Infrared solar absorption spectra of the atmosphere were taken during the CHEOPS III campaign in Esrange, Sweden in January and February 1990. The MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) instrument used has an unapodized spectral resolution of 0.015 cm −1 . Spectra were analysed by means of the SCAIS (Simulation Code for Atmospheric Infrared Spectra) level‐by‐level and line‐by‐line algorithm for radiative transfer through the atmosphere. The time series of the retrieved zenith column amounts of O 3 , H 2 O, CH 4 , N 2 O, HNO 3 , NO 2 , HCl, and HF are presented. The stratospheric HCl/HF ratio of 1.3 at the beginning of February, 1990 points towards the conversion of a considerable portion of gaseous HCl to other chemical or physical forms in the stratosphere in this period. A significant change in the HCl column amount between February 4, 1990 and February 8, 1990 was found; this can be correlated with a stratospheric warming between these two dates. From January 27, 1990 to February 11, 1990 the derived upper limits of NO 2 abundances were low (≤ 1.2 × 10 15 molecules/cm 2 ), whereas the column amounts of HNO 3 were very high with 2.9 × 10 16 molecules/cm 2 .
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Adrian et al. (1991) studied this question.