• Progress has been made in recognizing and addressing multistate transport of air pollution, especially for ozone and atmospheric haze and their precursors, in some parts of the nation. However, transport issues need to be identified and addressed more proactively and the scope broadened to include international transport.

  • For mobile sources, regulations for light-duty vehicles, light-duty trucks, and fuel properties have greatly reduced emissions per mile traveled. Gaps remain, however, in the ability to monitor, predict, and control vehicular emissions, especially from nonroad vehicles, heavy-duty diesel trucks, and malfunctioning automobiles.

  • Emission reductions from stationary sources (for example, power plants and large factories) have also been substantial. However, most of the reductions have been accomplished through regulations on new facilities, while many older higher-emitting facilities continue to be a substantial source of emissions.

  • In recent years, emissions cap and trade has provided an effective mechanism for achieving stationary-source emission reductions at reduced costs. However, cap-and-trade programs have been limited to relatively few pollutants, and the process of revising caps and targets in response to new technical and scientific knowledge has been cumbersome.

Assessing Status and Measuring Progress
  • With the exception of continuous emissions monitoring at some large stationary sources, the nation’s AQM system lacks a comprehensive and quantitative program to confirm the emission reductions claimed to have occurred as a result of AQM.

  • The air quality network in the United States is a national resource but is nevertheless inadequate to meet important objectives, especially that of tracking regional patterns of pollutant concentrations, transport, and trends (see Figure S-3).

  • The AQM system has not developed a program to track health and ecosystem exposures and effects and to document improvements in health and ecosystem outcomes achieved from improvements in air quality. Ecosystem effects have not been reliably and consistently accounted for in cost-benefit analyses.


Although the nation’s AQM system has been effective in addressing some of the most serious air quality problems, it has a number of limitations, as outlined above. In addressing how those limitations can best be remedied,

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