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

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 and city-level space scales. 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 to control the emissions of SO₂, NOx, 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, volatile organic compound (VOC) emissions 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/NOy. 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 NOx emission and further in ambient NOx/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 NOx (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 NOx and VOCs, an effective O₃ control is emerging in the YRD region.