Zhengtao Shen1,2, Fei Jin3, David O'Connor1, Deyi Hou1. 1. School of Environment , Tsinghua University , Beijing 100084 , China. 2. Department of Earth and Atmospheric Sciences , University of Alberta , Edmonton T6G 2E3 , Canada. 3. School of Engineering , University of Glasgow , Glasgow G12 8QQ , United Kingdom.