The behavior of mercury in the cryosphere: A review of what we know from observations

An unknown fraction of mercury that is deposited onto the cryosphere is emitted back to the atmosphere. Since mercury that enters the meltwater may be converted to highly toxic bioaccumulating methylmercury, it is important to understand the physical and chemical processes that control the ultimate fate of mercury in the cryosphere. In this study, we review deposition mechanisms as well as processes whereby mercury is lost from surface snow. We then discuss redox reactions involving cryospheric mercury. We address the conditions under which reduction and oxidation occur, the stabilizing effect of halides, and the reducibility of reactive gaseous mercury versus mercury associated with particles. We discuss physical processes including the aging of the snowpack, the penetration of insolation through the cryosphere, the vertical motion of gaseous elemental mercury molecules through the cryosphere, the melting of snowpacks, and the loss of mercury from snowpacks during snowmelt both to the atmosphere and with the meltwater's ionic pulse. These physicochemical processes are universally applicable. Variations in the behavior of cryospheric mercury between open high-latitude, open high-altitude, and forested regions, which are caused by differing environmental conditions, are also discussed. Finally, we review observed concentrations of mercury in surface snow, seasonal snowpacks, meltwater, and long-term cryospheric records. The information presented here can be used to develop a parameterization of the behavior of cryospheric mercury that is dynamically linked to environmental variables.