Photochemical Air Pollution in the Troposphere

VIII-2.1 The Earth s Atmosphere, 330 VIII-2.2 Atmospheric Air Pollution, 332 VIII-2.3 Photochemical Air Pollution in the Troposphere, 332 VIFI-2.4 Air Pollution in the Stratosphere, 340 VI11-3 Photochemistry of the Atmospheres of Other Planets, 352 VH1—3.1 Photochemistry of the Mars Atmosphere, 352 VII 1—3,2 Photochemistry of the Venus Atmosphere. 356 VIIl-3.3 Photochemistry of the Jovian Atmosphere, 357... 

Photochemical air pollution in the troposphere results from a complex interplay between sunlight and primary air pollutants emitted in ambient air that leads to the formation of ozone and other oxidizing and cye-irritaling agents. On the other hand, pollutants injected into the stratosphere by such human activities as supersonic transports (SST s) and release ofchlorofiuoro-methancs in air by their use as aerosol propellants and refrigerants may eventually reduce the protective layer of ozone from harsh solar ultraviolet radiation. Although the full impact of injected air pollutants in the stratosphere is not apparent at present, various model calculations show conclusively that the continuous future release of chlorofluoromethanes and NO (NO and N02) would result in substantial reduction of ozone in the stratosphere. 

Photochemical air pollution in the troposphere is induced by the action of solar ultraviolet radiation upon mixtures of NO (NO and N02), SO (S02 and sulfates), and reactive hydrocarbons (mostly olefins) emitted in the atmosphere by automobiles and power plants. 

Studies of the photochemical processes of small molecules are not only of intrinsic interest but also are important in understanding the photochemistry of isotope enrichment, of air pollution in the troposphere and stratosphere, and of the atmospheres of other planets. 

Photochemical air pollution of the earth s troposphere and stratosplu i. involves a series of complex reactions initiated by sunlight. Thanks to ili< large body of information accumulated in recent years, the main proas., leading to the formation of photochemical smog are well understood although the details of some reactions are still unknown. 

Ozone is a key component of photochemical smog. Although it is an essential UV screen in the upper atmosphere, it is an undesirable pollutant in the troposphere. It is extremely reactive and toxic, and breathing air that contains appreciable amounts of ozone can be especially dangerous for asthma sufferers, exercisers, and the elderly. We therefore have two ozone problems excessive amounts in many urban environments, where it is harmful, and depletion in the stratosphere, where it is vital. 

Certainly, photochemical air pollution is not merely a local problem. Indeed, spread of anthropogenic smog plumes away from urban centers results in regional scale oxidant problems, such as found in the NE United States and many southern States. Ozone production has also been connected with biomass burning in the tropics (79,80,81). Transport of large-scale tropospheric ozone plumes over large distances has been documented from satellite measurements of total atmospheric ozone (82,83,84), originally taken to study stratospheric ozone depletion. 

In addition to the criteria pollutants, a wide variety of trace gaseous and particulate species are present in the polluted troposphere (Finlayson-Pitts and Pitts, 1997). Table 2.9 shows some of these gaseous noncriteria pollutants identified in photochemical air pollution and gives typical concentrations under conditions ranging from those in remote areas to severely polluted urban air (see also Chapter 11). 

The above mentioned urban air pollution in Asian cities drives the tropospheric chemical reactions. This tropospheric chemistry is dominated by the oxidation of trace atmospheric components, as aresult ofwhich organic compounds such as methane and other hydrocarbons are converted into carbon dioxide and water. The consequences of these chemical transformations are known as photochemical smog (photosmog) and the associated problem of ground level ozone. Here we should consider also the effects of particulate matter, one of the major pollutants of urban air in Asia. 

The hydroxyl radical is central in tropospheric chemistry and photochemical smog formation. The hydroxyl radical OH is continually being formed and consumed in the troposphere and it has very short half-life due to its high reactivity, especially in urban polluted air. This species carries no charge, and is therefore chemically distinct from hydroxyl ion, OH", which has an additional electron. The major route for the formation of hydroxyl radical in the troposphere occurs by a complicated mechanism. 

Because PAN is in thermal equilibrium with NO2 and the peroxyacetyl radical, it can act as a means of transporting these more reactive species over long distances. The NO2 released by thermal decomposition of PAN is photolyzed rapidly in the troposphere to form O3 by Reaction 19.1 and Reaction 19.2. Ozone is a criteria air pollutant and is a major health concern. Thus, the PANs play important roles as a chemical means of transporting key species such as NO2 and formaldehyde to remote locations. As such, PANs are globally important atmospheric molecules, as well as urban air pollutants. Since the original observation of PANs in Los Angeles photochemical smog, PANs have been measured in every corner of the world. 

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