Cooperative Institute for Research in the Environmental Sciences, University of Colorado, Boulder, CO, USA

Department of Atmospheric and Oceanic Sciences, University of Colorado, Boulder, CO, USA

Cooperative Institute for Research in the Environmental Sciences, University of Colorado, Boulder, CO, USA

Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO, USA

Cooperative Institute for Research in the Environmental Sciences, University of Colorado, Boulder, CO, USA

Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO, USA

now at: Tofwerk AG, Thun, Switzerland

Department of Meteorology, Pennsylvania State University, University Park, PA, USA

Cooperative Institute for Research in the Environmental Sciences, University of Colorado, Boulder, CO, USA

Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO, USA

Chemical Sciences Division, NOAA Earth System Research Laboratory, Boulder, CO, USA

now at: Planning and Policy Program, Air Pollution Control Division, Colorado Department of Public Health and Environment, Denver, CO, USA

Cooperative Institute for Research in the Environmental Sciences, University of Colorado, Boulder, CO, USA

Chemical Sciences Division, NOAA Earth System Research Laboratory, Boulder, CO, USA

Cooperative Institute for Research in the Environmental Sciences, University of Colorado, Boulder, CO, USA

Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO, USA