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

Abstract

Radiocarbon (<sup>14</sup>C) 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 <sup>14</sup>C determination requires complete removal of interfering fractions with maximum recovery. The optimal strategy for <sup>14</sup>C-based source apportionment of carbonaceous aerosols should follow an approach to subdivide TC into different carbonaceous aerosol fractions for individual <sup>14</sup>C 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 <sup>14</sup>C-based source apportionment of EC. For the isolation of EC for <sup>14</sup>C 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 (O<sub>2</sub>) at 375 °C for separation of OC for untreated filters and water-insoluble organic carbon (WINSOC) for water-extracted filters; (2) S2 in O<sub>2</sub> 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 O<sub>2</sub> at 760 °C for recovery of the remaining EC. <br><br> 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 <sup>14</sup>C 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 <sup>14</sup>C sourc...

The Use of Levoglucosan and Radiocarbon for Source Apportionment of PM2.5Carbonaceous Aerosols at a Background Site in East China

“…As a result, the 14 C values of the EC fraction may be incorrect. 30 Although charring can be minimized by performing the water extraction step prior to the thermal treatment, 10,30,31 it is not adequate to separate all of the OC. 16,18 A longer separation duration for the elimination of the OC, for example, 12 and 24 h, was suggested at 375°C in air.…”

14C Analysis and Sample Preparation at the New Bern Laboratory for the Analysis of Radiocarbon with AMS (LARA)

“…Joint projects covered development of the gas ion source and the gas inlet system (Ruff et al 2007;Wacker et al 2013a), coupling of an elemental analyzer and other combustion instruments with the AMS (Perron et al 2010;Ruff et al 2010), development of sample pretreatment methods and instrumentation for routine 14 C dating (Němec et al 2010ab;) and applications to carbonaceous aerosols in the atmosphere and in glacier ice (Sigl et al 2009;Szidat 2009a,b;Zhang et al 2012). About >2000 unknown gas samples were measured within these projects at the ETH AMS systems until the end of 2012.…”

“…Separation and sample combustion instruments were directly coupled with the gas source (Perron et al 2010;Ruff et al 2010) with applications to carbonaceous aerosols in the atmosphere and in glacier ice (Sigl et al 2009;Szidat 2009a,b;Zhang et al 2012). A particular focus was on building and refining of an automated gas interface (Wacker et al 2013a), which enables the transfer of carbon dioxide to the gas ion source from different sources, such as sealed glass ampoules, an acidification device for carbonate samples (Wacker et al 2013b), as well as an elemental analyzer or other combustion instruments (Perron et al 2010).…”

Advanced source apportionment of carbonaceous aerosols by coupling offline AMS and radiocarbon size-segregated measurements over a nearly 2-year period

“…traffic exhaust emissions and coal burning) sources (Lemire et al, 2002;Szidat et al, 2004Szidat et al, , 2009. The measurement of the 14 C content of total carbon (TC), which comprises the elemental carbon (EC) originating from combustion sources and the organic carbon (OC), had been the subject of many studies (Schichtel et al, 2008;Glasius et al, 2011;Genberg et al, 2011;Zhang et al, 2012Zhang et al, , , 2016Zotter et al, 2014b;Bonvalot et al, 2016). Results have shown that in European sites, especially in Alpine valleys, the non-fossil sources play an important role during winter due to biomass burning and in summer due to biogenic sources (Gelencsér et al, 2007;Zotter et al, 2014b).…”