The role of ocean-atmosphere exchange in the global mercury cycle

Air-sea exchange of mercury (Hg) is a critical part of the global Hg cycle as it determines the response time of the biosphere to changes in mercury inputs. The cycling of Hg between the ocean and atmosphere is complex, principally because of the enhanced oxidation of elemental Hg (Hg$^0$), and the formation of reactive gaseous Hg (RGHg), in the marine boundary layer. We estimate that the dry deposition of RGHg to the ocean, which has not been previously considered in global budgets, is 35% of the total Hg input to the ocean. A further re-evaluation of the global Hg cycle suggests that there is a net transfer of Hg from the terrestrial environment to the ocean and that the deep ocean Hg concentration is increasing. Similarly, anthropogenic inputs on land have increased Hg in the Earth's surface layer with accumulation in the terrestrial environment accounting for nearly 80% of the net input from man's activities. Ultimately, because of the relatively short residence time of RGHg in the atmosphere, it is the oxidation of Hg$^0$ over the ocean, rather than RGHg transport from terrestrial sources, that is primarily contributing to oceanic RGHg deposition