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.

Automated summary

This study reveals that oxidized Hg at high altitudes exhibits even isotopic fractionation characteristics, while rainfall Hg shows both even and odd isotopic fractionation, possibly due to photo-reduction of Hg-II in clouds. Observations of MIF in free tropospheric Hg-0 indicate that oxidation of Hg-0 in high-altitude air masses produces opposite even- and odd-MIF in Hg-0 and oxidized Hg-II.