1. Departments of Environmental Science and Engineering & Chemical Engineering, California Institute of Technology, MC 210-41, Pasadena, California 91125, Schools of Earth and Atmospheric Sciences & Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0340, Department of Mathematics, University of Houston, Houston, Texas 77204-3476, and Department of Chemistry, California Institute of Technology, Pasadena, California 91125