1. Department of Environmental Science and Engineering, and Department of Chemical Engineering, California Institute of Technology, Mail Code 210-41, 1200 East California Boulevard, Pasadena, California 91125, Aerodyne Research, Inc., 45 Manning Road, Billerica, Massachusetts 01821, and Department of Chemistry & Biochemistry and CIRES, University of Colorado−Boulder, UCB 216, Boulder, Colorado 80309