1. Lecturer, College of Fundamental Science, Beijing Polytechnic, Beijing 100176, China.
2. Professor, School of Environment, Beijing Normal Univ., Beijing 100875, China (corresponding author).
3. Professor, School of Environment, Beijing Normal Univ., Beijing 100875, China.
4. Research Fellow, Institute for Energy, Environment and Sustainable Communities, Univ. of Regina, Regina, SK, Canada S4S 0A2.
5. Associate Professor, School of Labor Economics, Capital Univ. of Economics and Business, Beijing 100070, China.
6. Associate Professor, School of Environmental and Municipal Engineering, Xi’an Univ. of Architecture and Technology, Xi’an 710055, China.