Gaseous mercury distribution in interstitial air of snow pack in Station Nord, Greenland. Evidence of permanent mercury depletion event in the air of snow during polar sunrise

Gaseous Elemental Mercury (Hg°) concentrations have been measured in Station Nord, Greenland in the air of the snowpack from February 25$^{\rm th}$ to march 15$^{\rm th}$, 2002, during twilight and low solar irradiation periods. The concentration of Hg$^{\circ}$ decreases rapidly with depth in the snow pack air from ~1.5 ng/m$^3$ outside to ~0.1 ng/m$^3$ at 120 cm depth. Adsorption of mercury on snow could not explain such profile since temperature increases with depth (~-13 °C at 120 cm below the surface and ~-35 °C at the surface). Additionally calculations indicate that a poor fraction of Hg° could be adsorbed onto snow. A possible argument to explain such a decrease of Hg° with depth is to investigate homogeneous and/or heterogeneous chemistry occurring at the air/snow interface initiated by sun light. The Arctic snowpack is known to produce active bromine and chlorine species in the frame of interactions between sea-salts, ozone and acid species in the snow. The lifetime of Hg° is calculated on the basis of reactions with various reactive species in the gaseous phase existing in the interstitial air during twilight and under low solar irradiation periods, such as BrCI, Br$_2$, HOBr, Br, BrO and OH. We assume that the decrease of Hg° concentration with depth is the result of the reaction between Hg° and Br and/or BrO, which give a lifetime for Hg°$<$~0.8-8 hours based on known kinetic constants found in the recent literature. This decrease of mercury in the air of the snowpack indicates that during polar sunrise in the Arctic, mercury is probably accumulated in the snowpack on its oxidised form Hg (II), probably HgO. Therefore, the snowpack could be a sink of mercury in trie Arctic in spring where a permanent Mercury Depletion Event (M.D.E.) could exist at polar sunrise