Atmospheric Chemistry and Physics www.atmos-chem-phys.net 1680-7316 1680-7324 9 16 2009 10.5194/acp-9-6135-2009 http://www.atmos-chem-phys.net/9/6135/2009/ http://www.atmos-chem-phys.net/9/6135/2009/acp-9-6135-2009.html http://www.atmos-chem-phys.net/9/6135/2009/acp-9-6135-2009.pdf 6135 6155 2009-08-27 Factors controlling the distribution of ozone in the West African lower troposphere during the AMMA (African Monsoon Multidisciplinary Analysis) wet season campaign M. Saunois marielle.saunois@aero.obs-mip.fr C. E. Reeves C. H. Mari J. G. Murphy D. J. Stewart G. P. Mills D. E. Oram R. M. Purvis Université de Toulouse, UPS, LA (Laboratoire d'Aérologie), 14 avenue Edouard Belin, 31400 Toulouse, France CNRS, LA (Laboratoire d'Aérologie), 31400 Toulouse, France School of Environmental Sciences, University of East Anglia, Norwich, UK Department of Chemistry, University of Toronto, Toronto, Canada Facility for Airborne Atmospheric Measurement, National Centre for Atmospheric Science, Cranfield, UK now at: Facility for Ground Atmospheric Measurements, National Centre for Atmospheric Science, University of York, Heslington, York, UK Ozone and its precursors were measured on board the Facility for Airborne Atmospheric Measurements (FAAM) BAe 146 Atmospheric Research Aircraft during the monsoon season 2006 as part of the African Monsoon Multidisciplinary Analysis (AMMA) campaign. One of the main features observed in the west African boundary layer is the increase of the ozone mixing ratios from 25 ppbv over the forested area (south of 12° N) up to 40 ppbv over the Sahelian area. We employ a two-dimensional (latitudinal versus vertical) meteorological model coupled with an O₃-NO_x-VOC chemistry scheme to simulate the distribution of trace gases over West Africa during the monsoon season and to analyse the processes involved in the establishment of such a gradient. Including an additional source of NO over the Sahelian region to account for NO emitted by soils we simulate a mean NO_x concentration of 0.7 ppbv at 16° N versus 0.3 ppbv over the vegetated region further south in reasonable agreement with the observations. As a consequence, ozone is photochemically produced with a rate of 0.25 ppbv h^−1 over the vegetated region whilst it reaches up to 0.75 ppbv h^−1 at 16° N. 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