Stratospheric intrusion (SI) is an important natural source of tropospheric ozone (O₃​), yet its contribution to pollution in the Yangtze River Delta (YRD) remains poorly quantified. This study systematically investigates the spatiotemporal characteristics and mechanisms of spring SI impacts on near-surface O_3​ over the YRD during 2013–2025 using ERA5 reanalysis, multi-source observations, and backward trajectory models. Empirical Orthogonal Function (EOF) analysis of 300 hPa potential vorticity identifies three primary SI modes (E1–E3) over East Asia, representing distinct source regions. Composite results reveal a clear temporal lag: the YRD remains relatively clean on the intrusion day, followed by significant near-surface O₃​ enhancements 1–2 days later, averaging 4–10 μg/m³. Among the modes, E3 type intrusions, centered over southern Northeast China, exert the strongest influence in intensity and extent, while E2 type shows weak surface impacts due to prevailing mid-tropospheric ascending motion. Diagnostic analyses show that downward transport efficiency is strongly controlled by post-trough subsidence. Specifically, tilted descending branches associated with E1 and E3 types facilitate the downward transport of O₃-rich stratospheric air into the lower troposphere. Case studies demonstrate that individual SI events typically affect near-surface O₃​ for 9–12 hr. The E1 case exhibits moderate enhancement (~22 μg/m³) over broader inland areas, whereas the E3 case produces stronger enhancement (up to 71 μg/m³) concentrated along the coastal YRD. These results confirm mid-to-high latitude SI as a substantial driver of spring O₃​ episodes, underscoring the importance of explicitly accounting for SI processes in regional air quality forecasting.

Stratospheric intrusion; Yangtze River Delta; Near-surface ozone; Synoptic patterns