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To explore the seasonality in δ15N and δ18O of nitrate in Greenland snow, we describe a simple box model of local photochemistry. Isotope ratios of HNO3 are controlled by the nitrogen isotope fractionation between NO and NO2, the ratio of NO2 to NO, and seasonal variations in HNO3 production. The observed seasonal range in δ15N requires either a large net fractionation (∼70‰) associated with NOx cycling or a seasonal change in the δ15N of NOx sources. The observed range in δ18O of nitrate is smaller than that calculated from HNO3 production pathways, suggesting that seasonal transport may also be required to explain the seasonality in nitrate δ18O.
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Jarvis et al. (2008) studied this question.
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