has significant social and environmental impacts, from accidents and routine activities such as seismic exploration, drilling, and generation of polluting wastes.

Oil extraction is costly and sometimes environmentally damaging, although Dr. John Hunt of the Woods Hole

Oceanographic Institution pointed out in a 1981 paper that over 70% of the reserves in the world are associated with visible macroseepages, and many oil fields are found due to natural leaks. Offshore exploration and extraction of oil disturbs the surrounding marine environment.

But at the same time, offshore oil platforms also form micro-habitats for marine creatures. Extraction may involve dredging, which stirs up the seabed, killing the sea plants that marine creatures need to survive. Crude oil and refined fuel spills from tanker ship accidents have damaged ecosystems in Alaska, the Galapagos Islands, Spain, and many other places.

Burning oil releases carbon dioxide into the atmosphere, which contributes to global warming. Per energy unit, oil produces less CO2 than coal, but more than natural gas. However, oil's unique role as a transportation fuel makes reducing its CO2 emissions a particularly thorny problem; amelioration strategies such as carbon sequestering are generally geared for large power plants, not individual vehicles.

Renewable energy alternatives do exist, but given current technology, alternatives are uneconomical.

Solar, wind, geothermal, and other renewable electricity sources cannot directly replace high energy density liquid petroleum for transportation use; instead automobiles and other equipment must be altered to allow using electricity (in batteries) or hydrogen (via fuel cells or internal combustion) which can be produced from renewable sources.

Other options include using biomass-origin liquid fuels (ethanol, biodiesel). Any combination of solutions to replace petroleum as a liquid transportation fuel will be a very large undertaking.

Oil Spill is the unintentional release of liquid petroleum hydrocarbon into the environment as a result of human activity. The term often refers to marine oil spills, where oil is released into the ocean or coastal waters.

Oil can refer to many different materials, including crude oil, refined petroleum products (such as gasoline or diesel

fuel) or by-products, ships' bunkers, oily refuse or oil mixed in waste. They take months or even years to clean up.

Oil is also released into the environment from natural geologic seeps on the seafloor, as along the California coastline. Most man-made oil pollution comes from land-based activity, but public attention and subsequent regulation has tended to focus most sharply on seagoing oil tankers.

The fate, behavior and environmental effects of spilled oil can vary, depending upon the type and amount of material spilled. In general, lighter refined petroleum products such as diesel and gasoline are more likely to mix in the water column and are more toxic to marine life, but tend to evaporate more quickly and do not persist long in the environment.

Heavier crude or fuel oil, while of less immediate toxicity, can remain on the water surface or stranded on the shoreline for much longer.

Oil from the Exxon Valdez and Gulf War oil spills, while weathering over time, has persisted along the shoreline for years after the spill. By contrast, the Braer spill off the Shetland Islands in 1993 and the Sea Empress spill off Milford Haven in 1996 left almost no long-term environmental damage, despite both being twice as large as Exxon Valdez.

With large numbers of people living and depending on coastal areas for fishing and tourism throughout the world, the consequences of oil spills can be serious. Such possibilities have caused outcries for oil companies, ship owners, and shipbuilders to share the responsibility of preventing such a disaster.

As oil is lighter than water, and does not quickly decompose, it can remain on the surface for a long time. As it is also flammable, oil spills can fuel ocean fires.

Ships today are better equipped and built than ever before, with all new-build large tankers having double hulls. Ship-source pollution, which averaged over 400,000 tons a year at its peak in the late 1970s, averaged 27,000 tonnes per year from 2000-05, a fall of 93%.

Another way to mitigate the effects of an oil spill is through being able to contain and effectively treat spilled oil. A new vital step being developed in preventing oil damage is through installations of systems made for Fast Oil Recovery (FOR) of oil from wrecked ships.

Environmental effects

Studies of the Exxon Valdez oil spill have shown that the external damage caused by oil spills can be greater than was previously thought. It is now thought that the impacts to marine life can be less than one part per billion petroleum hydrocarbons[citation needed]. The lighter fractions of oil, such as benzene and toluene, are more toxic, but are more volatile and evaporate quickly.

Heavier components of crude oil, such as polynuclear aromatic hydrocarbons (PAHs) appear to cause the most damage; while they are less toxic, they persist in the environment much longer than volatile components.

