Contrasting Mechanisms of Heatwave‐Ozone Pollution Compound Events and Independent Heatwave Events in the Yangtze River Delta

Abstract Heat waves do not always coincide with ozone pollution, yet the mechanisms controlling their divergence remain insufficiently understood. Here, we investigate why some heat waves evolved into heatwave‐ozone pollution compound events (HWOP), whereas others remained isolated heat wave events (IHW), over the Yangtze River Delta during summer 2024. Using surface ozone observations and WRF‐CMAQ simulations, we identified HWOP and IHW events and examined their large‐scale circulation, lower‐tropospheric structure, and process‐level ozone tendencies. Both event types occurred under large‐scale warm anomalies associated with the westward extension of the Western Pacific Subtropical High and the eastward expansion of the South Asian High. Their ozone responses diverged, however, because of contrasting boundary‐layer meteorological conditions. Compared with HWOP, IHW was characterized by stronger ventilation and larger nighttime wind shear, which favored pollutant dispersion and more active vertical exchange. Process diagnostics further indicated that the divergence between HWOP and IHW was governed primarily by the balance between daytime photochemical production and nocturnal chemical loss. Compared to HWOP, IHW exhibited higher nocturnal near‐surface NO x and stronger ozone destruction, pointing to more effective nighttime titration and less favorable conditions for ozone persistence. In contrast, sustained ozone pollution during HWOP was associated with stronger daytime chemical production and weaker nocturnal removal. These results demonstrate that extreme heat alone is insufficient to produce ozone pollution; rather, the coupling between boundary‐layer structure and ozone chemical loss determines whether heat waves amplify surface ozone.