| Summary: | The troposphere above the West Antarctic Ice Sheet (WAIS) was sampled for hydroperoxides at 21 locations during 2-month-long summer traverses from 2000 to 2002, as part of the U.S. International Trans-Antarctic Scientific Expedition (US ITASE). First-time quantitative measurements using a high-performance liquid chromatography method showed that methylhydroperoxide (MHP) is the only important organic hydroperoxide occurring in the Antarctic troposphere and that it is found at levels 10 times those previously predicted by photochemical models. During three field seasons, means and standard deviations for hydrogen peroxide (H 2 O 2 ) were 321 ± 158 pptv, 650 ± 176 pptv, and 330 ± 147 pptv. While MHP was detected but not quantified in December 2000, levels in summer 2001 and 2002 were 317 ± 128 pptv and 304 ± 172 pptv. Results from firn air experiments and diurnal variability of the two species showed that atmospheric H 2 O 2 is significantly impacted by a physical snow pack source between 76° and 90°S, whereas MHP is not. We show strong evidence of a negative correlation between stratospheric ozone and H 2 O 2 at the surface. Between 27 November and 12 December in 2001, when ozone column densities dropped below 220 Dobson units (DU) (means in 2000 and 2001 were 318 DU and 334 DU, respectively), H 2 O 2 was 1.7 times that observed in the same period in 2000 and 2002, while MHP was only 80% of the levels encountered in 2002. Photochemical box model runs match MHP observations only when the production rate from CH 3 O 2 + HO 2 was increased to the upper limit of its estimated range of uncertainty. Model results suggest that NO and OH levels on WAIS are closer to coastal values, while Antarctic Plateau levels are higher, confirming that region to be a highly oxidizing environment. The modeled sensitivity of H 2 O 2 and particularly MHP to NO offers the potential to use atmospheric hydroperoxides to constrain the NO background and thus estimate the past oxidation capacity of the remote atmosphere using ice cores. Copyright 2005 by the American Geophysical Union.
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