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

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 25 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 30 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 around 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 35 exchange.