A heavy oil spill across the shore blankets rock-pools etc, preventing gas exchange and eliminating light as well as directly leaching toxins into the water; it can also become mixed deeply into pebble, shingle or sandy beaches, where it may remain for months or even years. Well-weathered heavy oil on inter-tidal rocks doesn't retain serious toxicity - for example, it will be grazed off by limpets without apparent ill-effect.

Seabirds are severely affected by spills as the oil penetrates and opens up the structure of their plumage, reducing the insulating ability of their feathers and making them more vulnerable to temperature fluctuations and much less buoyant in the water.

The oil also disrupts the birds flight abilities, limiting their ability to forage and escape from predators; they then ingest the oil as they attempt to preen causing kidney damage, altered liver function, and digestive tract irritation.

The limited foraging ability coupled with the ingestion of the oil quickly causes dehydration and metabolic imbalances. Most birds affected by an oil spill will die without human intervention.

The effects of oil spills on marine mammals such as sea otters and seals reduce their coat's natural insulation, leading to body temperature fluctuations and hypothermia. Ingestion of the oil has the same effects on marine mammals as it does on birds.

Largest oil spills

Oil Spills of over 100,000 tonnes or 30 million US gallons, ordered by Tonnes Spill / Tanker Location Date *Tonnes of crude oil link

Gulf War oil spill Persian Gulf 1991 January 23 5,000,000

Ixtoc I oil well Gulf of Mexico 1979 June 3 - 1980 March 23 454,000 - 480,000

Atlantic Empress / Aegean Captain Trinidad and Tobago 1979 July 19 287,000

Fergana Valley Uzbekistan 1992 March 2 285,000

Nowruz oil field Persian Gulf 1983 February 260,000

ABT Summer 700 nautical miles off Angola 1991 260,000

Castillo de Bellver Saldanha Bay, South Africa 1983 August 6 252,000

Amoco Cadiz Brittany, France 1978 March 16 223,000

Amoco Haven tanker disaster Mediterranean Sea near Genoa, Italy 1991 144,000

Odyssey 700 nautical miles off Nova Scotia, Canada 1988 132,000

Sea Star Gulf of Oman 1972 December 19 115,000

Torrey Canyon Scilly Isles, UK 1967 March 18 80,000 - 119,000

Irenes Serenade Navarino Bay, Greece 1980 100,000

Urquiola A Coruña, Spain 1976 May 12 100,000

(*) One tonne of crude oil is roughly equal to 308 US gallons, or 7.33 barrels.

Estimating the Volume of a Spill

By observing the thickness of the film and it's appearance on the surface of the water, it is possible to estimate the quantity of oil spilled. If the surface area of the spill is known, the total volume of the oil can be calculated from this information.

Film Thickness Quantity Spread

Appearance in mm gal/sq mi L/ha

Barely visible 0.0000015 0.0000381 25 0.365

Silvery sheen 0.0000030 0.0000762 50 0.731

First trace of color 0.0000060 0.0001524 100 1.461

Bright bands of color 0.0000120 0.0003048 200 2.922

Colors begin to dull 0.0000400 0.0010160 666 9.731

Colors are much darker 0.0000800 0.0020320 1332 19.463

Source: Metcalf & Eddy. Wastewater Engineering, Treatment and Reuse. 4th ed. New York: McGraw-Hill, 2003. 98.

Methods of cleaning an oil spill

A sheen can not be cleaned up. A sheen can be dispersed (but not cleaned up) with detergents which makes oil settle to the bottom and makes the seabed toxic. It is very difficult to clean up oils denser than water as they settle to the bottom; PCBs are an example of such a pollutant.

Some of the equipment used in cleaning up include:

Absorbent Boom, Sausage

Containment Boom (except for gasoline where confinement can cause dangerous levels of fume buildup)

Skimmers

Snare

Some of the methods used include:

Bioremediation: use of biological agents to remove oil

Burning: It can be done only when it is not windy, the oil has not dispersed and there is a calm sea.

Detergent: This is never advised as it makes the oil settle to the bottom and while it disperses oil and makes the surface look pretty, it does not clean oil up.

Do nothing: Sometimes it is better to do nothing and let the oil evaporate or break down on its own than make matters worse by attempting to clean up. Cleanup by detergents pollutes the seabed. Shoreline cleanup can further disturb the ecology by bleaching all life from the area.

