Comparative NEXAFS study on soot obtained from an ethylene/air flame, a Diesel engine, and graphite

Microstructure and molecular structure properties of different carbonaceous byproducts from combustion are fundamental to evaluate the radiative properties of such materials when combustion aerosols interact with solar radiation in atmosphere. Carbon near edge X-ray absorption fine structure (NEXAFS) spectroscopy is presented as an analytical technique for the carbon-specific characterization of various soot species. Ethylene soot, diesel soot, and carbon black NEXAFS spectra are discussed in relation to graphite as the most genuine model compounds of soot. Characteristic resonances in the spectra allow for the direct molecular speciation of the graphite-like solid core in soot, surface functional groups, and aromatic and aliphatic components, depending on the origin of the soot. Moreover, in the experimental case of ethylene soot samples probed at increasing distances from the burner nozzle, namely, at increasing residence times, NEXAFS experiments lead to the evidence that the soot collected near the nozzle appears to be more graphitized than soot sampled at larger heights-above-burner, in qualitative agreement with laser light scattering depolarization measurements reported here and previous findings obtained by electron microscopy characterization. The possible implications of the presented results on soot formation mechanism, combustion source attribution of carbon particulates, and UV-vis sunlight absorption by black carbon aerosols are briefly discussed.