No abstract

Mercury is deposited to the Polar Regions during springtime atmospheric mercury depletion events (AMDEs) but the relationship between snow and ice crystal formation and mercury deposition is not well understood. The objective of this investigation was to determine if mercury concentrations were related to the type and formation of snow and ice crystals. On the basis of almost three hundred analyses of samples collected in the Alaskan Arctic, we suggest that kinetic crystals growing from the vapor phase, including surface hoar, frost flowers, and diamond dust, yield mercury concentrations that are typically 2-10 times higher than that reported for snow deposited during AMDEs (∼80 ng/L). Our results show that the crystal type and formation affect the mercury concentration in any given snow sample far more than the AMDE activity prior to snow collection. We present a conceptual model of how snow grain processes including deposition, condensation, reemission, sublimation, and turbulent diffusive uptake influence mercury concentrations in snow and ice. These processes are time dependent and operate collectively to affect the retention and fate of mercury in the cryosphere. The model highlights the importance of the formation and postdeposition crystallographic history of snow or ice crystals in determining the fate and concentration of mercury in the cryosphere.

[1] Mechanisms of air-snow exchange of mercury (Hg) during and after atmospheric mercury depletion events (AMDEs) remain poorly constrained and this has limited our understanding of the arctic Hg cycle. We measured the Hg concentrations of surface snow through time and carried out flux chamber experiments during AMDE and non-AMDE conditions in the spring of 2006 near Barrow, Alaska. Clear skies, low-velocity onshore winds, and a stable boundary layer characterized the meteorology during this AMDE. Surface snow Hg concentrations (upper 1 cm) increased throughout a 9-day AMDE from background levels (4.1-15.5 ng/L) to elevated levels (147 and 237 ng/L) at two sampling sites and returned to near-baseline values within 2 days of AMDE cessation. The Hg concentrations of core samples from the full snowpack did not increase significantly during the AMDE and demonstrate that the Hg enhancement of surface snow resulted from deposition of atmospheric Hg to surface snow. We estimate that complete deposition of background Hg to a height of 200-450 m in the near-surface troposphere could account for the Hg gains to surface snow during this event. Snow incubated in field-based flux chambers emitted 4 to 7% of its total Hg content within 1 day and may represent an upper limit for the photo-reduction rate of ''easily'' reducible Hg in snow under post-AMDE conditions. Full-column snow core samples collected in the late springtime have comparable Hg loads to those observed during the AMDE season and imply that a significant fraction of the Hg deposited during the 3-month AMDE season was retained until snowmelt at this location.