Atmospheric Chemistry and Physics www.atmos-chem-phys.net 1680-7316 1680-7324 6 4 2006 10.5194/acp-6-897-2006 http://www.atmos-chem-phys.net/6/897/2006/ http://www.atmos-chem-phys.net/6/897/2006/acp-6-897-2006.html http://www.atmos-chem-phys.net/6/897/2006/acp-6-897-2006.pdf 897 905 2006-03-20 Observation of unusual chlorine activation by ground-based infrared and microwave spectroscopy in the late Arctic winter 2000/01 T. Blumenstock G. Kopp F. Hase G. Hochschild S. Mikuteit U. Raffalski R. Ruhnke Institute of Meteorology and Climate Research, Forschungszentrum Karlsruhe and Univ. Karlsruhe, Karlsruhe, Germany Swedish Institute of Space Physics, Kiruna, Sweden During the Arctic winter of 2000/01, ground-based FTIR and millimetre-wave measurements revealed significant amounts of ClO over Kiruna after the final warming in February 2001. In fact, column amounts of ClO were still increased in March 2001 when temperatures were about 20K above the PSC (Polar Stratospheric Clouds) threshold. At these temperatures, chlorine activation due to heterogeneous processes on PSCs is not possible even in the presence of strong lee wave effects. In order to discuss possible reasons of this feature, time series of other chemical species will be presented and discussed, too. Measurements of HF and COF₂ indicated that vortex air was still observed in mid-March 2001. Since the time series of HNO₃ column amounts do not give any evidence of a denitrification later than 11 February, chlorine activation persisting for several weeks after the presence of PSCs due to denitrification is rather unlikely. The photolysis of ClONO₂-rich air which had been formed at the end of February and beginning of March 2001 as well as chlorine activation due to the presence of an unusual aerosol layer are discussed as possible causes of the increased ClO column amounts after the final warming.