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Midlatitude stratospheric ClO at altitudes below 22 km has been measured for the first time. Measurements were made at latitudes between 27 and 48 deg N during three flights from Moffett Field, CA, in June and July of 1987, with a new instrument flown on the NASA ER-2 aircraft. The result from these flights is that the ClO mixing ratio increases from less than 0.5 pptv at 16.8 km to 2.0 pptv at 18.3 km and 10.1 pptv at 21 km. These altitude profiles agree with an extrapolated profile from a May 1986 balloon-borne experiment (Brune and Anderson, 1986).

Stratospheric chlorine monoxide concentrations were measured in situ using a new and improved observational method in which the detection package is lowered on a tether from a balloon-borne platform floating near 40 km altitude and then reeled back up. Two reel down and reel up sequences covering 38 to 31 km produced (ClO) profiles which agree with each other to + or - 12 percent. The measured (ClO) profile falls within the envelope of nine previous fast parachute drop results, but exhibits a different shape, peaking near 32 km as opposed to approximately 39 km for the average of the previous profiles. These results are lower than but consistent with the profiles derived from remote sensing methods, and indicate the need for simultaneous measurements of relevant species.