Freeze-Induced Halogen Redox Chemistry in Sea Ice and Snow Linking Oceanic Sources to Polar Aerosols

Sea ice and snow are key interfaces linking the ocean and atmosphere in polar regions, yet their role in driving biogeochemical coupling among halogens, aerosols, and clouds remains incompletely understood. Here, we show that ice-mediated chemical reactions provide an efficient pathway connecting sea ice and snowpack chemistry to atmospheric halogen activation, aerosol formation, and climate-relevant processes in polar environments.

Freezing processes induce strong solute enrichment, pH shifts, and microstructural heterogeneity within sea ice and snow, creating reactive interfacial environments that promote redox and photochemical transformations of iodine and bromine species. Laboratory experiments demonstrate that iodate, bromate, and halide species undergo accelerated reactions in ice involving natural organic matter, iron oxides, and nitrogen oxides, leading to the production of molecular halogens, reactive halogen intermediates, and organohalogen compounds. These ice-phase reactions are substantially enhanced relative to liquid systems and are further amplified under polar irradiation conditions.

Field observations reveal inorganic halogen speciation patterns in snow and sea-ice-influenced environments that are inconsistent with passive deposition alone, supporting the occurrence of active in-ice chemical processing. The resulting release of reactive halogen species facilitates air–ice exchange, contributes to boundary-layer halogen activation, and influences aerosol oxidation pathways, with potential impacts on cloud condensation nuclei and polar cloud formation.

Our findings highlight sea ice and snow as active biogeochemical reactors that couple oceanic halogen reservoirs to the atmosphere. As climate-driven changes in sea ice extent, snow cover, and freeze–thaw dynamics continue, ice-driven halogen chemistry is expected to modulate aerosol–cloud–ocean–sea ice interactions in polar regions, representing a previously underappreciated feedback in the polar climate system.