Free tropospheric halogen oxidation drives mercury deposition in the Asia Pacific region

Asian outflow represents 50% of global anthropogenic mercury emissions, but the atmospheric oxidation pathways controlling its deposition to the adjacent Pacific Ocean remain poorly constrained. Using multi-year snow and atmospheric mercury isotope measurements from Mt. Halla, Jeju Island, Korea, we delineate the mercury sources and oxidation pathways governing deposition to marginal Asia and the western Pacific. Mass-independent fractionation of even-number mercury isotopes (even-MIF; Δ 200 Hg) correlates with both air mass transport distance and trajectory altitude, indicating the importance of free tropospheric mercury photo-oxidation triggering even-MIF. Coupled anomalies between Δ 200 Hg and oxygen isotopes of nitrate (Δ 17 O(NO 3 - )) in snow indicate that reactive bromine, generated from debromination of local sea salt aerosol, sustains oxidative capacity in the free troposphere and drives active mercury deposition to Mt. Halla snow. Isotope mixing models quantify that halogen-mediated free tropospheric mercury oxidation (68%) exceeds near-surface oxidation (32%). Our results provide field-based evidence for a bromine-driven free tropospheric photo-oxidation mechanism regulating mercury deposition to Asia-Pacific ecosystems.