Hydrogen peroxide (H2O2) contributes to the atmospheres oxidizing capacity, which determinesthe lifetime of atmospheric trace species. Measured bidirectional summertime H2O2 fluxes fromthe snowpack at Summit, Greenland, in June 1996 reveal a daytime H2O2 release from thesurface snow reservoir and a partial redeposition at night. The observations also provide the firstdirect evidence of a strong net summertime H2O2 release from the snowpack, enhancing averageboundary layer H2O2 concentrations approximately sevenfold and the OH and HO2concentrations by 70% and 50%, respectively, relative to that estimated from photochemicalmodeling in the absence of the snowpack source. The total H2O2 release over a 12-day periodwas of the order of 5 * 10(13) molecules m(-2) s(-1) and compares well with observed concentrationchanges in the top snow layer. Photochemical and air-snow interaction modeling indicate thatthe net snowpack release is driven by temperature-induced uptake and release of H2O2 asdeposited snow, which is supersaturated with respect to ice-air partitioning, approachesequilibrium. The results show that the physical cycling of H2O2 and possibly other volatilespecies is a key to understanding snowpacks as complex physical-photochemical reactors and hasfar reaching implications for the interpretation of ice core records as well as for thephotochemistry in polar regions and in the vicinity of snowpacks in general.