Dynamics-aerosol-chemistry-cloud interactions in West Africa
Research Publications
27 August 2018
Aerosol distribution in the northern Gulf of Guinea: local anthroprogenic sources, long-range transport and the role of coastal shallow circulations
Publication: Atmos. Chem. Phys., 18, 12363-12389 ()
DOI Number: 10.5194/acp-18-12363-2018
Author: Cyrille Flamant, Adrien Deroubaix, Patrick Chazette, Joel Brito, Marco Gaetani, Peter Knippertz, Andreas H. Fink, Gaëlle de Coetlogon, Laurent Menut, Aurélie Colomb, Cyrielle Denjean, Rémi Meynadier, Philip Rosenberg, Regis Dupuy, Pamela Dominutti, Jonathan Duplissy, Thierry Bourrianne, Alfons Schwarzenboeck, Michel Ramonet, and Julien Totems
The complex vertical distribution of aerosols over
coastal southernWest 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 downwardlooking
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 regionalscale
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 100 km 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.