Funding for this research was provided by:
National Natural Science Foundation of China (81770020 to XY)
Haze Special Project of Hebei Province Finance Department (2014-1257)
Text and Data Mining valid from 2019-06-14
Received: 30 December 2018
Accepted: 5 June 2019
First Online: 14 June 2019
Ethics approval and consent to participate
: Data were collected from administrative departments rather than human subjects. No personal private information was involved.
: Not applicable.
: The authors declare that they have no competing interests.