the use of such models for all O3 nonattainment areas classified as serious and above.
Today, the scientific and engineering community has developed more than 20 3D CTMs for O3 in the United States and Europe, in addition to the UAM (see Russell and Dennis  for a detailed review). These models continue to grow and develop as new scientific information becomes available and as regulatory needs dictate. For example, O3 pollution was initially viewed as an urban problem, and the initial versions of the UAM were restricted to simulating individual urban areas. Today, it is recognized that O3 pollution extends over multistate airsheds, and the current version of the UAM and many other contemporary 3D photochemical models used to simulate O3 pollution are designed to treat this larger spatial domain.
Even as the UAM and other urban-scale CTMs were being developed and refined in the 1980s, the lessons learned from the application of these models were being applied to other types of CTMs. One of the most important of these efforts was the National Acid Precipitation Assessment Program’s development of a regional (or multistate airshed) CTM able to simulate acidic deposition in the eastern United States and Canada (NAPAP 1991b). The resulting model, the regional acid deposition model (RADM), included state-of-the-science representations of the physical and chemical processes leading to the formation and deposition of acidic species over multistate geographic areas (Chang et al. 1987). The model (together with similar tools, such as the ADOM model developed for Canada) provided important insights into the source-receptor relationships in the acid deposition problem in the United States and Canada. Although these models ultimately played a relatively minor role in the development of the Acid Rain Program enacted in the CAA Amendments of 1990,6 they represented a major step in the development of CTMs—namely, the expansion of the algorithms used in urban-scale CTMs for O3 into a more comprehensive regional-scale model able to simulate processes related to the formation, transport, and deposition of PM as well as gaseous pollutants.
Just as the development of urban-scale CTMs for O3 aided in the development of regional acid deposition models, such as RADM, the development of RADM provided the basis for significant advances in CTMs used for O3 pollution. Most present-day O3 models have regional rather than urban-scale coverage. Many have multiscale and nesting capabilities so that they can simulate a relatively large multistate area but also focus on
A more-detailed discussion of the Acid Rain Program is presented in Chapter 5.