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. Between June and September large amounts of biomass burningaerosol are released into the atmosphere from agricultural fires in centraland southern Africa. Recent studies have suggested that this plume iscarried westward over the Atlantic Ocean at altitudes between 2 and 4 km andthen northward with the monsoon flow at low levels to increase theatmospheric aerosol load over coastal cities in southern West Africa (SWA),thereby exacerbating air pollution problems. However, the processes by whichthese fire emissions are transported into the planetary boundary layer (PBL)are still unclear. One potential factor is the large-scale subsidencerelated to the southern branch of the monsoon Hadley cell over the tropicalAtlantic. Here we use convection-permitting model simulations with COSMO-ARTto investigate for the first time the contribution of downward mixinginduced by clouds, a process we refer to as downward cloud venting incontrast to the more common process of upward transport from a polluted PBL.Based on a monthly climatology, model simulations compare satisfactory withwind fields from reanalysis data, cloud observations, and satellite-retrieved carbon monoxide (CO) mixing ratio. For a case study on 2 July2016, modelled clouds and rainfall show overall good agreement with SpinningEnhanced Visible and InfraRed Imager (SEVIRI) cloud products and GlobalPrecipitation Measurement Integrated Multi-satellitE Retrievals (GPM-IMERG)rainfall estimates. However, there is a tendency for the model to producetoo much clouds and rainfall over the Gulf of Guinea. Using the COdispersion as an indicator for the biomass burning plume, we identifyindividual mixing events south of the coast of Cote d'Ivoire due tomidlevel convective clouds injecting parts of the biomass burning plume intothe PBL. Idealized tracer experiments suggest that around 15 % of the COmass from the 2–4 km layer is mixed below 1 km within 2 d over theGulf of Guinea and that the magnitude of the cloud venting is modulated bythe underlying sea surface temperatures. There is even stronger verticalmixing when the biomass burning plume reaches land due to daytime heatingand a deeper PBL. In that case, the long-range-transported biomass burningplume is mixed with local anthropogenic emissions. Future work shouldprovide more robust statistics on the downward cloud venting effect over theGulf of Guinea and include aspects of aerosol deposition.

Downward cloud venting of the central African biomass burning plume during the West Africa summer monsoon