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 3-D 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 Sep-Oct but such correlations are only seen at low latitudes in Feb-Mar. Observations further downwind over the Pacific show stronger 210Pb-O3 correlations in Feb-Mar than in Sep-Oct. The model reproduces these results and attributes the seasonal contrast to strong O3 production and vertical mixing over East Asia in Sep-Oct, seasonal shift of convection from China in Sep-Oct to Southeast Asia in Feb-Mar, and slow but sustained net O3 production in Asian outflow over the western Pacific in Feb-Mar. Seasonal biomass burning over Southeast Asia in Feb-Mar 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 Sep-Oct, 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.
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