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Volume 18, issue 16
Atmos. Chem. Phys., 18, 12363-12389, 2018
https://doi.org/10.5194/acp-18-12363-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Special issue: Results of the project "Dynamics–aerosol–chemistry–cloud...

Atmos. Chem. Phys., 18, 12363-12389, 2018
https://doi.org/10.5194/acp-18-12363-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 27 Aug 2018

Research article | 27 Aug 2018

Aerosol distribution in the northern Gulf of Guinea: local anthropogenic sources, long-range transport, and the role of coastal shallow circulations

Cyrille Flamant1, Adrien Deroubaix1,2, Patrick Chazette3, Joel Brito4, Marco Gaetani1, Peter Knippertz5, Andreas H. Fink5, Gaëlle de Coetlogon1, Laurent Menut2, Aurélie Colomb4, Cyrielle Denjean6, Rémi Meynadier1, Philip Rosenberg7, Regis Dupuy4, Pamela Dominutti4, Jonathan Duplissy8, Thierry Bourrianne6, Alfons Schwarzenboeck4, Michel Ramonet3, and Julien Totems3 Cyrille Flamant et al.
  • 1Laboratoire Atmosphères Milieux Observations Spatiales, Sorbonne Université, Université Paris-Saclay and CNRS, Paris, France
  • 2Laboratoire de Météorologie Dynamique, Ecole Polytechnique, IPSL Research University, Ecole Normale Supérieure, Université Paris-Saclay, Sorbonne Université, CNRS, Palaiseau, France
  • 3Laboratoire des Sciences du Climat et de l'Environnement, CEA, CNRS, Université Paris-Saclay, Gif-sur-Yvette, France
  • 4Laboratoire de Météorologie Physique, Université Clermont Auvergne, CNRS, Clermont-Ferrand, France
  • 5Institute of Meteorology and Climate Research, Karlsruhe Institute of Technology, Karlsruhe, Germany
  • 6Centre National de Recherches Météorologiques, Météo-France and CNRS, Toulouse, France
  • 7Institute of Climate and Atmospheric Science, School of Earth and Environment, University of Leeds, Leeds, UK
  • 8Helsinki Institute of Physics, University of Helsinki, Helsinki, Finland

Abstract. The complex vertical distribution of aerosols over coastal southern West Africa (SWA) is investigated using airborne observations and numerical simulations. Observations were gathered on 2 July 2016 offshore of Ghana and Togo, during the field phase of the Dynamics-Aerosol-Chemistry-Cloud Interactions in West Africa project. This was the only flight conducted over the ocean during which a downward-looking lidar was operational. The aerosol loading in the lower troposphere includes emissions from coastal cities (Accra, Lomé, Cotonou, and Lagos) as well as biomass burning aerosol and dust associated with long-range transport from central Africa and the Sahara, respectively. Our results indicate that the aerosol distribution on this day is impacted by subsidence associated with zonal and meridional regional-scale overturning circulations associated with the land–sea surface temperature contrast and orography over Ghana and Togo, as typically observed on hot, cloud-free summer days such as 2 July 2016. Furthermore, we show that the zonal circulation evidenced on 2 July is a persistent feature over the Gulf of Guinea during July 2016. Numerical tracer release experiments highlight the dominance of aged emissions from Accra on the observed pollution plume loadings over the ocean, in the area of aircraft operation. The contribution of aged emission from Lomé and Cotonou is also evident above the marine boundary layer. Given the general direction of the monsoon flow, the tracer experiments indicate no contribution from Lagos emissions to the atmospheric composition of the area west of Cotonou, where our airborne observations were gathered. The tracer plume does not extend very far south over the ocean (i.e. less than 100km from Accra), mostly because emissions are transported northeastward near the surface over land and westward above the marine atmospheric boundary layer. The latter is possible due to interactions between the monsoon flow, complex terrain, and land–sea breeze systems, which support the vertical mixing of the urban pollution. This work sheds light on the complex – and to date undocumented – mechanisms by which coastal shallow circulations can distribute atmospheric pollutants over the densely populated SWA region.

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This work sheds light on the complex mechanisms by which coastal shallow circulations distribute atmospheric pollutants over the densely populated southern West African region. Pollutants of concern are anthropogenic emissions from coastal cities, as well as biomass burning aerosol and dust associated with long-range transport. The complex vertical distribution of aerosols over coastal southern West Africa is investigated using airborne observations and numerical simulations.
This work sheds light on the complex mechanisms by which coastal shallow circulations distribute...
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