Groundbased differential optical absorption spectroscopic measurements of stratospheric trace gases are sensitive to stratospheric aerosol (PSC) layers as measured at Feb. 6, 1990, over northern Scandinavia. Mie extinction alters the intensity distribution of zenith scattered sunlight, in an extreme case presented here this can lead to 30%--40% changes of the airmass factor (AMF) for ozone. NO2 and OClO AMFs change less, O2 AMFs change dramatically. Ozone measurements at Kiruna, Sweden, for the time Feb. 4--8, 1990 are reported. No significant change in the O3 total column was observed during that period, when the effect of PSC's on the AMF was taken into account. ¿ American Geophysical Union 1993