Contrasting atmospheric boundary layer chemistry of methylhydroperoxide (CH3OOH) and hydrogen peroxide (H2O2) above polar snow

Atmospheric hydroperoxides (ROOH) were mea- sured at Summit, Greenland (72.97 N, 38.77 W) in sum- mer 2003 (SUM03) and spring 2004 (SUM04) and South Pole in December 2003 (SP03). The two dominant hy- droperoxides were H2O2 and CH3OOH (from here on MHP) with average (±1 ) mixing ratios of 1448 (±688) pptv, 204 (±162) and 278 (±67) for H2O2 and 578 (±377) pptv, 139 (±101) pptv and 138 (±89) pptv for MHP, respectively. In early spring, MHP dominated the ROOH budget and showed night time maxima and daytime minima, out of phase with the diurnal cycle of H2O2, suggesting that the organic peroxide is controlled by photochemistry, while H2O2 is largely influenced by temperature driven exchange between the atmosphere and snow. Highly constrained photochemical box model runs yielded median ratios between modeled and observed MHP of 52%, 148% and 3% for SUM03, SUM04 and SP03, respectively. At Summit firn air measurements and model calculations suggest a daytime sink of MHP in the upper snow pack, which decreases in strength through the spring season into the summer. Up to 50% of the estimated sink rates of 1-5◊10 11 molecules m 3 s 1 equivalent to 24- 96 pptv h 1 can be explained by photolysis and reaction with the OH radical in firn air and in the quasi-liquid layer on snow grains. Rapid processing of MHP in surface snow is expected to contribute significantly to a photochemical snow pack source of formaldehyde (CH2O). Conversely, summer levels of MHP at South Pole are inconsistent with the pre- vailing high NO concentrations, and cannot be explained cur- rently by known photochemical precursors or transport, thus suggesting a missing source. Simultaneous measurements of H2O2, MHP and CH2O allow to constrain the NO back- ground today and potentially also in the past using ice cores, although it seems less likely that MHP is preserved in firn and ice.