1. Laboratory of Atmospheric Chemistry, Paul Scherrer Institute, 5232 Villigen, Switzerland.
2. Tofwerk AG, 3600 Thun, Switzerland.
3. Department of Environmental Science, Stockholm University, Stockholm 11418, Sweden.
4. Institute of Meteorology and Climate Research, Karlsruhe Institute of Technology, 76344 Eggenstein-Leopoldshafen, Germany.
5. Institute for Atmospheric and Earth System Research, Faculty of Science, University of Helsinki, Helsinki 00014, Finland.