National estimates of outdoor air toxics concentrations

The Clean Air Act identifies 189 hazardous air pollutants (HAPs), or "air t oxics," associated with a nide range of adverse human health effects. The U .S. Environmental Protection Agency has conducted a modeling study with the Assessment System for Population Exposure Nationwide (ASPEN) to gain a gre ater understanding of the spatial distribution of concentrations of these H APs resulting from contributions of multiple emission sources. The study es timates year 1990 long-term outdoor concentrations of 148 air toxics for ea ch census tract in the continental United States, utilizing a Gaussian air dispersion modeling approach. Ratios of median national modeled concentrati ons to estimated emissions indicate that emission totals without considerat ion of emission source type can be a misleading indicator of air quality. T he results also indicate priorities for improvements in modeling methodolog y and emissions identification. Model performance evaluation suggests a ten dency for underprediction of observed concentrations, which is likely due, at least in part, to a number of limitations of the Gaussian modeling formu lation. Emissions estimates for HAPs have a high degree of uncertainty and contribute to discrepancies between modeled and monitored concentration est imates. The model's ranking of concentrations among monitoring sites is rea sonably good for most of the gaseous HAPs evaluated, with ranking accuracy ranging from 66 to 100%.