Abstract
AbstractAir‐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 (gradient‐based and relaxed eddy accumulation [REA]) and also simulated the flux with a gas exchange model. The coastal waters were typically supersaturated with Hg0 (mean ± 1σ = 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 fluctuation. 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 Hg0 transfer velocity compared to commonly used parameterizations. Although based on a limited data set, we suggest that the wind speed dependence of the Hg0 transfer velocity is more consistent with gases that have less water solubility than CO2 (e.g., O2). These pioneering flux measurements using micrometeorological techniques show that more such measurements would improve our understanding of air‐sea Hg exchange.
Highlights
On a global scale 3,800 Mg a−1 mercury (Hg) enter the ocean through atmospheric deposition and 300 Mg a−1 through riverine input (UNEP, 2019)
Environmental parameters used in the Hg0 air-sea exchange model include sea surface temperature (SST), sea surface salinity (SSS), wind speed (u10), atmospheric pressure (Pres), dissolved gaseous mercury (Hg0aq) (Figure 2) and gaseous elemental mercury (Hg0air) (Figure 3)
With this study we address the call of the Minamata Convention (UNEP, 2013) to improve our understanding of air-sea exchange of Hg0 by providing a comparison between modeled and measured Hg0 fluxes
Summary
On a global scale 3,800 Mg a−1 mercury (Hg) enter the ocean through atmospheric deposition and 300 Mg a−1 through riverine input (UNEP, 2019). Reduction of HgII to gaseous elemental mercury (Hg0) in the surface ocean leads to re-emission of Hg0 from the ocean of approximately 2,900 Mg a−1 (Horowitz et al, 2017). These current air-sea Hg exchange estimates are associated with high uncertainty and a better constraint on the Hg0 flux is crucial for two reasons: first, ocean emissions reduce the reservoir of HgII available for methylation in the water column and subsequent bioaccumulation in marine biota (Lavoie et al, 2013). The Hg0 flux is typically estimated based on a thin film gas exchange model (Liss & Merlivat, 1986; Wanninkhof, 1992) that uses in situ measurements of Hg0air and Hg0aq together with a wind speed dependent parameterization of k that was developed based on field experiments with volatile
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