1. Atmospheric Optics Group, University of Valladolid, Valladolid, Spain, and Meteorological Institute, University of Munich, Munich, Germany
2. Atmospheric Optics Group, University of Valladolid, Valladolid, Spain
3. Atmospheric Sounding Station El Arenosillo, INTA, Mazagón, Spain
4. Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado