| As a developing country,China's rapid development of industrialization and urbanization has caused serious air pollution problems.It has seriously affected social development and people's normal life.The main indicator of air pollutants is PM2.5.The exhaust pollutants from motor vehicles have been recognized as one of the main sources of PM2.5 in urban atmospheric environment.Therefore,it is of great practical significance to study the physical and chemical characteristics and aging behavior of motor vehicle emissions.This research investigated the morphology,size distribution,chemical compositions,mixing state and aging state of particles in different tunnels?two tunnels are given priority to diesel-powered vehicles and gasoline-powered vehicles,respectively?and road environments using transmission electron microscopy with an energy-dispersive X-ray spectrometry?TEM-EDX?,scanning electron microscopy with an energy-dispersive X-ray spectrometry?SEM-EDX?,atomic force microscope?AFM?,inductively coupled plasma mass spectrometry?ICP-MS?and ion chromatography?IC?.Tang-lang Hill?TLH?tunnel is given priority to diesel-powered vehilces,which accounted for 70.38% in all the vehicles passing through the tunnel,all the vehicles approximately 84 every ten minites.North-Ning Park?NNP?tunnel is given priority to gasoline-powered vehicles,which accounted for 92.62% in all the vehicles passing through the tunnel,all the vehicles approximately 486 every ten minites.Xueyuan Road in Beijing is also given priority to gasoline-powered vehicles,which accouted for 93.28% in all the passing vehicles,all the vehicles approximately 339 every ten minites.The analysis of mass concentration of PM2.5 in two tunnels showed that the concentration of PM2.5 in TLH tunnel was 197.2 ?g/m3,while the value was 85.9 ?g/m3 at the urban site in Shenzhen,indicating that the mass concentration of PM2.5 was much higher in the tunnel than at the correspongding urban site.The mass concentration of PM2.5 in exit sampling site in the NNP tunnel was 164.9 ?g/m3,while the value was 89.5 ?g/m3 in middle sampling site in the NNP tunnel,suggesting that the PM2.5 in NNP tunnel is comply with piston effect,which refers to the PM2.5 was transferred from the middle to the exit of the tunnel by the passing vehicles,resulting in the accumulation of PM2.5 in exit sampling site,so the mass concentraion of PM2.5 was higher in the exit than in the middle sampling site.Traffic congestion could result in higher mass concentration of PM2.5 in tunnels.It was also found that the the mass concentration of PM2.5 was much higher in the TLH tunnel than in the NNP tunnel,suggesting that there was clear relationship between the mass concentration of PM2.5 and vehicle types.Diesel-powered vehicles were likely reduce to higher mass concentration of PM2.5.The analysis of individual particles using TEM-EDX showed that the particles in tunnels were seperated into seven types according to particle morphologies,elemental compositions and stability under strong electron beams,namely,soot aggregates,organic particles,metal particles,sulfate particles,mineral particles,fly ash,and mixture particles of multiple components.The particles in tunnels could be apportioned into four groups according to their probable sources,namely,particles from tailpipe emissions,wear debris,road dust and secondary particles.Soot aggregates,organic particles,a part of metal particles and sulfate particles were emitted from tailpipe emissions.They represented primary particles emitted from gasoline and diesel engines.Some other metal particles were wear debris.Mineral particles and fly ash were classified into road dust.Secondary particles included mixture and other parts of sulfate particles.Sulfate,soot,mineral and mixture particles accounted for over 90% in total particles in the TLH tunnel,while sulfate,soot and mixture particles accounted for over 90% in total particles in NNP tunnel,suggesting that except for exhaust emissions,non-exhaust emissions also make a large contribution to particles in tunnels.Traffic congestion could lead to the the increase of the relative abundance of soot and mixture particles.The relative abundance of sulfate particles was higher in the exit sampling site than in the middle sampling site,whicle the relative abudance of soot particles was lower in the exit sampling site than in the middle sampling site.With the increase of particle size,the relative abundance of sulfate particles increased first and then decreased,while the relative abundacne of mixture particles