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):