The transport history of African biomass burning aerosols arriving in the remote Southeast Atlantic marine boundary layer and their impacts on cloud properties

Abstract. The transport of African biomass burning aerosols (BBAs) and their impacts on cloud formation and properties over the Southeast Atlantic (SEA) remain one of the largest sources of uncertainty in understanding climate effects across this region. In this study, vertical structures of thermodynamics, aerosol properties and cloud microphysics were characterized around Ascension Island during the CLARIFY-2017 (CLoud-Aerosol-Radiation Interactions and Forcing for Year 2017; August–September 2017) aircraft campaign, providing insights into the relationship between transported African BBAs and clouds in the marine boundary layer (MBL) over the remote SEA. The biomass burning (BB)-impacted MBL exhibited substantially enhanced aerosol number concentration (Nₐ, 0.1 – 3 µm) compared to the clean MBL around Ascension Island, leading to generally increased cloud droplet number concentration (N_d) but smaller cloud effective radius (Rₑ) in BB-impacted clouds compared to clean clouds. The cloud-layer mean N_d values were observed to be strongly correlated with aerosols below the cloud (sub-Nₐ) but were more weakly associated with aerosols immediately above the cloud. The increase in the sub-Nₐ is caused by entrained BBAs from above-cloud to below-cloud regions along long-range transport pathways and/or at the place of observation. We also explored possible simplifications to establish relationships between N_d and sub-Nₐ or Rₑ from in-situ measurements. Similar droplet activation fractions were observed in the clean and moderately BB-impacted (sub-Nₐ < 700 cm–³) clouds, while a greater variability was noted in more polluted clouds. The relationship between N_d and Rₑ remained consistent regardless of the levels of BB influence. Backward simulations were conducted using UK Met Office’s Numerical Atmospheric Modelling Environment (NAME), to track the sources and pathways of air parcels reaching Ascension Island. NAME results indicate that air parcels arriving in the Ascension Island MBL can originate from both the boundary layer (BL) and free troposphere (FT) during long-range transport, and entrainment mixing from the FT into the MBL over the SEA is likely to occur. BB pollution in the Ascension Island MBL could occur, when FT air parcels, primarily originating from the African continent (20° S – 0° N), carry BB smoke. By coupling NAME simulations with SEVIRI (Spinning Enhanced Visible and Infrared Imager) retrievals (aerosol and cloud fields) along the simulated transport path, the study suggests that efficient entrainment of African air parcels from the FT into the MBL occurs multiple days over the SEA before reaching the Ascension Island MBL, mainly in the region to the west of 0° E for examined cases. This study provides important aerosol and cloud parameterizations for climate models, and also provides observational constraints for evaluating the effects of transported BBAs on clouds and their subsequent radiative forcing over the SEA. Furthermore, the identified BBAs entrainment region may provide additional constraints for refining the vertical transport processes of African BBAs in models, thereby improving the representation of their vertical structures over the remote SEA.