1. Helsinki Institute of Physics; University of Helsinki; Helsinki Finland
2. Department of Physics; University of Helsinki; Helsinki Finland
3. Finnish Meteorological Institute; Helsinki Finland
4. Leibniz Institute for Tropospheric Research; Leipzig Germany
5. Institute for Atmospheric and Environmental Sciences; Goethe University Frankfurt; Frankfurt am Main Germany
6. Now at Department of Atmospheric Sciences; University of Washington; Seattle Washington USA
7. Division of Chemistry and Chemical Engineering; California Institute of Technology; Pasadena California USA
8. Center for Atmospheric Particle Studies; Carnegie Mellon University; Pittsburgh Pennsylvania USA
9. PH Department; CERN; Geneva Switzerland
10. CENTRA-SIM, F.C.U. Lisboa and U. Beira Interior; Covilhã Portugal
11. Laboratory of Atmospheric Chemistry; Paul Scherrer Institute; Villigen Switzerland
12. Now at Department Construction, Traffic and Environment, Canton of Aargau; Aarau Switzerland
13. Institute for Atmospheric and Climate Science; ETH Zurich; Zurich Switzerland
14. Institute for Ion and Applied Physics; University of Innsbruck; Innsbruck Austria
15. School of Earth and Environment; University of Leeds; Leeds UK
16. Atmospheric Research Centre of Eastern Finland; Finnish Meteorological Institute; Kuopio Finland
17. Aerosol Physics and Environmental Physics; University of Vienna; Vienna Austria
18. Université Versailles St-Quentin; Sorbonne Universités, UPMC Univ. Paris 06; CNRS/INSU, LATMOS-IPSL; Guyancourt France
19. Solar and Cosmic Ray Research Laboratory; Lebedev Physical Institute; Moscow Russia
20. Ionicon Analytik GmbH; Innsbruck Austria
21. Aerodyne Research Inc.; Billerica Massachusetts USA