Quantifying the nitrogen equilibrium and photochemistry-induced isotopic effects between NO and NO 2

Abstract. The nitrogen isotopic fractionations between NO and NO2 play a significant role in determining the nitrogen isotopic compositions (δ15N) of atmospheric NO2 and nitrate. This isotopic fractionation is controlled by a combination of equilibrium isotopic effect (EIE) and Leighton Cycle induced isotopic effect (LCIE), which are poorly constrained. We quantified this isotopic fractionation process by (1) measuring the isotopic fractionation factors of EIE and LCIE in a 10 m3 atmospheric simulation chamber and (2) mathematically calculating the relative importance of EIE and LCIE. Our results showed the enrichment factors of EIE and LCIE are 1.0275 ± 0.0012, and 0.9905 ± 0.005, respectively, at room temperature. We find that EIE is the dominate factor when NOx > 20 nmol mol−1, and LCIE is more important at low NOx concentrations (