Aqueous phase methylation as a potential source of methylmercury in wet deposition |
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| Institution: | 1. GRIL, Département de sciences biologiques, Université du Québec à Montréal, C.P. 8888, Succursale Centre-Ville, Montreal, Quebec H3C 3P8, Canada;2. GRIL, Département de sciences biologiques, Université de Montréal, C.P. 6128, Succursale Centre-Ville, Montréal, Quebec H3C 3J7, Canada;1. Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, MI 48109, United States;2. School of Earth and Ocean Sciences, Cardiff University, Cardiff CF10 3AT, United Kingdom;3. Water Research Institute, Cardiff University, Cardiff CF10 3AX, United Kingdom;4. Earth Research Institute, University of California Santa Barbara, Santa Barbara, CA 91306, United States |
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| Abstract: | The source of monomethylmercury (MMHg) in wet deposition is unknown. Volatilization of gaseous MMHg, evasion and demethylation of dimethylmercury, and methylation of Hg0 have been either proposed or tested unsuccessfully as potential sources. Here, we show that MMHg in precipitation, sampled across a wide geographical range in North America, is related positively to an operationally defined and measured reactive Hg species (HgR), but connected weakly to total Hg. The mean molar ratio of MMHg:HgR measured in continental precipitation (0.025±0.006) is comparable to the MMHg:Hg(II) ratio estimated from first-order rate constants for acetate-mediated Hg methylation and MMHg photolysis (0.025±0.002). This suggests MMHg may be formed in the atmosphere through a reaction between labile Hg(II) complexes and an unknown methylating agent(s), potentially acetate or similar molecules. Availability of Hg(II) appears to limit the reaction, and accordingly, increased atmospheric loadings of Hg could lead to enhanced MMHg in precipitation. |
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