Xuewu Fu, Martin Jiskra, Xu Yang, Nicolas Marusczak, Maxime Enrico, Jerome Chmeleff, Lars-Eric Heimburger-Boavida, Francois Gheusi, Jeroen Sonke 

Mass independent fractionation (MIF) of even mass number mercury (Hg) stable 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, 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 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 towards HgII deposition. We propose that Hg cycling models could accommodate the Hg-isotope constraints on emission and deposition fluxes.