Sources and Transformation Mechanisms of Atmospheric Particulate Bound Mercury Revealed by Mercury Stable Isotopes.

This study examined the isotopic composition of particulate bound mercury (PBM) in 10 Chinese megacities and explored the associated sources and transformation mechanisms. PBM in these cities was characterized by negative δHg (mean: -2.00 to -0.78‰), slightly negative to highly positive ΔHg (mean: -0.04 to 0.47‰), and slightly positive ΔHg (mean: 0.02 to 0.06‰) values. The positive PBM ΔHg signatures were likely caused by physiochemical reactions in aerosols. The ΔHg/ΔHg ratio varied from 0.94 to 1.39 in the cities and increased with the increase in the corresponding mean ΔHg value. We speculate that, in addition to the photoreduction of oxidized Hg, other transformation mechanisms in aerosols (e.g., isotope exchange, complexation, and oxidation, which express nuclear volume effects) also shape the ΔHg signatures in the present study. These processes are likely enhanced in the presence of strong gas-particle partitioning of gaseous oxidized Hg (GOM) and elevated levels of redox active metals (e.g., Fe), halides, and elemental carbon. Based on ΔHg data presented in this and previous studies, we estimate that large proportions (∼47 ± 22%) of PBM were sourced from the oxidation of gaseous elemental Hg followed by the partitioning of GOM onto aerosols globally, indicating the transformation of Hg(0) to PBM as an important sink of atmospheric Hg(0).