Evaluation of a two-step thermal method for separating organic and elemental carbon for radiocarbon analysis

Abstract

Abstract. We thoroughly characterized a system for thermal separation of organic carbon (OC) and elemental carbon (EC) for subsequent radiocarbon analysis. Different organic compounds as well as ambient aerosol filter samples were introduced into an oven system and combusted to CO 2 in pure O 2 . The main objective was to test which combustion times and temperatures are best suited to separate OC and EC. The final separation step for OC was combustion at 360 • C for 15 min. Combustion at this temperature proved enough to remove several organic test substances from the filter (including high molecular weight humic acid) but did not remove substantial amounts of EC. For isolation of EC, OC first needs to be completely removed from the filter. This was achieved by water extraction of the filter, followed by combustion of the water insoluble OC at 360 • C and combustion at an intermediate temperature step of 2 min at 450 • C. This last step removed the most refractory OC together with some EC. Finally, the remaining EC was combusted to CO 2 at 650 • C. The recovery of black carbon after the intermediate 450 • C step was approximately 80 %. Several tests provided strong evidence that OC was removed efficiently during the intermediate temperature step: (i) brown carbon, indicative of refractory OC, was removed; (ii) the fraction modern of EC did not decrease significantly if the temperature of the intermediate step was further increased. Based on tests with various organic compounds, we estimated that charred organic carbon could contribute 4-8 % to an elemental carbon sample that was isolated according to our method.

Sources and formation mechanisms of carbonaceous aerosol at a regional background site in the Netherlands: insights from a year-long radiocarbon study

“…From the deviation of the measured F 14 C from the nominal values, the contamination introduced by the combustion procedure can be estimated. This contamination is on average below 3 µg C/combustion (Dusek et al, 2014) and relatively small compared to typical sample amounts between 50 and 200 µg C.…”

“…EC is combusted after OC has been completely burned off the filter. To achieve complete OC removal, WSOC is first removed from the filter by water extraction to prevent charring of organic material (Dusek et al, 2014). Subsequently, most WIOC is removed by heating the filter piece at 360 • C in O 2 for 15 min.…”

“…Then the oven temperature is raised to 450 • C for 2 min and in this step a mixture of the most refractory OC and EC is burned off the filter in O 2 . The remaining EC is then combusted by heating at 650 • C. Zhang et al (2012) and Dusek et al (2014) estimate that after water extraction and flash combustion in O 2 , charred organic compounds can contribute roughly 5 % to the recovered EC. A mean bias for charring of 0.04 is therefore subtracted from the fraction modern of EC (F 14 C (EC) ) before source apportionment, assuming 5 % of the measured EC fraction consists of charred OC and that F 14 C (OC) is approximately 0.8.…”

“…The separation of OC and EC for all filter samples was performed on the aerosol combustion system (ACS) that was developed at the University of Utrecht and has been described in detail by Dusek et al (2014). Briefly, the ACS consists of a combustion tube, where aerosol samples are combusted at different temperatures in pure O 2 , and a purification line where the resulting CO 2 is isolated and separated from other gases, such as water vapor or NO x .…”

“…The carbon fraction considered representative of OC is combusted by heating of a different filter piece at 360 • C for 15 min in O 2 . For WIOC a water-extracted (see Dusek et al, 2014) filter piece is heated at 360 • C for 15 min in O 2 . EC is combusted after OC has been completely burned off the filter.…”