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.