Dredging: for oils dispersed with detergents and other oils denser than water.

Skimming:It can't be done if there is a rough sea

Solidifying

Recycling is the reprocessing of materials into new products.

Recycling prevents useful material resources being wasted, reduces the consumption of raw materials and reduces energy usage, and hence greenhouse gas emissions, compared to virgin production.

Recycling is a key concept of modern waste management

and is the third component of the waste hierarchy.

Recyclable materials, also called "recyclables" or "recyclates", may originate from a wide range of sources including the home and industry. They include glass, paper, aluminium, asphalt, iron, textiles and plastics. Biodegradable waste, such as food waste or garden waste, is also recyclable with the assistance of micro-organisms through composting or anaerobic digestion.

Recyclates need to be sorted and separated into material types. Contamination of the recylates with other materials must be prevented to increase the recyclates value and facilitate easier reprocessing for the ultimate recycling facility. This sorting can be performed either by the producer of the waste or within semi- or fully-automated materials recovery facilities.

There are two common household methods of helping increase recycling. In curbside collection (UK: kerbside collection) consumers leave presorted materials for recycling in front of their property, typically in boxes or sacks to be collected by a recycling vehicle.

Alternatively, with a "bring" or carry-in system, the householder may take the materials to collection points, such as transfer stations or civic amenity sites, where recyclates are placed into recycling bins based on the type of material.

Recycling does not include reuse where items retain their existing form for other purposes without the need for reproducing.

History

Recycling has been a common practice throughout human history. In pre-industrial times, scrap made of bronze and other precious metals were collected in Europe and melted down for perpetual reuse, and in Britain dust and ash from wood and coal fires was downcycled as a base material in brick making.

The main driver for these types of recycling was the economic advantage of obtaining recycled feedstock instead of acquiring virgin material, as well as a lack of public waste removal in ever more-populated sites.

Paper recycling began in Britain in 1921, when the British Waste Paper Association (now Confederation of Paper Industries) was established to encourage trade in waste paper recycling.

Resource shortages caused by the world wars, and other such world-changing occurrences greatly encouraged recycling. Massive government promotion campaigns were carried out in World War II in every country involved in the war, urging citizens to donate metals and conserve fiber, as a matter of significant patriotic importance.

Resource conservation programs established during the war were continued in some countries without an abundance of natural resources, such as Japan, after the war ended.

The next big investment in recycling occurred in the 1970s, due to rising energy costs (recycling aluminum uses only 5% of the energy required by virgin production; glass, paper and metals have less dramatic but very significant energy savings when recycled feedstock is used).

The passage of the Clean Water Act of 1977 in the USA created strong demand for bleached paper (office paper whose fibre has already been bleached white increased in value as water effluent became more expensive).

In 1973, the city of Berkeley, California began one of the first curbside collection programs with monthly pick ups of newspapers from residences. Since then several countries have started and expanded various doorstep collection schemes.

In 1987, the Mobro 4000 barge hauled garbage from New York to North Carolina, where it had been denied. It was then sent to Belize, where it was denied as well. Finally, the barge returned to New York and the garbage was incinerated. The incident led to heated discussions about waste disposal and recycling.

One event that initiated recycling efforts occurred in 1989 when Berkeley banned the use of polystyrene packaging for keeping McDonald's hamburgers warm. One effect of this ban was to raise the ire of management at Dow Chemical, the world's largest manufacturer of polystyrene, which led to the first major effort to show that plastics can be recycled. By 1999, there were 1,677 companies in the USA alone involved in the post-consumer plastics recycling business.

Benefits

Recycling is beneficial in two ways: it reduces the inputs (energy and raw materials) to a production system and reduces the amount of waste produced for disposal.

Some materials like aluminum can be recycled indefinitely as there is no change to the materials. Other recycled materials like paper require a percentage of raw materials (wood fibers) to be added to compensate for the degradation of existing fibers.

The resources being processed are purer, less energy is needed to process them and less energy is needed to transport from the place of extraction (e.g. bauxite/aluminium ore mines in Brazil or coniferous forests in Scandinavia).

This reduces the environmental, social, and usually the economic costs of manufacturing.

For example, bauxite mines in Brazil displace indigenous people, create noise pollution from blasting, machinery and transport, and create air pollution in the form of particulates (dust).

