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‰).

中文翻译：

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在偏远海洋边界层收集的硝酸盐气溶胶中的氮同位素：对大气活性氮物种中氮同位素分馏的影响

摘要 大气硝酸盐气溶胶的氮同位素组成 (δ15N) 由其前体的 δ15N、NOx 排放和大气 NOx 氧化过程中的同位素分馏决定。然而，后者尚未被实地观察很好地理解或量化。此外，这种同位素分馏的季节性变化尚未确定。为了更好地了解这一同位素分馏过程，在本研究中，我们分析了 2015 年 6 月 30 日至 2016 年 8 月 6 日在新西兰巴林海德收集的硝酸盐气溶胶的 δ15N，在那里对 NOx 的来源进行了深入研究。我们的研究结果表明，硝酸盐气溶胶中的 δ15N 值显示出明显的季节性变化，夏季（1 月至 3 月）δ15N 值较低（-12‰ 至 ∼ -9‰），而夏季（1 月至 3 月）δ15N 值较高（0‰–3‰）。冬季（6 月至 8 月），而 NOx 源的 δ15N 值变化范围很窄，从 -10.7 ± 1.4‰ 到 -9.8 ± 1.4‰。我们将这种差异归因于 NOx 氧化过程中显着且可变的同位素分馏。然后我们量化了以下过程中的同位素分馏：1) NO 和 NO2 之间的平衡和动力学同位素分馏；2) NO2氧化成硝酸盐。我们的计算表明，在 Baring Head，NO2 氧化途径的季节性变化是硝酸盐 δ15N 值季节性变化的主要驱动因素。此外，由两种模型（动力学分馏模型和平衡分馏模型）确定的氧化过程的总体同位素分馏因子在夏季普遍较低（从+6.3±1.7‰到+9.5±5.2‰），而在冬季较高（从 +15.8 ± 1。