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DESCRIPTION AND EVALUATION OF ATMOSPHERIC MERCURY SIMULATION USING THE CMAQ MODEL
Citation:
Impact/Purpose:
The goal of this research is to develop and test appropriate chemical and physical mechanisms for use in EPA's Models-3 chemical/transport models. These models will be addressing issues of tropospheric photochemistry, fine particles, toxic and semi-volatile substances, and acid deposition. As such, scientifically credible mechanisms for atmospheric gas- and aqueous-phase chemistry as well as heterogeneous chemistry, applicable to the particular pollutant regimes must be included in Models-3.
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.