Air Quality Research Division, Environment Canada, 4905 Dufferin Street, Downsview, ON M3H 5T4, Canada

now at: Space Sciences Laboratory, University of California, Berkeley, CA, 94720, USA

Department of Geography and Program in Planning, University of Toronto, 100 St. George Street, Toronto, ON M5S 3G3, Canada

Air Quality Research Division, Environment Canada, 4905 Dufferin Street, Downsview, ON M3H 5T4, Canada

Air Quality Research Division, Environment Canada, 4905 Dufferin Street, Downsview, ON M3H 5T4, Canada

Department of Earth and Space Science and Engineering, York University, Toronto, ON M3J 1P3, Canada

now at: GESTAR, Universities Space Research Association, Greenbelt, MD 20771, USA

Department of Earth and Space Science and Engineering, York University, Toronto, ON M3J 1P3, Canada

Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 21250, USA

Air Quality Research Division, Environment Canada, 4905 Dufferin Street, Downsview, ON M3H 5T4, Canada

Air Quality Research Division, Environment Canada, 4905 Dufferin Street, Downsview, ON M3H 5T4, Canada