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 Delta Hg-199, Delta Hg-201, and Delta Hg-200 and negative Delta Hg-204 signatures, similar to rainfall Hg, and we document rainfall Delta Hg-196 to be near zero. Cloud water and rainfall Hg show an enhanced odd-Hg MIF of 0.3 parts per thousand 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 Delta Hg-200 mass balance suggests that global Hg-II 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.