The Role of Chemical Boundary Conditions in Simulating Summer Ozone and Cross-Boundary Transport over China

Abstract. Regional chemical transport models are vital for diagnosing and forecasting tropospheric ozone (O₃) pollution. However, their accuracy is often limited by the simplified treatment of chemical boundary conditions (CBCs). This study provides a comprehensive evaluation of how different CBCs influence regional O₃ simulations over China using the WRF–CMAQ model. Four CBCs scenarios were assessed: a static BASE profile representing climatological conditions and three dynamic scenarios derived from H-CMAQ, GEOS-Chem, and CESM2.2. Model results were validated with surface networks, ozonesonde profiles, and satellite O₃ columns. The BASE scenario underestimated the average maximum daily 8-hour O₃ (avg-O3MDA8) and its 90th percentile by −5.7% and −13.1%, respectively, while dynamic CBCs substantially improved the accuracy. GEOS-Chem achieved the lowest bias (−0.3%) and highest agreement (0.85 and 0.83) for avg-O3MDA8 and its 90th percentile. H-CMAQ performed best in high-elevation northwestern regions, and CESM2.2 excelled in southern and southwestern areas. Vertically, all CBCs reasonably matched observations within the troposphere, but elevated lower-stratosphere biases were identified in BASE, H-CMAQ, and CESM2.2. A case study contrasting cyclone-scavenging and post-trough accumulation phases revealed that dynamic CBCs enhance cross-boundary transport efficiency, raising O₃ by 10–20% over eastern China through combined continental and stratospheric inflows. These results underscore the crucial role of synoptic circulation–driven transboundary transport in shaping regional O₃ concentrations and demonstrate the importance of realistic, time-varying CBCs for improving regional O₃ simulations, air quality forecasting, and transboundary pollution management in China.