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© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 21 Jun 2019

Submitted as: research article | 21 Jun 2019

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This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Sources and formation of carbonaceous aerosols in Xi'an, China: primary emissions and secondary formation constrained by radiocarbon

Haiyan Ni1,2,3, Ru-Jin Huang1, Junji Cao1, Jie Guo1, Haoyue Deng2, and Ulrike Dusek2 Haiyan Ni et al.
  • 1State Key Laboratory of Loess and Quaternary Geology, Key Laboratory of Aerosol Chemistry and Physics, Center for Excellence in Quaternary Science and Global Change, Institute of Earth Environment, Chinese Academy of Sciences, Xi'an, 710061, China
  • 2Centre for Isotope Research (CIO), Energy and Sustainability Research Institute Groningen (ESRIG), University of Groningen, Groningen, 9747 AG, the Netherlands
  • 3University of Chinese Academy of Sciences, Beijing, 100049, China

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, respectively. 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-souble 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.

Haiyan Ni et al.
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Short summary
We present a 1-year source apportionment record of carbonaceous aerosols in Xi'an, China. Biomass burning strongly increases in winter, while seasonal changes of coal and liquid fossil fuel combustion are moderate. We find strong evidence for fossil secondary OC formation during the warm period that is further enhanced in stagnant, polluted conditions due to longer atmospheric residence times. At the same time we find that water insoluble (primary) fossil is lost due to photochemical processing.
We present a 1-year source apportionment record of carbonaceous aerosols in Xi'an, China....