Summary of Photochemical Smog

Only a very small portion of the hundreds of reactions possible in polluted atmospheres have been presented, but these are representative of the types of mechanism being considered to account for photoinitiation, oxidation of NO and VOCs, and formation of noxious products associated with photochemical smog. Figure 5-12 shows a computer integration of the differential kinetic rate

45The oxidation of HOSO2. to H2SO4 in the troposphere is not simply the addition of another .OH radical. Rather, it is mostly by heterogeneous reactions taking place in cloud droplets, probably involving hydrogen peroxide, H2O2.

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FIGURE 5-12 Computer simulation of a generalized mechanism representing photochemical smog (see text). Redrawn from J. H. Seinfeld and S. N. Pandis, Atmospheric Chemistry and Physics: From Air Pollution to Climate Change. Copyright © 1998. New York: John Wiley & Sons, Inc. Used by permission of John Wiley & Sons, Inc.

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FIGURE 5-12 Computer simulation of a generalized mechanism representing photochemical smog (see text). Redrawn from J. H. Seinfeld and S. N. Pandis, Atmospheric Chemistry and Physics: From Air Pollution to Climate Change. Copyright © 1998. New York: John Wiley & Sons, Inc. Used by permission of John Wiley & Sons, Inc.

equations representing a generalized mechanism involving 20 steps—including many of the reactions and organic species given earlier—for a system initially containing equal concentrations (0.10 ppmv) of NO, H2CO, and organic species groups RH (alkanes) and RCHO (aldehydes), and a small amount (0.01 ppmv) of NO2.46 Compare this computer simulation with the photochemical smog chamber results (that however did not have initial amounts of H2CO and RCHO), where RH = C3H6, shown in Figure 5-11. Note the qualitative similarities between the two: the rapid conversion of NO to NO2 and the subsequent loss of NO2; the slower loss of RH (C3H in Figure 5-11) and buildup of ozone, after most of the NO has reacted; the buildup and decay of RCHO (CH3CHO in Figure 5-11); and the induction period in the

46J. H. Seinfeld and S. N. Pandis, Atmospheric Chemistry and Physics: From Air Pollution to Climate Change, pp. 292-298. Wiley, New York, 1998.

production of PAN. Applicable trends are also evident on the measurements of actual atmospheric conditions (Figure 5-9).

Although so far there have not been any large disasters directly attributable to photochemical smog, the quality of the environment is seriously degraded by its presence, and it is now widespread in the industrial world. Ambient ozone is one of the most pervasive environmental problems facing urban and regional areas. In the next chapter we review many of the remedies and legislative actions being undertaken to reduce the emissions of particulates, VOCs, carbon monoxide, and oxides of nitrogen (primarily from automobiles), and to suppress or limit secondary pollutants. There continues to be a great need for reliable kinetic data for the many chemical processes occurring in a polluted environment and for accurate models to simulate their physical and chemical behaviors.

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