Sources of Particulate Bound Mercury in the Northwest Pacific Constrained by Hg Isotopes

Atmospheric mercury (Hg) transport and transformation controls the global Hg budget. Of which, the particulate-bound mercury (PBM) sources and subsequent deposition are crucial, especially in the open ocean. Here, we present the first Hg isotope compositions of PBM in the Northwest Pacific during three cruises from 2018 to 2019. The PBM samples were less influenced by long-range transport of anthropogenic emissions according to the low PBM concentrations (19.7 ± 10.9, n = 52, 1SD). The mass-independent fractionation signatures are mainly controlled by photochemical reactions as the ratio of Δ¹⁹⁹Hg/Δ²⁰¹Hg was close to 1.0. The in situ Hg oxidation of Br radicals plays an important role in keeping PBM Δ²⁰⁰Hg values lower, while the background atmospheric Hg(II) pool, which is derived from Hg oxidation by O₂ in the free troposphere, provides highly positive Δ²⁰⁰Hg values for PBM. The common photoredox reactions and subsequent adsorption adjust the PBM isotopic composition within a moderate range. We used an isotope mixing model to estimate the contributions of each factor. Compared with PBM in other areas (e.g., urban, high-altitude), contributions from photochemical oxidation increased in the marine boundary layer of the open ocean. The results of this study have improved our understanding of the transport pathways of PBM in the atmosphere and highlighted the important use of Hg isotopes in process analysis and source tracing.