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. This work presents synergistic satellite, airborne and surface-based observations of a pocket of open cells (POC) in the remote south-eastAtlantic. The observations were obtained over and upwind of Ascension Island during the CLouds and Aerosol Radiative Impacts and Forcing (CLARIFY)and the Layered Smoke Interacting with Clouds (LASIC) field experiments. A novel aspect of this case study is that an extensive free-troposphericbiomass burning aerosol plume that had been transported from the African continent was observed to be in contact with the boundary layer inversionover the POC and the surrounding closed cellular cloud regime. The in situ measurements show marked contrasts in the boundary layer thermodynamicstructure, cloud properties, precipitation and aerosol conditions between the open cells and surrounding overcast cloud field.   The data demonstrate that the overlying biomass burning aerosol was mixing down into the boundary layer in the stratocumulus cloud downwind of thePOC, with elevated carbon monoxide, black carbon mass loadings and accumulation-mode aerosol concentrations measured beneath the trade-windinversion. The stratocumulus cloud in this region was moderately polluted and exhibited very little precipitation falling below cloud base. A rapidtransition to actively precipitating cumulus clouds and detrained stratiform remnants in the form of thin quiescent veil clouds was observed acrossthe boundary into and deep within the POC. The subcloud layer in the POC was much cleaner than that in the stratocumulus region. The clouds in thePOC formed within an ultra-clean layer (accumulation-mode aerosol concentrations of approximately a few  cm−3 ) in the upper region of the boundary layer,which was likely to have been formed via efficient collision–coalescence and sedimentation processes. Enhanced Aitken-mode aerosol concentrationswere also observed intermittently in this ultra-clean layer, suggesting that new particle formation was taking place. Across the boundary layerinversion and immediately above the ultra-clean layer, accumulation-mode aerosol concentrations were  ∼  1000  cm−3 . Importantly, theair mass in the POC showed no evidence of elevated carbon monoxide over and above typical background conditions at this location and time of year. Ascarbon monoxide is a good tracer for biomass burning aerosol that is not readily removed by cloud processing and precipitation, it demonstrates thatthe open cellular convection in the POC is not able to entrain large quantities of the free-tropospheric aerosol that was sitting directly on top ofthe boundary layer inversion. This suggests that the structure of the mesoscale cellular convection may play an important role in regulating thetransport of aerosol from the free troposphere down into the marine boundary layer.    We then develop a climatology of open cellular cloud conditions in the south-east Atlantic from 19 years of September Moderate Resolution Imaging Spectroradiometer (MODIS) Terra imagery. Thisshows that the maxima in open cell frequency ( u003e  0.25) occurs far offshore and in a region where subsiding biomass burning aerosol plumes mayoften come into contact with the underlying boundary layer cloud. If the results from the observational case study applied more broadly, then theapparent low susceptibility of open cells to free-tropospheric intrusions of additional cloud condensation nuclei could have some importantconsequences for aerosol–cloud interactions in the region.

Open cells exhibit weaker entrainment of free-tropospheric biomass burning aerosol into the south-east Atlantic boundary layer