High time-resolved measurement of stable carbon isotope composition in water-soluble organic aerosols: method optimization and a case study during winter haze in eastern China

Abstract

Water-soluble organic carbon (WSOC) is a significant fraction of organic carbon (OC) in atmospheric aerosols. WSOC is of great interest due to its significant effects on atmospheric chemistry, the Earth's climate and human health. The stable carbon isotope (δ 13 C) can be used to track the potential sources and investigate atmospheric processes of organic aerosols. However, the previous methods measuring the δ 13 C values of WSOC in ambient aerosols require a large amount of carbon content, are time-consuming and require labor-intensive preprocessing. In this study, a method of simultaneously measuring the mass concentration and the δ 13 C values of WSOC from aerosol samples is established by coupling the GasBench II preparation device with isotopic ratio mass spectrometry. The precision and accuracy of isotope determination is better than 0.17 ‰ and 0.5 ‰, respectively, for samples containing WSOC amounts larger than 5 µg. This method is then applied for the aerosol samples collected every 3 h during a severe wintertime haze period in Nanjing, eastern China. The WSOC values vary between 3 and 32 µg m −3 , whereas δ 13 C −WSOC ranges from −26.24 ‰ to −23.35 ‰. Three different episodes (Episode 1, Episode 2 and Episode 3) are identified in the sampling period, showing a different tendency of δ 13 C −WSOC with the accumulation process of WSOC aerosols. The increases in both the WSOC mass concentrations and the δ 13 C −WSOC values in Episode 1 indicate that WSOC is subject to a substantial photochemical aging during the air mass transport. In Episode 2, the decline of the δ 13 C −WSOC is accompanied by the increase in the WSOC mass concentrations, which is associated with regional-transported biomass burning emissions. In Episode 3, heavier isotope ( 13 C) is exclusively enriched in total carbon (TC) in comparison to WSOC aerosols. This suggests that the non-WSOC fraction in total carbon may contain 13 C-enriched components such as dust carbonate, which is supported by the enhanced Ca 2+ concentrations and air mass trajectory analysis. The present study provides a novel method to determine the stable carbon isotope composition of WSOC, and it offers a great potential to better understand the source emission, the atmospheric aging and the secondary production of water-soluble organic aerosols.

Carbonaceous Fractions Contents and Carbon Stable Isotope Compositions of Aerosols Collected in the Atmosphere of Montreal (Canada): Seasonality, Sources, and Implications

“…Carbon isotope compositions (δ 13 C WSOC ) were calibrated using four different internal standards overlapping the range of δ 13 C values expected in aerosol samples (e.g., Fisseha et al, 2009;Kirillova et al, 2013;Suto and Kawashima, 2018;Zhang et al, 2019): benzoic acid (δ 13 C = −29.80 ± 0.05 ), oxalic acid (δ 13 C = −6.46 ± 0.05 ), sucrose (δ 13 C = −11.76 ± 0.07 ), and levoglucosan (δ 13 C = −25.44 ± 0.07 ). δ 13 C of the internal standards were measured by EA/IRMS following the procedure described at section "Elemental Carbon + Organic Carbon and Carbonates."…”

“…While primary OC particles are mainly generated by combustion and/or biogenic sources, secondary organic carbon (SOC) particles are mainly produced through atmospheric reactions involving gaseous organic precursors (Jones and Harrison, 2005;Górka et al, 2014;Sudheer et al, 2016). The water-soluble organic carbon (WSOC) fraction can represent up to 80% of the total OC (Wang et al, 2010;Kirillova et al, 2014a;Bikkina et al, 2017;Zhang et al, 2019). WSOC can easily be incorporated into biological systems, contributing to serious health issues (Mills et al, 2009).…”

“…But while, to date, most studies have focused on the δ 13 C of the total and inorganic carbon aerosol fractions, less attention has been paid to measuring the carbon isotope compositions of the different organic carbon fractions present in urban aerosols, and specifically the δ 13 C of WSOC (Fisseha et al, 2009;Kirillova et al, 2014b;Pavuluri and Kawamura, 2017;Liu et al, 2018;Zhang et al, 2019). The lack of published δ 13 C WSOC data is mostly explained by analytical locks inherent to its isotope analysis.…”

“…This technique (LC/IRMS) allows an online and rapid (∼6 min) carbon isotope analysis of the WSOC fraction by means of a wet oxidation of all the aerosol organic compounds followed by the extraction of the formed CO 2 from the liquid phase (Krummen et al, 2004;Suto and Kawashima, 2018). This method has proved its suitability for the carbon isotope characterization of WSOC in PM compared to the traditional combustion method (Kirillova et al, 2010) as it shows a higher sensitivity, which in turn facilitates the analysis of low−concentration samples (Fisseha et al, 2006;Suto and Kawashima, 2018;Zhang et al, 2019).…”

Measurement report: Source characteristics of water-soluble organic carbon in PM<sub>2.5</sub> at two sites in Japan, as assessed by long-term observation and stable carbon isotope ratio

“…In the past few years, several new analytical methods for δ 13 CWSOC have been developed, such as wet oxidation/IRMS (Suto and Kawashima, 2018), GasBench/IRMS (Zhang et al, 2019), and total organic carbon analyzer/IRMS (Han et al, 2020).…”