The Seasonal Variability of Atmospheric Nitrogen Oxide (NO and NO2) Concentrations and Snow Emission Flux in Coastal Antarctica

Abstract:

The first year-round measurements of both nitrogen oxides (NO_x = NO + NO₂) in coastal Antarctica were carried out at Halley (75°35'S, 26°25'W, 35 m) from January to December 2007. The NO_x mixing ratios show a clear annual cycle; average (±1σ) NO and NO₂ mixing ratios during austral spring, summer and autumn were 1±2, 5±5, 2±2 pptv and 1±4, 4±5, 2±4 pptv, respectively. During winter darkness NO and NO₂ mixing ratios remained below the limit of detection of 2 and 4.6 pptv, respectively. Average (±1σ) NOx fluxes estimated for selected periods during austral spring (12th – 15th October 2007), summer (16th – 19th December 2007), autumn (21st – 24th February 2007) and winter (11th -13th June 2007) were 1.3±1.6 x10¹², 1.1±0.8 x10¹², 1.1±2 x10¹² and -1.2±0.6 x10¹² molecule m^-2 s^-1, respectively. Emissions of NOx from snow during the sunlit season are expected due to photolysis of nitrate present in surface snow. Surprisingly, the NO_x flux in summer was almost comparable to that in spring and fall even though measured nitrate photolysis rate coefficients during summer were 3 times those during the seasons with less available sunlight. Relatively small summer time NO_x flux is attributed to reduced upward mixing of snow emissions. Convective mixing during austral spring (February) and fall (October) resulted in a 2 to 4 fold increase in NO_x flux when compared to conditions when mixing was dominated by mechanical shear. In contrast, the near absence of convection due to cloudy skies during summer (December) reduces atmospheric turbulence and therefore, the snow to air NO_x flux, which is very likely a typical feature of the atmospheric boundary layer in coastal Antarctica.