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_TSPand PBM_2.5 exhibited positive odd and even mass-independent fractionation (MIF), with Δ¹⁹⁹Hg values in PBM_2.5 enriched by 0.25 ± 0.35‰ relative to PBM_TSP. The slopes between Δ¹⁹⁹Hg and Δ²⁰¹Hg, 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 Δ¹⁹⁹Hg 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.