We made the first measurements of the concentrations of hydroxyl radical (•
OH), a dominant environmental oxidant, in snow grains. Concentrations of •
OH in snow at Summit, Greenland, are comparable to values reported for midlatitude cloud and fog drops, even though impurity levels in the snow are much lower. At these concentrations, the lifetimes of organics and bromide in Summit snow are approximately 3 days and 7 h, respectively, suggesting that OH is a major oxidant for both species.