Constraining elemental mercury air–sea exchange using long-term ground-based observations

Abstract. Air–sea exchange of gaseous elemental mercury (Hg0) is a major component of the global mercury (Hg) biogeochemical cycle but remains poorly understood due to sparse in situ measurements. Here, we used long-term atmospheric Hg0 (Hgair0) observations combined with air mass back trajectories at four ground-based monitoring sites to study Hg0 air–sea exchange. The trajectories showed that all four sites sample mainly marine air masses. At all sites, we observed a gradual increase in mean Hgair0 concentration with air mass recent residence time in the marine boundary layer (MBL), followed by a steady state. The pattern is consistent with the thin-film gas exchange model, which predicts net Hg0 emissions from the surface ocean until the Hgair0 concentration normalised by Henry's law constant matches the surface ocean dissolved Hg0 (Hgaq0) concentration. This provides strong evidence that ocean Hg0 emissions directly influence Hgair0 concentrations at these sites. Using the observed relationship between Hgair0 concentrations and air mass recent MBL residence time, we estimated mean surface ocean Hgaq0 concentrations of 4–7 pg L−1 for the North Atlantic and Arctic oceans (AA) and 4 pg L−1 for the Southern, South Atlantic and south Indian oceans (SSI). Estimated ocean Hg0 emission fluxes ranged between 0.57–0.86 and 0.60–0.87 ngm-2h-1 for the AA and SSI, respectively, with a global extrapolated mean flux of around 2270 t yr−1 (1600–2900 t yr−1). This study demonstrates the applicability of long-term, ground-based Hgair0 observations in constraining Hg0 air–sea exchange.