HO₂NO₂ and HNO₃ in the coastal Antarctic winter night: a "lab-in-the-field" experiment

Abstract. Observations of peroxynitric acid (HO₂NO₂) and nitric acid (HNO₃) were made during a 4 month period of Antarctic winter darkness at the coastal Antarctic research station, Halley. Mixing ratios of HNO₃ ranged from instrumental detection limits to ~8 parts per trillion by volume (pptv), and of HO₂NO₂ from detection limits to ~5 pptv; the average ratio of HNO₃ : HO₂NO₂ was 2.0(± 0.6) : 1, with HNO₃ always present at greater mixing ratios than HO₂NO₂ during the winter darkness. An extremely strong association existed for the entire measurement period between mixing ratios of the respective trace gases and temperature: for HO₂NO₂, R² = 0.72, and for HNO₃, R² = 0.70. We focus on three cases with considerable variation in temperature, where wind speeds were low and constant, such that, with the lack of photochemistry, changes in mixing ratio were likely to be driven by physical mechanisms alone. We derived enthalpies of adsorption (ΔH_ads) for these three cases. The average ΔH_ads for HNO₃ was −42 ± 2 kJ mol⁻¹ and for HO₂NO₂ was −56 ± 1 kJ mol⁻¹; these values are extremely close to those derived in laboratory studies. This exercise demonstrates (i) that adsorption to/desorption from the snow pack should be taken into account when addressing budgets of boundary layer HO₂NO₂ and HNO₃ at any snow-covered site, and (ii) that Antarctic winter can be used as a natural "laboratory in the field" for testing data on physical exchange mechanisms.