What caused large ozone variabilities in three megacity clusters in eastern China during 2015–2020?

Research article Atmos. Chem. Phys., 24, 1607–1626, 2024 https://doi.org/10.5194/acp-24-1607-2024 © Author(s) 2024. This work is distributed under the Creative Commons Attribution 4.0 License. What caused large ozone variabilities in three megacity clusters in eastern China during 2015–2020? Tingting Hu1 , Yu Lin1 , Run Liu1,2 , Yuepeng Xu1 , Shanshan Ouyang1 , Boguang Wang1,2 , Yuanhang Zhang3 , and Shaw Chen Liu1,2 1 Institute for Environmental and Climate Research, Jinan University, Guangzhou, 511443, China Kong–Macau Joint Laboratory of Collaborative Innovation for Environmental Quality, Jinan University, Guangzhou, 511443, China 3 State Key Joint Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, Beijing, 100871, China 2 Guangdong–Hong Correspondence: Run Liu () and Shaw Chen Liu () Received: 23 May 2023 – Discussion started: 13 June 2023 Revised: 15 November 2023 – Accepted: 16 December 2023 – Published: 5 February 2024 Abstract. Due to a robust emission control policy, significant reductions in major air pollutants, such as PM2.5 , SO2 , NO2 , and CO, were observed in China between 2015 and 2020. On the other hand, during the same period, there was a notable increase in ozone (O3 ) concentrations, making it a prominent air pollutant in eastern China. The annual mean concentration of maximum daily 8 h average (MDA8) O3 exhibited alarming linear increases of 2.4, 1.1, and 2.0 ppb yr−1 (ppb is for parts per billion) in three megacity clusters: Beijing–Tianjin–Hebei (BTH), the Yangtze River Delta (YRD), and the Pearl River Delta (PRD), respectively. Meanwhile, there was a significant 3-fold increase in the number of O3 -exceeding days, defined as MDA8 O3 > 75 ppb. Our analysis indicated that the upward increases in the annual mean concentration of MDA8 were primarily driven by the rise in consecutive O3 -exceeding days. There were expansions of high O3 in urban centers to rural areas accompanied by a saturation effect so that MDA8 O3 concentrations at the high-O3 stations in 2015 remained nearly constant at 100 ppb. Last, we found a close association between O3 episodes with 4 or more consecutive O3 -exceeding days and the position and strength of tropical cyclones (TCs) in the northwest Pacific and the West Pacific subtropical high (WPSH). The TC and WPSH contributed to meteorological conditions characterized by clear skies, subsiding air motion, high vertical stability in the lower troposphere, increased solar radiation, and a positive temperature anomaly at the surface. These favorable meteorological conditions greatly facilitated the formation of O3 . Thus, we propose that the worsening O3 increases observed in the BTH, YRD, and PRD regions from 2015 to 2020 can be mostly attributed to enhanced photochemical O3 production resulting from an increased occurrence of meteorological conditions with high solar radiation and positive temperature anomalies under the influence of the WPSH and TCs. 1 Introduction Ozone (O3 ) is an important greenhouse gas, which can also have adverse effects on human health, vegetation, and a variety of materials (Bell et al., 2006; Cohen et al., 2017; Kalabokas et al., 2020; Nuvolone et al., 2018). Surface O3 is a secondary pollutant produced by photochemical reactions involving O3 precursors such as volatile organic compounds (VOCs), carbon monoxide (CO), and nitrogen oxide (NOx ) (Ma et al., 2012; Monks et al., 2015; Wang et al., 2017). In addition to O3 precursors, meteorological conditions are also crucial factors driving the O3 formation. Solar radiation, temperature, relative humidity, wind speed, and cloud cover have been found to be closely related to O3 formation (Dong et al., 2020; Han et al., 2020; Yin et al., 2019). Furthermore, large-scale circulations, such as the East Asian monsoon, West Pacific subtropical high (WPSH), and tropical cyclones (TCs) can influence O3 concentration as well Published by Copernicus Publications on behalf of the European Geosciences Union. 1608 T. Hu et al.: What caused ozone variabilities in eastern China during 2015–2020? (Lu et al., 2019; Rowlinson et al., 2019; Yang et al., 2014; Zhao and Wang, 2017). The concentrations of air pollutants SO2 , NOx , CO, PM10 , and PM2.5 in China have been significantly reduced since 2013 (M. Li et al., 2021; Li et al., 2022; Zhai et al., 2019), thanks to the implementation of the Air Pollution Prevention and Control Action Plan. However, the O3 concentration has dramatically increased and emerged as a major air pollutant in eastern China (Bian et al., 2019; Fu et al., 2019; Wang et al., 2020; Zheng et al., 2018). O3 concentrations are particularly high in the three megacity clusters in eastern China, namely Beijing–Tianjin–Hebei (BTH), the Yangtze River Delta (YRD), and the Pearl River Delta (PRD) (Gao et al., 2020; Guo et al., 2019; K. Li et al., 2021; Liu et al., 2018; Yang et al., 2019). Annual mean concentrations of maximum daily 8 h average (MDA8) O3 in the three megacity clusters are shown in Fig. 1. The linear increases in MDA8 O3 for the BTH, YRD, and PRD regions are 2.4, 1.1, and 2.0 ppb yr−1 (ppb is for parts per billion), respectively, during the period 2015– 2020. These increases are unusually large, compared to the increases in other parts of China, as well as the positive trends worldwide (Chen et al., 2020; Lu et al., 2018; Professional Committee of Ozone Pollution Control of Chinese Society for Environmental Sciences, 2022; Zhang et al., 2020). Thus, a crucial scientific question is as follows: what causes these large increases in the O3 concentration? Some recent studies suggested that changing photochemical processes induced by anthropogenic emissions are responsible for these increases (Li et al., 2019, 2022; Shao et al., 2021; Wang et al., 2020). However, in our analysis of the O3 increases at individual stations in eastern China during the period 2015–2020, we noticed that the interannual variations in the O3 concentration were strongly affected by the position and intensity of the WPSH and the presence of TCs in the western Pacific and South China Sea, consistent with the results of a number of recent studies (Chang et al., 2019; Mao et al., 2020; Ouyang et al., 2022; Zhao and Wang, 2017). These results suggest that transport or meteorological parameters associated with the WPSH and TCs may also play an important role in the large increases in MDA8 O3 . The significant impact of the WPSH on weather patterns and O3 concentrations over East China is widely recognized (Bachmann, 2015; Chang et al., 2019; Yin et al., 2019; Zhao and Wang, 2017). It is well established that the WPSH plays a critical role in controlling weather conditions, which in turn affects O3 concentrations. For example, the WPSH is known to contribute to the formation of the East Asian monsoon and influence precipitation patterns in the YRD. It also influences air temperature a (...truncated)