Stability and carbon isotope changes of soot and char materials during thermal oxidation: Implication for quantification and source appointment

The thermal-oxidative resistances of seven soot and char samples from various combustion processes were tested with a chemothermal oxidation (CTO) method. The residues of each soot/char oxidized at different temperature conditions were quantified for the remaining organic carbon contents and their stable carbon isotope compositions. The results showed that the soot/char samples had distinctly different resistance to CTO, with half-temperature values of 380–409 °C for diesel- and hexane-derived soots, 276–312 °C for biomass- and coal-derived soots, and 257–323 °C for the other two chars. The lower thermal resistance of the two biomass soots and coal soot suggested that they likely were formed at lower combustion temperatures and that they cannot be detected or quantified in environmental samples using the standard CTO-375 method. The stable carbon isotope data showed three different modes of δ13C shifts as the combustion temperature increased. Both diesel and hexane soots had no change of δ13C values during CTO treatment. The δ13C values of the coal soot shifted to more negative whereas the δ13C values of the other four biomass-derived soot/char samples became more positive. The data suggested that interpretation of δ13C data for residual BC materials after thermal treatment should be cautious.

► Thermal stability of seven soot/char samples was evaluated with a CTO method.
► The soot/char samples had distinctly different thermal resistance to CTO.
► The CTO375 method cannot detect soot BCs of lower thermal resistance.
► The seven soot/chars displayed three modes of δ13C shifts as combustion T increased.
► δ13C signatures of isotopically heterogeneous soots may change during CTO treatment.