JOHN H. SEINFELD, PHD, is Louis E. Nohl Professor at the California Institute of Technology. Dr. Seinfeld is a member of the U.S. National Academy of Engineering and the American Academy of Arts and Sciences, and a Fellow of the American Association for the Advancement of Science, the American Institute of Chemical Engineers, and the American Geophysical Union.

SPYROS N. PANDIS, PHD, is Professor of Chemical Engineering at the University of Patras, Greece, and EliasResearch Professor of Chemical Engineering and Engineering & Public Policy at Carnegie Mellon University.