Abstract
Abstract. Nitrogen isotope fractionations between nitrogen oxides (NO and NO2) play a significant role in determining the nitrogen isotopic compositions (δ15N) of atmospheric reactive nitrogen. Both the equilibrium isotopic exchange between NO and NO2 molecules and the isotope effects occurring during the NOx photochemical cycle are important, but both are not well constrained. The nighttime and daytime isotopic fractionations between NO and NO2 in an atmospheric simulation chamber at atmospherically relevant NOx levels were measured. Then, the impact of NOx level and NO2 photolysis rate on the combined isotopic fractionation (equilibrium isotopic exchange and photochemical cycle) between NO and NO2 was calculated. It was found that the isotope effects occurring during the NOx photochemical cycle can be described using a single fractionation factor, designated the Leighton cycle isotope effect (LCIE). The results showed that at room temperature, the fractionation factor of nitrogen isotopic exchange is 1.0289±0.0019, and the fractionation factor of LCIE (when O3 solely controls the oxidation from NO to NO2) is 0.990±0.005. The measured LCIE factor showed good agreement with previous field measurements, suggesting that it could be applied in an ambient environment, although future work is needed to assess the isotopic fractionation factors of NO+RO2/HO2→NO2. The results were used to model the NO–NO2 isotopic fractionations under several NOx conditions. The model suggested that isotopic exchange was the dominant factor when NOx>20 nmol mol−1, while LCIE was more important at low NOx concentrations (<1 nmol mol−1) and high rates of NO2 photolysis. These findings provided a useful tool to quantify the isotopic fractionations between tropospheric NO and NO2, which can be applied in future field observations and atmospheric chemistry models.
Highlights
IntroductionThe nitrogen isotopic composition (δ15N) of reactive nitrogen compounds in the atmosphere is an important tool in understanding the sources and chemistry of atmospheric NOx (NO + NO2–NO) (1 − f (NO2))
The nitrogen isotopic composition (δ15N) of reactive nitrogen compounds in the atmosphere is an important tool in understanding the sources and chemistry of atmospheric NOx (NO + NO2 isotopic fractionation. The (NO2–NO)) (1 − f (NO2))
There remain questions about how isotopic fractionations that may occur during photochemical cycling of NOx could alter the δ15N values as it partitions into NOy (NOy = atmospheric nitrate, NO3, N2O5, HONO, etc.; Chang et al, 2018; Freyer, 1991; Hastings et al, 2004; Jarvis et al, 2008; Michalski et al, 2005; Morin et al, 2009; Zong et al, 2017)
Summary
The nitrogen isotopic composition (δ15N) of reactive nitrogen compounds in the atmosphere is an important tool in understanding the sources and chemistry of atmospheric NOx (NO + NO2). There remain questions about how isotopic fractionations that may occur during photochemical cycling of NOx could alter the δ15N values as it partitions into NOy (NOy = atmospheric nitrate, NO3, N2O5, HONO, etc.; Chang et al, 2018; Freyer, 1991; Hastings et al, 2004; Jarvis et al, 2008; Michalski et al, 2005; Morin et al, 2009; Zong et al, 2017). Since the atmospheric chemistry of NOy varies significantly in different environments (e.g., polluted vs pristine, night vs day), the isotopic fractionations associated with NOy chemistry are likely to vary in different environments. These unknowns could potentially bias conclusions about NOx source apportionment reached when using nitrogen isotopes. Understanding the isotopic fractionations between NO and NO2 during photochemical cycling could improve our understanding of the relative role of sources versus chemistry for controlling the δ15N variations in atmospheric NO2 and nitrate
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