GEOPHYSICALRESEARCH LETTERS, VOL. 9, NO. 3, PAGES223-226 , MARCH1982 SIMULATION OF THE OCTOBER 23, 1980 STRATOPROBE FLIGHT W.F.J. Evans, C.T. McElroy, J.B. Kerr Atmospheric Environment Service, 4905 Dufferin Street, Downsview, Ontario M3H 5T4 J.C. McConnell York University, 4700 Keele Street, Downsview, Ontario M3J 1P3 Abstract. Measurements of ozone, water vapour, nitric acid and nitrogen dioxide taken during the STRATOPROBE flight of October 23, 1980 from Palestine Texasat 32øN are presented. The measured ratio of HN03 to NO2 is in good agreement with that of a one dimensional model using current photochemistry. The measured ratio is used to infer •proxy" hydroxyl densities. These OH densities are very low below 25 km, in agreement with the findings of earlier flights. Introduction In a joint experiment with NASAGoddard, the STRATOPROBE payload was flown together with the GSFC hydroxyl lidar payload in order to verify the low hydroxyl densities inferred from earlier STRATOPROBE flights (Evans et al; 1981). This paper presents the measurements of ozone, water vapour, nitric acid and nitrogen dioxide made on the STRATOPROBE flight of October 23, 1980. These measurements are similar to numer- ous other measurements of nitric acid and nitro- gen dioxide madeby other groups, many of which have been reviewed in the WMO/NASA report (Hud- son et al; 1981). However, the current measure- ment set is unique because of the accompanying measurements of water vapour and ozone which make it an excellent data set for photochemical simulation. Proxy hydroxyl densities are also inferred from the ratio of nitric acid to nitro- gen dioxide. Flight Conditions The 1980 STRATOPROBE flight was launched at 12:24 CDT from the NSBF at Palestine, Texas, 31.7øN, 95.7øW on October 23, 1980. Themeteor- logical conditions were measuredby a series of ozonesonde flights conducted during the flight of the main payload. The mean temperature and pressure of the atmosphere from several radio- sonde flights is shown in Table 1• The ozone mixing ratios are given in Figure 1. The Simulation Model The one-dimensional photochemical model used to simulate the experimental results has been described previously by Evans et al (1981). It employs the latest NASA-CODATA (1981) recom- Copyright 1982 by the American Geophysical Union. Paper number 1L1877. 0094-8276 /82/001L-1877 $01.00 mendations for stratospheric chemistry. In particular, the fast rates for OH + HNO 3 and OH + HO2NO 2 lead to reduced hydroxyl densities be- low 30 km as required by the experimental meas- urements. The model was run through periods of 20 to 45 days depending on the altitude of the calculation in order to reach equilibrium. Experimental Conditions The measured temperature and pressure pro- files in Table 1 were used in the simulation. These came from radiosonde flights close to the time of the STRATOPROBE flight. The measured ozone and water vapour profiles shown in Figure 1 were employed in the simulation. The ozone profile is from the average of four ECC ozone- sonde flights taken over several hours around the time of the main balloon launch. The ozone profile is estimated to have an error of about 10% at most altitudes. The water vapour profile was measured with the scanning radiometer which measures thermal emission in the 6.4 to 7.2 mic- ron region during the ascent of the balloon. A band model is then used to invert the ascent data into a profile. The details of this mea- surement are described by Evans (1981). The estimated error of the water vapour measurement is ñ25% at most altitudes of interest. The measured nitric acid profile is also shown in Figure 1; it has a much small peak mixing ratio of only 3.5 ppbv rather than the 6 ppbv usuallyobserved at 31øN in fall. The nitric acid profile was measured with the scan- ning radiometer instrument by observing thermal emission in the 11.3 micron feature of nitric acid during the balloon ascent; the profile is obtained by differentiating the observed rad- iance variation. The details of the nitric acid measurement technique are described by Evans et al (1976). The nitric acid measurement is esti- mated to have an error of ñ15% at most altitudes below 30 km. Above 30 km the error grows rapid- ly due to the small observed radiances. The nitrogen dioxide profile was measured by solar occultation during sunset with the spectrophotometer instrument; it represents a measurement at a solar zenith angleof 90 ø. The measurement technique has been described in de- tail by Kerr and McElroy (1976). The error as- sociated with the measurement is about 15% at altitudes of interest. In particular, the NO 2 profile is smaller than usually observed at 31øN in fall with peak mixing ratios of only 8 ppbv 223