Articles | Volume 16, issue 15
Research article
02 Aug 2016
Research article |  | 02 Aug 2016

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

Zeyuan Chen, Liang Chu, Edward S. Galbavy, Keren Ram, and Cort Anastasio

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.

Short summary
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.
Final-revised paper