Model overestimates of NOy in the upper stratosphere

Stratospheric NO y profiles from the Garcia‐Solomon two‐dimensional model are compared to observed profiles from the Upper Atmosphere Research Satellite (UARS) and the Atmospheric Trace Molecule Spectroscopy Experiment (ATMOS). The model consistently overestimates NO y in the upper stratosphere beyond the range of uncertainty in the satellite data. The relatively short lifetime of NO y in this region suggests a largely photochemical explanation for the discrepancy. Uncertainties in the photochemical production of NO y from N 2 O strongly influence stratospheric NO y profiles, but this influence decreases with increasing height in the upper stratosphere. Photochemical loss of NO y is controlled by the NO photolysis rate and the reaction rates of N + NO and N + O 2 . All these rates are subject to substantial uncertainties that probably contribute to the discrepancy between model and observations.