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