Beyond the tropopause hypothesis: Drivers of even mercury isotope fractionation unraveled by 3D modeling

Mercury (Hg) is a globally persistent contaminant with substantial health impact. The mass-independent fractionation of even mercury isotopes (even-MIF, denoted as Δ²⁰⁰Hg) is widely used to trace atmospheric Hg sources and fluxes. However, the chemical processes inducing even-MIF remain largely unknown. Here, we develop a three-dimensional atmospheric Hg isotope model linking Hg redox processes with even-MIF. The results show that the previous hypothesis of even-MIF occurring exclusively at the tropopause may not fully explain the global Δ²⁰⁰Hg patterns. We speculate OH-initiated reactions and photoreduction of Hg^II(p) as likely dominant drivers. Simulations show that even-MIF primarily originates in the free troposphere and propagates downward to surface air. Our results reveal distinct spatial Δ²⁰⁰Hg variations that were previously unaccounted for in tracing Hg sources. This study provides critical insights into even-MIF drivers and serves as a reference for using Δ²⁰⁰Hg to trace atmospheric Hg sources.