1. Department of Physics, University of Joensuu, P.O. Box 111, Fin-80101 Joensuu, Finland (K.-E.P., A.J.); Measurement and Sensor Laboratory, University of Oulu, Technology Park 127, Fin-87400 Kajaani, Finland (J.R.); Aerosol Technology Group, VTT Chemical Technology, P.O. Box 1401, Fin-02044 VTT, Finland (O.R., U.T., E.I.K.); and Observatory, University of Helsinki, P.O. Box 14, Fin-00014 University of Helsinki, Finland (K.L.)