Analysis Of Chemical Composition And Pollution Characteristics Of PM_2.5 In Northern Urban Areas Of Xi’an During COVID-19 Period

In the wake of the development of national economy,the acceleration of urbanization,the surge of energy consumption,the continuous growth of population density and the rapid rise of urban motor vehicle ownership,fine particle pollution in Xi’an is more serious.It is necessary to deeply analyze the chemical composition,pollution characteristics,formation mechanism and main pollution sources of PM_2.5.However,2019 novel coronavirus（COVID-19）outbreak suddenly broke out,and all levels of government adopted measures such as stoppage,shutting down,closing classes,traffic control and crowd restriction.The industrial structure and pollution emission structure changed greatly,which had a great impact on the sources and pollution characteristics of PM_2.5 components.Therefore,in order to understand the pollution sources under low anthropogenic pollution intensity and provide support for PM_2.5 control,PM_2.5 was studied during the outbreak of COVID-19 in Xi’an city.In this study,PM_2.5 samples were collected in the northern urban area of Xi’an from January to March 2020 by using online monitoring instruments.Combined with the ambient air quality data of national control observation points,the chemical composition,particle type and mass spectrum characteristics of PM_2.5 in the northern urban area of Xi’an were studied.On this basis,the positive definite factor matrix（PMF）method and the backward trajectory analysis（Trajstat）method are introduced to study the source of fine particulate matter,and the source of point atmospheric fine particulate matter is analyzed from two aspects of regional transport and local source.The major results of this study are as follows:（1）Compared to pre-COVID-Lockdown stages,the mass concentrations of PM_2.5,CO,SO₂,NO_X decreased by 10.56%,21.89%,13.40%,65.98%during COVID-Lockdown,respectively.（2）Due to the“epidemic”control measures,the source of pollutants has changed to a certain extent.NO₃^-、SO₄^2-and NH₄⁺were still the main components of water-soluble ions in the four stages,and the water-soluble ions were neutral and alkaline.（3）Through the analysis of OC/EC of fine particles in the northern urban area of Xi’an City,it was found that pre-COVID-Lockdown（PC）,COVID-Lockdown（DC）,gradual resumption of work（GW）,and almost complete resumption of work（AW）were all affected by coal combustion and automobile exhaust sources.（4）According to the analysis of chemical composition characteristics of mass spectrometry,EC particles accounted for the highest proportion,indicating that the northern urban area of Xi’an was mainly affected by primary pollution during the observation period.In DC stage,due to the influence of“epidemic”control measures,the proportion of EC and HM particles decreased significantly;OC and ECOC particles showed the same trend in the first three stages,indicating that their pollution sources were the same;at the same time,the most serious pollution in PC stage was due to the stable atmospheric stratification in winter and the inversion layer near the ground in Xi’an,which led to the accumulation of air pollution and the continuous aggravation of pollution.the particle size is mainly distributed in the range of 0.4-1.4μm,which is related to the aging degree of particles,particles with large particle size were more aged;During the lockdown of the epidemic,the amount of OC and Sulfate＿Al has changed significantly.It is due to the reduction of human activities that the source of pollution has changed.according to the time series,the organic acid-containing particles decreased significantly at this stage;Through the analysis of mixing state and wind dependent analysis,OC,Nitrate-related particles were mainly from the southeast where highways are located,sulfate-related were mainly from the west where a power plant is located,and dust-related particles were from residential areas and traffic.And main sources of organic acid-containing particles were biomass combustion and traffic emissions.（5）The PMF model was used to analyze the pollution sources in the northern urban area of Xi’an city.The results showd that the pollution sources in the northern urban area of Xi’an City during the period of COVID-19 mainly came from motor vehicle sources,coal combustion,industrial processes,secondary pollution,biological combustion,dust sources.（6）Based on the trajectory clustering results of the northern urban area of Xi’an,it was found that the PC and DC stages were mainly affected by the local short-distance air mass transmission and the northwest long-distance transmission,and affected by the adverse meteorological diffusion conditions and the northern winter heating.The local air flow from the central Shaanxi carries a large number of pollutants,which was easy to accumulate,resulting the concentration of PM_2.5highier.The GW and aw stages are mainly affected by local short distance transmission and air mass in Sichuan and Chongqing.The air mass that has influence on the four stages in the study period is from the southern part of Henan Province.By using the PSCF and CWT analysis,it was found that the northern urban area of Xi’an city is significantly affected by the external transport pollution during the study period,and the seasonal distribution characteristics of PSCF and CWT of PM_2.5 in Xi’an city are similar.