1. Doctor, Chinese Research Academy of Environmental Sciences, Beijing, China, 100012.
2. Professor, Chinese Research Academy of Environmental Sciences, Beijing, China, 100012 (corresponding author).
3. Doctor, School of Environment, Beijing Normal Univ., Beijing, China, 100875.
4. Professor, Centre for Studies in Energy and Environment, Univ. of Regina, Regina, SK, Canada S4S 0A2.