Investigating the Kinetics of Mercury Oxidation and the Contribution of Halogen Radicals from Field Measurements in Barrow, AK

Gaseous elemental mercury (GEM) is a ubiquitous atmospheric contaminant derived from both natural and anthropogenic sources. Due to its long atmospheric lifetime in mid-latitudes, elemental mercury undergoes long-range transport, including to remote polar regions. It is now well established that GEM is depleted from the atmosphere following polar sunrise in close correlation with ozone depletion events. The main driver of these mercury depletion events (MDEs) is hypothesized to be halogen radical species, causing oxidation to reactive gaseous mercury forms that are easily deposited to the snow and ice. Bromine radicals, including Br and/or BrO, are believed to be the primary oxidants driving MDEs, and numerous kinetic and thermodynamic laboratory studies in the literature suggest the greater importance of Br. However, modeling efforts show that BrO can have a significant effect on the rate of Hg depletion depending on the magnitude of rate constant for the BrO-Hg reaction. Chlorine has been generally dismissed as an unimportant player, due to the relatively low estimates of atomic chlorine concentrations in the Arctic. The OASIS (Ocean-Atmosphere-Sea Ice-Snowpack) campaign in Spring of 2009 in Barrow, AK has provided the unique opportunity to study the kinetics of mercury depletion events using high time-resolution field data of both Hg and numerous halogen species that were measured in-situ in the Arctic for the first time. Calculated (steady-state) time-resolved Cl and Br atom concentrations allow us to pursue the hypothesis that Br atom reaction represents the principal Hg oxidation pathway, through examination of the dependence of the observed rate of decay of Hg on [Br]. Here we will discuss the results of this analysis, including the extent to which the surprisingly large calculated [Cl] values can represent an important Hg sink.