**4. Conclusions**

In early 2020, the outbreak of the COVID-19 pandemic occurred. In order to control the spread of the pandemic, the Jiangsu provincial government took different levels of public health emergency responses. This paper studies the variations of air quality in response to different levels of PPC measures. In 2018, the Jiangsu provincial government issued the "Implementation Plan of Jiangsu Province's Three-Year Action Plan for Cleaner Air". Since then, the air quality has improved significantly. Therefore, this study focuses on analyzing the air quality during 2018–2020. By analyzing the relationships of air pollution data with PPC levels and meteorological conditions, the major conclusions are as follows.

The implementation of COVID-19 related PPC policies directly affected the concentrations of air pollutants. For AQI and pollutants of NO2, CO, PM10 and PM2.5, as long as the countermeasures were taken, the AQI and pollutant concentrations were reduced by

varied degrees regardless of the levels of PPC. However, there was no significant decrease in SO2 and O3 concentration during the period in response to PPC measures.

The air pollution reduction was closely related to the levels of PPC. In general, the higher the level of PPC measures, the greater was the reduction of air pollutant concentrations. Compared with the cities in northern China, the air quality in Jiangsu was more likely to be affected by restrictive policies, but the sensitivity of air quality to restrictive policies was slightly lower than that in the Guangdong–Hong Kong–Macao Greater Bay Area. This may be related to urban development and industrial structure in different regions.

NO2 was the pollutant most sensitive to PPC policies. The higher the level of PPC, the greater was the reduction of NO2 concentration. The CO and atmospheric particulate matter concentrations were most significantly reduced during the periods with first- and second-level PPC measures. With the recovery of production and living order, the pollutant concentrations gradually approached the average value in the same period of previous years.

There were great differences in the response speeds of urban air quality to the levels of PPC among cities with different levels of economic development and industrial structures. Southern Jiangsu, which has a higher level of economic development and is dominated by secondary and tertiary industries, had a faster response speed and a stronger responsiveness, indicating its high economic vitality.

Due to the complexity of air pollution change, in which many factors may play a key role in this process, this paper still has some limitations, which need to be discovered in further investigation.

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

**Funding:** This research was funded by the General Projects of Jiangsu Meteorological Bureau (KM202204), the Youth Fund of Jiangsu Meteorological Bureau (KQ202005), the Innovation Fund of the Public Meteorological Service Center of China Meteorological Administration (M2021010), the Open Research Fund of China Meteorological Administration Key Laboratory of Transportation Meteorology (BJG202206) and the East China Regional Meteorological Science and Technology Collaborative Innovation Fund Cooperation Project (QYHZ202109).

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

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available upon request from the corresponding author.

**Acknowledgments:** We thank Jiangsu Environmental Monitoring Center for the provision of air quality data. We are also very grateful to Shen Jinyou of Nanjing University of Science and Technology, Zhang Zhiwei of Jiangsu Meteorological Society and Chen Hao of Jiangsu Meteorological Observatory for their guidance and help to this paper.

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

#### **References**

