Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 5 3 2005 10.5194/acpd-5-3747-2005 http://www.atmos-chem-phys-discuss.net/5/3747/2005/ http://www.atmos-chem-phys-discuss.net/5/3747/2005/acpd-5-3747-2005.html http://www.atmos-chem-phys-discuss.net/5/3747/2005/acpd-5-3747-2005.pdf 3747 3771 2005-06-07 Influence of convective transport on tropospheric ozone and its precursors in a chemistry-climate model R. M. Doherty D. S. Stevenson W. J. Collins M. G. Sanderson Institute of Atmospheric and Environmental Science, University of Edinburgh, Edinburgh, UK Hadley Centre for Climate Prediction and Research, Met Office, Exeter, UK 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 NO_x to produce PAN, at the expense of NO_x. The combined effect of NO_x to PAN conversions and downward transport of lightning NO_x results in UT NO_x decreases. Convective lofting of NO_x from surface sources appears relatively unimportant. Despite UT NO_x decreases, UT O₃ production increases as a result of UT HO_x 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.