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Mass-independent fractionation (MIF) of stable even mass number mercury (Hg) isotopes is observed in rainfall and gaseous elemental Hg 0 globally and is used to quantify atmospheric Hg deposition pathways. The chemical reaction and underlying even-Hg MIF mechanism are unknown however and speculated to be caused by Hg photo-oxidation on aerosols at the tropopause. Here, we investigate the Hg isotope composition of free tropospheric Hg 0 and oxidized Hg II forms at the high-altitude Pic du Midi Observatory. We find that gaseous oxidized Hg has positive Δ 199 Hg, Δ 201 Hg, and Δ 200 Hg and negative Δ 204 Hg signatures, similar to rainfall Hg, and we document rainfall Hg Δ 196 Hg to be near zero. Cloud water and rainfall Hg show an enhanced odd-Hg MIF of 0.3‰ compared to gaseous oxidized Hg II , potentially indicating the occurrence of in-cloud aqueous Hg II photoreduction. Diurnal MIF observations of free tropospheric Hg 0 show how net Hg 0 oxidation in high-altitude air masses leads to opposite even- and odd-MIF in Hg 0 and oxidized Hg II . We speculate that even-Hg MIF takes place by a molecular magnetic isotope effect during Hg II photoreduction on aerosols that involves magnetic halogen nuclei. A Δ 200 Hg mass balance suggests that global Hg deposition pathways in models are likely biased toward Hg II deposition. We propose that Hg cycling models could accommodate the Hg-isotope constraints on emission and deposition fluxes.

Mass-Independent Fractionation of Even and Odd Mercury Isotopes during Atmospheric Mercury Redox Reactions