National inventory data and projections, by both EPRI and U.S. EPA, show that there was an overall ~50% drop in U.S. utility emissions of total mercury (Hg) between 2007 and 2010. Announced plant and unit retirements since 2010 indicate a continuing further reduction in U.S. Hg emissions. Because of uncertainties in speciation changes in the emitted mercury by these electric generating units (EGUs), it is however unclear how much U.S. deposition “should” have changed in this period. The primary question addressed here is therefore “Do the reductions in mercury emissions from EGUs (and other sources) in the United States, driven by MATS and other regulations, translate into observed changes in: a) mercury concentrations in precipitation and/or b) total mercury wet deposition.” As such, the NADP’s Mercury Deposition (MDN) data is analyzed for temporal and spatial trends. The last time such analyses were presented was with data until 2005¹. Hence, almost a decade of additional data is available for such spatial and temporal analysis, covering the aforementioned important Hg-emissions’ reduction period between 2007 and 2010. Additionally, new MDN sites have been added and some retired since 2005 which potentially affect the trend results. Preliminary results of these analyses are presented and discussed.

¹Prestbo, E.M., Gay, D. A. 2009. Wet deposition of mercury in the U.S. and Canada, 1996–2005: Results and analysis of the NADP mercury deposition network (MDN). Atmos. Environ. 43, 4223–4233.