**5. Conclusions**

In the panel data used in this study, the variables were all cointegrated. The Granger causality test results showed that economic growth, industrialization and urbanization were all long-term causalities of the changes of PM2.5 concentrations, and economic growth and industrialization also significantly affected changes in PM2.5 concentrations in the short term. The results of variance decomposition and the impulse response analysis showed that industrialization was the most important variable a ffecting PM2.5 concentrations. However, controlling only one socioeconomic factor to slow pollution growth is not feasible because there are either long-term or short-term and either bidirectional or unidirectional relationships among them. Though Liaoning Province has shown characteristics of economic and industrial transformation, it is also necessary to formulate more targeted policies to solve the problem of regional air pollution.

**Author Contributions:** Conceptualization, Y.H. and M.L.; formal analysis, T.S.; funding acquisition, Y.H; methodology, T.S., C.Z. and C.L. (Chunlin Li); project administration, Y.H.; validation, M.L. and C.L. (Chong Liu); writing—original draft, T.S.; writing—review and editing, M.L. and C.L. (Chunlin Li). All authors have read and agreed to the published version of the manuscript.

**Funding:** This study was financially supported by the National Natural Science Foundation of China (No. 41730647, 41671184 and 41671185).

**Acknowledgments:** The authors would like to thank Aaron van Donkelaar and Randall Martin of the Atmospheric Composition Analysis Group of Dalhousie University in Canada who o ffered the data used in this research.

**Conflicts of Interest:** The authors declare no conflict of interest.
