Abstract
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 (ΔHads) for these three cases. The average ΔHads 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.
Highlights
Peroxynitric acid (HO2NO2, written as HNO4) and nitric acid (HNO3) are acidic gases that are of increasing interest to polar tropospheric chemistry
Mixing ratios of HNO3 ranged from instrumental detection limits to ∼ 8 parts per trillion by volume and of HO2NO2 varied from detection limits to
These values are considerably lower than those observed at the South Pole in summer, where photochemical production is fuelled by emissions of NOx from the snowpack, and where mixing ratios of HNO3 and HO2NO2 were generally in the 10 s of pptv, and sometimes over 100 pptv, as discussed earlier
Summary
Peroxynitric acid (HO2NO2, written as HNO4) and nitric acid (HNO3) are acidic gases that are of increasing interest to polar tropospheric chemistry. The spatial and temporal distribution of HO2NO2 and HNO3 across the polar regions becomes important for understanding the overall atmospheric chemical system, and models require details of their sources, and any physical exchange process by which they move from one environmental compartment to another. The airborne measurements assessed the threedimensional distribution of HO2NO2 and HNO3 across the Antarctic Plateau during the ANTCI 2005 campaign (Slusher et al, 2010) They revealed significant vertical gradients in both species, with higher concentrations at the ground, consistent with a source associated with emissions from the snowpack. The data allow us to assess whether HNO3 and HO2NO2 are present in significant concentrations at other Antarctic locations and seasons than the Antarctic Plateau in summer They provide an opportunity to test laboratory-derived physical exchange parameters under semi-constrained, but genuine real-world conditions. Under these conditions of 24 h per day darkness, atmospheric photochemistry stalls, and trace gas concentrations are controlled entirely by either transport or physical air–snow exchange
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