1. Department of Chemistry, University of Colorado Boulder, Boulder, Colorado 80309, United States
2. Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, Boulder, Colorado 80309, United States
3. Department of Chemistry, Reed College, Portland, Oregon 97202, United States
4. Aerodyne Research Inc., Billerica, Massachusetts 01821, United States
5. Chemical Sciences Laboratory, National Oceanic & Atmospheric Administration, Boulder, Colorado 80305, United States