ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-10-9797-2010 The influence of biomass burning and transport on tropospheric composition over the tropical Atlantic Ocean and Equatorial Africa during the West African monsoon in 2006 Williams J. E. 1 Scheele M. P. 1 van Velthoven P. F. J. 1 Thouret V. 2 3 Saunois M. 2 3 5 Reeves C. E. 4 Cammas J.-P. 2 3 Royal Netherlands Meteorological Institute, De Bilt, The Netherlands Université de Toulouse, UPS, LA (Laboratorie d'Aérologie), 14 avenue Edouard Belin, 31400 Toulouse, France CNRS, LA (Laboratorie d'Aérologie), (UMR 5560), Observatoire Midi-Pyrénées, 31400 Toulouse, France School of Environmental Sciences, University of East Anglia, Norwich, UK now at: National Center for Atmospheric Research, Boulder, CO, USA 19 10 2010 10 20 9797 9817 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/10/9797/2010/acp-10-9797-2010.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/10/9797/2010/acp-10-9797-2010.pdf

Biomass burning (BB) in southern Africa is the largest emission source of CO and O<sub>3</sub> precursors within Africa during the West African Monsoon (WAM) between June and August. The long range transport and chemical processing of such emissions thus has the potential to exert a dominant influence on the composition of the tropical troposphere over Equatorial Africa (EA) and the Tropical Atlantic Ocean (TAO). We have performed simulations using a three-dimensional global chemistry-transport model (CTM) to quantify the effect that continental transport of such BB plumes has on the EA region. BB emissions from southern Africa were found to exert a significant influence over the TAO and EA between 10° S–20° N. The maximum concentrations in CO and O<sub>3</sub> occur between 0–5° S near the position of the African Easterly Jet – South as placed by the European Centre for Medium range Weather Forecasts (ECMWF) meteorological analysis data. By comparing co-located model output with in-situ measurements we show that the CTM fails to capture the tropospheric profile of CO in southern Africa near the main source region of the BB emissions, as well as the "extreme" concentrations of both CO and O<sub>3</sub> seen between 600–700 hPa over EA around 6° N. For more northerly locations the model exhibits high background concentrations in both CO and O<sub>3</sub> related to BB emissions from southern Africa. By altering both the temporal resolution and the vertical distribution of BB emissions in the model we show that changes in temporal resolution have the largest influence on the transport of trace gases near the source regions, EA, and in the outflow towards the west of Central Africa. Using a set of trajectory calculations we show that the performance of the CTM is heavily constrained by the ECMWF meteorological fields used to drive the CTM, which transport biomass burning plumes from southern Africa into the lower troposphere of the TAO rather than up towards the middle troposphere at 650 hPa. Similar trajectory simulations repeated using an updated meteorological dataset, which assimilates additional measurement data taken around EA, show markedly different origins for pollution events and highlight the current limitations in modelling this tropical region.

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