3.1.1. Interannual Variation Characteristics

According to the Ambient Air Quality Standard (GB 3095-2012), the first-level standard limit of the maximum 8-h average concentrations of O3 per day is 100 μg/m3, while the secondary level standard is 160 μg/m3. On this basis, our study considered the secondary standard limit concentrations as the threshold, implying that when O3-8h is >160 μg/m3, the standard is exceeded. The annual statistics for O3-8h are provided in Table 1. As seen, the annual average concentrations of O3 from 2016 to 2020 were 97.84 ± 4.55, 107.96 ± 5.40, 110.28 ± 5.82, 108.05 ± 4.94, and 104.04 ± 8.87 μg/m3, respectively.


**Table 1.** Statistics of O3-8h data in the NCP from 2016 to 2020. (Mean ± sd).

By ranking the average O3 concentrations in the NCP, the following order was identified: 2018 > 2019 > 2017 > 2020 > 2016. O3 concentrations exhibited an increasing trend since 2016, peaked in 2018, and then gradually decreased. We suggest that the change in O3 concentrations after 2018 is closely related to the following environmental policies. In 2017, China issued "The 2017 work plan for air pollution prevention and control in Beijing, Tianjin, Hebei, and surrounding areas", which urged Beijing–Tianjin–Hebei and surrounding cities to adjust their industrial structure; banned small, scattered polluting enterprises; and actively promoted the substitution of clean energy, such as electricity and natural gas, for coal. These measures bolstered the efficiency of treatment of industrial air pollution, strengthened the control of motor vehicle emissions, and tightened NOx emissions control.

The analysis of the statistical yearbook data indicated that the total NOx emissions in the study area remarkably decreased in 2018. Namely, the total NOx emissions decreased by 21.34% compared with 2017. The total NOx emissions in 2019 and 2020 also exhibited a downward trend, with decreases of 26.57% and 48.14%, respectively, compared with 2017. In 2018, China also issued "The Action Plan for Comprehensive Treatment of Air Pollution in Autumn and Winter from 2018 to 2019 in Beijing, Tianjin, Hebei, and surrounding areas", which put forward special treatment of VOCs in key industries for the first time and achieved its first tangible results by 2019 [30]. Given the efficient control of the emissions of O3 precursors, the average O3 concentrations decreased. Notably, O3 concentrations declined in 2020. We argue that this concurrent reduction was attributed to the reduction in industrial production and anthropogenic activities under the measures enforced to control and prevent the spread of COVID-19 in China in 2020 [31].
