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1999 Modeled Ambient Concentration Model
Note: EPA no longer updates this information, but it may be useful as a reference or resource.About the Model
To develop nationwide estimates of annual average ambient concentrations of air toxics, EPA is using the Assessment System for Population Exposure Nationwide (ASPEN) model. The scope of this national modeling effort is the United States, Puerto Rico and the Virgin Islands. The ASPEN model simulates the impacts of atmospheric processes (winds, temperature, atmospheric stability, etc.) on pollutants after they are emitted. The output of this air dispersion model is an estimate of the annual average ambient concentration of each air toxic pollutant at the centroid of each census tract within the geographic scope of the assessment. EPA used data from the 2000 census for this determination. For more information about the model, see the ASPEN model.