Constraints on the sources of tropospheric ozone from 210Pb-7Be-O3 correlations -: art. no. D07306

[1] The Pb-210-(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 Pb-210-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 Pb-210-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 Pb-210-O-3 correlations observed at low latitudes. The model reproduces the observed absence of Be-7-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 Be-7-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.