Sources and transformation mechanisms of atmospheric particulate bound mercury revealed by mercury stable isotopes

Abstract

This study examined the isotope composition of Particulate bound mercury (PBM) in ten Chinese megacities and explored the associated sources and transformation mechanisms. PBM in these cities were characterized by negative δ²⁰²Hg (means: -2.00 to -0.78‰), slightly negative to highly positive Δ¹⁹⁹Hg (means: -0.04 to 0.47‰), and slightly positive Δ²⁰⁰Hg (means: 0.02 to 0.06‰) values. The positive PBM Δ¹⁹⁹Hg signatures were likely caused by physiochemical reactions in aerosols. The Δ¹⁹⁹Hg/Δ²⁰¹Hg ratio varied from 0.94 to 1.39 in the cities and increased with the corresponding mean Δ¹⁹⁹Hg_PBM value. We speculate that, in addition to photoreduction of oxidized Hg, other transformation mechanisms in aerosols (e.g., isotope exchange, complexation, and oxidation that express nuclear volume effects) also shape the Δ¹⁹⁹Hg_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 Δ²⁰⁰Hg_PBM data presented in this and previous studies, we estimate that large proportions (~47 ± 22%) of PBM were sourced from oxidation of gaseous elemental Hg followed by partitioning of GOM onto aerosols globally, indicating transformation of Hg(0) to PBM as an important sink of atmospheric Hg(0).