This report’s recommendations are aimed at improving the nation’s AQM system to better integrate the tools and methods of the scientific and technological communities, and to provide an improved mechanism for assuring that the system and its components are achieving the intended public benefits.

The management of the nation’s air quality is a major and complex undertaking. AQM in the United States involves the work of tens of thousands of people who monitor the concentrations of various air pollutants at over a thousand sites, regulate thousands of different emission sources, and maintain a multimillion dollar research and development program to better understand the sources, fate, and effects of air pollutants. The CAA requires regulatory control of air pollutants that have widely varying properties. Some pollutants are rapidly removed from the atmosphere so that effects are largely limited to the immediate source area. Other pollutants (such as O₃ and PM) can be transported in significant amounts for hundreds to thousands of miles; therefore, their control requires cooperation between states and, in some instances, nations.

Implementation of the CAA has clearly contributed to the reduction in pollutant emissions in the United States. For example, over the past 30 years, the nation’s gross domestic product and total vehicle miles traveled increased by more than 2-fold, and its energy consumption increased by a factor of about 1.5. However, over the same period, EPA (2002a) reported that the total aggregate of emissions that directly affect the ambient concentrations of six criteria pollutants has decreased by 25% (see Figure 1-4). This trend suggests that the nation has been able to decouple, to some extent, pollutant emission rates from economic activity. EPA argues that the CAA played a major role in bringing about this decoupling. In the absence of the CAA, EPA estimated that emissions of CO, SO₂, NO_x, and PM in 1990 would have been larger by factors of about 2, 1.6, 1.4, and 3, respectively (EPA 1997). Others argued that factors other than environmental regulation (for example, increased income and technological advances) might be the main causes of the decrease in pollutant emissions (for example, Lomborg 2001; Pacala et al. 2003). However, Pacala et al. (2003) concluded that although a variety of factors contribute to observed benefits, regulation plays a prominent role. Although it is not possible to know precisely what the levels of pollutant emissions in the United States would be in the absence of a federally mandated AQM system, it is reasonable to conclude that this system has had an important role in controlling and lowering these emissions over the past 30 years.