Gas-phase isotopic exchange between NO and NO2 enriches the heavier 15N isotope in the more oxidized form. In the atmosphere the concentration of both gases, NO and NO2, is controlled during daytime by the Leighton relationship through the oxidation of NO with O3 and the photolytic reaction of NO2 to NO. For atmospheric concentrations (e.g., NOx, ~10 ppb), isotopic exchange and photolytic reaction are very fast with characteristic time constants of a few minutes compared to other removing reactions of atmospheric NOx via OH radicals (daytime reaction) and O3 (nighttime reaction) with time constants of some hours and more. We have found from 15N/14N measurements of atmospheric NO2 and NOx at J¿lich that both processes, isotopic exchange and photolytic reaction, interact and have an influence on the 15N/14N isotopic ratio. This interaction, together with seasonal variations of the NOx/O3 ratio at daytime and either complete oxidation of NO to NO2 or isotopic exchange between NO and NO2 at nighttime, explains the seasonal variation of the 15N/14N ratio of atmospheric NO2, which agrees with results from a simple model. ¿ American Geophysical Union 1993