1. Institute for Atmospheric and Climate Science; ETH Zurich; Zurich Switzerland
2. School of Engineering and Applied Science; Harvard University; Cambridge Massachusetts USA
3. Physikalisch-Meteorologisches Observatorium Davos and World Radiation Center; Davos Switzerland
4. Now at Swiss Federal Laboratories for Materials Testing and Research (Empa); Dübendorf Switzerland
5. Institute for Atmospheric and Environmental Sciences; Goethe University; Frankfurt Germany