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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

Volume: 18 | Issue: 4 | Pages: 957-968
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, Zi-Fang Liu5, Xian-Xiang Wang1, Xiao-Qing Zhao1

  • 1 College of Science, Sichuan Agricultural University, Ya’an 625014, 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
  • 5 Ya’an Environmental Monitoring Center, Ya’an 625014, China

Highlights

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.


Abstract

The rate of visibility deterioration in Ya’an city in the Sichuan Basin has been accelerating since the 2000s. Issues 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 till June 2014. The chemical compositions of the samples were determined. The 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 concentration of PM2.5, the highest bext, and the lowest VR were all 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, in total, accounted for 92.6% of the ambient bext. Rayleigh, elemental carbon, fine soil, nitrogen dioxide, and chloride salt accounted for a minor fraction (7.4%). Up to ~40.8% of the ambient bext was ascribed to relative humidity (RH), of which 26.4% and 14.0% were attributed 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 the occurrence of haze under a high average RH of 78.3 ± 10%, which significantly impacts visibility through various physico-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.

Keywords

Fine particulate matter Visibility Light extinction coefficient Chemical composition


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