Chemical characterization of laboratory-generated tar ball particles

Abstract

The chemical properties of laboratory-generated tar ball (Lab-TB) particles produced from dry distillate (wood tars) of three different wood species in the laboratory were investigated by analytical techniques that had never been used before for their characterization. The elemental compositions of laboratory-generated tar balls (Lab-TBs) from three tree species were very similar to one another and to those characteristic of atmospheric tar balls (TBs) collected from the savanna fire during the SAFARI 2000 sampling campaign. The O ∕ C and H ∕ C molar ratios of the generated Lab-TBs were at the upper limit characteristic of soot particles. The Fourier transform infrared spectroscopy (FT-IR) spectra of the generated Lab-TBs were very similar to one another as well and also showed some similarity with those of atmospheric humic-like substances (HULIS). The FT-IR measurements indicated that Lab-TBs have a higher proportion of aromatic structure than HULIS and the oxygen atoms of Lab-TBs are mainly found in hydroxyl and keto functional groups. Whereas Raman activity was detected in the starting materials of the Lab-TBs (wood tars) in the range of 1000–1800 cm−1, the Raman spectra of TBs were dominated by two pronounced bands with intensity maxima near 1580 (G band) and 1350 cm−1 (D band), indicating the presence of sp2-hybridized carbon structures and disorder in them, respectively. In the Py-GC-MS chromatograms of the Lab-TBs mostly aromatic compounds (aromatic hydrocarbons, oxygenated aromatics and heterocyclic aromatics) were identified in accordance with the results of Raman and FT-IR spectroscopy. According to organic carbon ∕ elemental carbon (OC ∕ EC) analysis using EUSAAR_2 thermal protocol, 22 % of the total carbon content of Lab-TBs was identified as EC, contrary to expectations based on the current understanding that negligible if any EC is present in this sub-fraction of the brown carbon family. Our results suggest that spherical atmospheric TBs with high C ∕ O molar ratios are closer to BC in many of their properties than to weakly absorbing HULIS.