Topographic Modulation of High-Temperature Ozone Suppression: Core-Periphery Threshold Structure in the Sichuan Basin

The response of tropospheric ozone (O₃) to warming is commonly interpreted through the "climate penalty" paradigm, but O₃ concentrations can stabilize or decline under extreme heat, a phenomenon known as high-temperature suppression (HTS). Its spatial heterogeneity and associated atmospheric processes in topographically enclosed regions remain poorly understood. We investigated HTS across the Sichuan Basin (SCB). Integrating 2020-2024 observations from 22 cities with ERA5 reanalysis, we applied robust locally weighted regression, piecewise cubic Hermite interpolation, and Random Forest models coupled with SHAP analyses to identify HTS thresholds and associated predictors. HTS was detected in 16 of the 18 analyzed cities (all except Leshan and Deyang). The critical suppression temperature (T_c) decreased from 36.8 °C in the basin interior to 33.8 °C in peripheral regions, forming a pronounced spatial gradient. In contrast, O₃ suppression intensity increased toward the basin interior, with the O₃ decline rate above T_c was exceeded reaching -9.65 μg m^-3 °C^-1 in the inner zone, approximately 2.1 times that in the outer zone. Machine-learning analyses indicated that O₃ variability under moderate temperatures was mainly associated with temperature and solar radiation, whereas the predictive contributions of boundary-layer, cloud, moisture, transport, and air-quality variables increased under extreme heat. The observed spatial heterogeneity was most consistently associated with differences in boundary-layer ventilation; cloud cover and basin-scale circulation may provide additional radiative and transport modulation. These results indicate that the contrasting HTS responses of basin-interior and peripheral cities are associated with terrain-constrained meteorological conditions rather than temperature alone. This basin-interior -periphery pattern provides a framework for interpreting nonlinear O₃- meteorology relationships in complex terrain and may inform regionally differentiated O₃ management under future warming scenarios.