Wet deposition of mercury (Hg) across the United States is influenced by changes in atmospheric conditions, domestic emissions and global background emissions. We examine trends in Hg precipitation concentrations at 47 Mercury Deposition Network (MDN) sites during 2004-2010 by using the GEOS-Chem nested-grid Hg simulation. We run the model with constant anthropogenic emissions and subtract the model results from the observations. This helps to remove the variability in observed Hg concentrations caused by meteorological factors, including precipitation. We find significant decreasing trends in Hg concentrations in precipitation at MDN sites in the Northeast (-4.1 +/- 0.49% yr(-1)) and Midwest (-2.7 +/- 0.68% y(r-1)). Over the Southeast (-0.53 +/- 0.59% yr(-1)), trends are weaker and not significant, while over the West, trends are highly variable. We conduct model simulations assuming a 45% decrease in Hg emissions from domestic sources in the modeled period and a uniform 12% decrease in background atmospheric Hg concentrations. The combination of domestic emission reductions and decreasing background concentrations explains the observed trends over the Northeast and Midwest, with domestic emission reductions accounting for 58-46% of the decreasing trends. Over the Southeast, we overestimate the observed decreasing trend, indicating potential issues with our assumption of uniformly decreasing background Hg concentrations.
