biological diversity (for greenhouse gases) or ocean food web stresses, and ultimately causing severe sight damage for many mammals (for stratospheric ozone depleters).

Table 4-5 also shows the change in emissions and sampled concentrations of EPA's six criteria pollutants from 1986 to 1995.11 Primarily as a result of the Clean Air Act and the Clean Air Act Amendments, emissions of the six primary pollutants have decreased substantially. For example, installing scrubbers and switching to low-sulfur coal caused a 19 percent decline in emissions from coal utility plants, which in turn resulted in an overall 18 percent decline in sulfur dioxide emissions from 1986 to 1995. A 16 percent decline in carbon monoxide emissions during the same period resulted primarily from a 20 percent decline in carbon monoxide emissions of on-road motor vehicles. Similarly, a 32 percent decline in lead emissions was primarily a result of the ban on leaded gasoline.

Declines in nitrogen dioxide (14 percent) and ground-level ozone emissions (6 percent) were less dramatic, but are expected to become more pronounced as the Clean Air Act Amendments of 1990 become effective. For example, reformulated fuel requirements (for oxygen and volatility) for on-road vehicles are likely to reduce carbon monoxide and ground-level ozone emissions. Similarly, the Acid Rain Program (Title IV) requires a 40 percent reduction in sulfur dioxide and a 10 percent reduction in nitrogen dioxide emissions from 1980 to 2010. Particulate matter may be more difficult to control given that almost 70 percent of anthropogenic-related emissions result from fugitive dust (e.g., unpaved roads), with an additional 20 percent coming from agriculture and forestry.

The declines in emissions are, of course, linked to lower concentrations of the six primary pollutants. Whereas emissions are estimated on the basis of industrial activity, technology, fuel consumption, and vehicle miles traveled, concentrations of pollutants are measured at selected monitoring sites across the country. Based on these measurements, estimated airborne concentrations of lead have fallen by 78 percent since 1986, while concentrations of airborne carbon monoxide, sulfur dioxide, and particulate matter have fallen by 37, 37, and 22 percent, respectively. Smaller declines occurred for ground-level ozone and nitrogen dioxide (6 and 14 percent, respectively).

Data on other air chemicals vary widely. Excellent data are available on emissions and concentrations of many of the greenhouse gases (particularly carbon dioxide) and stratospheric ozone destroyers. EPA pres-


Data prior to 1986 exist, but cannot be directly compared with data collected from 1986 on because of changes in data collection (see U.S. Environmental Protection Agency, 1996, for more details).

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