Measurements of H2O2in firn gas down to a 1.7‐m depth showed a consistent trend, with higher firn‐gas concentrations generally associated with higher concentrations in the firn at the same depth. However, firn to firn‐gas concentration ratios still exhibited a seasonal dependence, suggesting that for summer layers equilibrium has not yet been reached. The time to reach equilibrium between firn and firn gas is at least weeks. Snowfall and fog deposit several times more H2O2than the surface snow will retain at equilibrium, supporting the idea that surface snow is a temporary reservoir for H2O2. Thus from an equilibrium standpoint, the snow‐pack should be a source of atmospheric H2O2in the summer as well as fall, resulting in higher daytime concentrations than would occur based on just atmospheric photochemical reactions. But firn‐gas measurements reported here were generally near or lower than those in the atmosphere, suggesting that degassing is too slow to significantly influence atmospheric H
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