1. Environment Canada: AES, 4905 Dufferin Street, Downsview, Ontario M3H 5T4, Canada, Norwegian Institute for Air Research (NILU), P.O. Box 100, N-2007 Kjeller, Norway, Swedish Environmental Research Institute (IVL), P.O. Box 47086, S-402 58 Göteborg, Sweden, and Tekran Inc., 132 Railside Road, Unit 1, Toronto, Ontario M3A 1A3, Canada