**1. Introduction**

Ozone (O3) is one of the gas components in the atmosphere. More than 90% of all O3 is concentrated in the stratosphere, and less than 10% is distributed in the troposphere [1]. As is known, the ozone layer in the stratosphere can protect life on Earth by absorbing most of ultraviolet radiation from the sun. However, O3 in the troposphere is a secondary pollutant, which is the main driving force of atmospheric photochemical reactions and is one of the key factors in controlling atmospheric pollution [2]. As a characteristic product of photochemical smog, O3 is a strong oxidant that can threaten human health and vegetation. Excessive inhalation of O3 may cause respiratory infections, neurotoxic reactions or directly damage the human immune system [3]. High concentrations of O3 can inhibit the growth of plants, resulting in a reduction in crop yields [4]. Furthermore, tropospheric O3 is one of the most important greenhouse gases, and it may contribute to climate change. Therefore, tropospheric ozone pollution has attracted more and more attention, especially in recent years.

There are two main sources of tropospheric O3. One is from stratosphere through the stratospheric–tropospheric exchange. In the stratosphere, oxygen molecules may absorb ultraviolet radiation with a wavelength of less than 240 nm and decompose into oxygen atoms, which can combine with oxygen molecules to form O3, and this O3 may be transmitted down to the troposphere and become the source of tropospheric O3 [5]. The prospective O3 transmission from stratosphere to troposphere was studied using the most advanced chemical–climate model, and the results showed that the global average annual mass fluxes of stratospheric O3 into the troposphere were expected to increase by 53% from 2000 to 2100 [6]. It was reported that O3 in the mid-latitude stratosphere would intrude into the lower troposphere due to the convective activities over the tropical Pacific [7]. Similar stratosphere–troposphere interactions have been observed over the

**Citation:** Yu, R.; Lin, Y.; Zou, J.; Dan, Y.; Cheng, C. Review on Atmospheric Ozone Pollution in China: Formation, Spatiotemporal Distribution, Precursors and Affecting Factors. *Atmosphere* **2021**, *12*, 1675. https:// doi.org/10.3390/atmos12121675

Academic Editors: Duanyang Liu, Kai Qin and Honglei Wang

Received: 31 October 2021 Accepted: 7 December 2021 Published: 13 December 2021

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eastern Mediterranean [8,9]. Recent model studies and some studies based on observational constraints indicate that more than 10% of the ozone in the troposphere is transmitted from the stratosphere, while the rest is photochemically formed in the troposphere [10]. Previous observations indicate that the spring maximum in the lower troposphere over East Asia is contributed by stratospheric-to-tropospheric transport and regional photochemical O3 production [11]. That is, the tropospheric O3 can be generated by the photochemical reactions of primary pollutants such as volatile organic compounds (VOCs) and nitrogen oxides (NOx, mainly including NO and NO2). The main sources of NOx in the troposphere are the combustion of coal, vehicle exhausts and the burning of other fossil fuels. VOCs come from a wide range of sources, including natural sources such as plant emissions, and anthropogenic sources such as biomass combustion, coal combustion, solvent usage, and the chemical industry [12]. Under strong sunlight, NO2 may photolyze to generate atomic oxygen, which can react with oxygen molecules to generate ozone. The existence of massive VOCs in the air will hinder the decomposition of O3, resulting in tropospheric O3 accumulation [13]. Primary pollutants such as NOx and VOCs, as the precursors of O3, are closely related to the generation and change of tropospheric O3. Therefore, studying the correlation between O3 and its precursors is helpful to understand the changing pattern of tropospheric O3 pollution and to provide a scientific basis for creating effective measures to control the composite atmospheric pollution.

With the development of urbanization, industrialization, and traffic, tropospheric O3 pollution has become increasingly serious in many areas of China. According to ozone observation data from 74 Chinese cities, the mean daily maximum 8 h average mass concentration of O3 (O3-max-8 h) increased from 149 μg·m−<sup>3</sup> in 2013 to 161 μg·m−<sup>3</sup> in 2015 [14]. The atmospheric O3 concentration has the characteristics of spatiotemporal distribution, and can be affected by factors such as the precursors and meteorological factors. In recent years, the frequency of photochemical smog and the concentration of atmospheric O3 have been increasing year by year, which has increased the complexity of air pollution and the urgency of improving air quality. As a result, much attention has been paid to the formation mechanism of atmospheric O3, the pollution status and the influencing factors of tropospheric O3, and the sensitivity relationship between O3 and its precursors, which has become one of the research topics of current atmospheric environmental science [15]. Therefore, the formation mechanism of tropospheric O3, the spatiotemporal distribution characteristics of tropospheric O3 in some regions of China, the relationship between O3 and its precursors, and the factors affecting tropospheric O3 levels, were reviewed in this paper. Furthermore, some countermeasures for controlling tropospheric O3 pollution were put forward based on the actual situation in China.
