Springtime accumulation of atmospheric mercury in polar ecosystems

Abstract
Mercury and many of its compounds behave exceptionally in the environment due to their volatility, capability for methylation, and subsequent biomagnification in contrast with most of the other heavy metals. Long-range atmospheric transport of elemental mercury, its transformation to more toxic methylmercury compounds, the ability to undergo photochemical reactions and their bioaccumulation in the aquatic food chain have made it a subject of global research activities. Atmospheric Mercury Depletion Events (AMDEs) during polar springtime have been experimentally observed in the Arctic and in the Antarctic. During these events Hg⁰ and ozone concentrations are significantly depleted and well correlated, whereas concentrations of reactive gaseous mercury species (RGM) simultaneously increase. The main reaction mechanism and corresponding chemical and physical properties of involved species in polar regions are summarized in this work. Hg⁰ is removed from the atmosphere and deposited onto the underlying surface snow. This paper focused on the fast, photochemically driven, oxidation of boundary-layer Hg⁰, the influence of reactive halogen chemistry, and the resultant net input of mercury into the polar ecosystem during and after polar springtime. Several estimates of the size of the Arctic sink for newly deposited Hg range from 100 - 300 T/y, while estimates of the Antarctic sink are far more uncertain. The role of re-emission of elemental mercury from the snow surface is critically discussed.