Modeling Cumulative Concentrations of Air Toxics. D. A. Axelrad, U.S. Environmental Protection Agency, Office of Policy, Planning and Evaluation, 401 M St. SW (2123), Washington, DC 20460; A. S. Rosenbaum, Systems Applications International, Inc., 101 Lucas Valley Road, San Rafael, CA 94903; and T. J. Woodruff, U.S. Environmental Protection Agency, Office of Policy, Planning and Evaluation, 401 M St. SW (2123), Washington, DC 20460
Hazardous air pollutants (HAPs), commonly known as "air toxics," are emitted by a wide variety of sources, including large manufacturing facilities, waste incinerators, small commercial businesses such as dry cleaners, and mobile sources. Efforts to measure ambient concentrations of air toxics have been fairly limited. As a result, there is relatively little knowledge of the cumulative concentrations of HAPs resulting from the multiple emissions sources which may exist in any particular community. In the absence of monitoring data, a modeling approach is necessary to a gain a greater understanding of cumulative toxics concentrations and their geographic distribution. As part of EPA’s Cumulative Exposure Project, long-term cumulative concentrations of approximately 150 HAPs are being modeled at the census tract level for the continental United States. A long-term Gaussian dispersion modeling approach is used, along with an emissions inventory drawing on the Toxics Release Inventory and other EPA databases addressing the variety of source categories. Contributions to current concentrations from historic emissions of persistent pollutants and from non-anthropogenic sources are addressed with background values drawn from measurements in remote locations. To evaluate the performance of the emissions inventory and dispersion model, estimated concentrations of HAPs are compared with long-term measurement values, where available. This presentation will describe the modeling methodology and emissions inventory, model outputs, and the evaluation of model performance.
The views expressed in this presentation are those of the authors, and do not necessarily represent those of the U.S. Environmental Protection Agency.