**7. Summary and Recommendations**

High ozone concentrations are harmful to humans and the ecological environment, and atmospheric ozone pollution is becoming a major environmental problem that has been plaguing the economic development in China. There are significant regional differences in the distribution of O3-max-8 h in China. The overall level of O3-max-8 h in the NCP is higher than that of other regions. Due to economic development and dense population, the eastern region has a higher level of O3-max-8 h, and the high-value areas are distributed in flakes and bands. The high O3-max-8 h areas are mainly concentrated in the Beijing– Tianjin–Hebei region, the Yangtze River Delta and the Pearl River Delta. In general, the tropospheric ozone concentration is higher in summer while lower in winter, and higher in coastal areas and lower inland. The concentration of tropospheric O3 is related to its precursors, air temperature, solar radiation, air humidity, wind speed and the boundarylayer height. There is a highly nonlinear relationship between O3 and its precursors (NOx and VOCs), and the influencing mechanisms of NOx and VOCs in different regions are quite different. Generally speaking, the rural area is controlled by NOx and the urban area is controlled by VOCs. In addition, the influence of meteorological factors on tropospheric ozone concentration also has large regional differences. The control of atmospheric O3 should, first of all, be at a specific location. Secondly, the control of atmospheric O3 should be time-dependent, and largely depends on the meteorological conditions. Finally, in all cases, the optimal VOCs/NOx ratio for controlling emissions should be studied in detail. Through the coordinated control of O3 and other air pollution, such as PM2.5/PM10, we can realize people's high expectations of the air environment.

**Author Contributions:** Conceptualization, R.Y.; writing—original draft preparation, Y.L., J.Z.; writing—review and editing, R.Y.,Y.L., C.C. and Y.D.; supervision, R.Y.; project administration, R.Y. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Natural Science Foundation of China (No. 21777049) and the Graduate Tutor Team Project in Fujian Province, China (418-50300012). The APC was funded by both of the projects.

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

**Informed Consent Statement:** Not applicable.

**Acknowledgments:** We acknowledge Gongren Hu and Yu Yan for some comments on the manuscript as well as the anonymous reviewers and editors for helpful comments on the manuscript.

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