1. Institute for Tropospheric Research, Permoserstr. 15, D-04303 Leipzig, Germany;
2. Department of Atmospheric Science, University of Washington, Seattle, WA 98195, USA;
3. Department of Nuclear Physics, University of Lund, Solvegatan 14, S-22362 Lund, Sweden;
4. Department of Physics, University of Helsinki, SF-00170 Helsinki, Finland;
5. Department of Meteorology, Stockholm University, S-10691 Stockholm, Sweden