Investigation of the deposition and emission of mercury in arctic snow during an atmospheric mercury depletion event

Abstract

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.