Seasonal Shifts in Nitrogen Isotope Fractionation Regimes in the Leighton Cycle

Abstract

Abstract Nitrogen isotopes are used to trace nitrogen oxides (NO x = nitric oxide (NO) + nitrogen dioxide (NO 2 )) emissions and nitrate formation, but their interpretation depends on isotope redistribution during NO−NO 2 cycling. A key uncertainty is whether this partitioning is controlled by equilibrium exchange or Leighton cycle. We simultaneously measured δ 15 N(NO 2 ) and δ 15 N(NO x ) in Taiyuan, China, during winter and summer 2024. The isotopic difference between NO 2 and NO x reversed from (8.2 ± 6.5) ‰ in winter to (−6.1 ± 2.9) ‰ in summer. Winter data agree with theoretical equilibrium fractionation, and EIE‐corrected δ 15 N(NO x ) matched direct measurements, providing field validation that ambient δ 15 N(NO x ) can be reconstructed from δ 15 N(NO 2 ) under EIE dominated regime. In summer, 15 N‐depleted NO 2 showed that Leighton cycle isotopic effect (LCIE) can dominate NO−NO 2 partitioning even in polluted urban air. These results indicate that the magnitude and variability of LCIE require further constraints and validation for interpreting nitrogen isotopes in polluted atmospheres.