| Particulate matter is a main pollutant in China. Due to the bad effecs on human health, visibility and climate change, PM2.5(diameter < 2.5μm in particulate matter) is getting a lot of attention. Xi’an, as a typical city in the west, is severely influenced by PM2.5 pollution. Air pollution events happened with high-frequency during the International Horticulture Expo which held in 2011, Xi’an. In order to investigate the chemical composition and optical properties of PM2.5 in Xi’an during the Expo, Mini-vol sampler was used to collect continuous samples of PM2.5 from April 28 th to October 22 nd. Meanwhile, a Single Wavelength Integrating Nephelometer monitored the atmospheric scattering coefficient. Thermal/Optical Carbon Analyzer, Dionex-600 ion chromatograph, and Energy Dispersive X-Ray Fluorescence spectrometer were used respectively to analyse the mass concentration of organic carbon, elemental carbon, water soluble ions and elements in PM2.5 samples.The results show that the average PM2.5 mass concentration during the Expo was 118.1±57.3μg·m-3, with a seasonal change appearing as spring>autumn>summer. Meanwhile, PM2.5 in pollution days is higher than non-pollution days. The average mass concentration of OC and EC was 21.3μg·m-3 and 4.6μg·m-3, respectively covering 19.2% and 3.9% of the whole mass concentration, with a seasonal change trend of autumn>summer>spring. Autumn rising of the OC and EC concentration was mainly related to biomass burning. The OC/EC ratio of spring, summer and autumn was 6.7, 5.9, 5.0 respectively, showing that the spring source of OC and EC was complex while the autumn was from a singular source. The OC/EC ratio of pollution days and non-pollution days were 7.5 and 5.0. Through the calculation of secondary organic carbon aerosols, the average mass concentration of SOC was 14.3μg·m-3 during the Expo, covering 53.2% of OC mass concentration, with spring the highest and summer the lowest.During the Expo, SO42-, NO3- and NH4+ took up the highest proportion of total water soluble ions, 40.0%, 24.9% and 11.6% respectively. Spring has more dust storms, that is why Ca+ was higher than other seasons; SO42- had a significant rise in summer which is probably caused by the high temperature which advanced the transformation of SO2 and SO42-. NO3- growth in autumn with a higher proportion compared to other seasonsThe transformation of motor vehicle exhaust emission, frequent agricultural production activities(such as fertilization, etc.) in autumn might be the reason. Ion component distribution was generally similar when comparing pollution days and non-pollution days. On pollution days, the SO42- rate rose slightly, NO3- decreased while NH4 increased significantly.Inorganic elements are an important part of PM2.5, among which S, Ca, Zn, K are the highest, accounting to about 80% of the total elemental mass concentration; followed by CL, Fe, Ti, Mn, As, Mo, Br, Cr, Ni with a lower concentration. The elements change varied in different seasons and the concentration on pollution days was higher than non-pollution days. Through the Enrichment Factor method figure out that the EF values of Ti, Mn, K, Ca, Cr, Ni elements were below 10, indicating they were come from natural sources and less affected by human acitivies; the EF values of As, Br, Pb, Cl, Zn, S, Mo were all above 10, showing that they were less affected by natural sources, but rather mainly affected by anthropogenic sources. Soil dust calculation found that the soil dust of spring days and pollution days were higher than that of other seasons and non-pollution days.Extinction coefficient, showed a minimum level in spring(608.5 Mm-1), growing in summer(920.6 Mm-1), and reaching the highest in autumn(1144.1 Mm-1). Daily variation of scattering coefficient slightly differed in different seasons, and different pollution days, but the overall trend was consistent. It was closely related to the human activities and atmosphere boundary layer height. Using the IMPROVE formula to reconstruct scattering coefficient, it was found that(NH4)2SO4, OM and NH4NO3 contributed the most to scattering coefficient. In addition, correlated analysis found that the scattering coefficient had substantial positive correlation with PM2.5, NH4+, NO3-, SO42-, showing that they were the main components in PM2.5 that contribute to scattering coefficient. |
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