Abstract. It was discovered in 1995 that, during the spring time, unexpectedly low concentrations of gaseous elemental mercury (GEM) occurred in the Arctic air. This was surprising for a pollutant known to have a long residence time in the atmosphere; however conditions appeared to exist in the Arctic that promoted this depletion of mercury (Hg). This phenomenon is termed atmospheric mercury depletion events (AMDEs) and its discovery has revolutionized our understanding of the cycling of Hg in Polar Regions while stimulating a significant amount of research to understand its impact to this fragile ecosystem. Shortly after the discovery was made in Canada, AMDEs were confirmed to occur throughout the Arctic, sub-Artic and Antarctic coasts. It is now known that, through a series of photochemically initiated reactions involving halogens, GEM is converted to a Correspondence to: A. Steffen (alexandra.steffen@ec.gc.ca) more reactive species and is subsequently associated to particles in the air and/or deposited to the polar environment. AMDEs are a means by which Hg is transferred from the atmosphere to the environment that was previously unknown. In this article we review Hg research taken place in Polar Regions pertaining to AMDEs, the methods used to collect Hg in different environmental media, research results of the current understanding of AMDEs from field, laboratory and modeling work, how Hg cycles around the environment after AMDEs, gaps in our current knowledge and the future impacts that AMDEs may have on polar environments. The research presented has shown that while considerable improvements in methodology to measure Hg have been made but the main limitation remains knowing the speciation of Hg in the various media. The processes that drive AMDEs and how they occur are discussed. As well, the role that the snow pack and the sea ice play in the cycling of Hg is presented. It has been found that deposition of Hg from AMDEs occurs at marine coasts and not far inland and that a fraction of the Published by Copernicus Publications on behalf of the European Geosciences Union. deposited Hg does not remain in the same form in the snow. Kinetic studies undertaken have demonstrated that bromine is the major oxidant depleting Hg in the atmosphere. Modeling results demonstrate that there is a significant deposition of Hg to Polar Regions as a result of AMDEs. Models have also shown that Hg is readily transported to the Arctic from source regions, at times during springtime when this environment is actively transforming Hg from the atmosphere to the snow and ice surfaces. The presence of significant amounts of methyl Hg in snow in the Arctic surrounding AMDEs is important because this species is the link between the environment and impacts to wildlife and humans. Further, much work on methylation and demethylation processes has occurred but these processes are not yet fully understood. Recent changes in the climate and sea ice cover in Polar Regions are likely to have strong effects on the cycling of Hg in this envir...

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The oxidation of elemental mercury (Hg0) to the divalent gaseous mercury dibromide (HgBr2) has been proposed to account for the removal of Hg0 during depletion events in the springtime Arctic. The mechanism of this process is explored in this paper by theoretical calculations of the relevant rate coefficients. Rice-Ramsberger-Kassel-Marcus (RRKM) theory, together with ab initio quantum calculations where required, are used to estimate the following: recombination rate coefficients of Hg with Br, I, and O; the thermal dissociation rate coefficient of HgBr; and the recombination rate coefficients of HgBr with Br, I, OH, and O2. A mechanism based on the initial recombination of Hg with Br, followed by the addition of a second radical (Br, I, or OH) in competition with thermal dissociation of HgBr, is able to account for the observed rate of Hg0 removal, both in Arctic depletion events and at lower latitudes.

We develop an improved treatment of the surface ocean in the GEOS-Chem global 3-D biogeochemical model for mercury (Hg). We replace the globally uniform subsurface ocean Hg concentrations used in the original model with basin-specific values based on measurements. Updated chemical mechanisms for Hg 0 /Hg II redox reactions in the surface ocean include both photochemical and biological processes, and we improved the parametrization of particle-associated Hg scavenging. Modeled aqueous Hg concentrations are consistent with limited surface water observations. Results more accurately reproduce high-observed MBL concentrations over the North Atlantic (NA) and the associated seasonal trends. High seasonal evasion in the NA is driven by inputs from Hg enriched subsurface waters through entrainment and Ekman pumping. Globally, subsurface waters account for 40% of Hg inputs to the ocean mixed layer, and 60% is from atmospheric deposition. Although globally the ocean is a net sink for 3.8 Mmol Hg y -1 , the NA is a net source to the atmosphere, potentially due to enrichment of subsurface waters with legacy Hg from historical anthropogenic sources. IntroductionAnthropogenic mercury (Hg) sources have enriched atmospheric Hg deposition globally by at least a factor of 3 (1).Atmospheric Hg is predominantly the gaseous elemental form (Hg 0 ), and is oxidized to Hg II , which is then rapidly deposited. It is estimated that more than 80% of the Hg deposited to oceans is reemitted to the atmosphere as Hg 0 , driving the cycle of Hg through biogeochemical reservoirs (2). Aqueous reduction of divalent inorganic mercury (Hg II ) and subsequent loss of Hg 0 reduces the potentially bioavailable Hg II pool that may be converted to monomethylmercury, the most toxic species that poses health risks to fish consuming populations and wildlife (3).Previous efforts to model Hg air-sea exchange (2) and atmospheric transport (4-6) have been unable to reproduce high atmospheric concentrations observed in the Northern Hemisphere marine boundary layer (MBL) during ocean cruises (7-9). We hypothesize that this results from subsurface seawater Hg enrichment, reflecting the legacy of past anthropogenic inputs and controlling Northern Hemisphere MBL concentrations. Previous research comparing preindustrial and contemporary Hg budgets for different ocean basins indicates that anthropogenic enrichment of Hg reservoirs in the Atlantic Ocean and Mediterranean Sea is >50% (3). Other regions such as deep waters of the Pacific Ocean have seen negligible anthropogenic impacts while Hg concentrations in intermediate waters of the North Pacific (NP) appear to be increasing (10). These gradients in subsurface Hg across ocean regions (11, 12) have not been represented in models simulating atmospheric Hg. Here we investigate the potential effects of legacy anthropogenic Hg accumulation on oceanic air-sea exchange in the GEOSChem global model (1) by including the effects of variability in subsurface ocean concentrations in our simulation.Most marine surface waters...