ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-10-3027-2010 Cross-hemispheric transport of central African biomass burning pollutants: implications for downwind ozone production Real E. 1 13 Orlandi E. 2 Law K. S. 1 Fierli F. 2 Josset D. 1 Cairo F. 3 Schlager H. 6 Borrmann S. 4 5 Kunkel D. 4 Volk C. M. 7 14 McQuaid J. B. 8 Stewart D. J. 9 Lee J. 10 Lewis A. C. 10 Hopkins J. R. 10 Ravegnani F. 2 Ulanovski A. 11 Liousse C. 12 UPMC Univ. Paris 06, Univ. Versailles St-Quentin, CNRS/INSU, UMR 8190 LATMOS-IPSL, Paris, France Institute for Atmospheric Science and Climate, ISAC-CNR, Italy Istituto di Scienze dell'Atmosfera e del Clima, Roma, Italy Max Planck Institute for Chemistry, Particle Chemistry Department, Germany Institute for Atmospheric Physics, Johannes Gutenberg Univ., Mainz, Germany Deutsches Zentrum für Luft- und Raumfahrt (DLR), Oberpfaffenhofen, Institut für Physik der Atmosphäre, 82230 Wessling, Germany Institute for Atmospheric and Environmental Sciences, J. W. Goethe-Univ., Frankfurt, Germany Institute for Climate and Atmospheric Science School of Earth and Environment, University of Leeds, UK Department of Chemistry, Univ. of Reading, Reading, UK National Centre for Atmospheric Science Department of Chemistry, Univ. of York, York, UK CAO, Dolgoprudny, Russia Laboratoire d'Aérologie, Univ. de Toulouse, France now at: CEREA, ENPC/EDF, 20 rue Alfred Nobel 77455 – Champs sur Marne, France now at: Department of Physics, Univ. of Wuppertal, Germany 30 03 2010 10 6 3027 3046 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/10/3027/2010/acp-10-3027-2010.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/10/3027/2010/acp-10-3027-2010.pdf

Pollutant plumes with enhanced concentrations of trace gases and aerosols were observed over the southern coast of West Africa during August 2006 as part of the AMMA wet season field campaign. Plumes were observed both in the mid and upper troposphere. In this study we examined the origin of these pollutant plumes, and their potential to photochemically produce ozone (O<sub>3</sub>) downwind over the Atlantic Ocean. Their possible contribution to the Atlantic O<sub>3</sub> maximum is also discussed. Runs using the BOLAM mesoscale model including biomass burning carbon monoxide (CO) tracers were used to confirm an origin from central African biomass burning fires. The plumes measured in the mid troposphere (MT) had significantly higher pollutant concentrations over West Africa compared to the upper tropospheric (UT) plume. The mesoscale model reproduces these differences and the two different pathways for the plumes at different altitudes: transport to the north-east of the fire region, moist convective uplift and transport to West Africa for the upper tropospheric plume versus north-west transport over the Gulf of Guinea for the mid-tropospheric plume. Lower concentrations in the upper troposphere are mainly due to enhanced mixing during upward transport. Model simulations suggest that MT and UT plumes are 16 and 14 days old respectively when measured over West Africa. The ratio of tracer concentrations at 600 hPa and 250 hPa was estimated for 14–15 August in the region of the observed plumes and compares well with the same ratio derived from observed carbon dioxide (CO<sub>2</sub>) enhancements in both plumes. It is estimated that, for the period 1–15 August, the ratio of Biomass Burning (BB) tracer concentration transported in the UT to the ones transported in the MT is 0.6 over West Africa and the equatorial South Atlantic.<br> <br> Runs using a photochemical trajectory model, CiTTyCAT, initialized with the observations, were used to estimate in-situ net photochemical O<sub>3</sub> production rates in these plumes during transport downwind of West Africa. The mid-troposphere plume spreads over altitude between 1.5 and 6 km over the Atlantic Ocean. Even though the plume was old, it was still very photochemically active (mean net O<sub>3</sub> production rates over 10 days of 2.6 ppbv/day and up to 7 ppbv/day during the first days) above 3 km especially during the first few days of transport westward. It is also shown that the impact of high aerosol loads in the MT plume on photolysis rates serves to delay the peak in modelled O<sub>3</sub> concentrations. These results suggest that a significant fraction of enhanced O<sub>3</sub> in mid-troposphere over the Atlantic comes from BB sources during the summer monsoon period. According to simulated occurrence of such transport, BB may be the main source for O<sub>3</sub> enhancement in the equatorial south Atlantic MT, at least in August 2006. The upper tropospheric plume was also still photochemically active, although mean net O<sub>3</sub> production rates were slower (1.3 ppbv/day). The results suggest that, whilst the transport of BB pollutants to the UT is variable (as shown by the mesoscale model simulations), pollution from biomass burning can make an important contribution to additional photochemical production of O<sub>3</sub> in addition to other important sources such as nitrogen oxides (NO<sub>x</sub>) from lightning.

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