Findings of the African Combustion Aerosol Collaborative Intercomparison Analysis (ACACIA) Pilot Project to Understand the Optical Properties of Biomass Burning Smoke

Abstract. Africa is a critical source of biomass burning (BB) aerosols, and its importance is increasing. The African Combustion Aerosol Collaborative Intercomparison Analysis (ACACIA) Pilot Project set to optically characterize BB aerosol generated from sub-Saharan African fuels. We used a photoacoustic spectrometer as a reference instrument to determine the multiple-scattering correction factor C_λ for an AE33 aethalometer at three wavelengths, which produced weighted mean values of C₃₇₀ = 3.69, C₄₇₀ = 5.65, and C₅₂₀ = 6.39. C_λ increased with wavelength and C₃₇₀ was statistically independent of the others, suggesting a single C_λ is insufficient, especially in BB scenarios. While a dependence of C_λ on burning state was not found, C_λ was shown to strongly relate to particle single scattering albedo (SSA, ω). When C_λ was plotted against SSA, values slowly rose at low SSA values, followed by a sharp rise around an SSA of ~0.9; indicating a larger correction needed for less absorbing aerosol. A number of functions operating on either SSA or C_λ were explored and the best function was -C_λ/(1‒C_λ) = Aω+B. This is an important parametrization of C_λ specifically geared towards BB aerosol from African fuels under different aging states, and is of particular importance for future field work in that continent. An Ångström matrix plot shows that African BB aerosol can have values more akin to dust, which demonstrates that these fuels are distinct in their wavelength dependence from more typical BB aerosol. Lastly, we examined the mass extinction and absorbance cross sections for BB aerosol generated for the same fuels with two different tube furnace setups. Not only is this combustion method flexible, it was found to be reproducible between labs.