1. College of Urban and Environmental Sciences, Peking University, Bejing 100871, China. Phone: +86-10-62756137; Fax: +86-10-62756137; E-mail: yongping.li@urban.pku.edu.cn
2. Environmental Systems Engineering Program, Faculty of Engineering, University of Regina, Regina, Sask. S4S 0A2, Canada.
3. Chinese Research Academy of Environmental Science, Bejing Normal University, Bejing 100012-100875, China.