Direct in-situ molecular speciation of atmospheric oxidized mercury in polar regions

Mercury is a persistent environmental pollutant with strong impacts in polar regions, where atmospheric oxidation and subsequent deposition drive ecosystem loading and human exposure via methylmercury bioaccumulation. However, atmospheric mercury chemistry remains poorly constrained because gaseous oxidized mercury (Hg(II)) has rarely been resolved at the molecular level under ambient conditions. Most field observations rely on hours-to-days preconcentration techniques that provide limited speciation and are subject to sampling artefacts, leaving key oxidation pathways and deposition estimates largely unvalidated.

Here we present novel in-situ, online molecular measurements of individual oxidized mercury species in remote polar environments. We deployed nitrate-based chemical ionization atmospheric pressure interface time-of-flight mass spectrometry (NO₃⁻ CI-APi-TOF) to detect neutral Hg(II) compounds in real time, complemented by measurements of naturally charged ambient ions (APi-TOF). Observations were conducted in Antarctica at the Aboa station (austral summer 2014–2015) and in the central Arctic during the MOSAiC expedition (spring 2020, >80°N).

In Antarctica, we observe chemically diverse Hg(II) halides, including HgCl₂, HgBr₂, BrHgCl, and iodinated species (ClHgI, BrHgI, HgI₂), with episodic enhancements reaching several hundred pg Hg m⁻³ (reported as Hg mass concentration at STP). In the central Arctic, HgBr₂ is the only Hg(II) halide detected above the limit of detection during April 2020, with concentrations up to ~80 pg Hg m⁻³ and a decline to below detection by June. HgBr₂ maxima coincide with collocated Hg⁰ depletion and ozone variability, consistent with tight coupling to springtime halogen photochemistry.

Thermodynamic calculations support stable clustering of several mercuric halides with NO₃⁻ under inlet conditions, enabling selective detection of pure halides. While the ionization efficiency implies the derived concentrations represent lower limits, the observed magnitudes agree with other polar measurements, indicating that mercuric halides are major contributors to oxidized mercury in the polar boundary layer. The dominance of HgBr₂, and the presence of iodinated Hg(II) species in Antarctica, challenge current chemical transport models that typically predict HgCl₂ and Hg(OH)₂/HOHgBr as dominant oxidized forms. Because individual Hg(II) species differ strongly in photolysis rates, solubilities, and particle uptake, these new speciation constraints imply potentially substantial shifts in predicted mercury lifetime, transport, and deposition.

Our results demonstrate that real-time molecular speciation of oxidized mercury is now feasible in remote environments and provides a critical observational foundation for improving mercury redox chemistry in models and strengthening policy-relevant assessments of polar mercury deposition.