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| Titel |
On the isolation of OC and EC and the optimal strategy of radiocarbon-based source apportionment of carbonaceous aerosols |
| VerfasserIn |
Y. L. Zhang, N. Perron, V. G. Ciobanu, P. Zotter, M. C. Minguillón, L. Wacker, A. S. H. Prévôt, U. Baltensperger, S. Szidat |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 12, no. 22 ; Nr. 12, no. 22 (2012-11-16), S.10841-10856 |
| Datensatznummer |
250011602
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| Publikation (Nr.) |
 copernicus.org/acp-12-10841-2012.pdf |
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| Zusammenfassung |
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. |
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