University of California, Department of Atmospheric and Oceanic Sciences, Los Angeles, CA, USA

now at: UPMC Univ. Paris 06, Université Versailles St-Quentin, CNRS/INSU, UMR 8190, LATMOS-IPSL, Paris, France

University of California, Department of Atmospheric and Oceanic Sciences, Los Angeles, CA, USA

Earth and Atmospheric Sciences Department, University of Houston, Houston, TX, USA

School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia, USA

Air Quality Research Division, Science and Technology Branch, Environment Canada, Toronto, Ontario, Canada

Department of Earth and Space Science and Engineering, York University, Toronto, Ontario, Canada

Institute for the Study of Earth, Oceans and Space, University of New Hampshire, Durham, New Hampshire, USA

School of Environmental Sciences, University of East Anglia, Norwich, UK