Atmospheric mercury over sea ice during the OASIS-2009 campaign
Measurements of gaseous elemental mercury (GEM), reactive gaseous mercury (RGM) and particulate mercury (PHg) were collected on the Beaufort Sea ice near Barrow, Alaska, in March 2009 as part of the Ocean-Atmosphere-Sea Ice-Snowpack (OASIS) and OASIS-Canada International Polar Year programmes. These results represent the first atmospheric mercury speciation measurements collected on the sea ice. Concentrations of PHg averaged 393.5 pg m(-3) (range 47.1-900.1 pg m(-3)) and RGM concentrations averaged 30.1 pg m(-3) (range 3.5-105.4 pg m(-3)) during the two-week-long study. The mean concentration of GEM during the study was 0.59 ng m(-3) (range 0.01-1.51 ng m(-3)) and was depleted compared to annual Arctic ambient boundary layer concentrations. It is shown that when ozone (O-3) and bromine oxide (BrO) chemistry were active there is a positive linear relationship between GEM and O-3, a negative one between PHg and O-3, a positive correlation between RGM and BrO, and none between RGM and O-3. For the first time, GEM was measured simultaneously over the tundra and the sea ice. The results show a significant difference in the magnitude of the emission of GEM from the two locations, with significantly higher emission over the tundra. Elevated chloride levels in snow over sea ice are proposed to be the cause of lower GEM emissions over the sea ice because chloride has been shown to suppress photoreduction processes of RGM to GEM in snow. Since the snowpack on sea ice retains more mercury than inland snow, current models of the Arctic mercury cycle may greatly underestimate atmospheric deposition fluxes because they are based predominantly on land-based measurements. Land-based measurements of atmospheric mercury deposition may also underestimate the impacts of sea ice changes on the mercury cycle in the Arctic. The predicted changes in sea ice conditions and a more saline future snowpack in the Arctic could enhance retention of atmospherically deposited mercury and increase the amount of mercury entering the Arctic Ocean and coastal ecosystems.