1. Department of Physics, University of Helsinki, P.O. Box 64, 00014 Helsinki, Finland
2. Department of Chemistry, University of Helsinki, P.O. Box 55, 00014 Helsinki, Finland
3. Department of Atmospheric
Sciences, University of Washington, Seattle, Washington 98195, United States
4. Department of Chemistry, University of Copenhagen, Universitetsparken
5, 2100 Copenhagen
Ø, Denmark
5. Department of Applied Physics, University of Eastern Finland, 70211 Kuopio, Finland
6. Aerodyne Research Inc., 45 Manning Road, Billerica, Massachusetts 01821, United States