1. Department of Physical Sciences, University of Helsinki, Post Office Box 64, FI-00014, Helsinki, Finland.
2. Institute of Environmental Physics, University of Tartu, Ülikooli 18, EE-50090, Tartu, Estonia.
3. Division of Environmental Health and Risk Management, School of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK.
4. Department of Physics, University of Kuopio and Finnish Meteorological Institute, Kuopio Unit, Post Office Box 1627, 70211 Kuopio, Finland.
5. Finnish Meteorological Institute, Post Office Box 503, FI-00101, Helsinki, Finland.