Soluble reactive nitrogen oxides at South Pole during ISCAT 2000

The mist chamber/ion chromatography technique was used to measure soluble gaseous nitrite (NO2−) and nitrate (NO3−) within the lowermost atmosphere (2 cm up to 10 m above the snow) and in the firn air at South Pole during the last 2 weeks of December 2000. Collected NO2− and NO3− are attributed to nitrous (HONO) and nitric (HNO3) acids, respectively. Firn air mixing ratios of HONO were more than five times higher than those just above the snow. A single day of measurements showed HONO mixing ratios to be significantly lower at 10 m than 85 cm above the snow. These gradients suggest that HONO is produced in the snowpack and fluxes out into the overlying air. A strong and persistent local source of HONO is required to sustain mean mixing ratios of 30 ppt against very fast loss by photolysis under the intense 24 h sunlight at South Pole. For HNO3, measurements in the atmosphere were made with two independent techniques. A CIMS technique provided data at 1 min resolution, at a sampling height of 10 m. The mist chamber/ion chromatography technique integrated over nominally 30 min intervals, with most measurements made 85 cm above the snow. Mixing ratios of HNO3 were highest (mean 38 ppt) at 85 cm above the snow, compared to those both in the firn air and at 10 m above the snow. Comparing HNO3 mixing ratios between 85 cm and firn air suggests flux into the snow, while the comparisons between 85 cm and 10 m suggest fluxes in the opposite direction. We speculate that the maximum just above the snow surface reflects in situ production of HNO3, supported by fluxes of NOx and OH precursors (e.g., HONO, HCHO, HOOH) out of the snow. Comparison of NO3− concentrations in snow immediately upwind of the Atmospheric Research Observatory, 50 m away from the building and 15 km away from the station revealed no significant differences. This finding indicates that the very high NO mixing ratios observed during ISCAT 1998 and 2000 cannot be attributed to NO3− contamination of the snow surrounding the sampling location