Summer in the Yangtze River Delta (YRD) during the Meiyu season is typically characterized by persistent precipitation and suppressed solar radiation. However, within the Meiyu season, frequent “dry spells between rain events (hereafter ‘dry spells’)” between rain events often coincide with outbreaks of high-ozone pollution, whose formation mechanisms have not been systematically characterized. Focusing on the YRD, this study identifies high-ozone events during Meiyu dry spells using surface observations and ERA5 reanalysis. Under comparable regional pollution levels, we compare the spatiotemporal features of ozone pollution between Meiyu dry spells and non-Meiyu low-precipitation periods in summer, and analyze key meteorological variables and synoptic circulation patterns to elucidate similarities and differences in meteorological conditions and potential transport pathways between the two regimes. On this basis, we conduct a baseline GEOS-Chem simulation for Meiyu dry spell conditions and further design two sets of sensitivity experiments targeting biogenic volatile organic compounds (BVOCs) and regional transport. These experiments quantify the relative contributions of meteorology-driven BVOC variations and upwind emission contributions to ozone enhancement during Meiyu dry spells, thereby enabling a quantitative attribution of high-ozone sources under this background.

The results show that severe ozone pollution occurs over the central-eastern YRD during both Meiyu dry spells and non-Meiyu low-precipitation periods. Nevertheless, compared with non-Meiyu low-precipitation periods, Meiyu dry spells are more prone to producing high ozone concentrations in the northwestern YRD, forming a pronounced hotspot. Further analyses indicate that such high-ozone episodes during Meiyu dry spells are mainly governed by the synergistic effects of “enhanced radiation-elevated boundary layer-subsidence-induced stabilization.” A pronounced radiation window and reduced low cloud cover increase photochemical reaction efficiency and amplify local ozone production. Meanwhile, anomalous local subsidence stabilization suppresses vertical mixing, favoring pollutant accumulation. In addition, backward trajectory analysis and transport pathway diagnostics suggest that regional transport modulates the pollution enhancement in the northwestern YRD, such that upwind inflow and local photochemical amplification jointly contribute to the persistence and expansion of ozone maxima during Meiyu dry spells. Model evaluation indicates that the simulated standardized Ox (O₃+NO₂) anomalies averaged over the YRD in summers of 2021-2022 exhibit moderate agreement with observations (r≈0.59; CRMSE≈0.90), supporting the reliability of GEOS-Chem for process decomposition. Sensitivity experiments further confirm that, while regional transport cannot be neglected, meteorology-induced anomalies in local photochemical production (including BVOC emission responses) play a dominant role in enhancing ozone during Meiyu dry spells. This study provides both observational and modeling evidence for the coupled weather-chemistry mechanisms driving ozone maxima under Meiyu background conditions, and offers scientific support for air-quality forecasting and coordinated emission-control strategies during the Meiyu season.