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

[1] We investigate the mass dependent isotopic fractionation mechanisms, based on photolytic destruction and reaction with O(D-1), to explain the N-15/N-14 and O-18/O-16 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 [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 (NNO)-N-15-N-14-O-16. A simpler, semi-empirical approach by Blake et al. [2003] is able to model the magnitude of all the enrichments, including the one for (NNO)-N-15-N-14-O-16. 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 N2O by reaction with O(D-1) 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 (NNO)-N-14-N-14-O-16. From our 2-D model we infer the relative rates for isotopologues and isotopomers (NNO)-N-14-N-14-O-16 (446), (NNO)-N-14-N-15-O-16 (456), N-15 N-14 O-16 (546), (NN)-N-14-N-14 O-17 (447) and (NNO)-N-14-N-14-O-18 (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 12.0parts per thousand. If we do not distinguish between the (456) and ( 546) isotopomers, the mean isotopic fractionation for N-15 is 14.5%. If we assume that the mean tropospheric values for delta(456), delta(546), delta(15)N and delta(18)O are 16.35, - 2.35, 7.0 and 20.7parts per thousand, respectively, we infer the following isotopic signature for the integrated sources of N2O: delta(456) = - 2.9parts per thousand, delta(546) = - 11.7parts per thousand, delta(15)N = - 7.3parts per thousand and delta(18)O = 8.7parts per thousand.