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ATMOSPHERIC MERCURY TRANSPORT AND DEPOSITION
Citation:
BULLOCK, R. ATMOSPHERIC MERCURY TRANSPORT AND DEPOSITION. Presented at 2005 Air Toxics Workshop, RTP, NC, March 31, 2005.
Impact/Purpose:
The objectives of this task are to develop, improve, and evaluate EPA's Community Multiscale Air Quality (CMAQ) model, as an air quality management and NAAQS implementation tool. CMAQ is a multiscale and multi-pollutant chemistry-transport model (CTM) that includes the necessary critical science process modules for atmospheric transport, deposition, cloud mixing, emissions, gas- and aqueous-phase chemical transformation processes, and aerosol dynamics and chemistry. To achieve the advances in CMAQ, research will be conducted to develop and test appropriate chemical and physical mechanisms, improve the accuracy of emissions and dry deposition algorithms, and to develop and improve state-of-the-science meteorology models and contributing process parameterizations.
The model will be tested and evaluated to thoroughly characterize the performance of the emissions, meteorological and chemical/transport modeling components of the CMAQ system, with an emphasis on the chemical/transport model, CMAQ. Emissions-based models are composed of highly complex scientific hypotheses concerning natural processes that can be evaluated through comparison with observations, but not truly validated. Both operational and diagnostic evaluations, together with sensitivity analyses are needed to both establish credibility and build confidence within the client and scientific community in the simulation results for policy and scientific applications. The characterization of the performance of Models-3/CMAQ is also a tool for the model developers to identify aspects of the modeling system that require further improvement.
Description:
The current state of our scientific understanding the mercury cycle tells us that most of the mercury getting into fish comes from atmospheric deposition, but methylation of that mercury in aquatic systems is required for the concentrations in fish to reach harmful levels. We still do not have a full understanding of the behavior of mercury while it is in the atmosphere, partly due to the fact that we cannot yet resolve the individual chemical compounds of mercury in air and in atmospheric water. The US EPA has developed an advanced numerical simulation model for atmospheric mercury based on the same model it uses to describe photochemical oxidants, acid deposition, and particulate matter. This model, CMAQ, includes a number of chemical reactions of mercury, some of which remain somewhat controversial with respect to their true importance. Comparison of simulated wet deposition of mercury to observations of the same show that the model has considerable skill in that regard, but there are no such observations of dry deposition against which the model can be tested. Thus, the confidence with which we can apply traditional techniques for model evaluation is limited. In general, the science of atmospheric mercury is still evolving.