1. Atmospheric Sciences Program, Michigan Technological University, Houghton, MI 49931, USA.
2. Institute for Atmospheric Physics, Johannes Gutenberg University Mainz, 55128 Mainz, Germany.
3. Aerosol and Cloud Physics Group, Max Planck Institute for Chemistry, 55128 Mainz, Germany.
4. Department of Physics, Michigan Technological University, Houghton, MI 49931, USA.
5. National Center for Atmospheric Research, Boulder, CO 80307, USA.