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.
Highlights
Carbonaceous aerosols are of worldwide concern due to their effects on climate and air quality (Highwood and Kinnersley, 2006; Mauderly and Chow, 2008)
O2, 550, 150–580 O2, 700, 150–580 O2, 800, 150–580 a The temperatures in S2 in the Swiss 4S protocol are tested from 425–650 ◦C for optimisation. b The temperatures in S3 in the Swiss 4S protocol are tested from 550–850 ◦C for optimisation. c 2 % oxygen in helium. d The residence time at each temperature in the IMPROVE protocol depends on when the detector signal returns to the baseline to achieve well-defined carbon fractions
To minimise organic carbon (OC) charring, untimely removal of elemental carbon (EC) and the potential positive artefacts leading to co-evolution of EC with residual OC, the Swiss 4S protocol has been developed by optimising thermal-optical conditions, in particular the heating program and the choice of the carrier gas
Summary
Carbonaceous aerosols are of worldwide concern due to their effects on climate and air quality (Highwood and Kinnersley, 2006; Mauderly and Chow, 2008). It should be noted that the pre-treatment step of the water extraction is introduced in the THEODORE method to reduce charring further Both methods may remove substantial amounts of non-refractory EC (non-rEC) during the thermal treatment, so that the isolated EC much likely reflects only the most refractory EC (rEC) and cannot fully represent the total EC (tEC) spectrum. The filter transmittance is monitored by a 660-nm tuned-diode laser and the CO2 resulting from the sample analysis quantified by an NDIR cell placed upstream the instrument outlet This outlet is connected with a four-way valve to the CO2 cryogenic traps of the THEODORE system so as to recover the exhaust CO2 selectively corresponding to the desired fractions (Fig. 1). A voltage control valve and a gas flow sensor are installed for
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