The habitat loss and visual destruction is also negative both to the aesthetic qualities of the areas and the local environment. However, the mines do provide employment and revenue to the local population and economy, promoting development of the country as a whole.

Recycling aluminium saves 95% of the energy cost of processing new aluminium because the melting temperature is reduced from 900 °C to 600 °C. It is by far the most efficient material to recycle.

The most commonly used methods for waste disposal (landfill, pyrolysis, incineration) may be environmentally damaging and unsustainable. Therefore any way to reduce the volume of waste being disposed in this fashion may be beneficial. The maximum environmental benefit is gained by waste minimization (reducing the amount of waste produced), and reusing items in their current form such as refilling bottles.

Drawbacks

All recycling techniques consume energy for transportation and processing, and some also use considerable amounts of water.

There may also be drawbacks with the collection methods associated with recycling. Increasing collections of separated wastes adds to vehicle movements and the production of carbon dioxide. This may be negated however by centralized facilities such as some advanced material recovery facilities of mechanical biological treatment systems for the separation of mixed wastes.

However, this is almost never the case for urban areas, taking into account the massive number of workers, machines, and vehicles needed for the recycling process.

Perverse consequences from mercury recycling have been cited recently by the Wall Street Journal (April 20, 2006). The article traces mercury recovered from USA recycling programs into sales of mercury for alluvial mining activities in Brazil.

During the autumn of 2006, the EU banned the export of liquid mercury (Europe has no mercury mining, only recovery from recycling). A full life-cycle analysis prior to institution of recycling programs may reduce the risk of unintended environmental consequences.

Recycling techniques

Many different materials can be recycled but each type requires a different technique.

Aggregates & concrete

Concrete aggregate collected from demolition sites is put through a crushing machine, often along with asphalt, bricks, dirt, and rocks. Smaller pieces of concrete are used as gravel for new construction projects.

Crushed recycled concrete can also be used as the dry aggregate for brand new concrete if it is free of contaminants. This reduces the need for other rocks to be dug up, which in turn saves trees and habitats.

Batteries

The large variation in size and type of batteries makes their recycling extremely difficult: they must first be sorted into similar kinds and each kind requires an individual recycling process. Additionally, older batteries contain mercury and cadmium, harmful materials which must be handled with care.

Lead-acid batteries, like those used in automobiles, are relatively easy to recycle and many new lead-acid batteries contain a high percentage of recycled material.

Biodegradable waste

Biodegradable waste can be recycled into useful material by biological decomposition. There are two mechanisms by which this can occur. The most common mechanism of recycling of household organic waste is home composting or municipal curbside collection of green wastes sent to large scale composting plants.

Alternatively organic waste can be converted into biogas and soil improver using anaerobic digestion. Here organic wastes are broken down by anaerobic microorganisms in biogas plants.

The biogas can be converted into renewable electricity or burnt for environmentally friendly heating. Advanced technologies such as mechanical biological treatment are able to sort the recyclable elements of the waste out before biological treatment by either composting, anaerobic digestion or biodrying.

Electronics disassembly and reclamation

The direct disposal of electrical equipment — such as old computers and mobile phones — is banned in many areas due to the toxic contents of certain components.

The recycling process works by mechanically separating the metals, plastics and circuit boards contained in the appliance. When this is done on a large scale at an electronic waste recycling plant, component recovery can be achieved in a cost-effective manner.

Electronic devices, including audio-visual components (televisions, VCRs, stereo equipment), mobile phones and other hand-held devices, and computer components, contain valuable elements and substances suitable for reclamation, including lead, copper, and gold.

They also contain a plethora of toxic substances, such as dioxins, PCBs, cadmium, chromium, radioactive isotopes, and mercury. Additionally, the processing required to reclaim the precious substances (including incineration and acid treatments) release, generate and synthesize further toxic by-products.

In the United States, an estimated 70% of heavy metals in landfills come from discarded electronics.

Some regional governments are attempting to curtail the accumulation of electronics in landfills by passing laws obligating manufacturers and consumers to recycle these devices, but because in many cases safe dismantlement of these devices in accordance with first world safety standards is unprofitable, historically much of the electronic waste has been shipped to countries with lower or less rigorously-enforced safety protocols.

