1. Department of Physics, Division of Atmospheric Sciences and Geophysics, PO Box 64, FI-00014 University of Helsinki, Finland;
2. Department of Chemistry, Kent State University, Kent, Ohio 44242, USA;
3. Department of Environmental Physics, University of Tartu, 18 Ülikooli St., 50090 Tartu, Estonia;
4. Finnish Meteorological Institute, Research and Development, Erik Palmenin aukio 1, PO Box 503, FI-00101, Helsinki, Finland