On the isolation of OC and EC and the optimal strategy of radiocarbon-based source apportionment of carbonaceous aerosols

Abstract

Abstract. Radiocarbon (14C) measurements of elemental carbon (EC) and organic carbon (OC) separately (as opposed to only total carbon, TC) allow an unambiguous quantification of their non-fossil and fossil sources and represent an improvement in carbonaceous aerosol source apportionment. Isolation of OC and EC for accurate 14C determination requires complete removal of interfering fractions with maximum recovery. The optimal strategy for 14C-based source apportionment of carbonaceous aerosols should follow an approach to subdivide TC into different carbonaceous aerosol fractions for individual 14C analyses, as these fractions may differ in their origins. To evaluate the extent of positive and negative artefacts during OC and EC separation, we performed sample preparation with a commercial Thermo-Optical OC/EC Analyser (TOA) by monitoring the optical properties of the sample during the thermal treatments. Extensive attention has been devoted to the set-up of TOA conditions, in particular, heating program and choice of carrier gas. Based on different types of carbonaceous aerosols samples, an optimised TOA protocol (Swiss_4S) with four steps is developed to minimise the charring of OC, the premature combustion of EC and thus artefacts of 14C-based source apportionment of EC. For the isolation of EC for 14C analysis, the water-extraction treatment on the filter prior to any thermal treatment is an essential prerequisite for subsequent radiocarbon measurements; otherwise the non-fossil contribution may be overestimated due to the positive bias from charring. The Swiss_4S protocol involves the following consecutive four steps (S1, S2, S3 and S4): (1) S1 in pure oxygen (O2) at 375 °C for separation of OC for untreated filters and water-insoluble organic carbon (WINSOC) for water-extracted filters; (2) S2 in O2 at 475 °C followed by (3) S3 in helium (He) at 650 °C, aiming at complete OC removal before EC isolation and leading to better consistency with thermal-optical protocols like EUSAAR_2, compared to pure oxygen methods; and (4) S4 in O2 at 760 °C for recovery of the remaining EC. WINSOC was found to have a significantly higher fossil contribution than the water-soluble OC (WSOC). Moreover, the experimental results demonstrate the lower refractivity of wood-burning EC compared to fossil EC and the difficulty of clearly isolating EC without premature evolution. Hence, simplified techniques of EC isolation for 14C analysis are prone to a substantial bias and generally tend towards an overestimation of fossil sources. To obtain the comprehensive picture of the sources of carbonaceous aerosols, the Swiss_4S protocol is not only implemented to measure OC and EC fractions, but also WINSOC as well as a continuum of refractory OC and non-refractory EC for 14C source apportionment. In addition, WSOC can be determined by subtraction of the water-soluble fraction of TC from untreated TC. Last, we recommend that 14C results of EC should in general be reported together with the EC recovery.