ON THE INTERACTION OF ISOTOPIC EXCHANGE PROCESSES WITH PHOTOCHEMICAL-REACTIONS IN ATMOSPHERIC OXIDES OF NITROGEN

Gas-phase isotopic exchange between NO and NO2 enriches the heavier N-15 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., NO(x), approximately 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 NO(x) via OH radicals (daytime reaction) and O3 (nighttime reaction) with time constants of some hours and more. We have found from N-15/N-14 measurements of atmospheric NO2 and NO(x), at Julich that both processes, isotopic exchange and photolytic reaction, interact and have an influence on the N-15/N-14 isotopic ratio. This interaction, together with seasonal variations of the NO(x)/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 N-15/N-14 ratio of atmospheric NO2, which agrees with results from a simple model.