Mercury is a globally transported pollutant with significant impacts on ecosystems and human health, particularly in polar regions where bioaccumulation in marine food webs leads to elevated exposure. While atmospheric oxidation controls where mercury is deposited, the molecular identity of oxidized mercury species has remained largely unresolved due to limitations in measurement techniques.

In this study, we present the first direct, real-time observations of individual oxidized mercury compounds in the polar atmosphere using nitrate-based chemical ionization mass spectrometry (CI-APi-TOF). Measurements conducted in Antarctica and the central Arctic reveal that mercury exists predominantly as mercuric halides, with HgBr₂ identified as the dominant oxidized species in both regions.

These observations challenge current atmospheric models, which predict HgCl₂ and HOHgBr as the primary oxidized mercury forms. Instead, our results demonstrate a more complex speciation, including contributions from bromine-, chlorine-, and iodine-containing mercury compounds. The findings highlight the importance of halogen chemistry, particularly bromine, in driving mercury oxidation in polar environments.

By providing molecular-level insights into oxidized mercury, this work advances our understanding of mercury cycling and offers critical constraints for improving atmospheric chemistry models and global deposition estimates.