Physicochemical characteristics of particulate matter emitted by diesel blending with various aromatics

To evaluate the effects of aromatics structure on physicochemical features of diesel exhaust particles, particulate samples emitted from diesel fuel and its mixtures with benzene, m-xylene and tetralin were characterized using high-resolution transmission electron microscopy (HR-TEM), Raman spectroscopy (RS), X-ray photoelectron spectroscopy (XPS) and thermogravimetric analysis (TGA). Results showed that diesel soot had the least reactivity among four particle samples, followed by diesel/tetralin soot, which the latter was less reactive than diesel/m-xylene and diesel/benzene soot. In this aspect, diesel fuel with high proportion of aromatics, especially monoaromatics, demands lower thermal energy input for diesel particulate filter (DPF) regeneration. Higher reactivity of diesel/aromatics soot was in line with the less ordered nanostructure. Fringe parameters obtained from HRTEM showed that fringe length of soot particles decreased in the order: diesel > diesel/tetralin > diesel/m-xylene > diesel/benzene, while both fringe tortuosity and separation distance exhibited the opposite trend with fringe length. Notably, the effects of fuel formulations on fringe parameters were slight. Moreover, both I_D1/I_G and A_D1/A_G values revealed that diesel soot had a greater ordered structure than diesel/aromatics soot. The mean primary particle diameters of diesel/aromatics soot were larger than those of soot from pure diesel fuel. After mixing with aromatics, the mass fraction of volatile substances in particle samples increased while elemental carbon contents decreased. According to XPS, no relationship was observed between the surface oxygen content (O/C ratio) and the soot reactivity.