Nitrogen isotopes in nitrate aerosols collected in the remote marine boundary layer: Implications for nitrogen isotopic fractionations among atmospheric reactive nitrogen species

摘要

The nitrogen isotopic composition (delta N-15) of atmospheric nitrate aerosols is determined by both the delta N-15 of its precursor, NOx emissions, and the isotopic fractionations during the atmospheric oxidation of NOx. However, the latter has not been well-understood nor quantified by field observations. In addition, the seasonal variations of this isotopic fractionation have not been determined. To better understand this isotopic fractionation process, in this study, we analyzed the delta N-15 of nitrate aerosols collected from June 30, 2015 to August 6, 2016 at Baring Head, New Zealand, where the sources of NOx are well-studied. Our results showed that the delta N-15 values in nitrate aerosols display a clear seasonal variation, with lower delta N-15 values (-12 parts per thousand to similar to-9 parts per thousand) in the summer (January to March) and higher delta N-15 values (0 parts per thousand to 3 parts per thousand) in the winter (June-August), while the delta N-15 values of NOx sources exhibit a narrow range of variation from-10.7 +/- 1.4 parts per thousand to-9.8 +/- 1.4 parts per thousand. We attribute this discrepancy to the significant and variable isotopic fractionations during the oxidation processes of NOx. We then quantified the isotopic fractionation during 1) the equilibrium and kinetic isotopic fractionations between NO and NO2; and 2) the oxidation of NO2 to nitrate. Our calculations suggest that at Baring Head, the seasonal variations in the oxidation pathways of NO2 are the main driver of the seasonal variations of nitrate delta N-15 values. Furthermore, the overall isotopic fractionation factors of the oxidation process determined by two models (Kinetic fractionation model and Equilibrium fractionation model) are generally lower in the summer (from +6.3 +/- 1.7 parts per thousand to +9.5 +/- 5.2 parts per thousand)and higher in the winter (from +15.8 +/- 1.9 parts per thousand to +17.0 +/- 2.4 parts per thousand).