Places like Guiyu, China dismantle tons of electronics every year, profiting from the sale of precious metals, but at the cost of the local environment and the health of its residents.

Mining to produce the same metals, to meet demand for finished products in the west, also occurs in the same countries, and the United Nations Conference on Trade and Development (UNCTAD) has recommended that restrictions against recycling exports be balanced against the environmental costs of recovering those materials from mining. Hard rock mining in the USA produces 45% of all toxics produced by all USA industries (2001 US EPA Toxics Release Inventory).

Printer ink cartridges & toners

Printer ink cartridges can be recycled. They are sorted into different brands and models which are then resold back to the companies that created these cartridges. The companies then refill the ink reservoir which can be sold back to consumers.

Toner cartridges are recycled the same way as ink cartridges, using toner instead of ink. This method of recycling is highly efficient as there is no energy spent on melting and recreating the recycled object itself.

Ferrous metals

Iron and steel are the world's most recycled materials, and among the easiest materials to recycle, as they can be separated magnetically from the waste stream. Recycling is via a steelworks: scrap is either remelted in an Electric Arc Furnace (90-100% scrap), or used as part of the charge in a Basic Oxygen Furnace (around 25% scrap).

Any grade of steel can be recycled to top quality new metal, with no 'downgrading' from prime to lower quality materials as steel is recycled repeatedly. 42% of crude steel produced is recycled material.

Non-ferrous metals

Aluminium is shredded and ground into small pieces or crushed into bales. These pieces or bales are melted in an aluminium smelter to produce molten aluminium. By this stage the recycled aluminium is indistinguishable from virgin aluminium and further processing is identical for both.

Due to the high melting point of aluminum ore, large amounts of energy are required to extract aluminum from ore, making the environmental benefits of recycling aluminium enormous.

Approximately 5% of the CO2 is produced during the recycling process compared to producing raw aluminium (and an even smaller percentage when considering the complete cycle of mining and transporting the aluminium). Also, as open-cut mining most often used for obtaining aluminium ore, mining destroys large sections of natural land.

An aluminium can is 100% recyclable every time it is recycled, it saves enough energy to watch television for about three hours (compared to mining and producing a new can).

Glass

Glass bottles and jars are accumulated via curbside collection schemes and bottle banks, where the glass may be sorted into color categories. The collected glass cullet is taken to a glass recycling plant where it is monitored for purity and contaminants are removed.

The cullet is crushed and added to a raw material mix in a melting furnace. It is then mechanically blown or molded into new jars or bottles. Glass cullet is also used in the construction industry for aggregate and glassphalt. Glassphalt is a road-laying material which comprises around 30% recycled glass. Glass can be recycled indefinitely as its structure does not deteriorate when reprocessed.

Paper

Recycled paper is made from waste paper, usually mixed with fresh wood pulp. If the paper contains ink, it must be deinked. This also removes fillers, clays, and fiber fragments.

Almost all paper can be recycled today, but some types are harder to recycle than others. Kraft paper, papers coated with plastic or aluminum foil, and papers that are waxed, pasted, or gummed are usually not recycled because the process is too expensive. Gift wrap paper also cannot be recycled.

Different types of paper are usually sorted before recycling, such as newspapers and cardboard boxes.

Different grades of paper are recycled into different types of new products. Old newspapers are usually made into new newsprint, egg cartons, or paperboard. Old corrugated boxes are made into new corrugated boxes or paperboard. High-grade white office paper can be made into almost any new paper product: stationery, newsprint, magazines, or books.

Sometimes recyclers ask for the removal of the glossy inserts from newspapers because they are a different type of paper. Glossy inserts have a heavy clay coating that some paper mills cannot accept. Since the paper is weighed down by the clay coating, a paper mill gets more recyclable fibers from a ton of pure newsprint.

Paper can only be recycled a finite number of times due to the shortening of paper fibers making the material less versatile. Often it will be mixed with a quantity of virgin material, referred to as downcycling.

This does not however exclude the material from being used in other processes such as composting or anaerobic digestion, where further value can be extracted from the material in the form of compost or biogas.

Plastic

Plastic recycling is the process of recovering scrap or waste plastics and reprocessing the material into useful products. Compared to glass or metallic materials, plastic poses unique challenges - because of the massive number of types of plastic, they each carry a resin identification code, and must be sorted before they can be recycled.

