Snow-to-air exchanges of mercury in an Arctic seasonal snow pack in Ny-Ålesund, Svalbard
Résumé
The study of mercury (Hg) cycle in Arctic regions is a major subject of concern due to the dramatic increases of Hg concentrations in ecosystem in the last few decades. The causes of such increases are still in debate, and an important way to improve our knowledge on the subject is to study the exchanges of Hg between atmosphere and snow during springtime. We organized an international study from 10 April to 10 May 2003 in Ny-Ålesund, Svalbard, in order to assess these fluxes through measurements and derived calculations. Snow-to-air emission fluxes of Hg were measured using the flux chamber technique between 0 and 50 ng m−2 h−1. A peak in Gaseous Elemental Mercury (GEM) emission flux from the snow to the atmosphere has been measured just few hours after an Atmospheric Mercury Depletion Event (AMDE) recorded on 22 April 2004. Surprisingly, this peak in GEM emitted after this AMDE did not correspond to any increase in Hg concentration in snow surface. A peak in GEM flux after an AMDE was observed only for this single event but not for the four other AMDEs recorded during this spring period. In the snow pack which is seasonal and about 40 cm depth above permafrost, Hg is involved in both production and incorporation processes. The incorporation was evaluated to 5-40 pg m2 h. Outside of AMDE periods, Hg flux from the snow surface to the atmosphere was the consequence of GEM production in the air of snow and was about 15-50 ng m−2 h−1, with a contribution of deeper snow layers evaluated to 0.3-6.5 ng m−2 h−1. The major part of GEM production is then mainly a surface phenomenon. The internal production of GEM was largely increasing when snow temperatures were close to melting, indicating a chemical process occurring in the quasi-liquid layer at the surface of snow grains.