1. Department of Applied Physics, University of Eastern Finland, Kuopio, Finland
2. Department of Environmental and Biological Sciences, University of Eastern Finland, Kuopio, Finland
3. Department of Chemistry, University of Eastern Finland, P. O. Box 111, 80101 Joensuu, Finland
4. Finnish Meteorological Institute, Kuopio, Finland
5. Department of Mathematics and Statistics, University of Jyvaskyla, Jyvaskyla, Finland