Characteristics Of PM_2.5 At Three Sites In Wuhan

Since the comparative study on the PM_2.5 pollution in different functional zones in Wuhan is relatively scarce, PM_2.5 samples were collected simultaneously at the campus of HuaZhong Agricultural University（HZ）, the Hao Gou portion of South Luo Shi Road（HG）, and the DongHu scenic area（DH） using the MiniVol samplers. The sampling was conducted in March, July, October of 2015 and January 2016. Ion Chromatography and Atomic Absorption Spectroscopy were employed to determine the contents of water soluble ions（Na+, NH4+, K+, Mg2+, Ca2+, Cl-, NO3-, SO42-） and some selected metallic elements（including Fe, Mn, Cu, Zn, Ni, Cr, Cd, and Pb）, respectively. Results showed that the ensemble average of PM_2.5 concentration over all the monitoring period of three sites were 108.72±54.34, 103.30±37.32, 98.27±32.88μg/m3, for HG, DH and HN,respectively. During the season of spring and winter, HG site had the highest concentrations of 110.50 and 137.52μg/m3. HN site has the highest concentration of130.60μg/m3 for autumn. In summer, the concentrations of three sites are close to each other, with the value of 77.54, 73.06, 68.36μg/m3, respectively. Although the daily average concentrations of three sites were different, they shared the same variation trend during the sampling time.Of all the measured metallic elements, Fe always had the highest proportion of more than 50%, followed by Zn, Pb, and Cu. Both the lowest concentration of Cr and highest concentration of Ni appeared in spring. On the other hand, the highest concentrations for the remaining elements were observed in autumn and winter, and the lowest in summer. Regarding the difference at three sites, the highest concentration of Pb and Zn appeared in the HG site. This might be caused by the vehicle emission as HG site is located at the curbside of a busy road. The estimates of enrichment factor（EF） showed that the EF values of all elements in the three sites were quite similar. Mn and Cr had the EF values of lower than 10 at all the three sites, which suggested that the emission ofnatural sources is predominant. The EF values for Cd, Zn, Cu, Ni and Pb were all higher than 10, indicating that these elements mainly come from anthropogenic emissions.The average concentration of total water soluble ions was 37.22μg/m3,which constituted 36.1% of PM_2.5.The three main water soluble ions, SO42-, NO3- and NH4+,made up to more than 80% of the total value. For all the three sites, NH4+ had the highest concentration in summer, while the others shared a common variation trend, with highest in autumn and winter, and lowest in summer. Results of correlation analysis showed that the correlations among Mg2+, K+, Ca2+were significant, and the same applied for Cl-,K+,and NH4+. In addition, significant correlation was also observed between SO42- and NO3-.The average ratio of NO3- to SO42- for the three sites were all lower than 1. However,NO3- /SO42- ratios were close to 1, being relatively higher in autumn and winter. This suggested that contribution of mobile sources increased in autumn and winter.The coefficients of discrepancy between the three sites were all close to 0, which indicated that the difference of chemical components of PM_2.5 among the three sites are insignificant.