The emissions inventory and modeling methodologies in the national-scale assessment are used to estimate long-term outdoor concentrations of air toxics attributable to 1996 anthropogenic emissions, within 50 kilometers of each source. For many toxic air pollutants, however, outdoor concentrations should include "background" components attributable to long-range transport, resuspension of historical emissions, and nonanthropogenic sources. To accurately estimate 1996 outdoor concentrations of air toxics, it is necessary to account for these background concentrations that are not represented by atmospheric modeling of 1996 anthropogenic emissions.
In this assessment, except for diesel PM, background concentrations are based on monitored values identified in the Cumulative Exposure Project> (study which estimated 1990 ambient concentrations of air toxics). From that study, EPA obtained background concentration values for 13 of the toxic air pollutants which are added to the modeled concentrations for these pollutants.
The total estimated concentration for each pollutant in each census tract is determined by summing the estimated background concentrations and the modeled concentrations. Because the available data are insufficient to address any possible geographic variations in background, background concentrations are assumed to be constant across all census tracts. For pollutants whose background concentration values could not be identified in the technical literature, the background concentrations are assumed to be zero. This may result in underestimation of outdoor concentrations for some toxic air pollutants. (See list of the background concentration estimates for the 13 air toxics.)
For diesel PM, instead of using monitored air quality data to estimate background concentrations, a modeling-based approach was used to provide a rough approximation of concentrations due to transport from sources located between 50 km and 300 km from the receptors. Consequently, the values for the background concentration estimate vary across the country.