The preservation of atmospheric nitrate in snow at Summit, Greenland

There is great interest in using nitrate (NO3−) isotopic composition in ice cores to track the history of precursor nitrogen oxides (NOx = NO + NO2) in the atmosphere. NO3−, however, can be lost from the snow by surface processes, such as photolysis back to NOx upon exposure to sunlight, making it difficult to interpret records of NO3− as a tracer of atmospheric NOx loading. In a campaign consisting of two field seasons (May–June) at Summit, Greenland, high temporal frequency surface snow samples were collected and analyzed for the oxygen isotopic composition of NO3−. The strong, linear relationship observed between the oxygen isotopes of NO3−, in both 2010 and 2011, is difficult to explain in the presence of significant postdepositional processing of NO3−, unless several unrelated variables change in concert. Therefore, the isotopic signature of NO3− in the snow at Summit is most feasibly explained as preserved atmospheric NO3− deposition.