ferentiated by effective stack height and by location because the height of emissions affects the dispersion patterns from these sources. Emissions from point sources with an effective height of less than 250 m are aggregated to the county level, as are emissions from point sources with an effective height of 250 to 500 m. In contrast, point sources with an effective height of greater than 500 m, such as certain power plants and other large industrial facilities, are modeled individually—that is, APEEP does not aggregate emissions from these sources; they are modeled separately for each facility.
The air-quality models in APEEP use the emission data provided by EPA to estimate corresponding ambient concentrations in each county in the coterminous states. The accuracy of the estimated pollution levels produced by the APEEP model has been statistically tested against the Community Multiscale Air Quality (CMAQ) model (Byun and Schere 2006), which is considered the state-of-the-art air-quality model. The results of these statistical comparisons are shown in the accompanying materials to Muller and Mendelsohn (2006).
APEEP can be used to compute the marginal damage of emissions on a source-specific basis. This approach isolates the source-specific damage per ton for each of the six pollutants covered by the model. To calculate marginal damages, APEEP uses the following algorithm: First, APEEP estimates total damages due to all sources in the model, producing its baseline (observed) emissions (EPA 2009); next, APEEP adds 1 ton of one pollutant from one source and recomputes total damages. The marginal damage is the damage that occurs after adding 1 ton of pollutant minus the damages due to the baseline emissions. The algorithm isolates the contribution of a single ton of emissions from each source to total national damages. This approach captures the formation of secondary pollutants, such as sulfates and nitrates (constituents in PM2.5) as well as tropospheric ozone (O3) that are formed by the emissions of other substances. APEEP attributes the damage due to such secondary pollutants back to the source of emissions. As shown in Equation 1, the marginal damage is computed by adding the changes in damages across the complete set of receptor counties. (Receptor counties are those counties that receive emissions from a source.)
MDi,p = damage per ton of an emission of pollutant (p) from source (i).
Dr = total dollar damage that occurs at receptor county (r).
bp = 2002 baseline emissions of p.
ep = 2002 baseline emissions plus 1 ton of p from i.