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 δ 202 Hg (mean: −2.00 to −0.78‰), slightly negative to highly positive Δ 199 Hg (mean: −0.04 to 0.47‰), and slightly positive Δ 200 Hg (mean: 0.02 to 0.06‰) values. The positive PBM Δ 199 Hg signatures were likely caused by physiochemical reactions in aerosols. The Δ 199 Hg/Δ 201 Hg ratio varied from 0.94 to 1.39 in the cities and increased with the increase in the corresponding mean Δ 199 Hg PBM 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 Δ 199 Hg PBM 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 Δ 200 Hg PBM 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).