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During the Arctic winter of 2000/01, ground-based FTIR and millimetre-wavemeasurements revealed significant amounts of ClO over Kiruna after the finalwarming in February 2001. In fact, column amounts of ClO were stillincreased in March 2001 when temperatures were about 20K above the PSC(Polar Stratospheric Clouds) threshold. At these temperatures, chlorineactivation due to heterogeneous processes on PSCs is not possible even inthe presence of strong lee wave effects. In order to discuss possiblereasons of this feature, time series of other chemical species will bepresented and discussed, too. Measurements of HF and COF2 indicatedthat vortex air was still observed in mid-March 2001. Since the time seriesof HNO3 column amounts do not give any evidence of a denitrificationlater than 11 February, chlorine activation persisting for several weeksafter the presence of PSCs due to denitrification is rather unlikely. Thephotolysis of ClONO2-rich air which had been formed at the end ofFebruary and beginning of March 2001 as well as chlorine activation due tothe presence of an unusual aerosol layer are discussed as possible causes ofthe increased ClO column amounts after the final warming

Observation of unusual chlorine activation by ground-based infrared and microwave spectroscopy in the late Arctic winter 2000/01