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 (Hg⁰_air) 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 Hg⁰_air 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 Hg⁰_air concentration normalised by Henry’s law constant matches the surface ocean dissolved Hg⁰ (Hg⁰_aq) concentration. This provides strong evidence that ocean Hg⁰ emissions directly influence Hg⁰_air concentrations at these sites. Using the observed relationship between Hg⁰_air concentrations and air mass recent MBL residence time, we estimated mean surface ocean Hg⁰_aq 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 0.58–0.75 and 0.47–0.66 ng m^-2 h^-1 for the AA and SSI, respectively, with a global extrapolated mean flux of 1900 t y^-1 (1200–2600 t y^-1). This study demonstrates the applicability of long-term, ground-based Hg⁰_air observations in constraining Hg⁰ air-sea exchange.