Contribution of Highway and Nonroad Mobile Source Categories to Ambient Concentrations of 20 Hazardous Air Pollutants in 1996

Ambient concentration modeling was conducted to characterize the relative contribution of various mobile source categories to ambient concentrations of hazardous air pollutants (HAPs), and to more effectively set priorities for additional research. The contribution of highway and nonroad mobile source categories to ambient air concentrations of 20 hazardous air pollutants (HAPs) in 1996 were estimated using the Assessment System for Population Exposure Nationwide (ASPEN) dispersion model. In general, the largest mobile source contributors are onroad gasoline vehicles, followed by nonroad 2-stroke and 4-stroke gasoline equipment. However, for carbonyl compounds the contributions of nonroad diesel engines, aircraft, and onroad heavy duty diesel vehicles are relatively greater.