Open cells exhibit weaker entrainment of free-tropospheric biomass burning aerosol into the south-east Atlantic boundary layer
Authors: Abel Steven J. , Barrett Paul A. , Zuidema Paquita , Zhang Jianhao , Christensen Matt , Peers Fanny , Taylor Jonathan W. , Crawford Ian , Bower Keith N. , Flynn Michael 
Published: April 2020

Abstract. This work presents synergistic satellite, airborne and surface-based observations of a pocket of open cells (POC) in the remote south-east
Atlantic. 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-tropospheric
biomass burning aerosol plume that had been transported from the African continent was observed to be in contact with the boundary layer inversion
over the POC and the surrounding closed cellular cloud regime. The in situ measurements show marked contrasts in the boundary layer thermodynamic
structure, 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 the
POC, with elevated carbon monoxide, black carbon mass loadings and accumulation-mode aerosol concentrations measured beneath the trade-wind
inversion. The stratocumulus cloud in this region was moderately polluted and exhibited very little precipitation falling below cloud base. A rapid
transition to actively precipitating cumulus clouds and detrained stratiform remnants in the form of thin quiescent veil clouds was observed across
the 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 the
POC 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 concentrations
were also observed intermittently in this ultra-clean layer, suggesting that new particle formation was taking place. Across the boundary layer
inversion and immediately above the ultra-clean layer, accumulation-mode aerosol concentrations were ∼ 1000 cm−3. Importantly, the
air mass in the POC showed no evidence of elevated carbon monoxide over and above typical background conditions at this location and time of year. As
carbon monoxide is a good tracer for biomass burning aerosol that is not readily removed by cloud processing and precipitation, it demonstrates that
the open cellular convection in the POC is not able to entrain large quantities of the free-tropospheric aerosol that was sitting directly on top of
the boundary layer inversion. This suggests that the structure of the mesoscale cellular convection may play an important role in regulating the
transport 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. This
shows that the maxima in open cell frequency (> 0.25) occurs far offshore and in a region where subsiding biomass burning aerosol plumes may
often come into contact with the underlying boundary layer cloud. If the results from the observational case study applied more broadly, then the
apparent low susceptibility of open cells to free-tropospheric intrusions of additional cloud condensation nuclei could have some important
consequences for aerosol–cloud interactions in the region.