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

Abstract

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.

Organic Compounds, Radiocarbon, Trace Elements and Atmospheric Transport Illuminating Sources of Elemental Carbon in a 300‐Year Svalbard Ice Core

“…(2014) and further refined in Rauber et al. (2022). In brief, 14 C of EC was measured using a Sunset OC/EC analyzer (Model 5L, Sunset Laboratory, USA) coupled with the accelerator mass spectrometer Mini Carbon Dating System (MICADAS) at the Laboratory for the Analysis of Radiocarbon (LARA; Szidat et al., 2014).…”

“…EC was quantified during the EC steps of the Swiss_4S protocol and the volatilized CO 2 was directed to the MICADAS system for 14 C analyses. F 14 C(EC) was extrapolated to 100% EC yield and corrected for pyrolyzed OC with a thermal‐desorption model (Rauber et al., 2022; Rauber & Salazar, 2022).…”

“…Prior to the 14 C measurement, the filters were treated 3 times for carbonate removal under a fume hood by adding 50 μL of 1M HCl followed by air drying (Uglietti et al., 2016, T. Singer, personal communication). A further water extraction of the filters for removal of the water‐soluble organic carbon to minimize the positive artifact from OC charring (such as done in Rauber et al., 2022; Y. L. Zhang et al., 2012) was not necessary, as this step was already included during filtration of the melted ice samples (see Section 2.1). The extracted filters were thermally desorbed in the OC/EC analyzer during the first three steps of the Swiss_4S protocol, volatilizing the remaining water‐insoluble organic carbon from the filter (Y. L. Zhang et al., 2012).…”