1. Norwegian Institute for Air Research (NILU), Instituttveien 18, P.O. Box 100, N-2027, Kjeller, Norway, Department of Chemistry, University of Oslo, P.O. Box 1033, Oslo, Norway, Institute for Coastal Research/Physical and Chemical Analysis, G.K.S.S. Research Centre Geesthacht, D-21502 Geesthacht, Germany, and Laboratoire de Glaciologie et Géophysique de l'Environnement du C.N.R.S., 54 rue Molière, BP 96, 38402 Saint Martin d'Hères, France