Stratospheric Intrusions Over Hong Kong: Impact and Seasonality Based on Long‐Term Ozonesonde Records

Stratospheric intrusions (SI) is a natural source of tropospheric O 3 , yet quantifying their contribution remains challenging in the Pearl River Delta (PRD) of China, a region characterized by high anthropogenic emissions, distant from the stratosphere‐to‐troposphere transport (STT) hotspots, but vulnerable to the topographic effect of the Tibetan Plateau (TP). Combining ozonesondes, reanalysis and trajectory simulations, we find that tropopause foldings are the primary trigger for SI, facilitating the injection of stratospheric airmasses, while the TP promotes their downstream transport. Furthermore, we establish an SI climatology by detecting the distinct SI chemical features from long‐term ozonesondes with sufficient constraints. SI events are most frequent and deepest in winter, but induce larger O 3 enhancements in the upper troposphere in summer. SI seasonality is jointly controlled by large‐scale STT background and local conditions. These findings can advance our understanding of stratospheric impact on city clusters distant from STT hotspots.