The diurnal variability of atmospheric nitrogen oxides (NO and NO2 above the Antarctic Plateau driven by atmospheric stability and snow emissions

first_imgAtmospheric nitrogen oxides (NO and NO2) were observed at Dome C, East Antarctica (75.1� S, 123.3� E, 3233 m), for a total of 50 days, from 10 December 2009 to 28 January 2010. Average (±1�) mixing ratios at 1.0m of NO and NO2, the latter measured for the first time on the East Antarctic Plateau, were 111 (±89) and 98 (±89) pptv, respectively. Atmospheric mixing ratios are on average comparableto those observed previously at South Pole, but incontrast show strong diurnal variability: a minimum aroundlocal noon and a maximum in the early evening coincidewith the development and collapse of a convective boundarylayer. The asymmetric diurnal cycle of NOx concentrationsand likely any other chemical tracer with a photolyticsurface source is driven by the turbulent diffusivityand height of the atmospheric boundary layer, with the former controlling the magnitude of the vertical flux and the latter the size of the volume into which snow emissionsare transported. In particular, the average (±1�) NOx emission flux from 22 December 2009 to 28 January 2010, estimated from atmospheric concentration gradients, was 8.2(±7.4)×1012 moleculem−2 s−1 belongs to the largest valuesmeasured so far in the polar regions and explains the3-fold increase in mixing ratios in the early evening whenthe boundary layer becomes very shallow. Dome C is likelynot representative for the entire East Antarctic Plateau but illustratesthe need of an accurate description of the boundary layer above snow in atmospheric chemistry models. A simple nitrate photolysis model matches the observed median diurnal NOx flux during the day but has significant low bias during the night. The difference is significant taking into account the total random error in flux observations and model uncertainties due to the variability of NO− 3 concentrations in snow and potential contributions from NO− 2 photolysis. This highlights uncertainties in the parameterization of the photolytic NOx source in natural snowpacks, such as the poorly constrained quantum yield of nitrate photolysis. A steadystate analysis of the NO2 : NO ratios indicates that peroxy (HO2 +RO2) or other radical concentrations in the boundary layer of Dome C are either higher than measured elsewhere in the polar regions or other processes leading to enhanced NO2 have to be invoked. These results confirm the existence of a strongly oxidising canopy enveloping the East Antarctic Plateau in summer.last_img read more