Citation:

Luecken, D J. AND W T. Hutzell. CONCENTRATIONS OF TOXIC AIR POLLUTANTS IN THE U.S. SIMULATED BY AN AIR QUALITY MODEL. Presented at 27th NATO/CCMS International Technical Meeting on Air Pollution Modeling and its Application, Banff, Canada, October 25-29, 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:

As part of the US National Air Toxics Assessment, we have applied the Community Multiscale Air Quality Model, CMAQ, to study the concentrations of twenty gas-phase, toxic, hazardous air pollutants (HAPs) in the atmosphere over the continental United States. We modified the Carbon Bond 4 chemical mechanism to explicitly account for the additional production and decay of these HAPs, while retaining the full chemistry and radical cycling of the mechanism. Because risk assessments of toxic air pollutants require concentrations over a long period of time, we simulated the full year of 2001 using meteorological data from MM5. We obtained emission inventories of all twenty HAPs from the latest national inventories, and merged them with the inventories of other VOCs and NOx, in order to fully characterize the reactivity of the atmosphere.

Most previous assessments of risks from HAPs have used Gaussian dispersion models to obtain concentrations, so these numerical modeling results are unique because they take into account the full atmospheric chemistry, including the time-varying changes in radical concentrations that affect the ambient concentrations of HAPs. Several HAPs, including formaldehyde and acetaldehyde, can be produced in the atmosphere in greater quantities than they are emitted; so this is the first time that the production of these compounds has been accounted for over a large scale and for a long time period. We present modeled concentrations of formaldehyde, benzene, perchloroethylene, and chloroform, comparison with monitored data, and calculations of risk that result from these concentrations. In addition, we discuss the differences between the results from using a Gaussian dispersion model and CMAQ in the risk assessments of HAPs.

Although this work was reviewed by EPA and approved for publication, it may not necessarily reflect official Agency policy.

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