Elemental mercury in the marine boundary layer of North America: Temporal and spatial patterns

Abstract Gaseous elemental mercury (Hg0) comprises the majority of atmospheric mercury (Hg) and results in long-range transport of Hg. Secondary emissions from the marine surface mixed layer result in spatial and temporal Hg0 variability, which subsequently determine marine pools of divalent mercury (HgII) available for methylation. We measured atmospheric Hg0 concentrations in the marine boundary layer (MBL) around North America in regions of the Atlantic, Pacific, and Arctic Oceans (6.7°-77.3° N, 52.3°-150.0° W). Observed spatial Hg0 distributions across these regions match patterns revealed from current published biogeochemical models. These models have supported marine processes as the major factors controlling MBL Hg0. We observe differences in variability between ocean basins. Asian emissions appear to be well-mixed in the Eastern Pacific MBL, with stable measurements across a 3200 km transect in the North Pacific. Hg0 concentrations decrease but remain relatively stable in tropical regions of the Pacific and Atlantic. In contrast, we measured several 100–200 km regions of elevated Hg0 above North Atlantic surface waters. Repeated offshore sampling of Arctic Hg0 reveals strong seasonal trends that are similar to those collected at coastal long-term Arctic monitoring stations. These similarities suggest marine processes control seasonal Arctic trends, which are linked to sea ice formation and melt. Together our observations are consistent with marine controls on MBL Hg0 concentrations although further study is needed to fully distinguish between marine and atmospheric controls.