Department of Chemistry, University of California Berkeley, Berkeley, CA, USA

now at: Chemical Sciences Division, Earth Systems Research Laboratory, National Oceanic and Atmospheric Administration, Boulder, CO, USA

Department of Chemistry, University of California Berkeley, Berkeley, CA, USA

now at: Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO, USA

Department of Chemistry, University of California Berkeley, Berkeley, CA, USA

Atmospheric Chemistry Division, National Center for Atmospheric Research, Boulder, CO, USA

Atmospheric Chemistry Division, National Center for Atmospheric Research, Boulder, CO, USA

School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA, USA

School of Engineering and Applied Science, Harvard University, Cambridge, MA, USA

Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA, USA

Division of Geology and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA

Atmospheric Chemistry Division, National Center for Atmospheric Research, Boulder, CO, USA

Institut für Ionenphysik & Angewandte Physik, University of Innsbruck, Innsbruck, Austria

Department of Earth and Planetary Sciences, University of California Berkeley, Berkeley, CA, USA

Department of Chemistry, University of California Berkeley, Berkeley, CA, USA