Elemental mercury air–sea exchange - insights from long-term, ground-based atmospheric observations

Abstract

Mercury (Hg) is a globally distributed pollutant due to the long atmospheric residence time of elemental mercury (Hg0), which enables long-range transport before deposition to terrestrial and ocean surfaces. In the ocean, deposited Hg can be converted to monomethylmercury (MMHg), a highly toxic compound with severe implications for human and ecosystem health. Air–sea exchange of Hg0 plays a pivotal role in the global Hg cycle by both supplying and removing the Hg in the ocean, thereby influencing the substrate available for methylation. Despite its significance, this exchange remains poorly constrained, due to a scarcity of direct observations and an incomplete understanding of the underlying mechanisms. This dissertation uses indirect observations, that is, ground-based atmospheric Hg0 (Hg0air) measurements from long-term coastal monitoring sites to study Hg0 air–sea exchange. These measurements, from Mace Head (Irish west coast), Cabo Verde Observatory (sub-tropical North Atlantic Ocean), Cape Point (southwestern tip of South Africa) and Amsterdam Island (remote Indian Ocean), are combined with back trajectory analyses as well as a diverse set of meteorological and oceanographic datasets to provide new constraints on Hg0 air–sea exchange. Observed at all sites is a gradual increase in mean Hg0air concentration with time air masses recently spent in the Marine Boundary Layer (MBL), consistent with direct influence of ocean Hg0 emissions. Based on this relationship, a new net global Hg0 evasion flux of 2270 t y-1 (95 % confidence interval: 1600–2900 t y-1) is derived, which is lower than current chemical transport model (CTM) estimates, suggesting that these models overestimate the flux. Despite evidence of ocean emissions, Hg0air concentrations at the sites exhibit weak seasonal and diurnal variability, contrasting expectations of strong environmental control by variables such as sea surface temperature (SST), wind speed and solar radiation. Unexpected negative Hg0air–SST correlations suggest that, while warmer waters may induce evasion on a local scale, the large-scale influence of SST may be minor or masked by competing processes associated with phytoplankton activity, for example Hg0air uptake and surfactant-driven evasion suppression, as well as hemispheric-scale terrestrial vegetation Hg0air uptake. Moreover, the phytoplankton-driven processes, which have been largely overlooked, may strongly influence mean interannual variability in Hg0 air–sea exchange fluxes and resulting Hg0air concentrations in the MBL. Overall, this work demonstrates the value of long-term, ground-based Hg0air observations in constraining Hg0 air–sea exchange and bridging knowledge gaps, while also highlighting the need for extended measurements for a more comprehensive understanding of the global Hg cycle.