In order to gain further insight into the role of interstitial air within natural surfacesnow layers, simultaneous measurements of key species in the firn air wereperformed during the summer field season 2000 at the Summit EnvironmentalObservatory in Greenland at the top of the ice sheet. Measurements of H2O2 andHCHO in the firn air at various depths with different temperatures and radiationlevels indicate lower concentrations of H2O2 in the firn air compared to ambientconcentrations, while HCHO concentrations are always enhanced. Nevertheless,gradients and fluxes measured above the snowpack show strong diurnal variationswith emission from and deposition to the snow pack for both compounds. Theseresults can indicate that snow-enhanced photochemistry for the atmosphere-to-snowtransfer of both compounds is important. Recent experiments have demonstratedthat the snowpack can act as a photochemical reactor, influencing concentrations ofa wide variety of important tropospheric trace gases including ozone and nitrogenoxides. Although firn air comprises only a small portion of the boundary layer-snowsystem, it is probably of great importance for the investigation of the transfer oftrace gases between atmosphere and ice because it is the air that enables exchange ofreactive chemical species between the atmospheric boundary layer and the snow.