Application of the ECT9 protocol for radiocarbon-based source apportionment of carbonaceous aerosols

Abstract

Abstract. Carbonaceous aerosol is mainly composed of organic carbon (OC) and elemental carbon (EC). Both OC and EC originate from a variety of emission sources. Radiocarbon (14C) analysis can be used to apportion bulk aerosol, OC, and EC into their sources. However, such analyses require the physical separation of OC and EC. Here, we apply of ECT9 protocol to physically isolate OC and EC for 14C analysis and evaluate its effectiveness. Several reference materials are selected, including two pure OC (fossil “adipic acid” and contemporary “sucrose”), two pure EC (fossil “regal black” and “C1150”), and three complex materials containing contemporary and/or fossil OC and EC (“rice char”, NIST urban dust standards “SRM1649a” and “SRM8785”, i.e., fine fraction of resuspended SRM1649a on filters). The pure materials were measured for their OC, EC, and total carbon (TC) mass fractions and corresponding carbon isotopes to evaluate the uncertainty of the procedure. The average accuracy of TC mass, determined via volumetric injection of a sucrose solution, was approximately 5 %. Ratios of EC/TC and OC/TC were highly reproducible, with analytical precisions better than 2 % for all reference materials, ranging in size from 20 to 100 µg C. Consensus values were reached for all pure reference materials for both δ13C and fraction modern (F14C), with an uncertainty of < 0.3 ‰ and approximately 5 %, respectively. The procedure introduced 1.3 ± 0.6 µg of extraneous carbon, an amount compatible to that of the Swiss_4S protocol. In addition, OC and EC were isolated from mixtures of pure contemporary OC (sucrose) with pure fossil EC (regal black) and fossil OC (adipic acid) with contemporary EC (rice char EC) to evaluate the effectiveness of OC and EC separation. Consensus F14C values were reached for all OC (∼ 5–30 µg) and EC (∼ 10–60 µg) fractions with an uncertainty of ∼ 5 % on average. We found that the ECT9 protocol efficiently isolates OC or EC from complex mixtures. Based on δ13C measurements, the average contribution of charred OC to EC is likely less than 3 % when the OC loading amount is less than 30 µg C. Charring was further assessed by evaluating thermograms of various materials, including aerosol samples collected in the Arctic and from tailpipes of gasoline or diesel engines. These data demonstrate that the ECT9 method effectively removes pyrolyzed OC. Thus, the ECT9 protocol, initially developed for concentration and stable isotope measurements of OC and EC, is suitable for 14C-based apportionment studies, including µg C-sized samples from arctic environments.

Radiocarbon (14C) Analysis of Carbonaceous Aerosols: Revisiting the Existing Analytical Techniques for Isolation of Black Carbon

“…Here, we discuss six different analytical techniques (ATs; Table 1) that have been used for isolating EC (BC) for radiocarbon analysis: AT 1 (Gustafsson et al, 2009;Andersson et al, 2015), AT 2 (Zhang et al, 2012), AT 3 (Dusek et al, 2014;Zenker et al, 2017), AT 4 (Bernardoni et al, 2013), AT 5 (Liu et al, 2017), AT 6 (Huang et al, 2021).…”

“…Prior to that, suitable (EC) reference materials for 14 C analysis on aerosols need to be defined. This is because, presently, only ambient filter samples are used in 14C inter-comparisons where the "true" f m is not known (Liu et al, 2017;Zenker et al, 2017;Huang et al, 2021). Therefore, despite similar or different results, the accuracy of all methods still remains impossible to decide.…”

“…Hence, we conducted a review of all such analytical techniques with the sole intention of making the user aware of their associated caveats to be able to make an informed decision regarding the choice of a technique. Here, we review approaches that build on the commercially available thermo-optical OC-EC analyzer (Model 4L, Sunset Laboratory Inc., Tigard, OR, United States) for BC isolation, which has been the community standard for such analysis (Zhang et al, 2012;Andersson et al, 2015;Liu et al, 2017;Huang et al, 2021), as well as include approaches using inhouse developed systems (e.g., Bernardoni et al, 2013;Dusek et al, 2014). We highlight the need for reporting specific ancillary information with Δ 14 C results for improving transparency in the conditions in which BC was isolated.…”