Fractionation of N2O Isotopomers in the Simulated Stratospheric Sink Reactions

Nitrous oxide (N₂O) is an important trace gas in the atmosphere since it is radiatively active in the troposphere and also a precursor of nitric oxide which catalytically destroys ozone in the stratosphere. Natural isotope abundance in N₂O has been studied to understand its complex geochemical cycle. We previously reported that composition of N₂O isotopomers (¹⁴N¹⁴N¹⁶O, ¹⁵N¹⁴N¹⁶O, ¹⁴N¹⁵N¹⁶O, ¹⁴N¹⁴N¹⁸O) has a unique vertical profile in the stratosphere such that heavier isotopomers are enriched and the intramolecular site preference of ¹⁵N for the center position increases with altitude. This enrichment was generally expected from the simulation experiments of photolysis at wavelengths of about 200 nm and theoretical calculations based on photo-induced isotopic fractionation effects. However, the detailed analyses of the vertical profiles showed several differences between observation and photolytic simulation/calculation. Here we simulated another sink reaction, photo-oxidation of N₂O with excited oxygen atom, O(¹D), to evaluate the fractionation of isotopomers in the process. Mixture of N₂O and O₃ was irradiated by the light of 254 nm to initiate the photo-oxidation reaction, and isotopomer ratios of remaining N₂O were determined on the modified isotope-ratio-monitoring mass spectrometer that can analyze masses of molecular ion (N₂O⁺) and fragment ion (NO⁺). Factors controlling the stratospheric distribution of N₂O isotopomers will be discussed from fractionation factors during the sink reactions and stratospheric observations.