Hydroxyl radical in/on illuminated polar snow: formation rates, lifetimes, and steady-state concentrations

Abstract. While the 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 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 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 that the polar snows we studied in the laboratory are affected by contamination, despite significant efforts to prevent this; our results suggest similar contamination may be a widespread problem in laboratory studies of ice chemistry. Steady-state concentrations of ^•OH in clean snow studied in the field at Summit, Greenland, range from (0.8 to 3) × 10⁻¹⁵ M, comparable to values reported for midlatitude 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 near-surface, summer Summit snow of approximately 4 × 10⁻¹⁵ M. At this concentration, the ^•OH-mediated lifetimes of organics and bromide in Summit snow grains are approximately 3 days and 7 h, respectively, suggesting that hydroxyl radical is a major oxidant for both species.