Temperature and sunlight controls of mercury oxidation and deposition atop the Greenland ice sheet

Abstract

We conducted the first ever mercury speciation measurements atop the Greenland ice sheet at Summit Station (Latitude 72.6° N, Longitude 38.5° W, Altitude 3200 m) in the Spring and Summer of 2007 and 2008. These measurements were part of the collaborative Greenland Summit Halogen-HO_x experiment (GSHOX) campaigns investigating the importance of halogen chemistry in this remote environment. Significant levels of BrO (1–5 pptv) in the near surface air were often accompanied by diurnal dips in gaseous elemental mercury (GEM), and in-situ production of reactive gaseous mercury (RGM). While halogen (i.e. Br) chemistry is normally associated with marine boundary layers, at Summit, Greenland, far from any marine source, we have conclusively detected bromine and mercury chemistry in the near surface air. The likely fate of the formed mercury-bromine radical (HgBr) is further oxidation to stable RGM (HgBr₂, HgBrOH, HgBrCl...), or thermal decomposition. These fates appear to be controlled by the availability of Br, OH, Cl, etc. to produce RGM (Hg(II)), versus the lifetime of HgBr by thermal dissociation. At Summit, the production of RGM appears to require a sun elevation angle of >5 degrees, and an air temperature of <&minus;15 °C. Possibly the availability of Br, controlled by photolysis J(Br₂), requires a sun angle >5 degrees, while the formation of RGM from HgBr requires a temperature <&minus;15 °C . A portion of the deposited RGM is readily photoreduced and re-emitted to the air as GEM. However, a very small fraction becomes buried at depth. Extrapolating core samples from Summit to the entire Greenland ice sheet, we calculate an estimated net annual sequestration of ~13 metric tons Hg per year, buried long-term under the sunlit photoreduction zone.