Model overestimates of NO_y 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 N₂O 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 + O₂. All these rates are subject to substantial uncertainties that probably contribute to the discrepancy between model and observations.