An automated method for thermal-optical separation of aerosol organic/elemental carbon for C analysis at the sub-μgC level: A comprehensive assessment.

We describe and thoroughly evaluate a method for C analysis in different fractions of carbonaceous aerosols, especially elemental carbon (EC). This method combines a Sunset thermal-optical analyzer and an isotope ratio mass spectrometer (IRMS) via a custom-built automated separation, purification, and injection system. Organic carbon (OC), EC, and other specific fractions from aerosol filter samples can be separated and analyzed automatically for C based on thermal-optical protocols (EUSAAR_2 in this study) at sub-μgC levels. The main challenges in isolating EC for C analysis are the possible artifacts during OC/EC separation, including the premature loss of EC and the formation of pyrolyzed OC (pOC) that is difficult to separate from EC. Since those artifacts can be accompanied with isotope fractionation, their influence on the stable isotopic composition of EC was comprehensively investigated with various test compounds. The results show that the thermal-optical method is relatively successful in OC/EC separation for C analysis. The method was further tested on real aerosols samples. For biomass-burning source samples, (partial) inclusion of pOC into EC has negligible influence on the C signature of EC. However, for ambient samples, the influence of pOC on the C signature of EC can be significant, if it is not well separated from EC, which is true for many current methods for measuring C on EC. A case study in Xi'an, China, where pOC is enriched in C compared to EC, shows that this can lead to an overestimate of coal and an underestimate of traffic emissions in isotope-based source apportionment.