1. Cooperative Institute for Research in Environmental Sciences (CIRES) and Department of Chemistry and Biochemistry, 216 UCB, University of Colorado-Boulder, Boulder, Colorado 80309-0216, School of Earth, Atmospheric and Environmental Science, Main Building, Sackville Street, University of Manchester, Manchester M60 1QD, U.K., and Aerodyne Research Inc., Billerica, Massachusetts 01821-3976