FEBRUARY 2004: Constraints on the sources of tropospheric ozone from 210Pb-7Be-O3 correlations

Lead-210 is a tracer of continental influence, while 7Be is a tracer of stratospheric influence and subsidence. One may therefore expect 210Pb-7Be-O3 correlations to provide information on the origin of O3. We simulate the 210Pb-7Be-O3 relationships observed in three aircraft missions over the western Pacific (PEM-West A and B, TRACE-P) with the GEOS-CHEM model, and interpret the results in terms of the constraints that they offer on sources of tropospheric O3.

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Shown above are scatterplots of O3 against 210Pb and 7Be during PEM-West A (Sep-Oct 1991) and B (Feb-Mar 1994) aircraft missions in three altitude bins (0-3, 3-8, 8-12km) near Asia and over the remote Pacific, respectively. GEOS-CHEM model results (red open circles and dashed lines) are compared with the observations (dark crosses and solid lines). The lines of best fit are calculated if the Pearson correlation coefficient is greater than 0.3. 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. This work was led by Hongyu Liu and a full account is given in Liu et al. [2003].