Mercury stable isotopes reveal the sources and transformations of atmospheric Hg in the high Arctic

The Arctic plays a critical role in the global Hg cycle as both a sink of Hg and, in response to rapid global warming, an emerging source of Hg due to release from permafrost and glaciers. Human activities have led to significant mercury (Hg) contamination in the Arctic over many decades, potentially impacting the health of Arctic ecosystems and indigenous people. Thus, it is crucial to understand the sources and transformations of Hg in the Arctic in order to predict how Hg behaves as emissions and the global climate change. Here we examine Hg stable isotopes in atmospheric particulate Hg (PHg), gaseous elemental Hg (GEM) and surface snow in the high Arctic (Alert, Canada) during polar spring from 2011 to 2015, in order to trace the sources of atmospheric Hg and its fate in Arctic surface environment. Isotope signatures of GEM (delta Hg-202 = 0.67 +/- 0.24 parts per thousand, Delta Hg-199 = -0.23 +/- 0.04 parts per thousand, Delta Hg-200 = -0.06 +/- 0.04 parts per thousand, 2SD) are similar to the average Northern Hemisphere background, suggesting that GEM in the high Arctic atmosphere is well mixed with those from lower latitudes. Surprisingly, the isotope composition of PHg has negative Delta Hg-199, which is similar to GEM but distinct from the positive Delta Hg-199 typically observed for oxidized Hg species elsewhere. Furthermore, Hg in surface snow shows more negative MIF than PHg, indicating post-depositional loss of Hg via photoreduction. We suggest that PHg is primarily sourced from in situ oxidation of GEM and subsequent scavenging by particles, and thus inherited the isotope composition of GEM when the oxidation is near-complete. The photoreduction re-emission of Hg from snow is strongly affected by Hg speciation, but the overall extent of re-emission (20 +/- 31%) is lower than previous estimations for other locations, suggesting potentially a greater loading of Hg to Arctic ecosystem via snowmelt.