Recent atmospheric evolution of Gaseous Elemental Mercury concentrations revealed by firn air archive at Summit Station, Greenland.

Atmospheric Gaseous Elemental Mercury (GEM) has been measured systematically in various locations for the last ten years. Moreover, there are scarce data available since the end of the seventies, but no measurements have been done before. A better understanding of past evolution of GEM concentrations in the atmosphere is nevertheless essential to quantify the impact of human activities on the environment since it is the major form emitted by anthropogenic sources. An increase by a factor estimated between 2 and 5 is likely since the beginning of the Industrial Revolution. The ice archives are probably the only way to reconstruct atmospheric GEM story: the air extracted from the firn ice caps provides a record of past atmospheric composition. Field works (during July 2005 and may 2006) have been conducted in order to measure GEM in the firn air, from the surface to the close-off (75 meters depth), at Summit Station, Central Greenland. GEM atmospheric levels were stable during summer about 1.7 ng.m-3. An active reactivity has been observed in the first meters of the snowpack, involving both photochemical oxydation and reduction processes leading to gaseous mercury incorporation and/or production in the interstitial air. Finally, firn air pumping was achieved so as to evaluate GEM content in the 75 meters depth firn. These firn measurements have been associated with firn-diffusion modelling work (Rommaelere et al., 1997) so as to describe the last 25 years change in atmospheric composition for GEM. First results show that a decrease of GEM atmospheric levels is recorded since the early 1990s as shown by atmospheric monitoring of GEM in the Northern Hemisphere (Slemr et al., 2003). Looking at these combined modeled-field results, it seems that GEM information is conserved through the firn. We could then consider that air bubbles trapped in ice could store GEM and the analysis of the air in ice samples could inform us about past concentration for GEM.