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

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.