Influence of relative humidity and aging on the optical properties of organic aerosols from burning African biomass fuels.

Biomass burning (BB) is a major source of atmospheric fine carbonaceous aerosols, which play a significant, yet uncertain, role in modulating the Earth's radiation balance. However, accurately representing their optical properties in climate models remains challenging due to factors such as particle size, mixing state, combustion type, chemical composition, aging processes, and relative humidity (RH). In our study, we investigated BB organic-rich aerosols generated from smoldering sub-Saharan African biomass fuels. Fuel samples were collected in Africa and aerosols generated in the laboratory. We quantified key optical parameters, including mass cross-sections for extinction (2.04 ± 0.32 - 15.5 ± 2.48 m²/g), absorption (0.04 ± 0.01-0.3 ± 0.1 m²/g), and scattering (1.9 ± 0.68-15.3 ± 5.5 m²/g). Wavelength-dependent properties were used to determine absorption and scattering Ångström exponents. The single scattering albedo of these aerosols ranged from 0.8 ± 0.03 to 1.0 ± 0.04 and we observed a wavelength-dependent behavior. Extinction emission factors were determined at a wavelength of 550 nm, with values ranging from 42 ± 5 to 293 ± 32 m²/kg. Notably, optical properties exhibited fuel-type dependence, with differences observed between hardwood samples and other fuels, such as grass and animal dung. Aging increased mass extinction and scattering cross-sections at 550 nm, while humidity had the opposite effect across all fuels. Nitrate radical oxidation, both in photo and dark aging conditions, also influenced these properties. The findings are expected to close the gap in our understanding of optical properties of BB aerosol emissions in one of the least studied regions of the world - Africa - providing information to climate and air quality models for the region.