Atmospheric nitrogen oxides (NO and NO₂) at Dome C, East Antarctica, during the OPALE campaign

Abstract

Abstract. Mixing ratios of the atmospheric nitrogen oxides NO and NO2 were measured as part of the OPALE (Oxidant Production in Antarctic Lands & Export) campaign at Dome C, East Antarctica (75.1° S, 123.3° E, 3233 m), during December 2011 to January 2012. Profiles of NOx mixing ratios of the lower 100 m of the atmosphere confirm that, in contrast to South Pole, air chemistry at Dome C is dominated by strong diurnal cycles in solar irradiance and atmospheric stability. Depth profiles of mixing ratios in firn air suggest that the upper snowpack at Dome C holds a significant reservoir of photolytically produced NO2 and is a sink of gas phase ozone (O3). First-time observations of BrO at Dome C suggest 2–3 pptv near the ground, with higher levels in the free troposphere. Assuming steady-state, observed mixing ratios of BrO and RO2 radicals are too low to explain the large NO2 : NO ratios found in ambient air. A previously not considered interference with pernitric acid (HNO4) may explain part of this inconsistency. During 2011–2012 NOx mixing ratios and flux were larger than in 2009–2010 consistent with also larger surface O3 mixing ratios resulting from increased net O3 production. Large NOx mixing ratios arose from a combination of changes in mixing height and NOx snow emission flux FNOx. During 23 December 2011–12 January 2012 median FNOx was twice that during the same period in 2009–2010 due to significantly larger atmospheric turbulence and a slightly stronger snowpack source. A tripling of FNOx in December 2011 was largely due to changes in snow pack source strength caused primarily by changes in NO3− concentrations in the snow skin layer, and only to a secondary order by decrease of total column O3 and associated increase in NO3− photolysis rates. Systematic changes in the quantum yield of NO3− photolysis over time may contribute to the observed FNOx variability.