1. Department of Atmospheric, Oceanic, and Space Sciences, 1520 Space Research Building, 2455 Hayward Street, University of Michigan, Ann Arbor, Michigan, 48109-2143, and Dipartimento di Chimica Generale ed Organica Applicata, Università di Torino, Corso Massimo D'Azeglio 48, I-10125 Torino, Italy