Isotopic composition of mercury deposited via snow into mid-latitude ecosystems

Highlights: • Contrasting atmospheric processes control the Hg isotopic composition in snow. • Polar vortex snow progresses towards more negative odd-MIF when aged in sunlight. • Photoreduction of Hg(II) drives odd-MIF in mid-latitude snow. • Isotopic analysis of snowmelt is important for ecosystem Hg source identification. Atmospheric deposition of mercury (Hg) to terrestrial and aquatic ecosystems has significant implications for human and animal exposure. Measurements of Hg isotopic composition can be utilized to trace sources of Hg, but outside of the Arctic there has been little Hg isotopic characterization of snow. To better understand deposition pathways at mid-latitudes, five time series of snowfall were collected at two sites (Dexter and Pellston, Michigan, USA) to investigate the Hg isotopic composition of snowfall, how it changes after deposition, and how it compares to rain. The Hg isotopic composition of a subset of fresh snow samples revealed the influence of reactive surface uptake of atmospheric Hg(0). The first time series collected at Dexter occurred during a polar vortex, demonstrating Hg isotopic fractionation dynamics similar to those in Arctic snow, with increasingly negative Δ¹⁹⁹Hg as snow aged with exposure to sunlight. All other time series revealed an increase in Δ¹⁹⁹Hg as snow aged, with values reaching up to 3.5‰. This characterization of Hg isotopes in snow suggests a strong influence of oxidants and binding ligands in snow that may mediate Hg isotope fractionation. Additionally, isotopic characterization of Hg in snow deposited to natural ecosystems at mid-latitudes allows for better understanding of atmospheric mercury sources that are deposited to lakes and forests and that may become available for methylation and transfer to food webs.