1. Department of Physics, P.O. Box 64, FIN-00014 University of Helsinki, Finland;
2. Finnish Institute of Occupational Health, Topeliuksenkatu 41 a A, FIN-00250 Helsinki, Finland
3. Department of Meteorology, Stockholm University, S-10691 Stockholm, Sweden;
4. Fraunhofer-Institute for Atmospheric Environmental Research, Kreuzeckbahnstrasse 19, D-82467 Garmisch-Partenkirchen, Germany;
5. Institute of Spectrochemistry and Applied Spectroscopy (ISAS), P.O. Box 101352, D-44013 Dortmund, Germany;
6. Stockholm University (ITML), Air Pollution Laboratory, Institute of Applied Environmental Research, Stockholm University, S-10691 Stockholm, Sweden;
7. Finnish Meteorological Institute (FMI), Air Quality Research, Sahaajankatu 20 F, FIN-00810 Helsinki, Finland;
8. Department of Applied Physics, University of Kuopio, P.O. Box 1627, FIN-70211 Kuopio, Finland;
9. University of Sunderland, Centre for Marine and Atmospheric Sciences, School of the Environment, Benedict Building, St. George’s Way, Sunderland, SR2 7BW, UK;