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THE IMPORTANCE OF EMISSIONS SPECIATION TO THE ATMOSPHERIC TRANSPORT AND DEPOSITION OF MERCURY
Citation:
Bullock, O R. THE IMPORTANCE OF EMISSIONS SPECIATION TO THE ATMOSPHERIC TRANSPORT AND DEPOSITION OF MERCURY. Presented at EPA Conference on Assessing and Managing mercury from Historic and Current Mining Activities, San Francisco, CA, November 28-30, 2000.
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 atmospheric pathway of the global mercury cycle is believed to be the main source of mercury contamination to aquatic eco-systems throughout the United States and in most other nations where direct disposal of mercury to water has been largely eliminated. Although the spatial scope of transport and deposition is certainly global for atmospheric mercury as a whole, we now have compelling evidence that certain forms of mercury are very quickly scavenged from air by both wet and dry atmospheric processes and are not likely to travel very long distances through the atmosphere. Reactive gaseous mercury (RGM) and particulate mercury (HgP), while together making up only a small fraction of the total atmospheric burden of mercury, are thought to represent the majority of atmospheric mercury deposited to the surface. Mercury can be emitted to air in one of these forms, and travel only a short distance before deposition, or it can be emitted as elemental mercury gas (Hg0) and move through the atmosphere for weeks or months, diffusing into the global atmosphere. Based on recent modeling and field studies, it now appears that most of the elemental mercury that is eventually deposited to the surface may do so through chemical conversion to RGM and/or particulate mercury. It also appears that emissions of RGM and HgP can be converted to Hg0 by these same atmospheric processes under different conditions. While there remains considerable uncertainty about these chemical and physical conversion mechanisms, the expected deposition pattern for mercury from any source is certainly dependent on the chemical and physical form of that mercury when emitted. The term "speciation" is generally used to describe the determination of the fraction of a pollutant that occurs in various important forms. Mercury emission speciation is necessary for an assessment of mercury contamination from any source of atmospheric mercury.