Characteristics Of PM_2.5 Pollution And Evolution Of Fossil Fuel Combustion And Other Source In Autumn And Winter At Xi’an

Xi’an,the central city of the Fenwei Plain,particulate air pollution is one of the most important environmental issues confronting it with the rapid economic and social development during autumn and winter in the past 20 years.The characteristics of airborne PM,source apportionments,and major chemical species were investigated for the three autumn and winter periods during 2003-2004,2013-2014 and 2019-2020,respectively.The causes of severe PM air pollution in autumn and winter periods were summarized.In order to understand the formation and elimination process of air pollution in different areas of Xi’an,PM_2.5 samples were collected at five sites,including Gaoxing,Shaanxi University of Science and Technology,Xi’an Jiaotong University,Shaanxi Normal University and Fengdong New Town in autumn and winter 2020,respectively.The chemical components of PM_2.5 were obtained,and major sources of PM_2.5 were investigated by using the PMF receptor model.PM_2.5 and the chemical composition showed high relative spatial uniformity during the sampling period.Carbonaceous aerosol,NO₃^-,SO₄^2-,and NH₄⁺were the most abundant components.Besides the crustal elements,large enrichment factor（EFs）of Cu,As,Zn,Pb,Br,Cl,S,and Se were observed during the sampling periods,which indicated that the elements were affected by anthropogenic activities.The variations of chemical components and the species ratios showed that the contributions of primary particulate emissions were significant.The average concentrations of PM_2.5 were 215.0μg m^-3,181.4μg m^-3,and 75.7μg m^-3in Xi’an during the periods of 2003-2004,2013-2014 and 2019-2020,respectively.PM_2.5 concentration decreased significantly.The highest contributions were obseved with organic matter（～30%）;the percentage of nitrates increased significantly（from～9%to～21%）;the proportion of components showed decreasing trends for dust（～20%to～9%）and sulfates（～15%to～10%）,respectively.The major sources of PM_2.5 were fossil fuel combustion sources（including coal combustion and motor vehicle emissions）,biomass burning,dust,industrial emission,and secondary sources in autumn and winter.In the past 20 years,the contribution of secondary sources accounted for the largest proportion,and showed an increasing trend;the contribution percentages of fossil fuel combustion sources were still high（～30%,2013-2014 years）.The contribution percentages of dust and biomass burning showed a decreasing trend.The contribution of motor vehicle emission accounted for a relatively low percentages in the autumn and winter of 2019-2020,which attributed to the prevention and control of the epidemic.Considering the characteristics and source contributions of PM_2.5,the adjustments of industrial structure,energy structure,transportation structure,and land using structure should be strengthened,especially the control of fossil fuel combustion and biomass burning in autumn and winter.The strategy of clean energy heating should be accelerated.Finally,regional prevention and control measures should be developed to improve the air quality in the region.