The diurnal variability of atmospheric nitrogen oxides (NO and NO2) above the Antarctic Plateau driven by atmospheric stability and snow emissions
1British Antarctic Survey, Natural Environment Research Council, Cambridge, UK
2Scottish Association for Marine Science, Oban, Argyll, Scotland, PA37 1QA, UK
3Department of Earth Sciences, Royal Holloway, University of London, Egham, UK
4CNRM-GAME, URA 1357, Météo France CNRS, Toulouse, France
5Université Joseph Fourier – Grenoble 1/CNRS-INSU, Laboratoire de Glaciologie et Géophysique de l'Environnement, St. Martin d'Hères, France
Abstract. Atmospheric nitrogen oxides (NO and NO2) were observed at Dome C, East Antarctica (75.1° S, 123.3° E, 3233 m) during austral summer 2009–2010. Average (±1σ) mixing ratios at 1.0 m of NO and NO2, the latter measured for the first time on the East Antarctic Plateau, were 111 (±89) and 102 (±88) pptv, respectively. Atmospheric mixing ratios are on average comparable to those observed previously at South Pole, but in contrast show strong diurnal variability, with a minimum around local noon and a maximum in the early evening. The asymmetry in the diel cycle of NOx concentrations and likely any other chemical tracer with a photolytic surface source is driven by the diffusivity and height of the atmospheric boundary layer, with the former controlling the magnitude of the vertical flux and the latter the size of the volume snow emissions are diffusing into. In particular, the NOx emission flux estimated from concentration gradients was on average (±1σ) of 6.9 (±7.2) ×1012 molecule m−2 s−2 and is consistent with the 3-fold increase in mixing ratios in the early evening when the atmospheric boundary layer becomes very shallow. Dome C is likely not representative for the entire East Antarctic Plateau but illustrates the need of accurate descriptions for atmospheric boundary layer physics in atmospheric chemistry models. Calculated mean potential NO2 production rates from nitrate (NO3−) photolysis are only about 62% of the observed NOx flux and highlight uncertainties in the parameterization of the photolytic NOx snow source above Antarctica. A steady-state analysis of the NO2:NO ratios indicates high concentrations of peroxy radicals (HO2 + RO2) in the air above the snow and confirms the existence of a strongly oxidising canopy enveloping the East Antarctic Plateau in summer.