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. 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 theisotope effects occurring during the NOx photochemical cycle areimportant, but both are not well constrained. The nighttime and daytimeisotopic fractionations between NO and NO2 in an atmospheric simulationchamber at atmospherically relevant NOx levels were measured. Then, theimpact of NOx level and NO2 photolysis rate on the combinedisotopic fractionation (equilibrium isotopic exchange and photochemicalcycle) between NO and NO2 was calculated. It was found that theisotope effects occurring during the NOx photochemical cycle can bedescribed using a single fractionation factor, designated the Leighton cycleisotope effect (LCIE). The results showed that at room temperature, thefractionation factor of nitrogen isotopic exchange is 1.0289±0.0019 ,and the fractionation factor of LCIE (when O3 solely controls theoxidation from NO to NO2 ) is 0.990±0.005 . The measured LCIEfactor showed good agreement with previous field measurements, suggestingthat it could be applied in an ambient environment, although future work isneeded to assess the isotopic fractionation factors of NO + RO 2 / HO 2 → NO 2 . The results were used to model theNO– NO2 isotopic fractionations under several NOx conditions. Themodel suggested that isotopic exchange was the dominant factor when NOx>20  nmol mol −1 , while LCIE was more important at low NOx concentrations (  nmol mol −1 ) and high rates of NO2 photolysis. These findings provided a useful tool to quantify theisotopic fractionations between tropospheric NO and NO2 , which can beapplied in future field observations and atmospheric chemistry models.

atmospheric chemistry and physics

Quantifying the nitrogen isotope effects during photochemical equilibrium between NO and NO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;: implications for &amp;lt;i&amp;gt;δ&amp;lt;/i&amp;gt;&amp;lt;sup&amp;gt;15&amp;lt;/sup&amp;gt;N in tropospheric reactive nitrogen