A Born‐Oppenheimer photolysis model of N 2 O fractionation

The isotopically light N 2 O produced by microbial activity is thought to be balanced by the return of heavy stratospheric nitrous oxide. The Yung and Miller [1997] method that first explained these trends yields photolytic fractionation factors ∼half those observed by experiment or predicted quantum mechanically, however. To address these issues, we present here a Born‐Oppenheimer photolysis model that uses only commonly available spectroscopic data. The predicted fractionations quantitatively reproduce laboratory data, and have been incorporated into zonally averaged atmospheric simulations. Like McLinden et al. [2003], who employ a three‐dimensional chemical transport model with cross sections scaled to match laboratory data, we find excellent agreement between predictions and stratospheric measurements; additional processes that contribute to the mass independent anomaly in N 2 O can only account for a fraction of its global budget.