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[ID: 305] Hg Ostergarnsholm
PI: Wei Zhu
Air-sea exchange of gaseous elemental mercury (Hg0) is not well constrained, even though it is a major component of the global Hg cycle. Lack of Hg0 flux measurements to validate parameterizations of the Hg0 transfer velocity contributes to this uncertainty. We measured the Hg0 flux on the Baltic Sea coast using micrometeorological methods (two gradient-based and relaxed eddy accumulation (REA)) as well as a gas exchange model. We found that coastal waters were typically supersaturated with Hg0 (mean ± 1 = 13.5 ± 3.5 ng m-3; ca. 10 % of total Hg) compared to the atmosphere (1.3 ± 0.2 ng m-3). The Hg0 flux calculated using the gas exchange model ranged from 0.1 – 1.3 ng m-2 h-1 (10th and 90th percentile) over the course of the campaign (May 10 – June 20, 2017) and showed a distinct diel variation. The mean coastal Hg0 fluxes determined with the two gradient-based approaches and REA were 0.3, 0.5 and 0.6 ng m-2 h-1, respectively. In contrast, the mean open sea Hg0 flux measured with REA was larger (6.3 ng m-2 h-1). The open sea Hg0 flux indicated a stronger wind speed dependence for the transfer velocity of Hg0 compared to commonly used parameterizations. Although based on a limited data set, we suggest that the wind speed dependence of the Hg0 transfer velocityis more in line with gases that have a lower water solubility than CO2 (e.g. O2). More land-based Hg0 flux measruements using direct micrometerological methods could significantly improve our understanding of air-sea Hg exchange.