Structural changes of CAST soot during a thermal–optical measurement protocol
Thermal–optical measurement techniques are widely used to classify carbonaceous material. The results of different methods for total carbon are comparable but can vary by >44 % for elemental carbon. One major cause of variation is the formation of pyrolyzed carbon during the heating process which occurs mainly in samples with a high amount of brown carbon (BrC). In this study the structural changes of two different CAST (combustion aerosol standard) aerosol samples caused by the heating procedure in a thermal–optical instrument were investigated with UV–VIS and Raman spectroscopy, the integrating-sphere technique (IS) and transmission electron microscopy. All analysis techniques showed significant structural changes for BrC-rich samples at the highest temperature level (870 ∘C) in helium. The structure of the heated BrC-rich sample resembles the structure of an unheated BrC-poor sample. Heating the BrC-rich sample to 870 ∘C increases the graphitic domain size within the material from 1.6 to 2 nm. Although the Raman spectra unambiguously show this increase in ordering only at the highest temperature step, UV–VIS and IS analyses show a continuous change in the optical properties also at lower temperatures. The sample with a negligible amount of BrC, however, did not show any significant structural changes during the whole heating procedure.