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Quantifying nitrite and peroxynitrite formation from the photolysis of nitrate in/on ice


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Nitrate is a ubiquitous contaminant in polar snow whose concentrations in ice cores serve as a history of nitrogen oxide pollution in the atmosphere. It was once thought that nitrate deposited in snow was essentially inert, but a decade ago researchers in Greenland discovered that snow nitrate is broken down by sunlight to form products such as nitrogen oxides and nitrous acid. These species are emitted from the snow into the atmosphere, where they can significantly change atmospheric composition and chemistry. For example, nitrogen oxides from snow can lead to the formation of ozone in the lowest layers of the atmosphere, while nitrous acid is an important source of hydroxyl radical, a key oxidant that determines the lifetimes of atmospheric pollutants. There is significant uncertainty about the efficiency with which nitrate photodecomposition forms two of its products, nitrite and peroxynitrite. Studying the formation of these products is important because they are major sources of important atmospheric gases in polar regions. The goal of the work is to quantify the formation of nitrite and peroxynitrite from illumination of nitrate in/on ice so that the budgets of reactive nitrogen and oxidants in the Arctic can be better constrained. The investigators would accomplish this goal by: (1) Measuring the efficiencies for nitrite and peroxynitrite formation during nitrate illumination in ice; (2) Characterizing the impacts of organic molecules, which are ubiquitous constituents in snow, on nitrite and peroxynitrite formation; (3) Examining whether yields of nitrite and peroxynitrite vary if nitrate is present in different locations in/on the ice; and (4) Using the results to calculate rates of nitrite and peroxynitrite formation from nitrate photolysis in Arctic snow. Overall, these results should significantly improve understanding of the chemistry of nitrogen oxides, ozone, and hydroxyl radical in the Arctic snowpack and should help clarify the impacts of nitrate reactions on the overlying atmospheric boundary layer. The work would be carried out using well­defined laboratory ice samples, a variety of analytical techniques, and simple numerical modeling to determine the impact of the results on the chemistry in the Arctic snowpack and atmosphere. Two graduate students and several undergraduate students will be trained and mentored through the research.