1. Meteorological Research Department, Danish Meteorological Institute (DMI), Lyngbyvej 100, DK-2100 Copenhagen Ø, Denmark, Fachbereich CPhysikalische Chemie, Bergische Universität Wuppertal, Gaussstrasse 20, 42097 Wuppertal, Germany, and Department of Chemistry, H. C. Ørsted Institute, University of Copenhagen, DK-2100 Copenhagen Ø, Denmark