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The Role of Regional Atmospheric Transport in Controlling Mercury Wet Deposition over the Northeastern United States

Peter Weiss-Penzias
Department of Environmental Toxicology,
University of California,
Santa Cruz, CA USA

We present an investigation of the regional influence of large coal-combustion and other industrial mercury sources on mercury concentrations in wet deposition in the northeastern United States. Mercury concentrations in weekly integrated precipitation samples were obtained from three Mercury Deposition Network (MDN) sites that are progressively down-wind of numerous coal-fired power plants (CFPPs) in the Ohio River Valley region: Millford, Pennsylvania (PA72), Huntington, New York (NY20), and Bridgton, Maine (ME02). Seasonal and week-to-week variability in mercury concentrations at each site were investigated using ensembles of atmospheric back trajectories from each precipitation event during 2004-2005 and by calculating the residence time of each trajectory in an emissions area encompassing a ~500 km x ~500 km region with the most CFPPs. At all three sites, trajectory residence times were at a maximum in the month of June, which generally matches the season cycle in mercury concentrations. However, only PA72 produced consistently significant linear correlations between weekly mercury concentration and trajectory residence time (r = 0.52, n = 85 weekly samples, P < 0.0001), suggesting that the spatial extent of direct mercury pollution transport is < 700 km. The trajectory residence time/mercury concentration relationship was strongest at PA72 during March-May (r = 0.77, n = 31, P < 0.0001), and weakest in September-November (r = -0.06, n = 26). Volume-weighted mercury concentrations at PA72 are elevated compared to NY20 and ME02 by 10-25% in the summer, fall, and winter, and by 60% during the spring. Due to the strong linear relationship with trajectory residence time, this excess is attributed to direct pollution transport from the CFPP region. Additionally, we find that the 2001-2006 annual downward trend in volume-weighted mercury concentration at PA72 for the spring and summer months (the time of year with the most transport from the CFPP region) is statistically significant (-1 ng/L/year, P < 0.05), whereas the annual trend in the fall and winter months is not statistically significant. This suggests that further emissions reductions at CFPPs during the spring and summer would have a proportionally larger effect of lowering mercury in wet deposition at PA72 than emissions reductions at other times of the year