1. Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115, Department of Chemical Engineering, Environmental Engineering Program, Yale University, New Haven, Connecticut 06520-8286, Department of Chemistry, Clarkson University, Potsdam, New York 13699-5814, and Institute of Terrestrial Ecology, Swiss Federal Institute of Technology, ETH-ITO, Grabenstrasse 3, CH-8952 Schlieren, Switzerland