This can be costly - while metals can be sorted using electromagnets, no such 'easy sorting' capability exists for plastics. In addition to this, while labels do not need to be removed from bottles for recycling, lids are often made from a different kind of non-recyclable plastic.

Plastics recycling rates lag far behind those of other items, such as newspaper and aluminium; consumers are typically unsure of how to recycle plastics, and compared to paper and metals fewer recycling facilities exist.[citation needed]

Finally, recycled plastic is less appealing to manufacturers than new plastic.

Ship breaking

A form of metal recovery associated to recycling is "ship breaking". This is the process of breaking a ship into smaller, recyclable pieces of metal. It often has a number of major drawbacks to the local community and the local environment where ship breaking occurs.

Ship breaking tends to occur in poor countries where lack of or insufficient safety standards, labor laws and wage agreements makes them a lucrative area for demolition work. India, Pakistan, Turkey and Bangladesh make up the majority of these countries.

Toxic material in the form of metals, gas, fumes and exhaust often contaminate a large area surrounding the ship breaking yards, including nearby villages and sleeping quarters for the workers, which are commonly located near the yards.

Material such as paint, electrical equipment, wire, anodes and coatings are often burned or simply dumped in the dismantling process. This releases metals such as mercury, lead, arsenic and chromium.

Polychlorinated organic compounds are another source of toxic material that can be found in transformers and cable insulation often burned or dumped in and around the ship breaking yard.

It is believed that many of the social, economical and environmental drawbacks in shipbreaking could be alleviated greatly by adhering to safe handling of the recycling process, or the ship owner decontaminating the toxins from the ship before it gets sent to be demolished.

Textiles

When considering textile recycling one must understand what the material consists of. Most textiles are composites of cotton (biodegradable material) and synthetic plastics. The textile's composition will affect its durability and method of recycling.

Workers sort and separate collected textiles into good quality clothing and shoes which can be reused or worn. These sorting facilities are in a trend of being moved from developed countries such as the UK to developing countries.[9]

Damaged textiles are further sorted into grades to make industrial wiping cloths and for use in paper manufacture or material which is suitable for fibre reclamation and filling products. If textile reprocessors receive wet or soiled clothes however, these may still end up being disposed of in landfill, as the washing and drying facilities are not present at sorting units.

Fibre reclamation mills sort textiles according to fibre type and colour. Colour sorting eliminates the need to re-dye the recycled textiles. The textiles are shredded into "shoddy" fibres and blended with other selected fibres, depending on the intended end use of the recycled yarn.

The blended mixture is carded to clean and mix the fibres and spun ready for weaving or knitting. The fibres can also be compressed for mattress production. Textiles sent to the flocking industry are shredded to make filling material for car insulation, roofing felts, loudspeaker cones, panel linings and furniture padding.

Timber

Recycling timber has become popular due to its image as an environmentally friendly product, with consumers commonly believing that by purchasing recycled wood the demand for green timber will fall and ultimately benefit the environment. Greenpeace also view recycled timber as an environmentally friendly product, citing it as the most preferable timber source on their website.

The arrival of recycled timber as a construction product has been important in both raising industry and consumer awareness towards deforestation and promoting timber mills to adopt more environmentally friendly practices.

Criticism

Many areas of recycling have come under criticism or scrutiny, most notably the claimed benefits that recycling saves energy, reduces greenhouse gas emissions and creates jobs.

International Universal Recycling Codes

The communication and identification are laid out in International Universal Recycling Codes. These codes outline what material an item is made from, to facilitate easier reprocessing.

The UN Environment Programme (or UNEP) coordinates United Nations environmental activities, assisting developing countries in implementing environmentally sound policies and encourages sustainable development through sound environmental practices.

It was founded as a result of the United Nations Conference on the Human Environment in December 1972 and is headquartered in Gigiri, Nairobi, Kenya. UNEP also has six regional offices and various country offices.

Its activities cover a wide range of issues regarding the atmosphere, marine and terrestrial ecosystems.

It has played a significant role in developing international environmental conventions, promoting environmental science and information and illustrating the way those can work in conjunction with policy, working on the development and implementation of policy with national governments and regional institution and working in conjunction with environmental Non-Governmental Organizations (NGOs). UNEP has also been active in funding

and implementing environmentally related development projects.

