An Optimized Method for Determination of Water Soluble Organic Carbon and its Isotopic Composition in Aerosol Samples

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. Stable carbon isotope (δ 13 C) can be used to track the potential sources and investigate atmospheric processes of organic aerosols. In this study, a method of simultaneously measuring the mass concentration and δ 13 C values of WSOC from aerosol samples is established by coupling the Gas Bench 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 carbon larger than 5 μg. This method is then applied for the high time-resolution aerosol samples during a severe wintertime haze period in Nanjing, East China. WSOC varies between 3–32 μg m −3 , whereas δ 13 C-WSOC ranges from −26.24 ‰ to −23.35 ‰. Three different episodes (e.g., namely the Episode 1, the Episode 2, the 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 the Episode 1 indicate that WSOC is subject to a substantial photochemical aging during the air mass transport. In the 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 the Episode 3, heavier isotope ( 13 C) is exclusively enriched in total carbon (TC) compares to WSOC aerosols. This suggests that water-insoluble carbon may contain 13 C-enriched components such as dust carbonate which is supported by the enhanced Ca 2+ concentrations and air mass trajectories analysis. The present study provides a novel method to determine 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.