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 Hg^II 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 Hg^II, potentially indicating the occurrence of in-cloud aqueous Hg^II 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 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 Δ²⁰⁰Hg mass balance suggests that global Hg 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.