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.