Study On Pollution Characteristics Of PM_2.5 In North Of Xi’an By Single Particle Aerosol Mass Spectrometer

Atmospheric fine particulate matter has great impacts on ambient air quality,urban regional,climate change and human health.Physical and chemical properties of fine particles,such as particle size distribution,chemical composition,mixing state,and moisture absorption have profund effects on environment.The time resolution of tradition manual sampling is poor,the experimental analysis is complex,and the sample is easy to be lost in the process of storage and transportation,which makes the certain error of the analysis result.SPAMS has a high temporal resolution（1h）,which can effectively reduce the potential of human factors,obtain particle size distribution and chemical composition information simultaneously.This research mainly uses the Single Particle Aerosol Mass Spectrometer（SPAMS）to study the atmospheric fine particulate in different seasons and typical pollution processes from July 2015 to May2016 in Xi ￠ an,besides the single particle size distribution,chemical composition characteristics,mixed states,sources and the aging process analysis.This study is mainly discussed from the following aspects:（1）With the huge amount of data collected by SPAMS,this study use Art-2a algorithm to analyze these data combined with the positive and negative spectra characteristics,total partieles will be divided into nine major categories including organic carbon particles（OC）,macronmolecule carbon particles（HOC）,elemental carbon particles（EC）,elemental-organic carbon particles（ECOC）,levoglucanparticles（LEV）,mineral dust particles（Mdust）,heavy metals particles（HM）,rich-K particles（K）,rich-Na particles（Na）.（2）The characteristics of the chemical composition and size distribution of fine particulate matter in the typical pollution episodes in Xi’an.In this study,we use Single Particle Aerosol Mass Spectrometer（SPAMS）online observation to classify 3,492,815 atmospheric fine particles obtained during January 4 to 20,2016 when heavy pollution took place in Xi’an City and to analyze such characteristics as the proportion of main particle types,size distribution,humidity effect and sources analysis.The results show that the PM_2.5 size mainly ranges from 0.4 μm to 1.4 μm during heavy pollution of winter and main particle types include OC,ECOC and K.Compared with the pollution,the cleaning episodes show an increase of OC and LEV,a decline of EC,ECOC and K,and slight change of Mdust and HM.When the relative humidity is greater than 60%,the abundance of EC,ECOC and HM increases,while HOC is not sensitive to humidity change.In a low humidity environment,the abundance of K,Na and Mdust improves.Therefore,fine particles of Xi’an City in winter are mainly from fuel combustion and industrial emissions,and high humidity and low temperature easily lead to the formation of secondary particles.In this study,we selected a typical dust episode during 16 to 19 in August,2015,the variation characteristics and size distribution of the fine particulate matter during and before the dust storm were analyzed.The results showed that the particle size distribution mainly concentrated between 0.4 1.6 μm.EC were the main particle types before dust storm.The proportion of Mdust was significantly increased the during dust storm,OC and LEV were the main component after dust strom.The proportion of characteristic ions such as Al+,Ca+,Mg+,K+,SiO3-has increased with different degrees during pollution.The change of chemical composition of fine particles was attributed to Xi’an local sources and dust carry the external pollution adsorption and chemical reaction.（3）25 sampling sites,which covered basically the whole Xi’an City,were setfor collecting the road dust samples,using the scanning electron micrescopy（SEM）to scan the road dust,it was found that the road dust samples were mostly irregular particles with large size.The chemical composition and aerodynamic diameter of the road dust were measured simultaneously by the SPAMS,35 722 fine particles with diameter between 0.6².5 μm in both the positive and negative spectra were detected.The results showed that the analysis rate of samples were,in order,main road,secondary road,slip road and expressway.The mass spectra of road dust particles was similar to those of the soil dust and cement dust.The source of road dust was the dust source,traffic source and biomass burnning source.