Understanding mercury oxidation and air–snow exchange on the East Antarctic Plateau: a modeling study

Overview & Synopsis

Distinct diurnal and seasonal variations of mercury (Hg) have been observed in near-surface air at Con-cordia Station on the East Antarctic Plateau, but the processes controlling these characteristics are not well understood. Here, we use a box model to interpret the Hg 0 (gaseous elemental mercury) measurements in thes year 2013. The model includes atmospheric Hg 0 oxidation (by OH, O 3 , or bromine), surface snow Hg II (oxidized mercury) reduction, and air-snow exchange, and is driven by meteorological fields from a regional climate model. The simulations suggest that a photochemically driven mercury diurnal cycle occurs at the air-snow interface in austral summer. The fast oxidation of Hg 0 in summer may be provided by a two-step bromine-initiated scheme, which is favored by low temperature and high nitrogen oxides at Concordia. The summertime diurnal variations of Hg 0 (peaking during daytime) may be confined within several tens of meters above the snow surface and affected by changing mixed layer depths. Snow re-emission of Hg 0 is mainly driven by photoreduction of snow Hg II in summer. Intermittent warming events and a hypothesized reduction of Hg II occurring in snow in the dark may be important processes controlling the mercury variations in the non-summer period, although their relative importance is uncertain. The Br-initiated oxidation of Hg 0 is expected to be slower at Summit Station in Greenland than at Con-cordia (due to their difference in temperature and levels of nitrogen oxides and ozone), which may contribute to the observed differences in the summertime diurnal variations of Hg 0 between these two polar inland stations.