Snowpack nitrate photolysis drives the summertime atmospheric nitrous acid (HONO) budget in coastal Antarctica

Measurements of atmospheric nitrous acid (HONO) amount fraction and flux density above snow were carried out using a long-path absorption photometer at Halley station in coastal Antarctica between 22 January and 3 February 2022. The mean +/- 1 sigma HONO amount fraction was (2.1 +/- 1.5) pmolmol(-1) and showed a diurnal cycle (range of 1.0-3.2 pmolmol(-1)) with a maximum at solar noon. These HONO amount fractions are generally lower than have been observed at other Antarctic locations. The flux density of HONO from the snow, measured between 31 January and 1 February 2022, was between 0.5 and 3.4 x 10(12) m(-2)s(-1) and showed a decrease during the night. The measured flux density is close to the calculated HONO production rate from photolysis of nitrate present in the snow. A simple box model of HONO sources and sinks showed that the flux of HONO from the snow makes a >10 times larger contribution to the HONO budget than its formation through the reaction of OH and NO. Ratios of these HONO amount fractions to NOx measurements made in summer 2005 are low (0.15-0.35), which we take as an indication of our measurements being comparatively free from interferences. Further calculations suggest that HONO photolysis could produce up to 12 pmolmol(-1) h(-1) of OH, approximately half that produced by ozone photolysis, which highlights the importance of HONO snow emissions as an OH source in the atmospheric boundary layer above Antarctic snowpacks.