Relationship between the soot absorption function, the mobility size distribution, the absorption Angström exponent and the chemical composition of soot issued from diesel B7 combustion in a mini-CAST burner: Effect of catalytic stripping

The present study aims to relate the optical properties and chemical composition of the soot particles generated in a mini-CAST burner supplied with diesel B7 over several operating points (OPs) with an overall equivalence ratio in the range 0.10–1.54. The extinction spectra are acquired in a custom 1 m long cell and analyzed using the Rayleigh-Debye Gans for fractal aggregates theory to extract the soot absorption function E(m), its spectral dependence, the absorption Angström exponent (AAE) and the single scattering albedo (SSA). The chemical composition of soot is determined using a surface diagnostic (time-of-flight secondary ions mass spectrometry) from which the C/H ratio is determined, while the gas phase is analyzed by gas chromatography, yielding the total hydrocarbon content. Analyses performed on untreated soot (as sampled) and soot treated with a catalytic stripper are compared, to reach a detailed understanding of the impact of the stripping process on both the optical properties and the chemical composition of the emissions. Untreated samples show a clear dependence of AAE on the C/H ratio that is suggested to be affected by the post-sampling deposition of high-<i>m</i>/<i>z</i> species on the soot particles. Samples treated with a catalytic stripper are almost completely depleted of high-<i>m</i>/<i>z</i> species: their AAE is significantly lower than untreated samples, and no correlation is found between AAE and the C/H ratio. The AAE decreases as the geometric diameter of the soot increases. The methodology presented in this work can be transposed in principle to any liquid fuel. Copyright © 2025 American Association for Aerosol Research