**1. Introduction**

Photochemical pollution is one of the major types of atmospheric environmental pollution in China. With rapid urbanization and industrialization, the problem of ozone pollution has recently become acute, thereby attracting the attention of atmospheric and environmental researchers [1,2]. As a secondary photochemical pollutant, near-ground ozone (O3) is mainly formed by photochemical reactions of precursors such as nitrogen oxides (NOx) and volatile organic compounds (VOCs) [3,4]. Unlike particulate pollution (e.g., aerosol pollution), O3 pollution is insidious; it occurs even in sunny weather, and its detection and quantification are challenging. These challenges must be alleviated as near-ground O3 concentrations, exceeding a certain threshold, can cause a series of

**Citation:** Wang, X.; Zhao, W.; Zhang, T.; Qiu, Y.; Ma, P.; Li, L.; Wang, L.; Wang, M.; Zheng, D.; Zhao, W. Analysis of the Characteristics of Ozone Pollution in the North China Plain from 2016 to 2020. *Atmosphere* **2022**, *13*, 715. https://doi.org/ 10.3390/atmos13050715

Academic Editors: Duanyang Liu, Kai Qin, Honglei Wang and Begoña Artíñano

Received: 19 April 2022 Accepted: 27 April 2022 Published: 30 April 2022

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adverse effects on human health, the ecological environment and crops [5,6]. At present, O3 has replaced fine particulate matter as the primary pollutant, inherently affecting the number of good air days in spring and summer [7,8]. O3 concentrations have already directly affected the ranking of cities based on urban air quality. Therefore, identifying the pollution characteristics and the related drivers is quintessential for formulating air pollution prevention and control strategies.

In the past, scholars usually studied O3 based on satellite data due to the lack of large-scale and long-term monitoring networks. For instance, Liu et al. [9] used OMI satellite data to analyze the trend of O3 changes in east-central China from 2005 to 2014. Zhang et al. [10] used satellite observations of tropospheric O3 data to assess the spatial distribution of O3 in China. Since 2013, the Ministry of Environmental Protection has been monitoring the pollutant on a large scale in China. Many studies are exploring O3 concentrations and distribution in various regions of China based on O3 data from monitoring stations, yielding a multitude of results. For instance, Meng [11] conducted O3 monitoring in 74 Chinese cities over 3 consecutive years. In particular, it has been previously reported that O3 concentrations had been increasing annually, and O3 pollution exhibited distinct regional patterns. Zhang et al. [12] obtained O3 concentrations data for the Chengdu-Chongqing urban agglomeration from 2015 to 2019 using monitoring station data to discuss the spatio-temporal variation patterns of O3 in the study region. In addition, the chemical transport model has been implemented to analyze O3 formation and transport. For instance, Li et al. [13] used the WRF/CMAQ model to simulate the response of PM2.5 and O3 to emission reduction policies in the Yangtze River Delta region. Guo et al. [14] examined the characteristics of O3 pollution, elucidating its sensitivity to emissions using isolines. They showed that NOx drives O3 concentrations in most areas of China, hinting that a reduction in NOx can substantially reduce the concentration of O3. Fundamentally, O3 pollution is not only affected by anthropogenic emissions but also by meteorological factors. On a meteorological scale, Wang et al. [15] argued that O3 concentrations were closely associated with meteorological conditions. High concentrations of O3 pollution were usually observed under strong solar radiation and low wind speed. Li et al. [16] concluded that O3 concentrations negatively correlated with relative humidity by analyzing the correlation between O3 and meteorological factors in 74 Chinese cities. Cui et al. [17] integrated O3 concentrations with dynamic meteorological factors in the Beijing–Tianjin–Hebei region and found that the O3 concentrations positively correlated with temperature and evaporation; however, there was a distinct regional difference in the wind direction and the wind speed.

The NCP is one of the most densely populated areas in the world. The Beijing–Tianjin– Hebei region has a large population and represents China's political and cultural center, where the air pollution issue has become increasingly urgent [18]. The Jiangsu–Anhui– Shandong–Henan region in the southern part of the NCP is the connecting belt of air pollution in the Beijing–Tianjin–Hebei region and the Yangtze River Delta [19]. Only a few studies described the O3 pollution in the above areas, and the progress toward improving air quality is slow. With its rapid economic development, the NCP suffers from severe air pollution due to anthropogenic emissions, accumulation, and regional transmission [20]. Thus far, the lack of long-term research on the spatio-temporal characteristics of O3 constrains the progress in this regard. To address this gap, this study investigated the spatio-temporal distribution and drivers of O3 pollution in the NCP using data on O3 concentrations (from 230 air monitoring stations), meteorology (from 54 meteorological stations), and social factors between 2016 and 2020 to provide a reference for the management of O3 pollution.

#### **2. Materials and Methods**
