Results are presented from an October 17, 1988, flight of the NASA ER-2 stratospheric research aircraft. The flight sampled a nearly constant air mass of 20 km altitude, near 39¿N latitude, from before sunrise until near noon. The instrumentation on board simultaneously measured NO, ClO, O3, temperature, and pressure. The measurements are combined with modeled photodissociation coefficients and known reaction kintics to infer abundances of other important species, and the results are compared to previous estimates as a test for consistency in our understanding of the photochemical processes governing the species distributions. NO and ClO are observed to increase after sunrise, consistent with photolysis of nighttime reservoirs. The derived values for NO2, ClONO2, and N2O5 near midday are 320 (¿190) pptv, 170 (¿120) pptv, and 200 (¿150) pptv, respectively, and are in good agreement with most previous measurements and model estimates. The slope of the NOx (=NO+NO2) change with time is a sensitive function of the shape of the JNO2 versus zenith angle curve at zenith angles above about 75¿. This makes the derived value for N2O5 dependent on the model method used to compute the solar flux, with a detailed treatment of multiple scattering required for good results. Subtracting the sum of the measured and derived values for the nitrogen species from the average of measurements of total reactive nitrogen (NOy) on other flights, we obtain an estimate for the mixing ratio of HNO3 of 3.3 (¿1.7) ppbv. This value for HNO3 and the HNO3/NOy fraction are in good agreement with values measured by the ATMOS experiment. The HNO3 and NO2 values are further used to estimate a daytime average OH mixing ratio of 0.6 pptv, which is consistent with extrapolation of previous measurements at higher altitudes. ¿ American Geophysical Union 1990