1. Key Laboratory of Aerosol Chemistry and Physics State Key Laboratory of Loess and Quaternary Geology (SKLLQG) Institute of Earth Environment Chinese Academy of Sciences Xi'an 710061 P. R. China
2. Center of Excellence in Quaternary Science and Global Change Chinese Academy of Sciences Xi'an 710061 P. R. China
3. University of Chinese Academy of Sciences Beijing 100049 P. R. China
4. School of Chemistry and Materials Science University of Science and Technology of China Hefei 230026 P. R. China
5. Institute of Atmospheric Physics Chinese Academy of Sciences Beijing 100190 P. R. China
6. School of Physics and Information Technology Shaanxi Normal University Xi'an 710062 P. R. China
7. Department of Science and Environmental Studies The Education University of Hong Kong Hong Kong P. R. China