1. Department of Physical Sciences, University of Helsinki, P.O. Box 64, FI-00014 University of Helsinki, Finland, Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen Ø, Denmark, Finnish Meteorological Institute, Erik Palménin Aukio 1, P.O. Box 503, FI-00101 Helsinki, Finland, and Department of Applied Physics, University of Kuopio, and Finnish Meteorological Institute, P.O. Box 1627, FI-70211 Kuopio, Finland