The Clean Air Act (CAA) provides a legal framework for promoting public health and public welfare² by pursuing five major air quality goals (see Box 1). For the first goal, the CAA authorizes the U.S. Environmental Protection Agency (EPA) to set maximum allowable atmospheric concentrations of six major “criteria” pollutants by establishing National Ambient Air Quality Standards (NAAQS). Individual states then develop state implementation plans (SIPs) that show how, with the assistance of national control programs, they will meet these standards. Such efforts, as well as those in pursuit of the other CAA goals, seek to regulate emissions from a variety of stationary and mobile sources through the nation’s air quality management (AQM) system (see Figure ES-1). Since passage of the CAA Amendments of 1970, the nation has devoted significant efforts and resources to AQM, and substantial progress has been made.

The Committee on Air Quality Management was formed by the National Research Council to examine the role of science and technology in the implementation of the CAA and to recommend ways in which the scientific and technical foundations for AQM in the United States can be enhanced. Over a 2-year period, the committee heard briefings from experts