Concentrations of Toxic Air Pollutants in the U.S. Simulated by an Air Quality Model

The U.S. Environmental Protection Agency is examining the concentrations and deposition of air pollutants that are known or suspected to cause cancer or other serious health effects in humans. These “air toxics” or “hazardous air pollutants” (HAPs) include a large number of chemicals, ranging from non reactive (i.e. carbon tetrachloride) to reactive (i.e. formaldehyde), exist in gas, aqueous, and/or particle phases and are emitted from a variety of sources. Some HAPs, such as formaldehyde and xylene, also play an important role in the production of ozone and particulate matter. In addition, concentrations of air toxics are required over both shorter (days) as well as longer (a year) time scales in order to analyze health risks resulting from exposure to these compounds. These requirements challenge the current capabilities of numerical air quality models beyond their needs for other pollutants, such as ozone.