1. Institute of Space and Atmospheric Studies, University of Saskatchewan, Saskatoon, SK S7N 5E2, Canada.
2. Department of Environmental Sciences, Rutgers University, New Brunswick, NJ 08901, USA.
3. National Center for Atmospheric Research, Boulder, CO 80307, USA.
4. Department of Atmospheric Science, University of Wyoming, Laramie, WY 82071, USA.