Overestimation of Stratospheric Intrusion Impacts on Surface Ozone in China.

Several recent studies suggest stratospheric intrusion (SI) substantially influences surface ozone (O3) in China, sparking widespread discussion. Using observations from over 1,400 national stations (predominantly urban sites susceptible to strong nitric oxide titration) and multiple data sets (including ERA5 meteorology and stratospheric O3 tracers of EAC4 and WACCM) from 2015 to 2024, this study systematically investigates and quantifies the impact of SI on surface O3 in China. Over the past decade, we identified 21,306 SI-to-surface (SITS) events (mean: 1.5 events/station/year; max: 12.5 events/station/year). SI signals are more prominent during winter and spring or under conditions of weak local photochemistry (e.g., nighttime). Notably, only 0.6% of total O3 pollution events (frequency) nationwide are induced by SITS. The average stratospheric O3 load and contribution to surface O3 during pollution periods are approximately 5 ppbv and 6%, respectively. Furthermore, SITS impact varies substantially with altitude: increasing significantly as altitude rises (<200 m (plains): 5 ppbv, >3000 m: 15 ppbv). Overall, SITS events are rare and mostly insufficient to trigger O3 pollution. The influence of SI on surface O3 has been significantly overestimated in previous research. This study quantifies SI's actual role in surface O3 pollution, providing a scientific basis for relevant policy formulation.