substantially since the early 1980s; the decrease in NOx emissions is estimated to be more modest (see Table 6-1, part A). These emission decreases can be reasonably ascribed to the promulgation of emission controls related to the nation’s implementation of the CAA requirements. Because the importance of NOx for ozone (O3) control was not recognized until the late 1980s or early 1990s, the slower pace of NOx emission decreases, compared with other pollutants, is probably due to the later implementation of NOx controls.

The decline of pollutant emissions during a period of substantial growth in population, energy consumption, and gross domestic product in the United States is cited by EPA and others as evidence of the substantial progress of the AQM system. However, the trends listed in Table 6-1, part A, have been developed from inherently uncertain emission inventories (see Chapter 3), so significant uncertainties must also be attached to the emission trends portrayed in EPA’s reports. Because of such uncertainties, a technically robust AQM system should have mechanisms in place that could

TABLE 6-1 Summary of EPA’s Trends in Estimated Nationwide Pollutant Emissions and Average Measured Concentrations




A. Changes in Estimated Pollutant Emissions, %a














No trend data available








B. Changes in Measured Ambient Pollutant Concentrations, %




O3 1-hr



O3 8-hr







No trend data available



No trend data available








aNegative numbers indicate reductions in emissions and improvements in air quality.

bIncludes only directly emitted particles.

cBased on percentage change from 1985.

dThe trends in NO2 should be viewed cautiously because of potential artifacts from the instrumentation. Also, because NO is readily converted to NO2 in the atmosphere, ambient monitoring data are reported as NO2.

eBased on percentage change from 1999.

SOURCE: Adapted from EPA 2003.

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