1. Institute for Ion Physics and Applied Physics, University of Innsbruck, Innsbruck, Austria
2. Faculty of Physics, University of Vienna, Wien, Austria
3. Department of Physics, University of Helsinki, Helsinki, Finland
4. Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, Colorado, USA
5. National Oceanic and Atmospheric Administration (NOAA), Earth System Research Laboratory, Chemical Sciences Division, Boulder, Colorado, USA