be ineffective in reducing the pollutants that are responsible for the health effects. An important question is whether that is the case in the approach to ozone and airborne particulate matter (PM), of which the latter seems to have greater health effects (see Chapter 2).

Most ozone in the troposphere is not directly emitted to the atmosphere (although there are minor sources of such ozone, including some indoor air cleaners) (e.g., NRC 1991). Rather, it is formed from a complex series of photochemical reactions of the primary precursors: nitrogen oxides (NOx where [NOx] indicates the sum of [NO] and [NO2]), volatile organic compounds (VOCs), and to a smaller extent other pollutants, such as carbon monoxide (CO) (Figure 3-1) (e.g., NRC 1991). The specific reactions that form most of the tropospheric ozone are the photolysis of NO2 followed by the combination of the released oxygen atom with the abundant oxygen molecules (O2):



FIGURE 3-1 Source and chemical links between ozone and PM formation. Major precursors are shown in boxes with thick sides. Secondary particle components are shown in boxes with thin solid sides. Mobile sources (cars, trucks, and off-road vehicles) and plants are major sources of VOCs, and mobile sources and electricity-generating units are dominant sources of NOx, but myriad smaller sources also contribute. Trace species, such as OH, are crucial to the formation of ozone, sulfate, nitrate, and organic-carbon particulate matter. Ozone also leads to the oxidation of SO2 and NO2. Biologic activity and fertilizer use dominate ammonia (NH3) emissions. Source: Modified from NARSTO 2004. Reprinted with permission; copyright 2004, Cambridge University Press.

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