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SIMULATING URBAN AIR TOXICS OVER CONTINENTAL AND URBAN SCALES
Citation:
Hutzell, W T., D J. Luecken, AND J.K S. Ching. SIMULATING URBAN AIR TOXICS OVER CONTINENTAL AND URBAN SCALES. Presented at 2004 Models-3 Conference, Chapel Hill, NC, October 18 - 20, 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 US EPA is evaluating a version of the CMAQ model to support risk assessment for the exposure to Hazardous Air Pollutants (HAPs). The model uses a variant of the CB4 chemical mechanism to simulate ambient concentrations of twenty HAPs that exist primarily as gaseous compounds such as formaldehyde, acetaldehyde, 1,3-butadiene and benzene. Our evaluation uses results from annual simulations at two horizontal resolutions, 36X36 km2 and 4X4 km2, for the year 2001. The former resolution has a domain that covers the Continental United States. The latter resolution's domain nests inside the continental domain and focuses on the Philadelphia metropolitan area. The comparison examines observations versus a subset of predicted HAPs that include formaldehyde, acetaldehyde, benzene, perchloroethylene, and chloroform. Both simulations show an agreement with observations generally within a factor of two. The 4X4 km2 simulation appears to not have a better agreement in terms of root mean error and correlation coefficient by can have a lower bias and better match how observations vary over time. For both resolutions, the comparisons between time series show that agreement to observations depends on season and location. We propose that these behaviors result from the boundary conditions, emission inventory and the chemistry of a HAP. Differences between the simulations indicate that spatially resolving emissions, transport, and chemistry also explains differences from observations because the smaller resolution better captures the temporal variability of observed concentrations.
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.