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Atmospheric mercury is a toxic air and water pollutantthat is of significant concern because of its effects on human health andecosystems. A mechanistic representation of the atmospheric mercury cycle isdeveloped for the state-of-the-art global climate-chemistry model, CAM-Chem(Community Atmospheric Model with Chemistry). The model simulates theemission, transport, transformation and deposition of atmospheric mercury(Hg) in three forms: elemental mercury (Hg(0)), reactive mercury (Hg(II)),and particulate mercury (PHg). Emissions of mercury include those fromhuman, land, ocean, biomass burning and volcano related sources. Landemissions are calculated based on surface solar radiation flux and skintemperature. A simplified air–sea mercury exchange scheme is used tocalculate emissions from the oceans. The chemistry mechanism includes theoxidation of Hg(0) in gaseous phase by ozone with temperature dependence,OH, H<sub>2</sub>O<sub>2</sub> and chlorine. Aqueous chemistry includes both oxidationand reduction of Hg(0). Transport and deposition of mercury species arecalculated through adapting the original formulations in CAM-Chem. TheCAM-Chem model with mercury is driven by present meteorology to simulate thepresent mercury air quality during the 1999–2001 period. The resultingsurface concentrations of total gaseous mercury (TGM) are then compared withthe observations from worldwide sites. Simulated wet depositions of mercuryover the continental United States are compared to the observations from 26Mercury Deposition Network stations to test the wet deposition simulations.The evaluations of gaseous concentrations and wet deposition confirm astrong capability for the CAM-Chem mercury mechanism to simulate theatmospheric mercury cycle. The general reproduction of global TGMconcentrations and the overestimation on South Africa indicate that modelsimulations of TGM are seriously affected by emissions. The comparison towet deposition indicates that wet deposition patterns of mercury are moreaffected by the spatial variability of precipitation. The sensitivityexperiments show that 22% of total mercury deposition and 25% of TGMconcentrations in the United States   result  from domestic anthropogenicsources, but only 9% of total mercury deposition and 7% of TGMconcentrations are contributed by transpacific transport. However, thecontributions of domestic and transpacific sources on the western UnitedStates levels of mercury are of comparable magnitude

Model analyses of atmospheric mercury: present air quality and effects of transpacific transport on the United States