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 Hg0 globally used to quantify atmospheric Hg deposition pathways. The chemical reaction underlying even-Hg MIF mechanism are unknown however speculated be caused by photo-oxidation on aerosols at the tropopause. Here, we investigate isotope composition free tropospheric oxidized HgII forms high-altitude Pic du Midi Observatory. We find that has positive Δ199Hg, Δ201Hg, Δ200Hg negative Δ204Hg signatures, similar Hg, document Δ196Hg near zero. Cloud water show an enhanced odd-Hg 0.3‰ compared HgII, potentially indicating occurrence in-cloud aqueous photoreduction. Diurnal observations how net oxidation air masses leads opposite even- odd-MIF HgII. speculate takes place a molecular magnetic effect during photoreduction involves halogen nuclei. A balance suggests global pathways models likely biased toward deposition. propose cycling could accommodate Hg-isotope constraints emission fluxes.