Quantifying the nitrogen isotope effects during photochemical equilibrium between NO and NO 2: implications for δ15N in tropospheric reactive nitrogen

Li, Jianghanyang; Zhang, Xuan; Orlando, John; Tyndall, Geoffrey; Michalski, Greg

Nitrogen isotope fractionations between nitrogen oxides (NO and inline-formulaNO2) play a significant role in determining the nitrogen isotopic compositions (inline-formulaδ15N) of atmospheric reactive nitrogen. Both the equilibrium isotopic exchange between NO and inline-formulaNO2 molecules and the isotope effects occurring during the inline-formulaNOx photochemical cycle are important, but both are not well constrained. The nighttime and daytime isotopic fractionations between NO and inline-formulaNO2 in an atmospheric simulation chamber at atmospherically relevant inline-formulaNOx levels were measured. Then, the impact of inline-formulaNOx level and inline-formulaNO2 photolysis rate on the combined isotopic fractionation (equilibrium isotopic exchange and photochemical cycle) between NO and inline-formulaNO2 was calculated. It was found that the isotope effects occurring during the inline-formulaNOx 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 inline-formula1.0289±0.0019, and the fractionation factor of LCIE (when inline-formulaO3 solely controls the oxidation from NO to inline-formulaNO2) is inline-formula0.990±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 inline-formula M18inlinescrollmathml chem normal NO + normal RO normal 2 / normal HO normal 2 normal NO normal 2 115pt14ptsvg-formulamathimgf6194210be8418583ae77a87ec1da66c acp-20-9805-2020-ie00001.svg115pt14ptacp-20-9805-2020-ie00001.png . The results were used to model the NO–inline-formulaNO2 isotopic fractionations under several inline-formulaNOx conditions. The model suggested that isotopic exchange was the dominant factor when inline-formulaNOx>20 nmol molinline-formula−1, while LCIE was more important at low inline-formulaNOx concentrations (inline-formula<1 nmol molinline-formula−1) and high rates of inline-formulaNO2 photolysis. These findings provided a useful tool to quantify the isotopic fractionations between tropospheric NO and inline-formulaNO2, which can be applied in future field observations and atmospheric chemistry models.

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Li, Jianghanyang / Zhang, Xuan / Orlando, John / et al: Quantifying the nitrogen isotope effects during photochemical equilibrium between NO and NO2: implications for δ15N in tropospheric reactive nitrogen. 2020. Copernicus Publications.

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