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The influence of biomass burning (BB)-derived organic aerosol (OA) emissions on solar radiationviaabsorption and scattering is related to their physicochemical properties and can change upon atmospheric aging. 

The fractal dimension is a key parameter in quantifying the morphology of aerosol aggregates, which is necessary to understand their radiative impact. Here we used Transmission Electron Microscopy (TEM) images to determine 2D fractal dimensions using the nested square and box-grid method and used two different empirical equations to obtain the 3D fractal dimensions. The values ranged from 1.70 ± 0.05 for pine to 1.82 ± 0.07 for Eucalyptus, with both methods giving nearly identical results using one of the empirical equations and the other overestimated the 3D values significantly when compared to other values in the literature. The values we obtained are comparable to the fractal dimensions of fresh aerosols in the literature and were dependent on fuel type and combustion condition. Although these methods accurately calculated the fractal dimension, they have shortcomings if the images are not of the highest quality. While there are many ways of determining the fractal dimension of linear features, we conclude that the application of every method requires careful consideration of a range of methodological concerns. 

Biomass burning (BB) aerosols contribute to climate forcing, but much is still unknown about the extent of this forcing, owing partially to the high level of uncertainty regarding BB aerosol optical properties. A key optical parameter is the refractive index (RI), which influences the absorbing and scattering properties of aerosols. This quantity is not measured directly, but it is obtained by fitting the measured scattering cross section and extinction cross section to a theoretical model using the RI as a fitting parameter. We used the Rayleigh–Debye–Gans (RDG) approximation to retrieve the complex RI of freshly emitted BB aerosol from two fuels (eucalyptus and olive) from Africa in the spectral range of 500–580 nm. Experimental measurements were carried out using cavity ring-down spectroscopy to measure extinction over the range of wavelengths of 500–580 nm and nephelometry to measure scattering at three wavelengths of 450, 550, and 700 nm for size-selected BB aerosol particles. The fuels were combusted in a tube furnace at a temperature of 800 °C, which is representative of the flaming stage of burning. Filter samples were collected and imaged using tunneling electron microscopy to obtain information on the morphology and size of the particles, which was used in the RDG calculations. The mean radii of the monomers were 27.8 and 31.5 nm for the eucalyptus and the olive fuels, respectively. The components of the retrieved complex RI were in the range of 1.31 ≤ n ≤ 1.56 and 0.045 ≤ k ≤ 0.468. The real and complex parts of the RI increase with increasing particle mobility diameter. The real part of the RI is lower, and the imaginary part is higher than what was recommended in literature for black carbon generated by propane or field measurements from fires of mixed wood samples. Fuel dependent results from controlled laboratory experiments can be used in climate modeling efforts and to constrain field measurements from biomass burning. 

Abstract. An accurate measurement of the optical properties of aerosol is critical for quantifying the effect of aerosol on climate. Uncertainties persist and results of measurements vary significantly. Biomass burning (BB) aerosol has been extensively studied through both field and laboratory environments for North American fuels to understand the changes in opticaland chemical properties as a function of aging. There is a need for a widersampling of fuels from different regions of the world for laboratory studies. This work represents the first such study of the optical andchemical properties of wood fuel samples commonly used for domestic purposes ineast Africa. In general, combustion temperature or modified combustionefficiency (MCE) plays a major role in the optical properties of the emitted aerosol. For fuels combusted with MCE of 0.974±0.015, which is referred to as flaming-dominated combustion, the single-scattering albedo (SSA) values were in the range of 0.287 to 0.439, while for fuels combusted with MCE of 0.878±0.008, which is referred to as smoldering-dominated combustion, the SSA values were in the range of 0.66 to 0.769. There is a clear but very small dependence of SSA on fuel type. A significant increase in the scattering and extinction cross section (with no significant change inabsorption cross section) was observed, indicating the occurrence of chemistry, even during dark aging for smoldering-dominated combustion. Thisfact cannot be explained by heterogeneous oxidation in the particle phase,and we hypothesize that secondary organic aerosol formation is potentiallyhappening during dark aging. After 12 h of photochemical aging, BB aerosolbecomes highly scattering with SSA values above 0.9, which can be attributedto oxidation in the chamber. Aging studies of aerosol from flaming-dominatedcombustion were inconclusive due to the very low aerosol number concentration. We also attempted to simulate polluted urban environments byinjecting volatile organic compounds (VOCs) and BB aerosol into the chamber, but no distinct difference was observed when compared to photochemical aging in the absence of VOCs.