Mass‐Independent Fractionation of Mercury Stable Isotopes Reveals Atmospheric Transport Impact on Particulate‐Bound Mercury

Atmospheric particulate‐bound mercury (PBM), adsorbed onto aerosols like TSP and PM 2.5 (PBM TSP and PBM 2.5 ), is crucial to the Hg biogeochemical cycle. However, the mechanisms underlying the formation and transportation of PBM remain unclear. This study aims to explore these mechanisms by utilizing Hg isotopes and collecting PBM TSP and PBM 2.5 samples at five distinct sites in North China. Both PBM TSP and PBM 2.5 exhibited positive odd and even mass‐independent fractionation (MIF), with Δ 199 Hg values in PBM 2.5 enriched by 0.25 ± 0.35‰ relative to PBM TSP . The slopes between Δ 199 Hg and Δ 201 Hg, along with sulfur MIF and backward trajectories, suggest that intense photoreduction reactions during long‐distance and high‐altitude transport are the primary driver of the elevated Δ 199 Hg in fine particulate matter. Importantly, significant positive MIF signatures can occur not only in remote areas but also in anthropogenic Hg emission regions, underscoring the underestimated role of atmospheric transport, particularly on clean days.