Simultaneous in situ measurements of NO, NO2, HNO3, O3, N2O, pressure, and temperature at 30 km have been made from Palestine, Texas (32¿N) on September 13, 1988, using the Jet Propulsion Laboratory Balloon-Borne Laser In-Situ Sensor (BLISS) instrument, with the NOAA dual-channel balloon UV ozone spectrometer on the same gondola. Using tunable diode laser absorption spectroscopy over a long path length, measurements were made during a 24-hour flight of the daytime concentrations of NO, NO2, O3 and of the diurnal variation in the concentration of NO2, Postsunset measurements of NO2, made every half minute throughout much of the night, show the NO2 mixing ratio falling from a sunset value of 10.5 ppbv to 5.2 ppbv at night's end. According to the simple identity (d/dt)2>= -2k2>3> for the rate of decay of the NO2 concentration, where k is the rate constant for the reaction between NO2 and O3, an exponential fit to the observed decay agrees with that calculated to better than 10%, with a standard deviation of the fit of only ¿3%. Photochemical model calculations constrained to the measured temperature and O3 values, and incorporating both N2O5 and ClONO2 chemistry, predict in detail the observed NO2 diurnal behavior.

From the sunset/sunrise difference in the volume mixing ratio of NO2 is derived a value of 2.7¿0.4 ppbv for the sunrise N2O5 mixing ratio, in excellent agreement with the model predictions of 2.9 ppbv at this latitude. The measured daytime NO2/NO ratio was found to be in good agreement with model predictions at 30 km. The measured presunset sum NO+NO2 of 10.1¿0.8 ppbv agreed well with the measured postsunset NO2 amount of 10.5¿0.8 ppbv. Simultaneous measurements of the mixing ratios of HNO3 and postsunset NO2 allow an estimate of total reactive nitrogen, approximated at this time by NO2+HNO3+2(N2O5)+ClONO2, of 16.4¿1.2 ppbv at 30 km, and (from an earlier flight) of 13.7¿1.7 ppbv at 37 km. Using model predictions incorporating corrections for non-steady state and for diurnal chemistry, an OH mixing ratio of 8¿4 ppbv is derived from the measured HNO3/NO2 ratio of 0.72¿0.17 at 30 km. ¿ American Geophysical Union 1990