1. NILU Norwegian Institute for Air Research, P.O. Box 100, N-2007 Kjeller, Norway, WECC Wania Environmental Chemists Corp., 280 Simcoe Street, Suite 404, Toronto, Ontario, Canada M5T 2Y5, and Department of Chemical Engineering and Applied Chemistry, University of Toronto, 200 College Street, Toronto, Ontario, Canada M5S 3E5