This study investigates the characteristics and formation mechanisms of surface ozone pollution during Meiyu dry spells over the Yangtze River Delta (YRD). Using surface ozone observations from 252 national monitoring sites during 2013-2022, together with GPCP precipitation data, ERA5 reanalysis, 72 h backward trajectories, and GEOS-Chem simulations with emission sensitivity experiments, we examined the spatiotemporal features, meteorological controls, transport pathways, and source contributions of high-ozone episodes during Meiyu dry spells. The results show that ozone pollution during the Meiyu season has increased significantly over the YRD, and that high-ozone events occur frequently during dry spells embedded within the rainy season. These episodes are not associated with a more favorable mean meteorological background, but are instead linked to short-term favorable anomalies, including enhanced solar radiation, reduced cloud cover, higher temperature, lower humidity, and a deeper boundary layer. Synoptic and trajectory analyses further indicate that organized regional transport plays a key role, particularly under circulation patterns featuring enhanced low-level southeasterly flow and mid-tropospheric subsidence. Model diagnostics and sensitivity experiments show that anthropogenic emissions from the eastern YRD provide the dominant support for high oxidant levels, while regional transport is crucial for linking source and downwind receptor regions. These findings improve the understanding of ozone formation under Meiyu conditions and provide scientific support for coordinated regional ozone control in the YRD.

Yangtze River Delta; Meiyu season; Ozone pollution; Chemical transport model; Emission sensitivity