Abstract
Abstract. To investigate the sources and formation mechanisms of carbonaceous aerosols, a major contributor to severe particulate air pollution, radiocarbon (14C) measurements were conducted on aerosols sampled from November 2015 to November 2016 in Xi'an, China. Based on the 14C content in elemental carbon (EC), organic carbon (OC) and water-insoluble OC (WIOC), contributions of major sources to carbonaceous aerosols are estimated over a whole seasonal cycle: primary and secondary fossil sources, primary biomass burning, and other non-fossil carbon formed mainly from secondary processes. Primary fossil sources of EC were further sub-divided into coal and liquid fossil fuel combustion by complementing 14C data with stable carbon isotopic signatures. The dominant EC source was liquid fossil fuel combustion (i.e., vehicle emissions), accounting for 64 % (median; 45 %–74 %, interquartile range) of EC in autumn, 60 % (41 %–72 %) in summer, 53 % (33 %–69 %) in spring and 46 % (29 %–59 %) in winter. An increased contribution from biomass burning to EC was observed in winter (∼28 %) compared to other seasons (warm period; ∼15 %). In winter, coal combustion (∼25 %) and biomass burning equally contributed to EC, whereas in the warm period, coal combustion accounted for a larger fraction of EC than biomass burning. The relative contribution of fossil sources to OC was consistently lower than that to EC, with an annual average of 47±4 %. Non-fossil OC of secondary origin was an important contributor to total OC (35±4 %) and accounted for more than half of non-fossil OC (67±6 %) throughout the year. Secondary fossil OC (SOCfossil) concentrations were higher than primary fossil OC (POCfossil) concentrations in winter but lower than POCfossil in the warm period. Fossil WIOC and water-soluble OC (WSOC) have been widely used as proxies for POCfossil and SOCfossil, respectively. This assumption was evaluated by (1) comparing their mass concentrations with POCfossil and SOCfossil and (2) comparing ratios of fossil WIOC to fossil EC to typical primary OC-to-EC ratios from fossil sources including both coal combustion and vehicle emissions. The results suggest that fossil WIOC and fossil WSOC are probably a better approximation for primary and secondary fossil OC, respectively, than POCfossil and SOCfossil estimated using the EC tracer method.
Highlights
Carbonaceous aerosols are an important component of PM2.5, constituting typically 20 %–50 % of PM2.5 mass in many urban ar-Published by Copernicus Publications on behalf of the European Geosciences Union.H
The (WIOC/elemental carbon (EC))fossil during wintertime averaged 1.6 ± 0.1, which is closer to the primary organic carbon (OC)/EC ratios for coal combustion than those for vehicle emissions (Fig. 8b), suggesting that coal combustion is one important fossil source in winter other than vehicle emissions
This study presents the first source apportionment of various carbonaceous aerosol fractions, including EC, OC, water-insoluble OC (WIOC) and water-soluble OC (WSOC) in Xi’an, China, based on radiocarbon (14C) measurement in four seasons for the year 2015–2016. 14C analysis shows that non-fossil sources are an important contributor to OC fractions throughout the year, accounting for 58 ± 6 % WSOC, 53 ± 4 % OC and 45 ± 5 % WIOC, whereas fossil sources dominated EC, with non-fossil sources contributing 18±6 % EC on the yearly average
Summary
Carbonaceous aerosols are an important component of PM2.5 (particles with aerodynamic diameter < 2.5 μm), constituting typically 20 %–50 % of PM2.5 mass in many urban ar-H. EC is emitted as primary aerosols from incomplete combustion of biomass (e.g., wood, crop residues and grass) and fossil fuels (e.g., coal, gasoline and diesel). In addition to these combustion sources, OC has other non-combustion sources, for example, biogenic emissions, cooking, etc. Unlike EC that is exclusively emitted as primary aerosols, OC includes both primary and secondary OC (POC and SOC, respectively), where SOC is formed in the atmosphere by chemical reaction and gas-to-particle conversion of volatile organic compounds (VOCs) from non-fossil (e.g., biomass burning, biogenic emissions and cooking) and fossil sources (Jacobson et al, 2000; Kanakidou et al, 2005; Hallquist et al, 2009). OC can be separated into water-soluble OC (WSOC) and water-insoluble OC (WIOC), according to water solubility of OC
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