UNEP has aided in the development of guidelines and treaties on issues such as the international trade in potentially harmful chemicals, transboundary air pollution, and contamination of international waterways.

The World Meteorological Organization and the UNEP established the Intergovernmental Panel on Climate Change (IPCC) in 1988. UNEP is also one of several Implementing Agencies for the Global Environment Facility (GEF).

Structure

UNEP's Governing Council consists of a total of 58 member states which serve three-year terms. These seats are allocated according to geographical regions. The Governing Council is the primary developer of policy guidelines for UN environmental programs and plays a diplomatic role in promoting cooperation between UN member states on environmental issues.

The UNEP secretariat consists of 890 staff members, roughly 500 of which are international staff while the remaining are hired locally. The Secretariat is the body which oversees the implementation of UNEP policies and programs and is responsible for the annual budget which totals around $105 million (US) and is almost entirely earned from member states.

Executive Director

UNEP's current Executive Director is Achim Steiner, who succeeded previous director Klaus Töpfer in 2006. Dr Töpfer served two consecutive terms, beginning in February 1998.

On 15 March 2006, the United Nations Secretary-General, Kofi Annan, nominated Achim Steiner, former Director General of the IUCN to the position of Executive Director. One day later, the UN General Assembly followed Annan's proposal and elected him.

The position was held for 17 years (1975-1992) by Dr. Mostafa Kamal Tolba, who was instrumental in bringing environmental considerations to the forefront of global thinking and action. Under his leadership, UNEP's most widely acclaimed success - the historic 1988 agreement to protect the ozone layer - the Montreal Protocol was negotiated.

Awards

UNEP established a Global 500 Roll of Honour in 1987 to recognize notable contributions by individuals and organizations. This scheme closed in 2003 and has been replaced by Champions of the Earth.

International Years

2007: (International) Year of the Dolphin

The year 2007 has been declared as (International) Year of the Dolphin - (http://www.yod2007.org) by the United Nations and UNEP (United Nations Environment Programme).

The UN Convention on Migratory Species, together with its specialized agreements on dolphin conservation ACCOBAMS and ASCOBANS and the WDCS (Whale and Dolphin Conservation Society) have proposed 2007 as the Year of the Dolphin ('YOD'))

(International) Patron of the Year of the Dolphin is H.S.H. Prince Albert II of Monaco.

Reform

Following the publication of Fourth Assessment Report of the Intergovernmental Panel on Climate Change in February 2007, a "Paris Call for Action" read out by French President Chirac and supported by 46 countries, called for the United Nations Environment Programme to be replaced by a new and more powerful 'United Nations Environment Organization' (UNEO), to be modelled on the World Health Organization.

The 46 countries included the European Union nations, but notably did not include the United States, China, Russia, and India, the top four emitters of greenhouse gasses.

Specific objectives of the organization:

mapping current activities and establish a forum for information exchange on SBE initiatives, so that gaps and overlaps may be reduced and common standards established increasing awareness of existing SBE initiatives and issues amongst the international buildings and

construction community;

taking action on fields not covered by existing organisations and networks

Main activities:

networking - helping specialists and generalists to get to know each others’ abilities and needs

managing and providing technical support for the international Sustainable Building Challenge (SBC) process, which involves over 15 countries in the development and testing of a rating tool for buildings. This work has led to the development of a performance rating tool called SBTool (GBTool before), which is unique in its ability to adopt to local needs and conditions

managing The Sustainable Building Information System (SBIS), a web-based database of international R&D information relating to sustainable building

running a web-based database called the Skills Registry, which features a searchable file of the skills and experience of individuals and organizations.

iiSBE and CIB are major co-sponsors of the SB conference series, which began in Maastricht in 2000, then went on to Oslo in 2002 and, in September 2005, in Tokyo. The venue for SB08 will be Melbourne, Australia.

In partnership with CIB and UNEP, iiSBE also sponsored a series of SB conferences in 2004 in developing regions and in Central/Eastern Europe, all linked structurally to the Tokyo SB05 event. These events were held in Sao Paolo, Stellenbosch (South Africa) Shanghai, Warsaw, Kuala Lumpur and Athens, and the regional coordinators presented regional summaries of sustainable building in their regions at SB05. A similar series will take place in 2007.