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 condition (CBC). This study provides a comprehensive evaluation of how different CBC influence regional O3 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 O3 columns. The BASE scenario underestimated the average maximum daily 8 h O3 (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 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 CBC for improving regional O3 simulations, air quality forecasting, and transboundary pollution management in China.