1. Ph.D. Student, College of Environmental Sciences, Peking Univ., Beijing 100871, China. E-mail: yongliu@pku.edu.cn
2. Professor, College of Environmental Sciences, Peking Univ., Beijing 100871, China (corresponding author). E-mail: hcguo@pku.edu.cn
3. Ph.D. Student, College of Environmental Sciences, Peking Univ., Beijing 100871, China. E-mail: jardon.zhou@gmail.com
4. Ph.D. Student, Faculty of Engineering, Univ. of Regina, Regina, Sask., Canada S4S 0A2. E-mail: xiaoshengqin@env.uregina.ca
5. Ph.D. Student, College of Environmental Sciences, Peking Univ., Beijing 100871, China. E-mail: khuang@pku.edu.cn