Modeling Dynamics of Mercury on the Antarctic Plateau: Roles of Photochemistry and the Underlying Snowpack

Abstract:

Understanding chemical cycling of mercury in the Antarctic plateau can provide important constraints on the global biogeochemical cycling of mercury, but there remain numerous uncertainties in mercury chemistry and behavior in remote Polar Regions. Recent research has recognized the atmosphere of the Antarctic plateau as a highly oxidizing area with elevated concentrations of hydroxyl radicals (OH ~ 3 × 10⁶ molecule cm^-3 during austral summer) and other atmospheric oxidants. Some chemical species (e.g. NO_x) may undergo multiple recycling between the atmospheric boundary layer and the underlying snowpack.

We use a box model along with measurements of mercury to investigate the chemistry and dynamics of mercury in the boundary layer of Concordia station (3320 m above sea level). Information on atmospheric dynamics (e.g. atmospheric boundary layer height) is obtained from a regional climate model which has been validated against meteorological observations at the same station. We evaluate the proposed two-step oxidation mechanism of Hg⁰ in which HgBr is the intermediate, as well as several other oxidation mechanisms used in current chemical transport models. The deposition of oxidized Hg and its subsequent photoreduction in the surface snow increase the snowpack pool of Hg⁰ and serve as an important step in the atmosphere-snow mercury recycling.

We constrain our box model with measurements of mercury from the Antarctic plateau. Gaseous elemental mercury (GEM, Hg⁰) has been continuously measured at Concordia station in the past several years. In 2013, measurements of Hg⁰, ozone and NO_x were conducted in both snow interstitial air and the overlying atmosphere. Hg⁰ concentrations were found to slowly decrease during the austral winter (perpetual night); whereas during the austral summer (perpetual day), Hg⁰ concentrations showed a clear diurnal cycle and high day-to-day fluctuations. This suggests that photochemistry plays an important role in controlling the reactivity of mercury on the Antarctic plateau.