1. Centre for Atmospheric Research Experiments, Environment Canada, 6248 Eighth Line, Egbert ON, L0L 1N0, Canada, Chemical Engineering and Applied Chemistry, University of Toronto, 200 College St., Toronto, Ontario, M5S 3E5 Canada, and Norwegian Institute for Air Research, Polar Environmental Centre, No-9296 Tromsø, Norway; Department of Environmental Assessment, Swedish University of Agricultural Sciences, P.O. Box 7050, SE-705-07, Uppsala, Sweden