Land-sea breeze contribution to pollutant dispersion from the Guinea Coastal cities of West Africa

Abstract. Urbanization in Africa is often associated with increased air pollution, which affects not only the main cities but also remote regions depending on the prevailing meteorological conditions. Here, we investigate the role of the land-sea breeze (LSB) circulation for the dispersion of pollutants originating from five major coastal cities along the Guinea Coast in West Africa (Abidjan, Accra, Lomé, Cotonou, and Lagos). The study is based on a passive tracer emission experiment using the ICOsahedral Non-hydrostatic model coupled to the Aerosol and Reactive Trace gases module (ICON-ART) for a representative dry-season situation between 08 and 10 January 2021. Pollutants are emitted between 2 and 50 m from the ground from 0600 to 1800 UTC (close to local time) on the first day of simulation, from where they spread horizontally and vertically. The simulation reveals that the LSB starts to intensify near the coast around noon and propagates inland, reaching its maximum latitude at 7° N (approximately 200 km from the coasts) around 2100 UTC. Pollutants are first swiftly transported inland by the southwesterly wind of the LSB. As the planetary boundary layer deepens, particularly above the convergence zone near the LSB front, pollutants can reach the 875–800 hPa layer before being carried towards the coast and the Atlantic Ocean by the prevailing northeasterly return flow. Interestingly, these returning pollutants do not mix strongly down to the surface. Therefore, a well-developed LSB along the coast appears to contribute to attenuating urban coastal pollution by supporting a rapid dispersion of pollutants.