We present volume mixing ratio profiles of NO, NO2, HNO3, HNO4, N2O5, and ClNO3 and their composite budget (NOy), from 20 to 39 km, measured remotely in solar occultation by the Jet Propulsion Laboratory MkIV Interferometer during a balloon flight from Fort Summer, New Mexico (35 ¿N), on September 25, 1993. In general, observed profiles agree well with values calculated using a photochemical steady state model constrained by simultaneous MkIV observations of long-lived precursors and aerosol surface area from the Stratospheric Aerosol and Gas Experiment II. The measured variation of concentrations of NOx (=NO+NO2) and N2O5 between sunrise and sunset reveals the expected ~2:1 stoichiometry at all altitudes. Despite relatively good agreement between theory and observation for profiles of NO and HNO3 the observed concentration of NO2 becomes progressively higher than model values below 30 km, with the discrepancy reaching ~30% at 22 km. This suggests an incomplete understanding of factors that regulate the NO/NO2 and NO2/HNO3 ratios below 30 km. Observations obtained during September 1990, prior to the June 1991 eruption of Mount Pinatubo, as well as during April 1993 and September 1993 provide a test of our understanding of the affect of aerosol surface area on the NOx/NOy ratio at midlatitudes. The observations reveal a decrease in the NOx/NOy ratio for increasing aerosol surface area that is consistent with the heterogeneous hydrolysis of N2O5 being the dominant sink of NOx between altitudes of 18 and 24 km for the conditions encountered (e.g., surface areas as high as 14 μm2 cm-3 and temperatures from 209 to 219 K). ¿ 1998 American Geophysical Union