Optical properties of incipient soot

The exact knowledge of the optical properties of soot nanoparticles is fundamental for several aspects including the correct determination of the soot concentration in combustion environments using optical diagnostics and the correct estimation of the environmental impact of the emitted particles. Although extensive researches over the years have led to a substantial agreement on the optical properties of mature soot particles, the optical properties of the incipient soot nanoparticles are still uncertain. From the particle inception point to the formation of large and more mature soot particles, the evolution of the optical properties must account for variations due to the size and the physicochemical transformation of the investigated particles. This work aims to determine the refractive index and optical properties of inception particles formed in lightly sooting flames. A previous determination based on in-situ light absorption and scattering measurements is revisited taking advantage of particle size measurements by differential mobility analysis. The spectral dependencies of the optical properties are derived by the Kramers-Kronig analysis of the ex-situ VUV-NIR light absorption measurements. Results confirm the strong decrease in the absorptivity in the vis-NIR region of inception particles with unimodal size distribution and d-3 nm, and confirm a strong size dependency of soot optical properties. The thermal-optical analysis of the sampled particles shows that the particle mass absorption coefficient also correlates with organic carbon content.