Chemical Components And Source Apportionment Of PM_2.5 In Xiangtan,Central South China

Xiangtan,South China,is a typical valley city characterized by year-round high relative humidity and very low wind speeds.Under the influence of this special topographical and meteorological conditions,atmospheric PM_2.5 pollution is rather severe.In order to find out the atmospheric PM_2.5 pollution level,chemical composition characteristics and their contributions to PM_2.5,PM_2.5 samples were collected at YT and KD sites from April 2016 to February 2017.The contents of water-soluble ions,OC/EC and inorganic elements were determined by ion chromatography,OC/EC analyzer and ICP-MS method,respectively,and the chemical mass balance（CMB）and positive matrix factorization（PMF）were carried out for source apportionment of PM_2.5.The following results were obtained:（1）During the sampling period,the mass concentration of PM_2.5 can be as high as 217μg/m³,indicating that PM_2.5 pollution was rather severe.From pollution level PM_2.5 at two sites,it was characteristics of regional pollution.The seasonal variation of PM_2.5 mass concentration was characterized by the highest in winter,the second in autumn and the lowest in spring.（2）The concentrations of the major water-soluble ions in PM_2.5 at YT and KD sites were44.6 and 40.9μg/m³,accounting for 59.2%and 57.7%of the PM_2.5 mass,respectively.The concentration of the secondary inorganic ions(SIAs:SO₄^2-、NO₃^-、NH₄⁺)was the highest,accounting for more than 70%of the total concentration of inorganic water-soluble ions.（3）The average concentrations of OC and EC in PM_2.5.5 were 12.3 and 5.4μg/m³,accounting for 15.5%and 6.9%of the PM_2.5 mass,respectively.The concentrations of OC and EC in PM_2.5 at YT site were higher than that at the KD site.In addition,the concentrations of OC and EC in PM_2.5.5 collected in autumn and winter were higher than that collected in spring.The secondary organic carbon content in PM_2.5 can be as high as 8.35μg/m³,accounting for 50%of the OC concentration.（4）The concentrations of K,Fe,Al,Sb and Zn in the atmospheric PM_2.5 are the highest（i.e.,>300 ng/m³）,followed by Ca,Mg,Pb,Ba,As and Mn（i.e.,>40 ng/m³）.The results of enrichment factor analysis showed that the EF values of Co,Cu,Zn,As,Se,Cd,Sb,Tl and Pb elements were all above 100,indicating that they were mainly from anthropogenic source.（5）The results showed that the concentrations of PM_2.5 and its major chemical components in the winter of 2016 were lower than those in the winter of 2013,indicating that the effects of air pollution control in Xiangtan in the last several years was obvious.Compared with the concentrations of PM_2.5.5 and its major chemical components on clean and polluted days in the two winters,it was found that SIAs were mainly responsible for PM_2.5pollution episodes in Xiangtan.（6）Based on the analysis results of CMB and PMF model,the contributions of the main pollution sources to PM_2.5 mass were secondary inorganic aerosol（24.8%）,vehicle exhaust（20.2%）,coal combustion（20.0%）,fugitive dust（17.2%）,steel industry（8.5%）,and industrial emission（7.5%）.Thus,it is recommended that secondary inorganic aerosols,coal combustion,and vehicles are the primary targets in order to reduce PM_2.5 pollution in Xiangtan.