The Seasonal Variability of Atmospheric Nitrogen Oxide (NO and NO2) Concentrations and Snow Emission Flux in Coastal Antarctica
Monday, 14 December 2015
Poster Hall (Moscone South)
The first year-round measurements of both nitrogen oxides (NOx = NO + NO2) in coastal Antarctica were carried out at Halley (75°35'S, 26°25'W, 35 m) from January to December 2007. The NOx 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 NOx 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 NOx 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 NOx 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 NOx flux, which is very likely a typical feature of the atmospheric boundary layer in coastal Antarctica.