Drivers of Increasing Ozone during the Two Phases of Clean Air Actions in China 2013–2020

[Image: see text] In response to the severe air pollution issue, the Chinese government implemented two phases (Phase I, 2013–2017; Phase II, 2018–2020) of clean air actions since 2013, resulting in a significant decline in fine particles (PM(2.5)) during 2013–2020, while the warm-season (April–September) mean maximum daily 8 h average ozone (MDA8 O(3)) increased by 2.6 μg m(–3) yr(–1) in China during the same period. Here, we derived the drivers behind the rising O(3) concentrations during the two phases of clean air actions by using a bottom-up emission inventory, a regional chemical transport model, and a multiple linear regression model. We found that both meteorological variations (3.6 μg m(–3)) and anthropogenic emissions (6.7 μg m(–3)) contributed to the growth of MDA8 O(3) from 2013 to 2020, with the changes in anthropogenic emissions playing a more important role. The anthropogenic contributions to the O(3) rise during 2017–2020 (1.2 μg m(–3)) were much lower than that in 2013–2017 (5.2 μg m(–3)). The lack of volatile organic compound (VOC) control and the decline in nitrogen oxides (NO(x)) emissions were responsible for the O(3) increase in 2013–2017 due to VOC-limited regimes in most urban areas, while the synergistic control of VOC and NO(x) in Phase II initially worked to mitigate O(3) pollution during 2018–2020, although its effectiveness was offset by the penalty of PM(2.5) decline. Future mitigation efforts should pay more attention to the simultaneous control of VOC and NO(x) to improve O(3) air quality.