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

Abstract The nitrogen isotopic composition (δ15N) of atmospheric nitrate aerosols is determined by both the δ15N 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 δ15N 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 δ15N values in nitrate aerosols display a clear seasonal variation, with lower δ15N values (−12‰ to ∼ -9‰) in the summer (January to March) and higher δ15N values (0‰–3‰) in the winter (June–August), while the δ15N values of NOx sources exhibit a narrow range of variation from −10.7 ± 1.4‰ to −9.8 ± 1.4‰. 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 δ15N 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‰ to +9.5 ± 5.2‰)and higher in the winter (from +15.8 ± 1.9‰ to +17.0 ± 2.4‰).