Trends in Mobile Source-Related Ambient Concentrations of Hazardous Air Pollutants, 1996 to 2007

The EPA has the responsibility for monitoring the progress of our nation towards reducing emissions and ambient concentrations of air toxics. To help set priorities for data collection in the future and to measure progress, we have estimated county-level emissions in 2007 for 18 mobile source air toxics. Mobile source contributions to ambient concentrations are estimated for a number of these hazardous air pollutants using the same methods employed in the National Scale Air Toxics Assessment. Between 1996 and 2007, emissions and concentrations of gaseous hazardous air pollutants from mobile sources are projected to decrease between 24 and 50 percent, depending on the pollutant. Increases in trace metal emissions and concentrations are projected as a result of increased vehicle miles traveled for onroad sources and increased activity for nonroad sources. Trace metal emissions are likely to decline beyond 2007, however, as new technologies for heavy duty engines are phased in.