Investigation of Air-snow Exchanges of Mercury at Summit: Proof of Concept for Automated Sampling at the Snowtower Facility

Mercury (Hg), a persistent and toxic element, is found both naturally and as an anthropogenically-produced compound in the environment. In the atmosphere, gaseous elemental mercury (Hg0 or GEM) is the predominant form of mercury (>95%). GEM can be converted to divalent mercury species (Hg(II)) by oxidation processes. Divalent mercury is subject to rapid wet and dry deposition to snow surfaces. Hg exchanges between atmosphere and cryosphere are still poorly quantified, leading to a lack of understanding about the role of the cryosphere in the global mercury cycle. A GEM analyzer (model 2537B Tekran) was coupled during July 2009 with the snowpack sampling platform (i.e., the snowtower) operated at the GeoSummit camp (Greenland) FLUX facility. We observed a diel pattern for GEM concentrations in snow interstitial air with production occurring during low irradiation periods (concentrations in the snow air exceeding atmospheric levels) and destruction during high irradiation periods (concentrations in the snow air lower that atmospheric levels). In the uppermost 10-30 cm of the snowpack, GEM production is likely driven by photoreduction of Hg(II) species and competes with GEM oxidation. Deeper, GEM oxidation becomes the predominant processes. Interestingly, GEM and ozone show concurrent destruction in the snow interstitial air. In the polar marine boundary layer, destruction of GEM is known to be the result of photochemically initiated reactions involving bromine species. Similarly, Br radical chemistry is possibly the cause of losses of GEM in the snow interstitial air at Summit, despite the much lower sea-salt concentrations at this continental site. These preliminary data represent a proof of concept that automated sampling of GEM concentrations in the lower atmosphere and snow interstitial air at Summit is feasible all year round, and that this research can yield valuable new insight into snow interstitial air oxidation chemistry. Potential questions that could be address by such GEM sampling include: (i) how perennial snow surfaces of Greenland impact the tropospheric budget of mercury?, (ii) how post-deposition redox chemistry occurring at the surface of the ice sheet may affect the long term records of mercury preserved in ice cores?, and (iii) if GEM concentrations in the snow air can be used as a new proxy to characterize the oxidation chemistry driven by halogen radicals within the Summit snowpack?