Finnish Meteorological Institute, P.O. Box 503, Helsinki, Finland

Department of Physical Sciences, P.O. Box 64, 00014 University of Helsinki, Finland

Geophysical Institute, University of Alaska, P.O. Box 757320, Fairbanks, AK 99775-7320, USA

Department of Physical Sciences, P.O. Box 64, 00014 University of Helsinki, Finland

Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, CA 92093-0356, USA