84 86 88 90 92 94 Year

FIGURE 16.43 Trends in PM1(I in the South Coast air basin in California (Los Angeles area) from 1985 to 1994. The averages for six sites having complete data throughout this time period are shown as well as the maximum concentrations. The 1994 data are for January to October (kindly provided by Dr. Shankar Prasad, California Air Resources Board).

1988 1989 1990 1991 1992 1993 1994 1995 Year

FIGURE 16.44 Concentrations of PM,0 normalized to 1988 as the base year in the United States for urban, suburban, and rural areas (adapted from Darlington et al., 1997).

However, as expected from the chemistry discussed in Chapter 7.E, simultaneous control of NH3 has a significant effect on particle nitrate formation, since the formation of ammonium nitrate is a major mechanism for conversion of gaseous HN03 to particulate nitrate. Thus, Meng et al. (1997) predict that a 50% reduction in both NOx and NH3 would give about the same reduction in particulate nitrate.

Not only does the chemistry generating 03 affect the formation of particles, but the reverse is true; i.e., the presence of particles is thought to affect ozone formation. The major reason for this is the effect of aerosol particles on light scattering and hence the actinic flux and photolysis rates. For example, scattering of light by particles in effect increases the path length through the atmosphere (see Chapters 3.C.2f and 14.C), leading to increased rates of photolysis. On the other hand, absorption of light by particles decreases the effective actinic flux. For example, Dicker-son et al. (1997) used the Urban Airshed Model to examine the effects of increased light scattering by aerosol particles in the boundary layer in the northeastern United States. Using a refractive index typical of particles in that region; they predict that the amount of ozone generated increased by about 30% due to the increased rates of photolysis, particularly NOz. The opposite effect was predicted for absorbing aerosol particles. Similarly, Jacobson (1998) predict decreased 03 formation (by 5-8%) in the Los Angeles area due to reduction in photolysis rates from UV-absorbing aerosol particles.

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