Surface ozone (O₃) increased by ~20% in the Yangtze River Delta (YRD) region of China during 2014-2020, but the aggravating trend is highly variable on interannual time scale and city-level space scale. Here, we employed multiple air quality observations and numerical simulation to describe the increasing but variable trend of O₃ and to reveal the main driving factors behind it. In 2014-2017, the governmental air pollution control action plan was mostly against PM_2.5 (mainly control the emissions of SO₂, NO_x and primary PM_2.5) and effectively reduced the PM_2.5 concentration by 18-45%. However, O₃ pollution worsened in the same period with an increasing rate of 4.9 μg·m^-3 yr^-1, especially in the Anhui province where the growth rate even reached 14.7 μg·m^-3 yr^-1. After 2018, owing to the coordinated prevention and control of both PM_2.5 and O₃, the VOCs emission in the YRD region has also been controlled with a great concern, and the O₃ aggravating trend in the same period has been obviously alleviated (1.1 μg·m^-3 yr^-1). We further combined the precursor concentration and the corresponding O₃ formation regime to explain the observed trend of O₃ in 2014-2020. The leading O₃ formation regime in 2014-2017 is diagnosed as VOC-limited (21%) or mix-limited (58%), with the help of a simulated indicator HCHO/NO_y. Under such condition, the decreasing NO₂ (2.8% yr^-1) and increasing VOCs (3.6% yr^-1) in 2014-2017 led to a rapid increment of O₃. With the continuous reduction in NO_x emission and further in ambient NO_x/VOCs, the O₃ production regime along the Yangtze River has been shifting from VOC-limited to mix-limited, and after 2018 the mix-limited regime has become the dominant O₃ formation regime for 55% of the YRD cities. Consequently, the decreases of both NO_x (3.3% yr^-1) and VOCs (7.7% yr^-1) in 2018-2020 obviously slowed down the aggravating trend of O₃. Our study argues that with the implementation of coordinated regional reduction of NO_x and VOCs, an effective O₃ control is emerging in the YRD region.

ozone,YRD,O3 reformation regime,WRF-Chem