The ²¹⁰Pb-⁷Be-O₃ relationships observed in three aircraft missions over the western Pacific (PEM-West A and B, TRACE-P) are simulated with a global three-dimensional chemical tracer model (GEOS-CHEM) driven by assimilated meteorological observations. Results are interpreted in terms of the constraints that they offer on sources of tropospheric ozone (O₃). Aircraft observations of fresh Asian outflow show strong ²¹⁰Pb-O₃ correlations in September–October, but such correlations are only seen at low latitudes in February–March. Observations further downwind over the Pacific show stronger ²¹⁰Pb-O₃ correlations in February–March than in September–October. The model reproduces these results and attributes the seasonal contrast to strong O₃ production and vertical mixing over east Asia in September–October, seasonal shift of convection from China in September–October to Southeast Asia in February–March, and slow but sustained net O₃ production in Asian outflow over the western Pacific in February–March. Seasonal biomass burning over Southeast Asia in February–March is responsible for the positive ²¹⁰Pb-O₃ correlations observed at low latitudes. The model reproduces the observed absence of ⁷Be-O₃ correlations over the western Pacific during September–October, implying strong convective and weak stratospheric influence on O₃. Comparison of observed and simulated ⁷Be-O₃ correlations indicates that the stratosphere contributes less than 20–30% of O₃ in the middle troposphere at northern midlatitudes even during spring.