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Observations of peroxynitric acid (HO2NO2) and nitric acid(HNO3) were made during a 4 month period of Antarctic winter darknessat the coastal Antarctic research station, Halley. Mixing ratios ofHNO3 ranged from instrumental detection limits to ~8parts per trillion by volume (pptv), and of HO2NO2 from detectionlimits to ~5 pptv; the average ratio ofHNO3 : HO2NO2 was 2.0(± 0.6) : 1, with HNO3 alwayspresent at greater mixing ratios than HO2NO2 during the winterdarkness. An extremely strong association existed for the entire measurementperiod 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, wherewind speeds were low and constant, such that, with the lack ofphotochemistry, changes in mixing ratio were likely to be driven by physicalmechanisms 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 inlaboratory studies. This exercise demonstrates (i) that adsorptionto/desorption from the snow pack should be taken into account whenaddressing budgets of boundary layer HO2NO2 and HNO3 at anysnow-covered site, and (ii) that Antarctic winter can be used as a natural"laboratory in the field" for testing data on physical exchangemechanisms

atmospheric chemistry and physics

HO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;NO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; and HNO&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt; in the coastal Antarctic winter night: a &amp;quot;lab-in-the-field&amp;quot; experiment