Chemical Composition And Source Analysis Of PM_2.5 In Four Cities In Southern Sichuan

In recent years,with the rapid development of China’s economy and the rapid growth of energy consumption,the situation of air pollution has become more and more serious,and the frequent occurrence of haze events has been widely concerned by people from all walks of life.In this study,PM_2.5 samples were collected from four representative cities（Luzhou,Neijiang,Yibin and Zigong）in southern Sichuan Basin,from September 2015 to August 2016.Firstly,the concentration level of PM_2.5 and its chemical components（water-soluble inorganic ions,carbonaceous components and inorganic elements）were analyzed.Secondly,the concentrations,seasonal and spatial variations and pollution sources of PM_2.5 and its chemical components in four cities of South Sichuan were discussed.Finally,the sources apportionment of PM_2.5,the transport track of air mass and the potential sources of PM_2.5 were analyzed by using positive matrix factorization model（PMF）,backward trajectory model and potential source contribution model（PSCF）.This study aimed at providing a theoretical basis for the prevention of atmospheric pollution.The main conclusions were as followed:（1）The annual mass concentration of PM_2.5 in southern Sichuan were 46.7-68.5μg·m^-3.Higher mass concentrations of PM_2.5 were observed in winter compared to other seasons,while the lowest mass concentrations of PM_2.5 were observed in summer.The PM_2.5 pollution levels were given priority to excellent weather and good weather,with the total percentages of 66.7-82.5%.In addition to Neijiang,the other three cities had heavy pollution.The PM_2.5 pollution was most serious at Zigong,followed by Yibin and Luzhou,and the lightest at Neijiang.（2）SO₄^2-,NH₄⁺and NO₃^-were the most sufficient ion species,with the highest concentration in winter and the lowest in summer.Secondary inorganic ions（SNA）accounted for 28.6-55.8%of PM_2.5,which indicated that the secondary air pollution in southern Sichuan was serious.High homology had been found between SO₄^2-and NO₃^-during winter.The dominant forms of NH₄⁺in PM_2.5 were NH₄NO₃,（NH₄）₂SO₄ and NH₄HSO₄.The total carbon aerosol（TCA）accounted for 29.32-43.13%of PM_2.5,which was an important part of PM_2.5.The concentrations of OC and EC were the highest in winter and the lowest in summer.The correlation between OC and EC was poor,and the sources of the two carbonaceous species showed great diversity,including vehicle exhaust,coal combustion and biomass burning.There were SOC pollutions in the atmosphere and SOC had high concentrations during winter.Inorganic elements accounted for 4.0-4.9%of PM_2.5,and the concentration of crustal elements is higher.According to the enrichment factor method（EF）,Mg in Luzhou and Zigong were originated from soil;Ca,Na,Fe,Ti,Mn,Ba,Ni,and Mg in Neijiang and Yibin were influenced by both natural and anthropogenic sources;K,Zn,Cr,Pb and Cu were mainly derived from anthropogenic sources.（3）The results of source apportionment indicated that PM_2.5 in southern Sichuan was mainly attributed to eight factors:secondary sulphate,biomass burning,industrial emission,secondary nitrate,vehicle exhaust,coal combustion,road dust,construction dust.Most of the PM_2.5 cluster tracks came from Yunnan,Guizhou,Chongqing,Tibet,Gansu and Sichuan,and the tracks with high PM_2.5 concentration came from Tibet,Chongqing,Northern Guizhou,Northeastern Yunnan and Sichuan.The main potential source areas of PM_2.5 were Chongqing,Northeast Yunnan,Guizhou and Sichuan.In winter,the potential source areas covered a wider range and the WPSCF value was higher.