Direct observations of atmospheric oxidized mercury speciation in polar areas

Mercury is a persistent pollutant with significant public health impacts in polar regions where fish consumption drives human exposure. Atmospheric oxidation pathways control where mercury deposits globally, but the lack of molecular-level observations of oxidized mercury products has hindered the validation of proposed chemical mechanisms. Here, we show the in-situ online detection of individual mercuric halides (HgCl2, BrHgCl, HgBr2, ClHgI, BrHgI, and HgI2) in the polar boundary layer using atmospheric pressure chemical ionization mass spectrometry. Our observations identify HgBr2 as the dominant oxidized mercury species at both poles, while HgCl2 and other halides were also observed in Antarctica. The observed speciation diverges from current model predictions, which favor HgCl2 and HOHgBr as dominant oxidized forms. Our results show that real-time molecular measurements can substantially advance global mercury monitoring and improve the chemical models used to assess environmental policies and predict deposition patterns.