Citation:

Luecken, D J. AND W T. Hutzell. USING THE AIR QUALITY MODEL TO ANALYZE THE CONCENTRATIONS OF AIR TOXICS OVER THE CONTINENTAL U.S. Presented at CRC Mobile Source Air Toxics Workshop, Scottsdale, AZ, November 30-December 2, 2004.

Impact/Purpose:

The objective of this task is to improve EPA's ability to accurately predict the concentrations and deposition of air pollutants in the atmosphere that are known or suspected to cause cancer or other serious health effects to humans, or adverse environmental effects. It is an essential component of EPA's National Air Toxics Assessment (NATA), which seeks to identify and quantify the concentrations and sources of those hazardous air pollutants which are of greatest potential concern, in terms of contribution to population risk. It is a major contributor to NERL's Air Toxics Research Program.

"Air toxics" or "hazardous air pollutants" (HAPs) is a category that covers a large variety of chemicals, which range from relatively non reactive to extremely reactive; can exist in the gas, aqueous, and/or particle phases; display a large range of volatilities; experience varying deposition velocities, including in some cases revolatilization; and are emitted from a wide variety of sources at a large variety of different scales. In addition, concentrations of air toxics are needed by regulators for both short (days) as well as long (up to a year) time scales. These requirements challenge our current capabilities in air quality models far beyond the needs for other pollutants, such as ozone. The specific work being done under this task involves 1.) developing and testing chemical mechanisms which are appropriate for describing the chemistry of air toxics; 2.) incorporating these chemical and physical mechanisms into EPA's CMAQ modeling system and applying the model at a variety of scales; and 3.) developing the methods for using models to predict HAPs concentrations at subgrid or neighborhood scales; and 4.) using these tools to assess the magnitude and variability of concentrations to which urban populations are exposed.

Description:

The U.S. Environmental Protection Agency is examining the concentrations and deposition of hazardous air pollutants (HAPs), which 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. Several HAPs, such as formaldehyde, acetaldehyde and acrolein, can be produced in the atmosphere in significant quantities, so it is important to adequately characterize this complex chemistry. To do this, we have modified a 3 D photochemical air quality model, the Community Multiscale Air Quality (CMAQ) modeling system, to simulate the concentration of toxic air pollutants over large spatial and temporal scales. The model has been applied to study 20, high priority HAPs that exist in the gas phase, including such compounds as benzene, formaldehyde, acrolein, acetaldehyde, and chloroform. We describe the development of chemical mechanisms appropriate for HAPs; the incorporation of the chemistry and physics of HAPs into a chemical transport model; and the simulation over an entire year and the continental U.S. The analysis of the model results presented here focuses on formaldehyde and acrolein. There is substantial temporal and spatial variability in HAP concentrations. The fraction of secondarily-produced formaldehyde and acrolein can be large, with the contribution of atmospheric chemistry varying in importance across the country and across seasons. The contribution of mobile sources to ambient concentrations depends on both the primary emissions from mobile sources as well as the amount that is formed in the atmosphere from mobile source emissions of other VOCs.

The research presented here was performed under the Memorandum of Understanding between the U.S. Environmental Protection Agency (EPA) and the U.S. Department of Commerce's National Oceanic and Atmospheric Administration (NOAA) and under agreement number DW13921548. Although it has been reviewed by EPA and NOAA and approved for publication, it does not necessarily reflect their policies or views.

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