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

Abstract

African biomass burning (BB) aerosols transported over the southeast Atlantic (SEA) strongly influence cloud properties but remain a major source of uncertainty in regional climate assessment. This study characterizes vertical profiles of thermodynamic conditions, aerosol properties, and cloud microphysics around Ascension Island during an aircraft campaign (August–September 2017). Backward-dispersion simulations evidence that observed pollution originated from long-range transported African BB plumes. In BB-polluted marine boundary layers (MBL), aerosol number concentrations (Na) were substantially elevated relative to the clean MBL, driving increased cloud droplet number concentrations (Nd) and reduced cloud effective radii (Re). Cloud-layer mean Nd correlated strongly with aerosols below the cloud (sub-Na) but weakly with free-tropospheric (FT) aerosols. Enhanced sub-Na was due to BB aerosols entrained from the FT into the MBL along long-range transport and/or locally. Droplet activation fractions were similar in clean and moderately BB-polluted (sub-Na < 700 cm−3) clouds, while a weaker Nd–Na correlation was observed in more polluted clouds. Region-specific Nd–Na parameterizations are necessary for representing BB aerosol-cloud interactions over the remote SEA. A robust inverse Nd–Re relationship was observed, regardless of BB influence. By coupling backward simulations with satellite retrievals, this study indicates that FT-to-MBL entrainment of African BB aerosols over the SEA occurs several days before arrival at Ascension Island, predominantly west of 0° E for examined cases. These findings provide unique observational constraints for representing aerosol-cloud interactions and vertical transport of African BB aerosols in climate models, offering improved assessments of African BB impacts over the SEA.