1. Department of Geoscience and Remote Sensing, Delft University of Technology, P.O. Box 5048, 2600 GA Delft, Netherlands.
2. Department of Environmental Sciences, Wageningen University, P.O. Box 47, 6700 AA Wageningen, Netherlands.
3. Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, Boulder, CO 80309, USA.
4. NOAA Chemical Sciences Laboratory, 325 Broadway, Boulder, CO 80305, USA.
5. Institut für Meteorologie, Ludwig-Maximilians-Universität, Theresienstrasse 37, 80333 München, Germany.
6. School of Earth and Environment, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UK.