Quantifying the nitrogen isotope effects during photochemical equilibrium between NO and NO2: Implications for δ15N in tropospheric reactive nitrogen

Both the equilibrium isotopic exchange between NO and NO2 molecules and the isotope effects occurring during the NO, 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 NO, 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 NO, 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 O-3 solely controls the oxidation from NO to NO2) is 0.990 +/- 0.005.