3.1.2. Seasonal Variations

The seasonal variations in the O3 concentrations from 2016 to 2020 are shown in Figure 2. As shown, the variation trend of O3 average concentrations was approximately the same in different years in the NCP. Moreover, O3 concentrations gradually increased during spring, peaked in summer (132.53–154.20 μg/m3), and decreased gradually in autumn and winter, reaching the lowest value in winter (53.81–61.75 μg/m3). The seasonal variation characteristics of O3 concentrations were ranked in the following order: summer > spring > autumn > winter. Overall, O3 pollution mainly developed in summer, as higher concentrations were identified in spring and summer, while lower concentrations were observed in autumn and winter.

The changes in O3 concentrations are fundamentally closely related to meteorological conditions. The photochemical reaction process can be formalized by Equations (3)–(5) (see below) [32,33]. The main process is NO oxidation by atmospheric oxidants to generate NO2, whose photolysis generates O3. Temperature and solar radiation play important roles in the formation and transport of O3. For instance, Xu, et al. [34] reported that O3 concentrations positively correlated with temperature and the intensity of solar radiation because intense solar radiation and high temperature promoted the photochemical reaction process. We found that the temperature in the NCP gradually increased from spring, while summer was characterized by the strongest solar radiation and highest temperature throughout the year. Therefore, this period was the most conducive for O3 formation, whose concentrations reached the highest level. In autumn and winter, the solar radiation

intensity and irradiation time gradually weakened. Thus, O3 concentrations gradually dwindled, reaching the lowest value in winter. We identified only a slight difference in temperature between spring and autumn. However, O3 concentrations in spring were significantly higher compared with those in autumn due to the influence of more precursors, drought, and less precipitation in spring:

$$\rm NO + O\_3 \rightarrow NO\_2 + O\_2 \tag{3}$$

$$\text{NO}\_2 + \text{hv} \rightarrow \text{NO} + \text{O} \tag{4}$$

$$\text{O} + \text{O}\_2 \rightarrow \text{O}\_3 \tag{5}$$

**Figure 2.** Seasonal changes in O3 concentrations in the NCP from 2016 to 2020.
