Global modeling of the isotopic analogues of N2O:: Stratospheric distributions, budgets, and the 17O-18O mass-independent anomaly -: art. no. 4233

[1] A three-dimensional chemical transport model (CTM) is used to study the stratospheric distributions and global budgets of the five most abundant isotopic analogues of N2O: (NNO)-N-14-N-14-O-16, (NNO)-N-14-N-15-O-16, (NNO)-N-15-N-14-O-16, (NNO)-N-14-N-14-O-18, and (NNO)-N-14-N-14-O-17. Two different chemistry models are used to derive photolysis cross sections for the analogues of N2O: (1) the zero-point energy shift model, scaled by a factor of 2 to give better agreement with recent laboratory measurements and (2) the time-dependent Hermite propagator model. Overall, the CTM predicts stratospheric enrichments that are in good agreement with most measurements, with the latter model performing slightly better. Combining the CTM-calculated stratospheric losses for each N2O species with current estimates of tropospheric N2O sources defines a budget of flux-weighted enrichment factors for each. These N2O budgets are not in balance, and trends of -0.04 to -0.06 parts per thousand/yr for the mean of (NNO)-N-14-N-15-O-16 and (NNO)-N-15-N-14-O-16 and -0.01 to -0.02parts per thousand/yr for (NNO)-N-14-N-14-O-18 are predicted, although each has large uncertainties associated with the sources. The CTM also predicts that (NNO)-N-14-N-14-O-17 and (NNO)-N-14-N-14-O-18 will be fractionated by photolysis in a manner that produces a nonzero mass-independent anomaly. This effect can account for up to half of the observed anomaly in the stratosphere without invoking chemical sources. In addition, a simple one-dimensional model is used to investigate a number of chemical scenarios for the mass-independent composition of stratospheric N2O.