1. INAR–Institute for Atmospheric and Earth System Research, P.O. Box 64, 00014 University of Helsinki, Finland.
2. Finnish Meteorological Institute, Erik Palménin aukio 1, 00560 Helsinki, Finland.
3. Laboratory of Environmental Physics, Institute of Physics, University of Tartu, Tartu 50090, Estonia.
4. CERN, CH1211 Geneva, Switzerland.
5. Aerodyne Research Inc., Billerica, MA 01821, USA.
6. Goethe University Frankfurt, Institute for Atmospheric and Environmental Sciences, 60438 Frankfurt am Main, Germany.