Diurnal cycle of coastal anthropogenic pollutant transport over southern West Africa during the DACCIWA campaign

During the monsoon season, pollutants emitted by large coastal cities and biomass burning plumes originating from central Africa have complex transport pathways over southern West Africa (SWA). The Dynamics-Aerosol-Chemistry-Cloud Interactions in West Africa (DACCIWA) field campaign has provided numerous dynamical and chemical measurements in and around the super-site of Savè in Benin (≈ 185km away from the coast), which allows quantification of the relative contribution of advected pollutants. Through the combination of in situ ground measurements with aircraft, radio-sounding, satellite, and high-resolution chemistry-transport modeling with the CHIMERE model, the source attribution and transport pathways of pollutants inland (here, NOx and CO) are carefully analyzed for the 1-7 July 2016 period. The relative contributions of different sources (i.e., emissions from several large coastal cities) to the air quality in Savè are characterized. It is shown that a systematic diurnal cycle exists with high surface concentrations of pollutants from 18:00 to 22:00UTC. This evening peak is attributed to pollution transport from the coastal city of Cotonou (Benin). Numerical model experiments indicate that the anthropogenic pollutants are accumulated during the day close to the coast and transported northward as soon as the daytime convection in the atmospheric boundary layer ceases after 16:00UTC, reaching 8°N at 21:00UTC. When significant biomass burning pollutants are transported into continental SWA, they are mixed with anthropogenic pollutants along the coast during the day, and this mixture is then transported northward. At night, most of the coastal anthropogenic plumes are transported within the planetary boundary layer (below about 500m above ground level), whereas the biomass burning pollutants are mostly transported above it, thus generally not impacting ground level air quality.