Optical investigations and characterization of soot of different morphology and maturity

Sammanfattning: The formation path from small poorly absorbing incipient soot to larger fractal-like strongly absorbing black soot is extensive, and along this path the optical and physicochemical properties of the soot evolve. Soot emitted into the atmosphere may originate from some stage of this process, which will result in a wide spectrum of carbonaceous aerosols in the atmosphere which may interact with the sun and influence the radiative balance of the earth. In this work, differently matured soot from a mini-CAST soot generator was studied in terms of optical properties and the relation to its physicochemical properties. Various optical diagnostic tools, mainly multi-wavelength extinction, elastic light scattering (ELS), and laser-induced incandescence (LII), but also complementary aerosol instrumentation, were used for these purposes. These tools have provided generic information about soot properties, and additionally the applicability of the methods for soot analysis has been evaluated.Soot from the mini-CAST was found to have properties which range from nm-sized soot with optical properties of brown carbon (BrC) to larger soot aggregates of black carbon (BC) type. It was shown that the BrC type of soot had a refractory soot core with properties similar to young soot. Hence, it was shown to not consist of a BC core with a BrC like coating. Also it was shown that upon heating during thermo-optical analysis in an inert atmosphere that the BrC soot transformed and became more absorbing. LII was used to study the optical properties of soot, and it was shown that the optical properties of mature soot agreed well with results from extinction measurements, but for young soot LII results indicated absorption of slightly more mature soot character. Further analysis of the temperature evolution of the soot in the low fluence regime allowed for estimation of the soot absorption efficiency. Results showed large differences in absorption efficiencies for the differently matured soot and values for the mature soot agreed well with values presented in the literature. Double-pulse LII experiments showed how rapid laser heating induced changes in soot of different maturity. It was shown that the absorption properties were enhanced as a result of thermal annealing for all soot with the strongest effect for young soot. Another effect for young soot (using LII at 532 nm excitation) was an increased fluorescence from vaporized fragments that potentially can interfere with the detection of LII signals. A nephelometer was used to study the elastic scattering by soot particles, and it was investigated if scattering theory could be used to solve the inverse problem and obtain information on the morphological properties. The method appeared feasible as tests revealed good results when compared to results based on micrograph image analysis. The method may be useful for estimation of morphological properties of fractal-like soot, as it provides a faster and less elaborate estimation than microscopy analysis. The findings of this work contribute to the understanding of how differently matured soot interact with electromagnetic radiation, especially for the laser-induced incandescence method. Hence information has been gained on how to optimise the diagnostic potential of LII as well as on limitations in the diagnostics of soot of different maturity.