MODEL OVERESTIMATES OF NOY IN THE UPPER-STRATOSPHERE

Stratospheric NOy profiles from the Garcia-Solomon two-dimensional mod el are compared to observed profiles from the Upper Atmosphere Researc h Satellite (UARS) and the Atmospheric Trace Molecule Spectroscopy Exp eriment (ATMOS). The model consistently overestimates NOy in the upper stratosphere beyond the range of uncertainty in the satellite data. T he relatively short lifetime of NOy in this region suggests a largely photochemical explanation for the discrepancy. Uncertainties in the ph otochemical production of NOy from N2O strongly influence stratospheri c NOy profiles, but this influence decreases with increasing height in the upper stratosphere. Photochemical loss of NO 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 con tribute to the discrepancy between model and observations.