3.2.3. Seasonal Variation of O3 Spatial Distribution

The O3 concentrations in the NCP significantly changed during all seasons from 2016 to 2020. Furthermore, the seasonal-scale spatial distribution of O3 is shown in Figure 7. In spring, the cities in the northern part of the NCP (such as Beijing and Tianjin) were characterized by the lowest O3 concentrations, with the lowest value of 111.43 μg/m3. O3 concentrations in the southern Anhui Province were also low. The high O3 concentrations areas were mainly concentrated in the southwest part of the Shandong Province, junction cities between the west of Shandong Province and the southeast of Hebei Province, with a maximum of 134.83 μg/m3. In summer, O3 concentrations in the study area were high, with widely distributed high-concentration areas. The high-value areas of O3 were mainly clustered in the Beijing–Tianjin–Hebei urban agglomeration, western Shandong Province, and northern Henan Province, with a maximum of 169.90 μg/m3. O3 concentrations in the Shandong Peninsula, Anhui, and Jiangsu provinces to the south of the NCP were relatively low, with the lowest value of 97.55 μg/m3. In autumn, O3 concentrations distribution exhibited the low concentrations pattern in the north and the high concentrations in the south. O3 concentrations in the Beijing–Tianjin–Hebei urban agglomeration were the lowest, with the lowest value of 64.13 μg/m3, while in southern Shandong Province and Anhui Province, they were higher, with a maximum of 116.90 μg/m3. In winter, O3 concentrations in the study area were lower than those in other seasons. Moreover, O3 concentration

generally exhibited a trend of low concentrations in the north and high concentrations in the south. From a regional perspective, O3 concentrations in eastern coastal cities were the highest, with a maximum of 75.63 μg/m3, while O3 concentrations in the Beijing–Tianjin– Hebei urban agglomeration were low, with the lowest value of 44.78 μg/m3. Overall, although the regional distribution range of high O3 concentrations exhibited a certain degree of seasonal variability, the central and western parts of Shandong Province were always characterized by high concentrations. This pattern is attributed to the developed industrial structure of the heavy industry in Shandong Province.

**Figure 7.** Seasonal spatial distribution map of O3 in the NCP from 2016 to 2020: (**a**) Spatial distribution of O3 in spring, (**b**) Spatial distribution of O3 in summer, (**c**) Spatial distribution of O3 in autumn, (**d**) Spatial distribution of O3 in winter.

If we consider only the impact of precursor emissions, the spatial distribution of O3 in different seasons reveals the same trend. However, due to the differences in the spatial distribution of O3 in the four seasons, meteorological conditions must be considered to analyze their impact on the regional differences in O3. The variations in O3 concentrations and meteorological conditions are shown in Figure 8. As shown, the annual variation trends in O3 concentrations, average temperature, and average air pressure in the NCP were nearly the same from 2016 to 2020. To study the relationship between meteorological conditions and O3, the daily O3 concentrations and excessive rate were statistically analyzed under different meteorological conditions using 2020 as a typical study year. As shown in Figure 9, when the temperature was greater than 30 ◦C, O3 concentrations were the highest, and the excessive rate reached 32%; when the temperature was less than 0 ◦C, O3 concentrations were 54.47 μg/m3, and the excessive rate was 0%. Particularly, the higher the temperature, the greater the O3 concentration and the excessive rate. When the air pressure was in

the range of 1000–1013.25 hPa, O3 concentrations were high, with a maximum value of 116.52 μg/m3, and the excessive rate reached 15%. When the air pressure was greater than 1020 hPa and less than 990 hPa, the O3 concentrations were lower, and the excessive rate was 0% at this time. In general, the O3 concentrations and excessive rate were rather higher under low air pressure meteorological conditions (Pa < 1013.25 hpa standard atmospheric pressure). This trend was driven by the following phenomenon: When the near-ground air pressure is low, O3 is horizontally transported from the surroundings to the low-pressure area. The O3 and its precursors converge in the low-pressure area, increasing the O3 concentration. At the high pressure near the ground, O3 diffuses into the surroundings. Moreover, the lower the air pressure, the higher the O3 concentration [40] and vice versa.

**Figure 8.** Variation in O3 concentrations and meteorological conditions in the NCP from 2016 to 2020.

**Figure 9.** (**a**) O3 concentrations and excessive rate under different temperature conditions, (**b**) O3 concentrations and excessive rate under different air pressure conditions.

To this end, we analyzed the meteorological conditions in the study area to elucidate the drivers behind the regional differences in O3 concentrations in different seasons. We found that, in spring, O3 concentrations were low in large areas, particularly in the northern, southern, and eastern coastal cities of the NCP. However, O3 concentrations were high in the central and western cities of the Shandong Province. At this time, there was only a minor difference in the average temperature and other meteorological conditions between the provinces in the study area. The regional difference was driven by abundant heavy industrial cities in central and southwest Shandong and due to large energy consumption. This, in turn, enhanced the emission of pollutants. At the same time, the central area of Shandong is a mountainous and hilly area, where the complex terrain is conducive to the accumulation of pollutants. Therefore, being affected by the industrial structure and topography of this region, O3 pollution has been more severe compared to other cities [41].

We found that, in summer, the concentration of O3 in the Beijing–Tianjin–Hebei urban agglomeration and its surrounding areas was high. On the one hand, as the core economic zone of China, the Beijing–Tianjin–Hebei urban agglomeration stands out with a large population density, rapid industrial development, high economic development [42,43], strong urban heat island effect, and higher temperature compared with other regions. Note that high temperatures are conducive to the generation of O3. On the other hand, some previous studies [44,45] argued that given the complex and regional characteristics of the Beijing–Tianjin–Hebei region, the regional transport of pollutants is prevented. In summer, the O3 concentrations in the Beijing–Tianjin–Hebei region and cities such as Jinan, Zibo, and Liaocheng in western Shandong Province are relatively high. At the same time, the southeast wind prevails in the northern cities, and the high O3 concentrations areas are more likely to spread to the northwest, which may lead to the formation of a high O3 concentrations cluster northwest of the NCP.

In autumn, given the difference in latitude, cities in the southern part of the study area experienced strong solar radiation and high temperatures, which provided favorable conditions for the generation of O3. The concentrations of O3 were generally high in the south and low in the north. In winter, the northern part of the study area was closer to the high-pressure center in Asia, and the weather conditions were stable. O3 generation is affected by the latitude and heat; the temperature and O3 generation rate in the northern part were lower than those in the southern region, and the O3 concentrations reached their lowest in a year. However, similar spatial distribution characteristics (low in the north and high in the south) were still evident. Geographically, the coastal areas of the Shandong Peninsula and the Yancheng City of the Jiangsu Province were characterized by the highest O3 concentrations, possibly caused by external transport and ship-driven pollution [46].
