Isotopic fractionation of nitrous oxide in the stratosphere: Comparison between model and observations

We investigate the mass dependent isotopic fractionation mechanisms, based on photolytic destruction and reaction with O( 1 D), to explain the 15 N/ 14 N and 18 O/ 16 O fractionation of stratospheric N 2 O and reconcile laboratory experiments with atmospheric observations. The Caltech/JPL two‐dimensional (2‐D) model is utilized for detailed studies of N 2 O 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 N 2 O. Although the Yung and Miller [1997] 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. [2001] provides a better fit to the magnitudes of the enrichments, with the notable exception of the enrichment for the 15 N 14 N 16 O. A simpler, semi‐empirical approach by Blake et al. [2003] is able to model the magnitude of all the enrichments, including the one for 15 N 14 N 16 O. The Blake et al. [2003] 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 N 2 O by reaction with O( 1 D) 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 14 N 14 N 16 O. From our 2‐D model we infer the relative rates for isotopologues and isotopomers 14 N 14 N 16 O (446), 14 N 15 N 16 O (456), 15 N 14 N 16 O (546), 14 N 14 N 17 O (447) and 14 N 14 N 18 O (448), to be 1, 0.9843, 0.9942, 0.9949, and 0.9900, respectively. Thus the destruction of N 2 O 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 15 N is 14.5‰. If we assume that the mean tropospheric values for δ 456 , δ 546 , δ 15 N and δ 18 O are 16.35, −2.35, 7.0 and 20.7‰, respectively, we infer the following isotopic signature for the integrated sources of N 2 O: δ 456 = − 2.9‰, δ 546 = −11.7‰, δ 15 N = −7.3‰ and δ 18 O = 8.7‰.