Source apportionment of carbonaceous aerosols in Xi'an, China: insights from a full year of measurements of radiocarbon and the stable isotope <sup>13</sup>C

Abstract

Sources of organic carbon (OC) and elemental carbon (EC) in Xi'an, China, are investigated based on 1year radiocarbon and stable carbon isotope measurements. The radiocarbon results demonstrate that EC is dominated by fossil sources throughout the year, with a mean contribution of 83 ± 5 % (7 ± 2 µg m −3 ). The remaining 17 ± 5 % (1.5 ± 1 µg m −3 ) is attributed to biomass burning, with a higher contribution in the winter (∼ 24 %) compared to the summer (∼ 14 %). Stable carbon isotopes of EC (δ 13 C EC ) are enriched in winter (−23.2 ± 0.4 ‰) and depleted in summer (−25.9 ± 0.5 ‰), indicating the influence of coal combustion in winter and liquid fossil fuel combustion in summer. By combining radiocarbon and stable carbon signatures, relative contributions from coal combustion and liquid fossil fuel combustion are estimated to be 45 % (median; 29 %-58 %, interquartile range) and 31 % (18 %-46 %) in winter, respectively, whereas in other seasons more than one half of EC is from liquid fossil combustion. In contrast with EC, the contribution of non-fossil sources to OC is much larger, with an annual average of 54 ± 8 % (12 ± 10 µg m −3 ). Clear seasonal variations are seen in OC concentrations both from fossil and non-fossil sources, with maxima in winter and minima in summer because of unfavorable meteorological conditions coupled with enhanced fossil and non-fossil activities in winter, mainly biomass burning and domestic coal burning. δ 13 C OC exhibited similar values to δ 13 C EC , and showed strong correlations (r 2 = 0.90) in summer and autumn, indicating similar source mixtures with EC. In spring, δ 13 C OC is depleted (1.1 ‰-2.4 ‰) compared to δ 13 C EC , indicating the importance of secondary formation of OC (e.g., from volatile organic compound precursors) in addition to primary sources. Modeled mass concentrations and source contributions of primary OC are compared to the measured mass and source contributions. There is strong evidence that both secondary formation and photochemical loss processes influence the final OC concentrations.

High contributions of fossil sources to more volatile organic aerosol

“…F 14 C bb for EC is slightly bigger than F 14 C nf for OC because, besides biomass burning, biogenic emissions also contribute to OC but have a smaller F 14 C than that of biomass burning. The estimation of F 14 C bb and F 14 C nf has been reported in our previous study (Ni et al, 2018). Then, f bb (EC) was used to determine EC mass concentrations from nonfossil biomass (EC bb ) and fossil fuel combustion (EC fossil ):…”

“…The r bb , r coal and r vehicle varies with the fuel types and properties, combustion conditions, sampling and analysis methods, etc. Best estimate of r bb (4 ± 1; average ± standard deviation), r coal (2.38 ± 0.44) and r vehicle (0.85 ± 0.16) is done through a literature search and described in our earlier study (Ni et al, 2018). p is the fraction of EC from coal combustion (EC coal ) in EC fossil .…”

“…For example, Huffman et al (2009b) measured the volatility of primary OA from biomass burning and found that the more volatile BBOA was generally dominated by smoldering combustion, while the less volatile BBOA was more influenced by flaming com-bustion. Paciga et al (2016) found that the BBOA was more volatile than HOA in an urban background site in Paris, with 50 % semi-volatile organic components (SVOCs with effective saturation concentrations C * of 1-100 µg m −3 range) and 30 % low-volatility organic compounds (LVOCs with C * of 10 −3 -0.1 µg m −3 ). Gaston et al (2016) found that levoglucosan (i.e., the biomass burning tracer) was semi-volatile and had the potential to form low-volatility products during the chemical aging of ambient biomass burning aerosol, applying high-resolution time-of-flight chemical ionization mass spectrometer (HRTOF-CIMS) coupled to a Filter Inlet for Gases and AEROsols (FIGAERO) instrument for aerosol in the northwestern US.…”

“…Source and ambient studies indicate that BBOA and HOA are generally more volatile than OOA. Meanwhile, the volatility of BBOA can be quite variable, depending on type of biomass and the combustion conditions, and either higher or lower than that of HOA (e.g., Grieshop et al, 2009c;Huffman et al 2009a;Paciga et al, 2016;Cao et al, 2018). For example, Huffman et al (2009b) measured the volatility of primary OA from biomass burning and found that the more volatile BBOA was generally dominated by smoldering combustion, while the less volatile BBOA was more influenced by flaming com-bustion.…”

Dual‐Isotope Constraints on Seasonally Resolved Source Fingerprinting of Black Carbon Aerosols in Sites of the Four Emission Hot Spot Regions of China

“…However, bottom‐up technology‐based emission inventories (EIs) are acknowledged to carry an uncertainty by factors of 2–4 for BC, with the uncertainty stemming largely from variations in BC emission factors (EF, mass of BC emitted per mass fuel consumed, or product produced) but also from uncertainties in activity estimates (e.g., energy consumption data; e.g., Bond et al, ; Lu et al, ; Wang et al, ; Zhao et al, ). Observationally based studies are combining to demonstrate that the relative contributions of biomass burning versus fossil fuel combustion estimated by bottom‐up EIs are systematically inconsistent with observationally‐based (i.e., top‐down) diagnostic source apportionment of BC in East Asia based on the 14 C‐BC fingerprint (Andersson et al, ; Chen et al, ; Fang et al, ; Li et al, ; Liu, Mo, et al, ; Ni et al, ; Zhang et al, ). These 14 C‐based source apportionments of BC showed that fossil source is dominant in East Asian urban areas (e.g., Beijing, Shanghai, and Guangzhou) and ~50% in Himalayan‐Tibetan Plateau.…”