You are here:
ATMOSPHERIC MERCURY SIMULATION USING THE CMAQ MODEL: FORMULATION DESCRIPTION AND ANALYSIS OF WET DEPOSITION RESULTS
Citation:
Bullock, O R. AND K. A. Brehme. ATMOSPHERIC MERCURY SIMULATION USING THE CMAQ MODEL: FORMULATION DESCRIPTION AND ANALYSIS OF WET DEPOSITION RESULTS. ATMOSPHERIC ENVIRONMENT 36(13):2135-2146, (2002).
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 Community Multiscale Air Quality (CMAQ) modeling system has recently been adapted to simulate the emission, transport, transformation and deposition of atmospheric mercury in three distinct forms; elemental mercury gas, reactive gaseous mercury, and particulate mercury. Emissions of mercury are currently defined from information developed for and published in EPA's Mercury Study Report to Congress. The atmospheric transport of these three forms of mercury is simulated in the same manner as for all other substances simulated by the CMAQ model to date. Transformations of mercury are simulated with four new chemical reactions within the standard CMAQ gaseous chemistry framework and a highly modified cloud chemistry mechanism which includes a compound-specific speciation for oxidized forms of mercury, seven new aqueous-phase mercury redox reactions, six aqueous mercury chemical equilibria, and a two-way mechanism for the sorption of dissolved oxidized mercury to elemental carbon particles. The CMAQ mercury model simulates the partitioning of reactive gaseous mercury between air and cloud water based on the Henry's constant for mercuric chloride, and a Henry's equilibrium is assumed for elemental mercury also. Particulate mercury is assumed to be incorporated into the aqueous medium during cloud nucleation. Wet and dry deposition is simulated for each of the three forms of mercury. Wet deposition rate is calculated based on precipitation information from the CMAQ meteorological processor and the physicochemical mercury speciation in the cloud chemistry mechanism. Dry deposition rate is calculated based on dry deposition velocity and air concentration information for each of the three forms of mercury. An evaluation of the performance of the CMAQ mercury model compared to weekly observations of mercury wet deposition is performed. The horizontal modeling domain for the evaluation covers the central and eastern United States and adjacent southern Canada. The evaluation results are described for two separate evaluation periods; April 4 to May 2 of 1995, and June 20 to July 18 of 1995.
The information in this document has been funded wholly or in part by the U.S. Environmental Protection Agency (EPA) and has been developed as part of an interagency agreement with the National Oceanic and Atmospheric Administration (NOAA). It has been subject to review by both EPA and NOAA, and has been approved for publication. Mention of trade name or commercial products does not constitute endorsement or recommendation for use.