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

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 maximum daily 8 h average ozone (MDA8 O₃) increased by 2.6 μg m^-3 yr^-1 during the same period. Here, we derived the drivers behind the rising O₃ 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 O₃ from 2013 to 2020, with the changes in anthropogenic emissions playing a more important role. The anthropogenic contributions to the O₃ 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 O₃ 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 O₃ 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 O₃ air quality.