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

• Published in: Atmospheric Chemistry and Physics
• Main Authors: Chen, Zeyuan, Chu, Liang, Galbavy, Edward S., Ram, Keren, Anastasio, Cort
• Format: Text
• Language: English
• Published: 2018
• Subjects: Greenland
• Online Access: https://doi.org/10.5194/acp-16-9579-2016; https://www.atmos-chem-phys.net/16/9579/2016/

While the hydroxyl radical ( <mo>•</mo> 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 <mo>•</mo> OH on/in snow and ice. Here we measure the formation rate, lifetime, and concentration of <mo>•</mo> OH for illuminated polar snow samples studied in the laboratory and in the field. Laboratory studies show that <mo>•</mo> 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 <mo>•</mo> OH in samples studied at Summit, Greenland, is 5 times lower than the average measured in the laboratory, while the average <mo>•</mo> 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 <mo>•</mo> 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 <mo>•</mo> OH to the snow grains will approximately double the steady-state concentration of snow-grain hydroxyl radical, leading to an average [ <mo>•</mo> OH] in near-surface, summer Summit snow of approximately 4 × 10 −15 M. At this concentration, the <mo>•</mo> 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.