National Estimates of Outdoor Air Toxics Concentrations

TLDR

The Clean Air Act lists 189 hazardous air pollutants (HAPs) linked to adverse health effects, and EPA’s ASPEN modeling study seeks to understand their spatial distribution amid high uncertainty in emission estimates. The study aims to provide a clearer picture of how these HAP concentrations vary across the United States by accounting for multiple emission sources. It estimates 1990 long‑term outdoor concentrations of 148 air toxics for every census tract in the continental U.S. using a Gaussian air‑dispersion model.

Abstract

The Clean Air Act identifies 189 hazardous air pollutants (HAPs), or "air toxics," associated with a wide 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 greater understanding of the spatial distribution of concentrations of these HAPs resulting from contributions of multiple emission sources. The study estimates year 1990 long-term outdoor concentrations of 148 air toxics for each census tract in the continental United States, utilizing a Gaussian air dispersion modeling approach. Ratios of median national modeled concentrations to estimated emissions indicate that emission totals without consideration of emission source type can be a misleading indicator of air quality. The results also indicate priorities for improvements in modeling methodology and emissions identification. Model performance evaluation suggests a tendency for underprediction of observed concentrations, which is likely due, at least in part, to a number of limitations of the Gaussian modeling formulation. Emissions estimates for HAPs have a high degree of uncertainty and contribute to discrepancies between modeled and monitored concentration estimates. The model's ranking of concentrations among monitoring sites is reasonably good for most of the gaseous HAPs evaluated, with ranking accuracy ranging from 66 to 100%.