Nighttime production of elemental gaseous mercury in interstitial air of snow at Station Nord, Greenland

The study of the global cycle of mercury in polar environments is of great importance as mercury appears to be highly accumulated in the snowpack before melting. Exchange rates between snow and atmosphere is of particular interest to better understand the dispersion of this pollutant in polar ecosystems. Continuous monitoring of gaseous mercury (Hg°) in the air of snow and in ambient air at Station Nord, Greenland were performed from 5 to 13 March, 2002. Hg° concentrations in the snow during the day were lower than atmospheric ones as a result of rapid oxidation of Hg° by bromine species (e.g. Br• and BrO•) to form Hg(II) species, subsequently deposited onto snow grains. During 4 successive nights, we recorded a Hg° production in the air of the snow at the depth of 20 cm below the snow surface. Moreover during the 5 following days, we also recorded a Hg° production in the air of the snow at the depth of 40 cm. During these periods, concentrations of Hg° in the snow started to increase after the sunset, and reached a maximum around 2:00 a.m., thereafter decreasing to a minimum just before sunrise. Resulting emission fluxes were however weak in the range of 0.06–0.40 ng m−2 h−1. We postulated that this phenomenon could be the result of the reduction of Hg(II) by HO2• produced in the snow at night.