HCl has been observed in the stratosphere from balloon borne spectroscopic measurements of a portion of the fundamental band in the 3 microns wavelength region. The observation of absorption lines using the sun as a source at solar zenith angles larger than 90° indicates an HCl volume mixing ratio equal to (3.8 ± 1.5) × 10−10 at 20 km increasing with altitude at least up to 30 km where the value is (1.4 ± 0.6) × 10−9. The total zenith amount above 21 km is found to be (10± 3.5) × 10−14 cm−2 in agreement with previous observations performed up to this altitude. A maximum number density equal to (7.2 ± 3) × 108 cm−3 is observed at 24 ± 2 km altitude.

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A grating monochromator has been flown from Aire sur l'Adour (Landes) on May 10, 1968, April 19, 1969 and October 3, 1969. The instrumentation and the experimental procedure have been described previously (Ackerman et al., 1968) as well as the results of the first flight. For the two subsequent flights the wavelength marking was improved by increasing the number of the telemetered wavelength marking pulses. For the last flight the FM-FM analog telemetry system was replaced by a PCM telemetry system in such a way that the data analysis was completely made by means of a computer. Float altitudes were respectively 34, 38 and 37 km. From 1850 Å to 3000 Å up to 90 spectra per flight of the solar radiation penetrating into the atmosphere were telemetered to the ground. The solar zenith distance was ranging from 30° to 60°. The equivalent slit width was equal to 19 Å in the case of the first experiment and was equal to 14.6 Å for the two subsequent flights.

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