1. Division of Atmospheric Sciences, Department of Physical Sciences, University of Helsinki, P.O. Box 64, FI-00014 University of Helsinki, Finland, Department of Chemistry, University of Oulu, P.O. Box 3000, FI-90014 University of Oulu, Finland, and Laboratory of Inorganic Chemistry, Department of Chemistry, University of Helsinki, P.O. Box 55, FI-00014 University of Helsinki, Finland