Analysis On The Pollution Charateristics And Source Apportionment Of PM_2.5 In Fall And Winter In Guanzhong Area

Since reforming and opening up,with rapid development of economy,urbanization and industrialization and enhancement of energy consumption in Guanzhong area day by day,which has been included in the key research areas of air pollution prevention and control,the air pollution caused by it has become more and more serious,especially in autumn and winter,which has aroused widespread concern in society.The persistent polluted weather in autumn and winter in Guanzhong area not only seriously restricts the sustainable development of social economy in this area,but also affects the health and life’s quality of the public.The control of air pollution in Guanzhong area has become the most serious environmental problem faced by the state,Shaanxi provincial government and the people.The study of PM_2.5 pollution characteristics,spatial-temporal variation and emission source in Guanzhong area can provide scientific basis for improving environmental air quality and pollution control.In this paper,single Particle Aerosol Mass Spectrometer（SPAMS）was used to collect atmospheric PM_2.5 samples in Guanzhong,in which five observation points were selected（Xi’an,Baoji,Weinan,Tongchuan and Xianyang）in autumn and winter from September 2017 to February 2018.Based on the above observation data and the corresponding PM_2.5 concentration,the temporal and spatial variations of the chemical composition and mass spectrometry characteristics of PM_2.5 in Guanzhong area and other influencing factors were studied.The Positive Matrix Factorization（PMF）was used to identify the local source apportionments of collected fine particles based on their characteristics and categories.Finally,air flow trajectories of five cities in Guanzhong area were clustered using Trajstat software and GDAS meteorological data to study the transport path and potential source area of particulate matter.The main conclusions are as follows:（1）During the observation period,the average mass concentrations of PM₁₀ and PM_2.5 in autumn and winter in Guanzhong area were 133.1μg/m³ and 78.0μg/m³.The average concentrations of PM_2.5 in Xi’an,Xianyang,Weinan,Baoji and Tongchuan in autumn and winter were（84.1±58.9）μg/m3,（91.3±64.8）μg/m3,（83.1±56.3）μg/m3,（69.0±44.4）μg/m3,（62.4±34.2）μg/m3,respectively.The ratios of PM_2.5/PM₁₀ in five points were 0.57,0.59,0.56,0.59 and 0.57,respectively.PM_2.5 accounts for nearly 60%of PM₁₀ concentration,indicating that the pollution characteristics of fine particulate matter in Guanzhong area are more significant.（2）The chemical composition of PM_2.5 showed that the main chemical components of PM_2.5 particles in Guanzhong area in autumn and winter were potassium,sodium,nitrate,sulfate,organic nitrogen,potassium salt,chloride,phosphate,silicate and ammonium salt.Compared with autumn,the chemical composition of fine particulate matter is more complex in winter.The increase of sulfate,nitrite,chloride,organic nitrogen and oxalic acid is obvious,which indicates that the influence from biomass combustion and coal combustion is increased in winter.In addition,the increase of aluminium and sodium salts is more obvious in winter,which is suspected to be related to coal-burning pollution.The contents of calcium,magnesium,silicon and phosphorus decreased significantly in winter,indicating that the influence of dust pollution was reduced significantly.Potassium,ammonium and other components increased significantly in winter,indicating a high degree of atmospheric mixing in winter,the impact of secondary pollution is obvious.（3）The results of PM_2.5 mean spectra showed that the average mass spectra of PM_2.5in winter and autumn have little change,and the main components of PM_2.5 are basically the same.But the average signal intensity of different components varies greatly in different seasons.Compared with autumn,the signals of C_n⁺,NO₂⁺,K⁺,Na⁺,etc.decreased significantly,while the signals of SO₄²⁺,organic matter increased significantly.（4）According to ART-2a classification results,the main chemical types of PM_2.5 can be divided into 10 categories:organic carbon particles（OC）,macromolecule organic carbon particles（HOC）,elemental carbon particles（EC）,mixed carbon particles（ECOC）,levodextran particles（LEV）,mineral dust particles（Mdust）,heavy metal particles（HM）,potassium-rich particles（K）,sodium-rich particles（Na）,and others.Generally speaking,carbon and potassium-rich particles in the atmosphere are the two largest types of particles in PM_2.5.The proportion of Lev and OC particles in winter is 26.9%and 17.6%respectively,which is related to coal-fired heating in winter.The proportion of EC particles in autumn is 44.6%,indicating that the contribution of motor vehicle exhaust is relatively prominent in autumn.（5）The result of PM_2.5 source apportionment in Guanzhong area:The results of orthogonal matrix factor model（PMF）source analysis showed that the contribution of PM_2.5 quality is industrial source（25.49%）,in which 7.41%of Pb/Li industry,6.36%of Ti industry,11.72%of other industries,19.62%of coal combustion,12.79%of dust pollution,including 8.24%of road dust,4.73%of crustal dust,secondary pollution.（12.43%）,biomass combustion（11.80%）,motor vehicle exhaust（10.60%）,cooking fume（5.63%）and other pollution（1.46%）.（6）Through cluster analysis of the trajectories about the five cities in Guanzhong area in autumn and winter,it was found that air pollution in autumn was mainly affected by local short-distance air mass transport,which should be related to the blocking cycles of air flow encountering the Qinling Mountains.Air pollution in both autumn and winter were affected by Northwest airflow.However,compared with autumn,air pollution in winter was mainly controlled by the cold high pressure from the central Eurasian continent.The Northwest Airflow trajectory was longer and higher,and the total trajectory proportion increases.In addition,due to the adverse weather diffusion conditions and the influence of northern winter heating,the local airflow from central Shaanxi Province carries a large amount of pollutants which are easy to accumulate,resulting in the high concentration of PM_2.5.（7）The results of PM_2.5 source apportionment in Xi’an:the results by PMF model showed that the main pollution sources are coal combustion,industrial pollution,motor vehicle exhaust,dust and secondary pollution,accounting for 28.5%,19.15%,13.06%,12.9%and 12.22%respectively,with a total of 85.83%.In autumn,the main pollution sources are dust,secondary pollution,industrial pollution,coal combustion and motor vehicle exhaust,which contributed 25.43%,21.37%,14.92%,7.86%and 6.24%respectively,totaling 75.82%.Potential source function（PSCF）and concentration weight trajectory（CWT）model were used to analyze the potential source area of PM_2.5.Results showed that the potential source areas of PM_2.5 in Xi’an were mainly concentrated in the Sichuan-Chongqing border,southern Shaanxi,Henan,Shanxi,northern Hubei and its surrounding areas.