14 N 14 N 16 O (446) 14 N 15 N 16 O (456) – N  15 N 14 N 16 O (546) -- N 

1. Optimized Evaluation of Isotopic Fractionation of N2O in the AtmosphereJason D. Weibel1, Run-Lie Shia2, and Yuk L. Yung21Chemistry Department, Shenandoah University, Winchester VA2Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena CA • Background: • An understanding of the isotopic fractionation of N2O is important for constraining the budget of its sources and sinks • Fundamental role in stratospheric chemistry • Extraordinary potency as a greenhouse molecule Results: Comparison of calculated and measured values for the enrichment due to photodissociation. Solid line = Optimized cross sections, dotted line = Morgan et al. 2004, Dashed line = Chen et al. 2010, Dash-dot line = Chen et al. 2008, X = von Hessberg et al. 2004, Diamonds = Röckmann et al. 200, Triangles = Turatti et al. 2000, and Squares = Zang et al. 2000 Changes in amount of N emitted to the atmosphere OC -- ocean LD -- land AN -- anthropogenic Total = OC+LD+AN SK -- sink TD -- trend Comparison of calculated and measured values for atmospheric fractionation • Chemistry: • Calculations performed using the Caltech/JPL model for simulating the distribution of N2O in the atmosphere. • The main removal pathway for N2O from the atmosphere is photodissociation, • Secondary loss mechanism (~10%) due to the photo-oxidation • reaction with O(1D), Representation of the general scheme for an isotope dependent reflection principle. • Photochemical Absorption Cross Sections: • Rate of photodissociation reactions is proportional to the photochemical absorption cross section. • Theoretical cross sections based on reflection principle • Most recent theoretical cross sections include contributions from multiple vibrational modes and improved ab initio excited state PES • Better agreement with measured cross sections • Improvements in cross sections have not lead to much greater improvement in corresponding stratospheric fractionation • Optimization of Calculated Cross Sections: • Optimize the isotope dependent photochemical absorption cross sections • Introduction of scaling factors to cross sections • Conclusions: • Improvements in calculated photochemical absorption cross sections • However, better agreement in cross sections has not led to much greater agreement in fractionation • Optimization of cross sections has led to better agreement with measured fractionation values • Optimized photochemical absorption cross sections can lead to an improved understanding of the behavior of N2O in the atmosphere. Standard notation for the isotopologues of N2O 14N14N16O (446) 14N15N16O (456) –N15N14N16O (546) --N 14N14N17O (447) –17O 14N14N18O (448) –18O (δ15N = ½ [δN + δN]) Scaling factors used in the optimization of the photochemical absorption cross sectdions • References: • Chen, W.C., S. Nanbu, R.A. Marcus, “IsotopomerFractonation in the UV photolysis of N2O: 3. 3D ab initio surfaces and anharmonic effects.”, J. Phys. Chem. A,114 (36) 9700-9708 (2010) • Chen, W.C., M.K. Prakash, and R.A. Marcus, “Isotopomer Fractionation in the UV Photolysis of N2O: 2. Further comparison of theory and experiment.” J. Geophys. Res., 113 (2008): D05309 • Morgan, C.G., M. Allen, M.C. Liang, R.L. Shia, G.A. Blake, and Y.L. Yung, “Isotopic fractionation of nitrous oxide in the stratosphere: Comparison between model and observations.”, J. Geophys. Res., 109 (2004) D04305 • Röckmann, T., C.A.M. Brenninkmeijer, M. Wollenhaupt, J.N. Crowley, and P.J. Crutzen, “Measurement of the isotopic fractionation of 15N14N16O, 14N15N16O, and 14N14N18O in the UV photolysis of nitrous oxide.” Gephys. Res. Lett., 27 (2000) 1399-1402 • Turatti, F., D. Griffith, S. Wilson, M. Esler, T. Rahn, H. Zang, and G. Blake, “Positionally dependent 15N fractionation factors in the UV photolysis of N2O determined by high resolution FTIR spectroscopy.”, Geophys. Res. Lett., 27 (2000) 2489-2492 • von Hessberg. P., J. Kaiser, M.B. Enghoff, C.A. McLinden, S.L. Sorensen, T. Röckmann, and M.S. Johnson, “Ultra-violet absorption cross sections of isotopically substituted nitrous oxide species: 14N14NO. 15N14NO, 14N15NO, and 15N15NO.”, Atmos. Chem. Phys., 4 (2004) 1237-1253 • Zhang, H., P.O. Wennberg, V.H. Wu, and A.G. Blake “Fractionation of 14N15N16O and 15N14N16O during photolysis at 213 nm.”, Geophys. Res. Lett., 27 (2000) 2481-2484