Atmospheric nitrogen oxides (NO and NO2) at Dome C, East Antarctica, during the OPALE campaign
Atmospheric Chemistry and Physics Discussions
Mixing ratios of the atmospheric nitrogen oxides NO and NO<sub>2</sub> 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 NO<sub>x</sub> 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
... of mixing ratios in firn air suggest that the upper snowpack at Dome C holds a significant reservoir of photolytically produced NO<sub>2</sub> and is a sink of gas phase ozone (O<sub>3</sub>). 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 RO<sub>2</sub> radicals are too low to explain the large NO<sub>2</sub> : NO ratios found in ambient air. A previously not considered interference with pernitric acid (HNO<sub>4</sub>) may explain part of this inconsistency. During 2011–2012 NO<sub>x</sub> mixing ratios and flux were larger than in 2009–2010 consistent with also larger surface O<sub>3</sub> mixing ratios resulting from increased net O<sub>3</sub> production. Large NO<sub>x</sub> mixing ratios arose from a combination of changes in mixing height and NO<sub>x</sub> snow emission flux <i>F</i><sub>NO<sub>x</sub></sub>. During 23 December 2011–12 January 2012 median <i>F</i><sub>NO<sub>x</sub></sub> 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 <i>F</i><sub>NO<sub>x</sub></sub> in December 2011 was largely due to changes in snow pack source strength caused primarily by changes in NO<sub>3</sub><sup>−</sup> concentrations in the snow skin layer, and only to a secondary order by decrease of total column O<sub>3</sub> and associated increase in NO<sub>3</sub><sup>−</sup> photolysis rates. Systematic changes in the quantum yield of NO<sub>3</sub><sup>−</sup> photolysis over time may contribute to the observed <i>F</i><sub>NO<sub>x</sub></sub> variability.