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Characteristics, Formation Mechanisms and Potential Transport Pathways of PM2.5 at a Rural Background Site in Chongqing, Southwest China

Category: Aerosol and Atmospheric Chemistry

Volume: 19 | Issue: 9 | Pages: 1980-1992
DOI: 10.4209/aaqr.2019.01.0010
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Chao Peng1,2, Mi Tian 1,7, Yang Chen1, Huanbo Wang1, Leiming Zhang3, Guangming Shi1,4, Yuan Liu1, Fumo Yang 1,4,5, Chongzhi Zhai6

  • 1 Research Center for Atmospheric Environment, Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences, Chongqing 400714, China
  • 2 University of Chinese Academy of Sciences, Beijing 100049, China
  • 3 Air Quality Research Division, Science and Technology Branch, Environment and Climate Change Canada, Toronto, Ontario, Canada
  • 4 National Engineering Research Center for Flue Gas Desulfurization, College of Architecture and Environment, Sichuan University, Chengdu 610065, China
  • 5 Institute of New Energy and Low-carbon Technology, Sichuan University, Chengdu 610065, China
  • 6 Chongqing Academy of Environmental Science, Chongqing 401147, China
  • 7 School of Urban Construction and Environmental Engineering, Chongqing University, Chongqing 400044, China


  • Chemical characteristics of PM2.5 were investigated at a rural background site.
  • Photochemical reactions and transport led to high SO42– concentrations in summer.
  • The role of NO3 formation was highlighted for the dramatic increase of PM2.5.
  • Ammonium-rich condition promoted NO3 formation under low temperature.


Daily PM2.5 samples were collected at a rural background station (JinYun) located in Chongqing across four consecutive seasons from October 2014 to July 2015. The major water-soluble inorganic ions (WSIIs), organic carbon (OC) and elemental carbon (EC) were analyzed, and their chemical characteristics, transport pathways and potential source regions were investigated. The average annual PM2.5 concentration was 56.2 ± 31.0 µg m–3, of which secondary inorganic aerosol (SNA) and carbonaceous aerosols composed 41.0% and 29.4%, respectively. Higher concentrations of and contributions from SO42–, which were likely caused by the secondary transformation of SO2 into SO42–, were observed in summer than in autumn and spring. Additionally, transportation from the urban area of Chongqing (Yubei) played an important role in elevating the SO42– during this season. Although the accumulation of PM2.5 during pollution episodes in winter was also due to aqueous-phase reactions, based on the entire year, NO3 formation may have been primarily driven by homogeneous gas-phase reactions. Furthermore, the aerosol environment was ammonium-rich, and NH4+ formation promoted the production of NO3 at lower temperatures. The carbonaceous component, which consisted of 81.0–84.6% OC, exhibited higher concentrations in winter than in the other seasons; 50.0–77.2% of the total OC, in turn, was contributed by primary organic carbon (POC). Potential source contribution function (PSCF) analysis suggests that the site was mainly affected by regional pollution originating in the southwestern and northern areas of Chongqing.


PM2.5 Rural background site Chemical transformation Potential transport pathways

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