1. Meteorological Service of Canada, 4905 Dufferin Street, Downsview, Ontario, M3H 5T4, Canada, Department of Chemical Engineering and Applied Chemistry, University of Toronto, 200 College Street, Toronto, Ontario, M5S 1A4, Canada, Swedish Environmental Research Institute, Box 47086, Gothenburg, SE-402 58, Sweden, and Finnish Meteorological Institute, Sahaajankayu 20E, FIN-00810, Helsinki, Finland