Constraints on the sources of tropospheric ozone from 210Pb‐7Be‐O3 correlations

The 210 Pb‐ 7 Be‐O 3 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 (O 3 ). Aircraft observations of fresh Asian outflow show strong 210 Pb‐O 3 correlations in September–October, but such correlations are only seen at low latitudes in February–March. Observations further downwind over the Pacific show stronger 210 Pb‐O 3 correlations in February–March than in September–October. The model reproduces these results and attributes the seasonal contrast to strong O 3 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 O 3 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 210 Pb‐O 3 correlations observed at low latitudes. The model reproduces the observed absence of 7 Be‐O 3 correlations over the western Pacific during September–October, implying strong convective and weak stratospheric influence on O 3 . Comparison of observed and simulated 7 Be‐O 3 correlations indicates that the stratosphere contributes less than 20–30% of O 3 in the middle troposphere at northern midlatitudes even during spring.