1. Finnish Meteorological Institute, Air Quality Research, Sahaajankatu 20E, FIN-00810 Helsinki, Finland;
2. Department of Meteorology, Arrhenius Laboratory, Stockholm University, S-106 91, Stockholm, Sweden;
3. Retired, earlier in Air Quality Measurements and Analysis Research Division, Atmospheric Environment Service, Environment Canada, 4905 DuVerin Street, Downsview, Ontario M3H 5T 4, Canada