| 27 Jul 2026
Supercooled nitrate solution on ice at 232 K: Enhanced HONO yields
Abstract. Snowpack nitrate photolysis is a major source of nitrogen oxides and nitrous acid that control atmospheric oxidants in polar environments. It remains unclear at what temperature nitrate salts crystalize in snow and how this impacts the photolysis product yields. Here we show, using near‑edge X‑ray absorption fine structure (NEXAFS) spectroscopy, that sodium nitrate does not precipitate at temperatures down to 23 K below the eutectic temperature at ice surfaces, but does so in absence of ice. This indicates that strong interfacial supercooling or liquid‑like solvation persists. Complementary snow photolysis experiments demonstrate that such liquid-like nitrate shows a higher HONO to NO2 emission ratio than precipitated nitrate, which is consistent with the enhanced role of secondary chemistry in the aqueous phase. These findings show that supercooled nitrate might be present over wide ranges of Arctic, Antarctic, and upper tropospheric temperatures with a significant impact on the HONO and NO2 fluxes and thus the composition and chemistry in the air above snow or in contact with ice clouds.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics.
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