Modeling Mercury Isotopic Fractionation in the Atmosphere

Mercury (Hg) stable isotopes analysis has become a powerful tool to identify Hg sources and to understand its biogeochemical processes. However, it is challenging to link the observed Hg isotope fractionation to its global cycling. Here, we integrate source Hg isotope signatures and process-based Hg isotope fractionation into a three-dimensional isotope model based on the GEOS-Chem model platform. Our simulated isotope compositions of total gaseous Hg (TGM) are broadly comparable with available observations across global regions. The isotope compositions of global TGM, potentially distinguishable over different regions, are caused by the atmospheric mixture of anthropogenic, natural, and re-emitted Hg sources, superimposed with competing processes, notably gaseous Hg(0) dry deposition and Hg redox transformations. We find that Hg(0) dry deposition has a great impact on the isotope compositions of global TGM and drives the seasonal variation of δ202Hg in forest-covered regions. The photo-reduction of Hg(II) dominates over Hg(0) oxidation process in driving the global Δ199 Hg (and Δ201 Hg) distribution patterns in TGM. We suggest that the reported fractionation factors for vegetation Hg(0) uptake might be underestimated at some forest sites and that the magnitude of isotope fractionation associated with gaseous Hg(II) reduction is likely close to aquatic Hg(II) reduction. Our model provides a vital tool for coupling the global atmospheric Hg cycle and its isotope fractionation at various scales, and advances our understanding of atmospheric Hg transfer and transformation mechanisms.