ATMOSPHERIC CHEMISTRY AND RISK ASSESSMENTS OF HAZARDOUS AIR POLLUTANTS

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 NO. and NH3 would give about the same reduction in particulate nitrate. 

ATMOSPHERIC CHEMISTRY AND RISK ASSESSMENTS OF HAZARDOUS AIR POLLUTANTS 

In addition to the role of atmospheric reactions in the fate of airborne FLAPs, atmospheric chemistry also plays a role in the formation of some of them, most notably formaldehyde and acetaldehyde. Thus, the potential formation of such compounds from the oxidation of precursors in the atmosphere must also be taken into account in their risk assessments. 

A detailed review of the sources, atmospheric chemistry, and fates of HAPs is beyond the scope of this book. Instead, we concentrate here on several organic HAPs to illustrate the role of atmospheric chemistry in risk assessments. For other species, reviews and summaries are available in the literature (e.g., see California Air Resources Board (1997) for summaries of 243 species and Lin and Pehkonen (1999) for a review of the atmospheric chemistry of mercury). 

The California Air Resources Board has prepared risk assessments for a number of toxic airborne compounds and mixtures, designated as toxic air contaminants, TACs (Table 16.15). For example, risk assessments for individual compounds such as benzene, benzo[a]pyrene (see Chapter 10), formaldehyde, and vinyl chloride have been carried out, in addition to complex mixtures such as diesel exhaust (California Air Resources Board, 1997a) and environmental tobacco smoke (California Environmental Protection Agency, 1997). These risk assessment documents form the basis for controls imposed as part of the risk management process (e.g., see Seiber, 1996). 

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G. ATMOSPHERIC CHEMISTRY AND RISK ASSESSMENTS OF HAZARDOUS AIR POLLUTANTS 927 TABLE 16.15 (continued) ... 

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