New source mechanism for airborne particulate mercury in the central Arctic

Understanding the mercury cycle in the Arctic is important due to the harmful bioaccumulation of its toxic form, methylmercury, in wildlife and ultimately Arctic residents. Gaseous elemental mercury (Hg(0)) is relatively well-mixed across the northern hemisphere atmosphere due to its long atmospheric lifetime. Hg(0) can be oxidized, especially in the Arctic spring during halogen-driven depletion events. The resulting gaseous oxidized mercury (Hg(II)) is relatively quickly deposited onto snow, either directly or via condensing onto particles, forming particulate mercury (PHg). It is generally understood that a large fraction of the deposited Hg(II) and PHg is photoreduced to Hg(0) and re-emitted to the atmosphere. However, mercury remaining in the snowpack till melt can become bioavailable through entering the ocean.

There is a severe lack of Hg(II) and PHg observations in the central Arctic, particularly over sea ice, limiting our understanding of the mercury cycle in that region and inhibiting us from quantifying mercury budgets in all environmental compartments and particularly where it unfolds its harmful neurotoxic effects. Moreover, most of the observational efforts aiming at creating process understanding focused on spring during mercury depletion events or the snow melt period, leaving large knowledge gaps for fall and winter.

Here, we show atmospheric observations of PHg during MOSAiC, measured with an aerosol mass spectrometer in fall and spring over the central Arctic pack ice. In both seasons, PHg concentrations correlate strongly with wind speed and chloride, suggesting a mechanical (wind-driven) process behind atmospheric PHg related partly to blowing snow. In addition, there are significant differences between fall and spring observations (e.g. no atmospheric mercury depletion events in fall), suggesting that various processes are at play.

This wind-driven process has hitherto not been reported and is different from observations at land-based stations as well as previous measurements over sea ice that ascribed the formation of PHg to adsorption of Hg(II) onto pre-existing aerosols or diamond dust rather than aerosolization from the snow pack. We hypothesize, based on snow chemical analyzes and literature, that the elevated halide content in snow on sea ice creates complexes of PHg, which are much harder to photoreduce than Hg(II), leading to a larger PHg content in snow. These processes of forming PHg and wind-driven aerosolization have implications for the mercury content of snow and the distances over which PHg is re-deposited after atmospheric transport given that the lifetime of PHg is about one order of magnitude larger than that of Hg(II) in the atmosphere.