Background :

1. An Optimized Evaluation of Isotopic Fractionation of N2O in the AtmosphereYuk L. Yung1, Jason D. Weibel2, and Run-Lie Shia11Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena CA2Chemistry Department, Shenandoah University, Winchester VA ABSTRACT This work examines the relationships between the isotopic photo-dissociation of N2O, agreement between experimental and theoretical photolytic absorption cross-sections and calculated enrichment values for the lower stratosphere. Improvements in the theoretical description of the isotope dependent photo-dissociation cross-sections for N2O have spurred a re-evaluation of previous calculations of stratospheric fractionation. However, except in the case of 15N14N16O, these improvements have not corresponded to much greater agreement between measured and calculated stratospheric enrichment. While overall agreement between experimental and theoretical photo-absorption cross-sections has improved, there are still some discrepancies. Therefore, we have optimized the agreement between calculated and measured isotopic stratospheric enrichment values by modifying the calculated photo-dissociation cross-sections. • Optimization of Calculated Cross Sections: • Optimize the isotope dependent photochemical absorption cross sections • Introduction of scaling factors to cross sections Results: Table 3. Comparison of calculated and measured values for atmospheric fractionation Figure 3. 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. 2001, Triangles = Turatti et al. 2000, and Squares = Zang et al. 2000 • 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, Table 2. Scaling factors used in the optimization of the photochemical absorption cross sections • Secondary loss mechanism (~10%) due to the photo-oxidation reaction with O(1D), • 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 Figure 1. Changes in amount of N emitted to the atmosphere Total = Ocean + Land + Anthropogenic Sink Land Ocean Anthropogenic Trend Figure 2. 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 Table 1. Standard notation for the isotopologues of N2O • 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. • References: • Chen et al (2010) J. Phys. Chem. A 114 9700–9708; Chen et al. (2008) J. Geophys. Res.113 D05309; Morgan et al (2004)J. Geophys. Res.109 D04305; Röckmann et al. (2000) Gephys. Res. Lett.271399–1402; Turatti et al. (2000) Geophys. Res. Lett., 272489–2492; von Hessberget al. (2004)Atmos. Chem. Phys. 4 1237–1253; Zhang et al. (2000) Geophys. Res. Lett.27 2481–2484