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

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.