Photochemical model calculations utilizing updated reaction rate constants and photo-cross section and constrained by Spacelab 3 (May 1985) Atmospheric Tace Molecule Spectrosocpy (ATMOS) experiment measurements of species abundances and atmospheric parameters yield NO/NO2 aboundance ratis in excellent agreement (>25--30% differences: the measurements uncertainties) with the ATMOS 30¿N sunset and 47¿S sunrise results between ~20 and ~50 km. Over this altitude range the NO/NO2 ratio is varying by a factor of 100. At 25 km and above ~40 km. difficulties in retrieving abundances from occulation spectra for the rapidly diurnally varying NO or NO2 may contribute to the large discrepancies with the model values. The agreement between model and observations for most of the stratosphere serves to confirm key aspects of current stratospheric chemical mdoels. In addition, models results for N2O5 at 47¿ sunrise reproduce the ATMOS measurements, supporting the current simple scheme in models for sequestering NOx in the N2O5 nightime reservoir. However, the model values at 30¿N sunset are as much as a factor of 3.5 larger than the corresponding ATMoS N2O5 observations. Agreement beween calculations and measurements occurs when the N2O5 photodissociation rate constants are increased by 50--75%. ¿American Geophyscial Union 1990