Internally mixed multicomponent soot: Impact of different salts on soot structure and thermo-chemical properties

An often referred feature of ambient and combustion aerosol samples is their ratio of elemental carbon (EC) to organic carbon (OC). Thermo-optical methods are commonly used to determine this ratio. As those methods generally consist of a heating step under inert atmosphere for OC quantification and a consecutive combustion step under oxidative atmosphere for EC quantification, combustion catalysts may drastically impact the measured EC/OC value. Such catalysts may be minerals, e.g. inorganic salts or oxides, that are mixed with the soot and may stem from various sources.In this study, the impact of varying content of different salts internally mixed with soot on soot structure and oxidation reactivity was studied. For this purpose, a novel method for preparation of model soot aerosols internally mixed with different inorganic salts (CaSO4, Ce(SO4)2/CeO2, Na2SO4, or NaCl) at different contents was applied by spraying aqueous salt solutions into a propane diffusion flame. Proof of production of internal mixtures of soot with the different salts was given by scanning electron microscopy (SEM). Raman microspectroscopy (RM) was utilized to characterize the soot structure. Soot oxidation reactivity was analyzed by temperature-programmed oxidation (TPO). It could be proven that doping of the soot with inorganic salts does not impact the soot structure. However, soot oxidation reactivity is strongly enhanced with increasing salt content resulting in a TPO emission maximum shifted by up to 200 K towards lower temperatures. Our results pose questions on the feasibility of thermo-optical methods for the determination of EC/OC values of carbonaceous aerosols in the presence of internally mixed minerals.

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