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Background Concentrations

The emissions inventory and modeling methodologies in the national-scale assessment are used to estimate long-term outdoor concentrations of air toxics attributable to both 1996 and 1999 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, unidentified emission sources, and natural emission sources. To accurately estimate outdoor total ambient concentrations of air toxics, it is necessary to account for these background concentrations that are not represented by atmospheric modeling of 1996 or 1999 anthropogenic emissions.

Approach in 1999 National-Scale Assessment

For the 1999 national-scale assessment, we have background concentrations for 13 air toxics that are based on available monitored data. For the remainder of the air toxics in the assessment, we used values reported in technical literature as identified in the Cumulative Exposure Project (CEP) (15 air toxics), or we assumed zero background (105 air toxics) if no values were reported in the CEP. The values taken from the CEP were based on technical literature and are not representative of any particular year. For all 28 air toxics with estimated background concentrations, each census tract in the county is assigned the same county-specific background concentrations. The total estimated concentration for each pollutant in each census tract is the sum of the background and the modeled concentrations. For a list of pollutants and background concentration distributions, see Table 1 (PDF 1 p., 25 KB).

The same approach was used for diesel PM in both the 1999 and 1996 assessments. 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.

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Approach in 1996 National-Scale Assessment

For the 1996 national-scale assessment, EPA assigned a background concentration for 12 of the 32 air toxics. For each of these 12 pollutants, we assumed a constant (one number) background concentration across the entire U.S. We used monitored values reported in technical literature dating back to the mid-80s (identified in the Cumulative Exposure Project). The background concentration values for these 12 toxic air pollutants were 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 were insufficient to address any possible geographic variations in background, background concentrations are assumed to be constant across all census tracts. For the remaining 20 air toxics 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 12 air toxics.)

The same approach was used for diesel PM in both the 1999 and 1996 assessments. 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.

Please see the technical report "Estimated Background Concentrations for the National-Scale Assessment" (PDF120pp., 2607 KB) for more details.

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