26 Aug 2022
26 Aug 2022

An Optimised OC/EC Fraction Separation Method for Radiocarbon Source Apportionment Applied to Low-Loaded Arctic Aerosol Filters

Martin Rauber1,2, Gary Salazar1,2, Karl Espen Yttri3, and Sönke Szidat1,2 Martin Rauber et al.
  • 1Department of Chemistry, Biochemistry and Pharmaceutical Sciences, University of Bern, Bern, Switzerland
  • 2Oeschger Centre for Climate Change Research, University of Bern, Bern, Switzerland
  • 3Department of Atmospheric and Climate Research, NILU – Norwegian Institute for Air Research, Kjeller, Norway

Abstract. Radiocarbon (14C) analysis of carbonaceous aerosols is used for source apportionment, separating the carbon content into fossil vs. non-fossil origin, and is particularly useful when applied to subfractions of total carbon (TC), i.e., elemental carbon (EC), organic carbon (OC), water-soluble OC (WSOC), and water-insoluble OC (WINSOC). However, this requires an unbiased physical separation of these fractions, which is difficult to achieve. Separation of EC from OC using thermal-optical analysis (TOA) can cause EC loss during the OC removal step and form artificial EC from pyrolysis of OC (i.e., so-called charring), both distorting the 14C analysis of EC. Previous work showed that water extraction reduces charring. Here, we apply a new combination of a WSOC extraction and 14C analysis method with an optimised OC/EC separation that is coupled with a novel approach of thermal-desorption modelling for compensation of EC losses. As water-soluble components promote the formation of pyrolytic carbon, water extraction was used to minimise the charring artefact of EC, and the eluate subjected to chemical wet oxidation to CO2 before direct 14C analysis in a gas-accepting accelerator mass spectrometer (AMS). This approach was applied to 13 aerosol filter samples collected at the Arctic Zeppelin Observatory (Svalbard) in 2017 and 2018, covering all seasons, which bear challenges for a simplified 14C source apportionment due to their low loading and the large portion of pyrolysable species. Our approach provided a mean EC yield of 0.87 ± 0.07 and reduced the charring to 6.5 % of the recovered EC amounts. The mean Fraction Modern (F14C) over all seasons was 0.85 ± 0.17 for TC, 0.61 ± 0.17 and 0.66 ± 0.16 for EC before and after correction with the thermal-desorption model, respectively, and 0.81 ± 0.20 for WSOC.

Martin Rauber et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2022-625', Anonymous Referee #2, 15 Sep 2022
  • RC2: 'Comment on egusphere-2022-625', Anonymous Referee #1, 21 Sep 2022

Martin Rauber et al.

Model code and software

COMPYCALC Martin Rauber, Gary Salazar

Sunset-calc Martin Rauber, Jan Strähl

Martin Rauber et al.


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Short summary
Carbon-containing aerosols from ambient air are analysed for the radioactive isotope radiocarbon to determine the contribution from fossil-fuel emissions. Light-absorbing soot-like aerosols were isolated by water extraction and thermal separation. This separation is affected by artefacts, for which we developed a new correction method. The investigation of aerosols from the Arctic shows that our approach works well for such samples, where many artefacts are expected.