Photoformation of hydroxyl radical on snow grains at Summit, Greenland

We measured the photoformation of hydroxyl radical (⋅OH) on snow grains at Summit, Greenland during the spring and summer. Midday rates of ⋅OH formation in the snow phase in the summer range from 130 to 610nmolL-1h-1, expressed relative to the liquid equivalent volume of snow. Calculated formation rates of snow-grain ⋅OH based on the photolysis of hydrogen peroxide and nitrate agree well with our measured rates during summer, indicating that there are probably not other major sources of ⋅OH under these conditions. Throughout both the spring and summer, HOOH is by far the dominant source of snow-grain ⋅OH; on average, HOOH produces approximately 100 times more ⋅OH than does NO3-. Rates of ⋅OH photoformation have a strong seasonal dependence and increase by approximately a factor of 10 between early spring and summer at midday. The rate of ⋅OH photoformation on snow grains decreases rapidly with depth in the snowpack, with approximately 90% of photoformation occurring within the top 10 cm, although ⋅OH formation occurs to depths below 20 cm. The formation of ⋅OH on snow grains likely initiates a suite of reactions in the snowpack, including the transformation of organic carbon (OC) and oxidation of halides. The reaction of ⋅OH with OC probably forms a number of volatile organic compounds (VOCs) that are potentially emitted into the atmospheric boundary layer. Indeed, our measured rates of ⋅OH photoformation on snow grains are large enough that they could account for previously reported fluxes of VOCs from the snowpack at Summit, although the relative importance of thermal desorption and photochemical production for most of these VOCs still needs to be determined.