Mass independent fractionation of even and odd mercury isotopes during free tropospheric mercury oxidation

Even mercury (Hg) isotope mass independent fractionation (MIF), observed in rainfall globally, is used to quantify atmospheric Hg deposition pathways. The underlying reaction and MIF mechanism are unknown however. Here we investigate the Hg isotope composition of free tropospheric gaseous elemental Hg0 and HgII forms. We find that gaseous oxidized HgII has positive Δ199Hg, Δ201Hg, Δ200Hg, and negative Δ204Hg signature, similar to rainfall HgII, and we document rainfall HgII Δ196Hg to be near-zero. Cloud water and rainfall HgII show enhanced odd MIF compared to gaseous HgII, indicating in-cloud HgII photoreduction. Hg MIF observations of free tropospheric Hg0 dynamics show how net Hg0 oxidation leads to opposite MIF in Hg0 and HgII. A Δ200Hg mass balance for Hg0 and HgII forms suggests that measurements and models underestimate the tropospheric HgII pool.