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

摘要

pstrongAbstract./strong While the hydroxyl radical (supmoa?￠/mo/supOH) in the snowpack is likely a dominant oxidant for organic species and bromide, little is known about the kinetics or steady-state concentrations of supmoa?￠/mo/supOH on/in snow and ice. Here we measure the formation rate, lifetime, and concentration of supmoa?￠/mo/supOH for illuminated polar snow samples studied in the laboratory and in the field. Laboratory studies show that supmoa?￠/mo/supOH 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 supmoa?￠/mo/supOH in samples studied at Summit, Greenland, is 5??times lower than the average measured in the laboratory, while the average supmoa?￠/mo/supOH 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 supmoa?￠/mo/supOH in clean snow studied in the field at Summit, Greenland, range from (0.8 to 3)a????a??10supa??15/supspan class="thinspace"/spanM, 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 supmoa?￠/mo/supOH to the snow grains will approximately double the steady-state concentration of snow-grain hydroxyl radical, leading to an average [supmoa?￠/mo/supOH] in near-surface, summer Summit snow of approximately 4a????a??10supa??15/supspan class="thinspace"/spanM. At this concentration, the supmoa?￠/mo/supOH-mediated lifetimes of organics and bromide in Summit snow grains are approximately 3??days and 7span class="thinspace"/spanh, respectively, suggesting that hydroxyl radical is a major oxidant for both species./p.