Research Highlights of the "Dynamics–Aerosol–Chemistry–Cloud Interactions (DACCIWA)” project in West Africa

The EU-funded DACCIWA (Dynamics-Aerosol-Chemistry-Cloud Interactions in West Africa, 2013−2018) project combined measurements in the field in southern West Africa (SWA) with extensive modeling activities and work with satellite data to advance our incomplete understanding of interactions between emissions, clouds, radiation, precipitation and regional circulations of the West African monsoon. To overcome the problem of data scarcity, high-quality observations of emissions, atmospheric composition and meteorological parameters were sampled during the main DACCIWA field campaign in June-July 2016. The campaign involved three research aircraft, three ground-based supersites, enhanced radiosonde launches, and intensive measurements at urban sites in Abidjan and Cotonou. These unique collection of data are freely available to the research community at http://baobab.sedoo.fr/DACCIWA/.

The present contribution highlights novel findings related to the dominant role of low-level clouds, sources of emissions, concentrations of pollutants, as well as to their impacts on clouds, radiation and the regional circulation. DACCIWA has provided extensive observational data focusing on the occurrence of low-level stratus decks that form at night in connection with the passage of a coastal front and the development of a low-level jet. These conditions dominate the meteorology of SWA during June-July. The radiative impact of these clouds is crucial for the regional energy balance and can significantly impact on SWA precipitation and the West African monsoon circulation.

DACCIWA measurements and modeling results also show that in SWA international air quality standards are frequently violated with respect to particles and in pollution hot spots also with respect to some trace gases. Current emission inventories, however, appear to underestimate some aerosol precursors, which creates a significant obstacle to modeling and assessing atmospheric chemistry and air pollution in the region. DACCIWA observations suggest that particles are both locally produced but also advected in significant amounts from the extensive biomass burning fires in equatorial Africa. Overall concentrations are so high that the susceptibility to cloud-aerosol interactions in the low stratus deck is moderate and does not strongly affect the regional meteorology. In contrast, aerosol radiative effects, enhanced by the swelling of particles in the very moist lower atmosphere, reduce surface solar radiation and thus impact on boundary layer development, stratus to cumulus transition, the inflow of Gulf of Guinea maritime air and likely also on convection.