Air-Sea Exchange and Boundary Layer Chemistry of Mercury over the Open Ocean

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The atmosphere is the most important source of mercury (Hg) to the marine environment and air-sea exchange is a major pathway for both the addition and removal of Hg from the atmosphere. Preliminary data suggests that the cycling of Hg at the air-sea interface is a complex process involving abiotic (photochemical) oxidation of elemental Hg (Hg0) and reduction of ionic Hg (HgII) in surface waters, as well as oxidation of Hg0 in the atmospheric boundary layer. A scientist from the University of Connecticut plans to assess the importance of these processes by determining the variation in the speciation of Hg and other ancillary measurements in atmospheric and surface water samples recovered from one site in the North Atlantic near Bermuda and another from Cape Point, South Africa. The resulting data set would be used to test the following three hypotheses: (1) photochemical oxidation of Hg0 in surface seawater occurs but the overall net reaction is HgII reduction; (2) organic matter is the primary dissolved constituent influencing the rate of HgII reductions in surface seawater; and (3) oxidation of Hg0 in the marine boundary layer and subsequent dry deposition of HgII compounds is substantial and rivals that of wet deposition. Controlled incubation experiments would also be carried out to determine redox transformations in surface waters. Lastly, the data from the project would be used to constrain and improve ocean and global Hg models, as well as incorporate their results into model simulations using the Harvard University GEOS-Chem modeling framework. As regards broader impacts, this study would further our understanding of the Hg cycle. This study would allow U.S. scientists to interact with their counterparts in South Africa and would continue to build international collaborations with South African institutions. One postdoc, one graduate student, and one undergraduate student would be supported and trained as part of this project.