Abstract
Mercury (Hg) is a globally persistent contaminant with substantial health impact. The mass-independent fractionation of even mercury isotopes (even-MIF, denoted as Δ(200)Hg) 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 Δ(200)Hg 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 Δ(200)Hg 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 Δ(200)Hg to trace atmospheric Hg sources.