Simulating atmospheric mercury with the Community Multi-scale Air Quality (CMAQ) model

Numerical air quality simulation models have been used since the 1970's to study the impacts of potential emissions control strategies on urban-scale ambient concentrations of air pollutants. During the 1980's they came to be used on broader scales to study ozone, particulate matter, and acid deposition across entire continents. As many of these models have demonstrated their accuracy and value as planning tools, they have been transferred from the research arena to operational status where they are used in regulatory analysis and governmental policy development. Examples of numerical air quality models developed through EPA-NOAA partnership and used in such policy analyses are the Urban Airshed Model, the Regional Oxidant Model, the Regional Acid Deposition Model, and most recently the Community Multiscale Air Quality (CMAQ) model. The CMAQ model brings the capabilities of multiple scales (urban through regional) and multiple pollutants (ozone, acid deposition, particulate matter, air toxics) within one comprehensive modeling system. This capability is now being used to address the problem of environmental mercury contamination.

In 2001, the CMAQ was first adapted to simulate the emission, transport, transformation and deposition of atmospheric mercury based on the relevant scientific and emission source information available at that time. Scientific research continues to supply new information about the sources and behavior of atmospheric mercury and the numerical formulations of special CMAQ modules for mercury simulation continue to be modified accordingly. Experimental simulations using the CMAQ mercury model have shown that a particular chemical reaction, the gaseous oxidation of elemental mercury by the hydroxyl radical, is leading to faster-than-expected conversion of mercury to its more rapidly deposited oxidized forms. The strength of this effect is strongly influenced by the physical form assumed for the reaction product. This finding has important ramifications for the generally accepted notion of an average atmospheric lifetime for elemental mercury of about one year. It also has a bearing on the ongoing controversy about the relative importance of local and distant emissions of mercury to observed mercury depositional flux.

Results from CMAQ mercury simulations will be shown illustrating the importance of gaseous mercury oxidation by the hydroxyl radical given various assumptions about the rate of the chemical reaction and the physical form of the reaction product. From these results arise a range of estimates for the expected decrease in atmospheric deposition of mercury following domestic emission reductions, and these will also be illustrated.

Disclaimer: 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.