Atmos. Chem. Phys. Discuss., 5, 3747-3771, 2005
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
Influence of convective transport on tropospheric ozone and its precursors in a chemistry-climate model
R. M. Doherty1, D. S. Stevenson1, W. J. Collins2, and M. G. Sanderson2
1Institute of Atmospheric and Environmental Science, University of Edinburgh, Edinburgh, UK
2Hadley Centre for Climate Prediction and Research, Met Office, Exeter, UK

Abstract. The impact of convection on tropospheric O3 and its precursors has been examined in a coupled chemistry-climate model. There are two ways that convection affects O3. First, convection affects O3 by vertical mixing of O3 itself. Convection lifts lower tropospheric air to regions where the ozone lifetime is longer, whilst mass-balance subsidence mixes O3-rich upper tropospheric (UT) air downwards to regions where the O3 lifetime is shorter. This tends to decrease UT ozone and the overall tropospheric column of O3. Secondly, convection affects O3 by vertical mixing of ozone precursors. This affects O3 chemical production and destruction. Convection transports isoprene and its degradation products to the UT where they interact with lightning NOx to produce PAN, at the expense of NOx. The combined effect of NOx to PAN conversions and downward transport of lightning NOx results in UT NOx decreases. Convective lofting of NOx from surface sources appears relatively unimportant. Despite UT NOx decreases, UT O3 production increases as a result of UT HOx increases driven by isoprene oxidation chemistry. However, UT O3 tends to decrease, as the effect of convective overturning of O3 itself dominates over changes in O3 chemistry. The changes in tropical UT O3 are transported polewards resulting in a 15% decrease in the global tropospheric O3 burden. These results contrast with an earlier study that uses a model of similar chemical complexity. Differences in chemistry schemes - in particular isoprene-driven changes, as well as differences in convection schemes themselves, are the most likely causes of such discrepancies. Further modelling studies are needed to constrain this uncertainty range.

Citation: Doherty, R. M., Stevenson, D. S., Collins, W. J., and Sanderson, M. G.: Influence of convective transport on tropospheric ozone and its precursors in a chemistry-climate model, Atmos. Chem. Phys. Discuss., 5, 3747-3771, doi:10.5194/acpd-5-3747-2005, 2005.
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