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Surface ozone (O 3 ) 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 and city-level space scales. Here, we employed multiple air quality observations and numerical simulation to describe the increasing but variable trend of O 3  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 to control the emissions of SO 2 , NO x , and primary PM 2.5 ) and effectively reduced the PM 2.5  concentration by 18%–45%. However, O 3  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 O 3 , volatile organic compound (VOC) emissions in the YRD region has also been controlled with a great concern, and the O 3  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 O 3  formation regime to explain the observed trend of O 3  in 2014–2020. The leading O 3  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 NO 2  (2.8% yr −1 ) and increasing VOCs (3.6% yr −1 ) in 2014–2017 led to a rapid increment of O 3 . With the continuous reduction in NO x  emission and further in ambient NO x /VOCs, the O 3  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 O 3  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 O 3 . Our study argues that with the implementation of coordinated regional reduction of NO x  and VOCs, an effective O 3  control is emerging in the YRD region.

Increasing but Variable Trend of Surface Ozone in the Yangtze River Delta Region of China