Stable Mercury Isotopes Revealing Photochemical Processes in the Marine Boundary Layer

The marine boundary layer (MBL) is an important transportation and reaction zone of atmospheric mercury on Earth. However, the transformation mechanisms of Hg in the MBL remain unclear. In this study, total suspended particle samples were collected in the MBL during two cruises, and the levels of particulate bound mercury (PBM) and mercury isotopes were analyzed. The results showed that (a) continental anthropogenic emissions have limited contribution to PBM in the MBL; (b) PBM likely experienced the oxidation of Hg⁰ by Br radicals and subsequent adsorption of Hg²⁺ on to particulate surfaces, as inferred from the significant negative δ²⁰²Hg; (c) the Δ¹⁹⁹Hg/Δ²⁰¹Hg ratio of ∼1.0 suggests that PBM underwent photoreduction, which was influenced by organic compounds, and the contribution of photoreduction to the extent of odd-number mass-independent fractionation in PBM in the MBL was more significant than that of oxidation triggered by Br atoms. This study provides insights into the photochemical processes influencing mercury in the MBL.