1. Division of Atmospheric Sciences, Department of Physical Sciences, University of Helsinki, P.O. Box FI-00014, Helsinki, Finland, and Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, California 92093-0356, and Geophysical Institute, University of AlaskaFairbanks, P.O. Box 757320, Fairbanks, Alaska 99775-7320