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Atmospheric chemistry

A scientific discipline concerned with the chemical composition of the Earth's atmosphere. Topics include the emission, transport, and deposition of atmospheric chemical species; the rates and mechanisms of chemical reactions taking place in the atmosphere; and the effects of atmospheric species on human health, the biosphere, and climate.

A useful quantity in atmospheric chemistry is the atmospheric lifetime, defined as the mean time that a molecule resides in the atmosphere before it is removed by chemical reaction or deposition. The atmospheric lifetime measures the time scale on which changes in the production or loss rates of a species may be expected to translate into changes in the species concentration. The atmospheric lifetime can also be compared to the time scales for atmospheric transport to infer the spatial variability of a species in the atmosphere; species with lifetimes longer than a decade tend to be uniformly mixed, while species with shorter lifetimes may have significant gradients reflecting the distributions of their sources and sinks.Atmospheric general circulation

The principal constituents of dry air are nitrogen (N2; 78% by volume), oxygen (O2; 21%), and argon (Ar; 1%). The atmospheric concentrations of N2 and Ar are largely determined by the total amounts of N and Ar released from the Earth's interior since the origin of the Earth. The atmospheric concentration of O2 is regulated by a slow atmosphere-lithosphere cycle involving principally the conversion of O2 to carbon dioxide (CO2) by oxidation of organic carbon in sedimentary rocks (weathering), and the photosynthetic conversion of CO2 to O2 by marine organisms which precipitate to the bottom of the ocean to form new sediment. This cycle leads to an atmospheric lifetime for O2 of about 4 million years.Photosynthesis

Water vapor concentrations in the atmosphere range from 3% by volume in wet tropical areas to a few parts per million by volume (ppmv) in the stratosphere. Water vapor, with a mean atmospheric lifetime of 10 days, is supplied to the troposphere by evaporation from the Earth's surface, and it is removed by precipitation. Because of this short lifetime, water vapor concentrations decrease rapidly with altitude, and little water vapor enters the stratosphere. Oxidation of methane represents a major source of water vapor in the stratosphere, comparable to the source contributed by transport from the troposphere.

The most abundant carbon species in the atmosphere is CO2. It is produced by oxidation of organic carbon in the biosphere and in sediments. The atmospheric concentration of CO2 is rising, and there is concern that this may cause significant warming of the Earth's surface because of the ability of CO2 to absorb infrared radiation emitted by the Earth (the greenhouse effect). The total amount of carbon present in the atmosphere is small compared to that present in the other geochemical reservoirs, and therefore it is controlled by exchange with these reservoirs. Equilibration of carbon between the atmosphere, biosphere, soil, and surface ocean reservoirs takes place on a time scale of decades.Greenhouse effect

Methane is the second most abundant carbon species in the atmosphere and an important greenhouse gas. It is emitted by anaerobic decay of biological carbon (for example, in wetlands, landfills, and stomachs of ruminants), by exploitation of natural gas and coal, and by combustion. It has a mean lifetime of 12 years against atmospheric oxidation by the hydroxyl (OH) radical, its principal sink.

Many hydrocarbons other than methane are emitted to the atmosphere from vegetation, soils, combustion, and industrial activities. The emission of isoprene [H2CC(CH3)CHCH2] from deciduous vegetation is particularly significant. Nonmethane hydrocarbons have generally short lifetimes against oxidation by OH (a few hours for isoprene), so that their atmospheric concentrations are low. They are most important in atmospheric chemistry as sinks for OH and as precursors of tropospheric ozone, organic nitrates, and organic aerosols.

Carbon monoxide (CO) is emitted to the atmosphere by combustion, and it is also produced within the atmosphere by oxidation of methane and other hydrocarbons. It is removed from the atmosphere by oxidation by OH, with a mean lifetime of 2 months. Carbon monoxide is the principal sink of OH and hence plays a major role in regulating the oxidizing power of the atmosphere.

Nitrous oxide (N2O) is of environmental importance as a greenhouse gas and as the stratospheric precursor for the radicals NO and NO2. The principal sources of N2O to the atmosphere are microbial processes in soils and the oceans; the main sinks are photolysis and oxidation in the stratosphere, resulting in an atmospheric lifetime for N2O of about 130 years.

About 90% of total atmospheric ozone (O3) resides in the stratosphere, where it is produced by photolysis of O2. The ultraviolet photons (λ < 240 nm) needed to photolyze O2 are totally absorbed by ozone and O2 as solar radiation travels through the stratosphere. As a result, ozone concentrations in the troposphere are much lower than in the stratosphere. Stratosphere Troposphere

Tropospheric ozone plays a central role in atmospheric chemistry by providing the primary source of the strong oxidant OH. It is also an important greenhouse gas. In surface air, ozone is of great concern because of its toxicity to humans and vegetation. Ozone is supplied to the troposphere by slow transport from the stratosphere, and it is also produced within the troposphere by a chain reaction involving oxidation of CO and hydrocarbons by OH in the presence of NOx. Ozone production by this mechanism is particularly rapid in urban areas, where emissions of NOx and of reactive hydrocarbons are high.

Sulfuric acid produced in the atmosphere by oxidation of sulfur dioxide (SO2) is a major component of aerosols in the atmosphere and an important contributor to acid deposition. Sources of SO2 to the atmosphere include emission from combustion, smelters, and volcanoes, and oxidation of oceanic dimethylsulfide [(CH3)2S] emitted by phytoplankton. It is estimated that about 75% of total sulfur emission to the atmosphere is anthropogenic. Air pollution

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From McGraw-Hill Concise Encyclopedia of Environmental Science. The Content is a copyrighted work of McGraw-Hill and McGraw-Hill reserves all rights in and to the Content. The Work is © 2008 by The McGraw-Hill Companies, Inc.
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