Hydroxyl Radical in/on Illuminated Polar Snow: Formation Rates, Lifetimes, and Steady-State Concentrations

While hydroxyl radical (OH) in the snowpack is likely a dominant oxidant for organic species and bromide, little is known about the kinetics or steady-state concentrations of OH on/in snow 10 and ice. Here we measure the formation rate, lifetime, and concentration of OH for illuminated polar snow samples studied in the laboratory and in the field. Laboratory studies show that OH kinetics and steady-state concentrations are essentially the same for a given sample studied as ice and liquid; this is in contrast to other photooxidants, which show a concentration enhancement in ice relative to solution as a result of kinetic differences in the two phases. The average production rate of OH in samples 15 studied at Summit, Greenland is 5 times lower than the average measured in the laboratory, while the average OH lifetime determined in the field is 5 times higher than in the laboratory. These differences indicate the polar snows we studied in the laboratory are affected by contamination, despite significant efforts to prevent this; our results suggest similar contamination might be a widespread problem in laboratory studies of ice chemistry. Steady-state concentrations of OH in clean snow studied in the field 20 at Summit, Greenland range from (0.8 to 3) × 10 M, comparable to values reported for mid-latitude cloud and fog drops, rain, and deliquesced marine particles, even though impurity concentrations in the snow samples are much lower. Partitioning of firn-air OH to the snow grains will approximately double the steady-state concentration of snow-grain hydroxyl radical, leading to an average [OH] in nearsurface, summer Summit snow of approximately 4 × 10 M. At this concentration, the OH-mediated 25