Formaldehyde and hydrogen peroxide in air, snow and interstitial air at South Pole

Average H2O2 (HCHO) mixing ratios measured above the snowpack at South Pole (SP) were 278 pptv (103 pptv) in December 2000 and between 4 and 43 times (1.4–2.6 times) the value estimated from gas-phase photostationary state (PSS) model calculations. The larger difference is realized if dry deposition of both species is included in the model. H2O2 and HCHO fluxes from the snowpack were independently determined from gradient measurements in the air above the snow surface, from firn-air measurements and from the temporal concentration changes in near-surface snow. On average, the snowpack at SP was releasing on the order of 1×1013 and 2×1012 molecules m−2 s−1 of H2O2 and HCHO, respectively, in December 2000. This is consistent with the volumetric fluxes needed for the PSS model to reproduce the observed atmospheric mixing ratios of both H2O2 and HCHO. The highly elevated levels of both species found in firn air further support the above estimates. In the case of HCHO, it was also shown that there was good agreement between the measured flux and the physical air–snow exchange model as driven by changes in snow temperature from winter to summer. Shading experiments suggest that the net production of HCHO within the snow by heterogeneous photochemical processes may have exceeded photochemical destruction by no more than 20% of the measured fluxes. The very rapid changes observed in atmospheric HCHO, which are also seen in NO and OH, can be understood in terms of dynamical processes that lead to rapid changes in the atmospheric mixing depth.