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Effects of Chemical Composition of PM2.5 on Visibility in a Semi-Rural City of Sichuan Basin

Category: Urban Air Quality

Accepted Manuscripts
DOI: 10.4209/aaqr.2017.08.0264
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Yun-Chun Li 1,2, Man Shu1, Steven Sai Hang Ho 3, Jian-Zhen Yu4, Zi-Bing Yuan4, Xian-Xiang Wang1, Xiao-Qing Zhao1

  • 1 College of Science, Sichuan Agricultural University, Ya’an 62504, China
  • 2 Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control (AEMPC), School of Environmental Sciences and Engineering, Nanjing University of Information Science and Technology, Nanjing 210044, China
  • 3 Key Lab of Aerosol Chemistry & Physics, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710075, China
  • 4 Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong, China


The rate of visibility deterioration in Ya’an has been accelerating.
Correlations between visibility, PM2.5, and meteorology were studied.
(NH4)2SO4, OM, and NH4NO3 totally accounted for 92.6% of the ambient bext.
RH contributed ~40.8% of the ambient bext.
More efforts are needed to reduce the average daily PM2.5 concentration.


The rate of visibility deterioration in Ya’an city in Sichuan basin has been accelerating since 2000s. The issue related to the air quality as well as meteorological conditions are reported in this study. Fine particulate matters (PM2.5) were collected in Ya’an from June 2013 to June 2014. Chemical compositions were determined on the samples. Annual average visual range (VR), PM2.5 concentrations, and ambient light extinction coefficient (bext) were 11.9 ± 9.2 km, 64.1 ± 41.6 µg m–3, and 452 ± 314 Mm–1, respectively. The highest PM2.5 and bext and the lowest VR were both seen in winter, followed by spring, autumn and summer. Organic matter (OM), ammonium sulfate [(NH4)2SO4], and ammonium nitrate [NH4NO3] were the major constituents, accounting for 32.8%, 28.3%, and 12.1%, respectively, of the total PM2.5 mass. The revised Interagency Monitoring of Protected Visual Environments (IMPROVE) equation was applied to estimate ambient bext. On an annual basis, (NH4)2SO4 was the most significant contributor (43.1%), followed by OM (27.1%) and NH4NO3 (22.4%), which totally accounted for 92.6% of the ambient bext. Rayleigh, elemental carbon, fine soil, nitrogen dioxide, and chloride salt accounted for minor fractions (7.4%). Up to ~40.8% of the ambient bext was ascribed to relative humidity (RH), of which 26.4% and 14.0% attributing to the hygroscopic growth of (NH4)2SO4 and NH4NO3, respectively. More efforts are needed to reduce the average daily PM2.5 concentration of < 59 µg m–3 to avoid occurrence of haze under a high average RH of 78.3 ± 10%, which significantly impacted on visibility through various physic-chemical processes. Emissions of precursor gases such as sulfur dioxide, nitrogen oxides, ammonia and volatile organic compounds should be reduced to improve the air quality and visibility in Ya’an.


Fine particulate matter Visibility Light extinction coefficient Chemical composition

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