1. Aeronomy Laboratory, National Oceanic and Atmospheric Administration, R/AL7, 325 Broadway, Boulder, Colorado 80305, Cooperative Institute for Research in Environmental Science, University of Colorado, Boulder, Colorado 80309, Atmospheric Chemistry Division, The National Center for Atmospheric Research, Boulder, Colorado 80303, and Rosenstiel School of Marine and Atmospheric Science, Division of Marine and Atmospheric Chemistry, 4600 Rickenbacker Causeway, Miami, Florida 33149