1. Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, M5S 3H6, Canada, Air Quality Research Branch, Meteorological Service of Canada, Environment Canada, 4905 Dufferin Street, Toronto, Ontario, M3H 5T4, Canada, and Environmental Technology Centre, Environmental Protection Service, Environment Canada, 335 River Road, Ottawa, Ontario, K1A 0H3, Canada