Photochemical mechanism–dependent ozone formation and precursor sensitivity under varying NOₓ conditions

Abstract. Photochemical mechanisms are core components of air quality models, yet differences among them introduce substantial uncertainty in ozone (O₃) simulations. Here, we systematically evaluate three widely used mechanisms (CB06, SAPRC07, and RACM2) using the CMAQ model during an O₃ pollution episode in Chengdu, China. To ensure consistency, a customized volatile organic compounds emission inventory was developed for RACM2. The results show pronounced inter-mechanistic differences in simulated O₃, radical chemistry, and precursor sensitivities under distinct NO_x conditions. SAPRC07 and RACM2 produced higher and comparable O₃ levels (~95 ppbv) with better performance in urban areas, whereas CB06 simulated lower O₃ (~80 ppbv) and performed better under low-NOₓ conditions. These differences are linked to variations in radical concentrations and reaction pathways, with RACM2 yielding the highest OH levels, SAPRC07 better capturing HO₂, and CB06 simulating lower RO₂. Despite these discrepancies, all mechanisms consistently reproduced the dominant O₃ formation pathway, with RO₂+NO contributing over 50 % of net production. However, radical sources and termination pathways differed substantially between urban and suburban environments, reflecting regime-dependent chemistry. Sensitivity analysis further shows that anthropogenic aromatics and alkenes dominate urban O₃ formation, whereas biogenic emissions dominate in suburban areas. Notably, O₃ sensitivity to biogenic emissions is higher in urban high-NO_x conditions, highlighting an amplified biogenic contribution. These findings demonstrate that mechanism choice fundamentally affects O₃ formation and precursor sensitivities, emphasizing the need for multi-mechanism approaches to improve model reliability and support effective emission control strategies.