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 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 and oxidized Hg forms at the high-altitude Pic du Midi Observatory. We find that gaseous oxidized Hg has positive ΔHg, ΔHg, and ΔHg and negative ΔHg signatures, similar to rainfall Hg, and we document rainfall Hg ΔHg to be near zero. Cloud water and rainfall Hg show an enhanced odd-Hg MIF of 0.3‰ compared to gaseous oxidized Hg, potentially indicating the occurrence of in-cloud aqueous Hg photoreduction. Diurnal MIF observations of free tropospheric Hg show how net Hg oxidation in high-altitude air masses leads to opposite even- and odd-MIF in Hg and oxidized Hg. We speculate that even-Hg MIF takes place by a molecular magnetic isotope effect during Hg photoreduction on aerosols that involves magnetic halogen nuclei. A ΔHg mass balance suggests that global Hg deposition pathways in models are likely biased toward Hg deposition. We propose that Hg cycling models could accommodate the Hg-isotope constraints on emission and deposition fluxes.