Regional differences and causes of ozone concentration exceeding standards in the Yangtze River Delta urban agglomeration

Based on hierarchical cluster analysis, this paper used the Kolmogorov–Zurbenko filter and the Hysplit model to analyze the causes of O 3 -8 h concentration exceedances and the potential source contribution areas across various sub-regions in the Yangtze River Delta urban agglomeration in China. The results indicated that the number of days affected by ozone exceedances in the Yangtze River Delta urban agglomeration averaged 168.1 ± 1.7 days·yr − 1 from 2015 to 2024, and its annual increase was 3.5 ± 0.4 days·yr − 1 . During the warm season, the phenomenon of exceedances in the regions surrounding the estuaries of the Yangtze River and Qiantang River, as well as the northern plains and the southern Anhui hilly areas, was primarily attributed to abnormal weather, anthropogenic emissions, or intensity of medium- to small-scale atmospheric circulation transport. Among them, the potential sources of excess ozone in the plains were not only locally concentrated but also concentrated in areas along the coastal economic belt of the North Yellow Sea. However, the formation of heavy pollution was mostly controlled by abnormal anthropogenic emissions during the local weather change process under low wind speeds. The potential sources in the latter were similar to those in the Zhejiang-Fujian hilly area. The local sources were the causes of small-scale ozone pollution formation, while the external input was not only the cause of large-scale pollution, but also the main cause of heavy pollution that exceeded the Grade II Standard. In the southeastern coastal area, the sea-land breeze could not only promote the diffusion of ozone pollutants controlled by local meteorological variables, but also promote the input of pollutants from the Zhejiang-Fujian hilly area along the westward path to form excessive ozone.