**5. Conclusions**

In this study, we explored the temporal and spatial trends of the concentrations of PM10 and PM2.5 in Xi'an, China, and the response to the meteorological factors and environmental regulation intensity. The concentrations of PM10 were higher than those of PM2.5, especially in spring and winter. The annual, seasonal, and monthly variations of PM10 and PM2.5 were striking. In terms of seasonal variations, the air quality is best in summer, and worst in winter. The spatial characteristics of air quality did not reveal any significant spatial differences due to the air monitoring stations being concentrated in the downtown area.

Among the meteorological factors, precipitation, relative humidity, and atmospheric temperature had impacts on atmospheric pollution. Conversely, wind speed affected concentration only in autumn and winter, and wind direction affected them only in spring and summer. In addition, the effects of environmental regulation intensity were prominent (i.e., environmental governance investment per unit of air pollutant emissions).

**Author Contributions:** Conceptualization, Y.T. and H.S.; methodology, Y.T. and L.Z.; software, Y.T. and Y.W.; validation, Y.T., L.Z. and H.S.; formal analysis, Y.T. and L.Z.; investigation, Y.T. and L.Z.; resources, Y.T., L.Z. and J.S.; data curation, Y.T., L.Z. and Y.W.; writing—original draft preparation, Y.T.; writing—review and editing, Y.T. and H.S.; visualization, Y.T. and L.Z.; supervision, J.S.; project administration, J.S.; funding acquisition, J.S. and H.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was funded by the Key Research and Development Program of Shaanxi, grant number 2019ZDLSF05-02; the Open Research Fund of Key Laboratory for Ecology and Environment of River Wetlands in Shaanxi Province, grant number SXSD202002.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

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