Composition and Chemistry
Isotopic fractionation of nitrous oxide in the stratosphere: Comparison between model and observations
Article first published online: 20 FEB 2004
Copyright 2004 by the American Geophysical Union.
Journal of Geophysical Research: Atmospheres (1984–2012)
Volume 109, Issue D4, 27 February 2004
How to Cite
2004), Isotopic fractionation of nitrous oxide in the stratosphere: Comparison between model and observations, J. Geophys. Res., 109, D04305, doi:10.1029/2003JD003402., , , , , and (
- Issue published online: 20 FEB 2004
- Article first published online: 20 FEB 2004
- Manuscript Accepted: 2 OCT 2003
- Manuscript Revised: 17 SEP 2003
- Manuscript Received: 10 JAN 2003
 We investigate the mass dependent isotopic fractionation mechanisms, based on photolytic destruction and reaction with O(1D), to explain the 15N/14N and 18O/16O fractionation of stratospheric N2O and reconcile laboratory experiments with atmospheric observations. The Caltech/JPL two-dimensional (2-D) model is utilized for detailed studies of N2O and its isotopologues and isotopomers in the stratosphere. We compare model results with observations of isotopic enrichment using three different methods of calculating photolytic cross-sections for each of the major isotopologues and isotopomers of N2O. Although the Yung and Miller  successfully modeled the pattern of enrichments for each isotopologue or isotopomer relative to each other, their approach underestimated the magnitude of the enrichments. The ab initio approach by Johnson et al.  provides a better fit to the magnitudes of the enrichments, with the notable exception of the enrichment for the 15N14N16O. A simpler, semi-empirical approach by Blake et al.  is able to model the magnitude of all the enrichments, including the one for 15N14N16O. The Blake et al.  cross-sections are temperature-dependent, but adjustments are needed to match the measurements of Kaiser et al. [2002a]. Using these modified cross-sections generally improves the agreement between model and mass spectrometric measurements. Destruction of N2O by reaction with O(1D) results in a small but nonnegligible isotopic fractionation in the lower stratosphere. On a per molecule basis, the rates of destruction of the minor isotopologues or isotopomers are somewhat less than that for 14N14N16O. From our 2-D model we infer the relative rates for isotopologues and isotopomers 14N14N16O (446), 14N15N16O (456), 15N14N16O (546), 14N14N17O (447) and 14N14N18O (448), to be 1, 0.9843, 0.9942, 0.9949, and 0.9900, respectively. Thus the destruction of N2O in the atmosphere results in isotopic fractionations of (456), (546), (447) and (448) by 19.4, 9.5, 5.5 and 12.0‰. If we do not distinguish between the (456) and (546) isotopomers, the mean isotopic fractionation for 15N is 14.5‰. If we assume that the mean tropospheric values for δ456, δ546, δ15N and δ18O are 16.35, −2.35, 7.0 and 20.7‰, respectively, we infer the following isotopic signature for the integrated sources of N2O: δ456 = − 2.9‰, δ546 = −11.7‰, δ15N = −7.3‰ and δ18O = 8.7‰.