Biomass burning (BB) is a major source of absorbing aerosols globally and accounts for about 40% of black carbon in the atmosphere. The Southern African region contributes approximately 35% of the planet’s BB aerosol emissions. During the austral winter and spring, smoke is transported westward towards the southeast Atlantic Ocean, where it overlies and interacts with a quasi-permanent stratocumulus (Sc) cloud deck. Aerosol-cloud-climate interactions contribute the largest uncertainty to model estimates of anthropogenic forcing. The SEA region thus exhibits a large model-to-model divergence of climate forcing due to aerosols. This makes studies in the region particularly valuable for understanding these interactions. Previous studies focusing on Southern Africa BB have explored the distribution of aerosol loading. However, changes in aerosol optical properties during transport are not well documented.

This study aims to use remotely sensed observations to investigate the evolution of BB aerosol optical properties after emission within continental Africa, during transport over land, and over the Atlantic Ocean. Measurements taken from a collection of remote-sensing instruments during the ORACLES campaign are combined with results from two regional models, the WRF-AAM and WRF-CAM5, to explore the changes in the optical properties of smoke plumes as they age. The aerosol age is modeled using tracers from the WRF-AAM configured over the region’s spatial domain (14 ºN – 41 ºS, 34 ºW – 51 ºE). The study conducted an analysis of extinction, single scattering albedo (SSA), and extinction Angstrom exponent (EAE) in relation to aerosol age. Additionally, observations from airborne 4STAR, ground-based AERONET were compared with model results using WRF-CAM5.

The analysis revealed that aerosol age varied distinctly with longitude and the physical and chemical processes associated with the transport drive changes in the optical properties. The aerosols sampled closest to the source exhibited lower SSA values relative to particles sampled along the coastline. Along the coastline, free tropospheric SSA peaked at about 5-6 days, before gradually decreasing over the ocean, with a minimum value observed after approximately 12 days. SSA was underestimated by WRF-CAM5, and the modeled values are constrained to a narrower range than observations highlighting the importance of improving the representation of mass absorption and extinction in regional climate models.