H2O2 is an important component of the atmospheric oxidizingcapacity, determining the lifetime of atmospheric tracespecies. Bi-directional summertime H2O2 fluxes from thesnowpack at Summit, Greenland, reveal a daytime release fromthe surface snow reservoir during the warm part of the dayand a partial re-deposition at night. The data also providethe first direct evidence of a strong net summertime H2O2release from the snowpack, increasing average boundary layerH2O2 concentrations ~ 7-fold and the OH and HO2concentrations by 70% and 50%, respectively. Changes in H2O2concentration in the snow combined with photochemical andair-snow interaction modeling show that the net snowpackrelease is driven by temperature induced desorption of H2O2as deposited snow, which is supersaturated with respect toice-air partitioning, approaches equilibrium. The resultsshow that the physical cycling of H2O2, and possibly othervolatile species is a key to understanding snowpacks ascomplex physical-photochemical reactors and has far reachingimplications for the interpretation of ice core records aswell as for the photochemistry in polar regions and in thevicinity of snowpacks in general.