The role of chemical boundary conditions in simulating summer ozone and cross-boundary transport over China

Regional chemical transport models are vital for diagnosing and forecasting tropospheric ozone (O 3 ) pollution. However, their accuracy is often limited by the simplified treatment of chemical boundary condition (CBC). This study provides a comprehensive evaluation of how different CBC influence regional O 3 simulations over China using the WRF–CMAQ model. Four CBC 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 O 3 columns. The BASE scenario underestimated the average maximum daily 8 h O 3 (avg-O3MDA8) and its 90th percentile by −5.7 % and −13.1 %, respectively, while dynamic CBC substantially improved the accuracy. GEOS-Chem achieved the lowest bias (−0.3 %) and highest agreement (IOA = 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 CBC 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 CBC enhance cross-boundary transport efficiency, raising O 3 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 O 3 concentrations and demonstrate the importance of realistic, time-varying CBC for improving regional O 3 simulations, air quality forecasting, and transboundary pollution management in China.