1. National Oceanic and Atmospheric Administration Aeronomy Laboratory, Boulder, CO 80303, and Cooperative Institute for Research in Environmental Science, University of Colorado, Boulder, CO 80309 Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309
2. National Oceanic and Atmospheric Administration Aeronomy Laboratory, Boulder, CO 80303, and Cooperative Institute for Research in Environmental Science, University of Colorado, Boulder, CO 80309. A. R. Ravishankara is also affiliated with the Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309