increased all the time.The geometric mean diameters of soot,organic and metal particles in tunnels were smaller,while the geometric mean diamters of mineral and mixture particles in tunnels were larger.The geometric mean diameter of all the particles in tunnels was smaller than that at the corresponing urban site.The geometric mean diameters of particles in the TLH tunnel which was given priority to diesel-powered vehicles was higher than that in the NNP tunnel which was given priority to gasoline-powered vehicles.Sulfate particles accounted for the largest proportion in all the particles in the two tunnels,and sulfate was a significant inorganic component in tunnel environment.The sulfate particles in tunnels were seperated into 6 types: single sulfate particles,long-striped sulfate particles,core-shelle structure sulfate particles,core-shell structure sulfate particles with mixed cores,mixed sulfate particles and CaSO4.The long-striped sulfate particles contained element potassium.Sulfate particles were more easily coated by organics in the TLH tunnel than at the coreesponding urban site,while the aging degree of sulfate particles in the TLH tunnel was weaker than that at the urban site,most likely due to the absence of photochemical aging process.The relative abundance of core-shell structure sulfate and core-shell structure sulfate with mixed cores was higher in the exit sampling site than in the middle sampling site in the NNP tunnel,while the relative abundance of single sulfate particles and long-srtiped sulfate particles were lower in the exit sampling site than in the middle sampling site in the NNP tunnel.Mixture particles in tunnels included irregular-shaped particles and core-shell structure?C-S?particles.The composition and morphologies of the cores of the C-S particles were similar to those of mineral particles,metal particles,and mixture particles of sulfate with mineral or metal components.The shells of C-S particles were mainly composed of organics.So the C-S particles in tunnels could be apportioned into three types with three different cores: core-shell structure particles with cores of sulfate,core-shell structure particles with cores of mixture of sulfate and mineral or metal and core-shell structure particles with cores of mixture of other particles without sulfate particles.The three kinds of core-shell structure particles were used to assess the aging degree of the particles in the semi-closed tunnel environement using core-shell ratio?R?.The smaller the R of a core-shell structure particle is,the more aged it becomes.The aging degree of the three kinds of core-shell structure particles in tunnels in descending order was: core-shell structure particles with cores of mixture of mineral/metal particles without sulfate particles,core-shell structure particles with cores of mixture of sulfate and mineral/metal particles,and core-shell structure particles with cores of sulfate.These data suggested that mineral and metal components have very likely accelerated the particle aging in the absence of photochemical reactions in the semi-closed tunnel environment.There was no siginicant difference in the aging degree of particles in the middle and exit sampling sites,indicating that the aging of particles in tunnels was primary aging.They would not to be more aged because of without solar radiation and photochemical process.Therefore,a three-stage evolution model of traffic-derived particles from tailpipe to the ambient atmosphere was established.Firstly,the particles coagulate and mixed after they were emitted,secondly,these particles primary aged in the semi-closed tunnel environment without sunlight and solar radiation,thirdly,they were further photochemical aged in the atmospheric environment.The analysis of heavy metal elements in PM2.5 in tunnels showed that the mass concentration of heavy metal elements in PM2.5 in tunnel was much higher than that at the corresponding urban site.Zn,Ti,Ba,Cu were the major heavy metal elements in PM2.5 in the two tunnels and Xueyuan Road.Besides Cu,Cd,Ti,Sb and Zn were likely sourced from wear debris from brake and tire through the enrichment factor.The analysis of water soluble ions in PM2.5 in tunnels showed that SO42-,NH4+ and NO3- ?SNA?accounted for a large proportion in all the water soluble ions.The mass concentration of water soluble ions in PM2.5 in tunnels was higher than that at the corresponding urban site.The concentration of SNA ions in PM2.5 was higher in the exit sampling site than the middle sampling site,indicating that SNA ions was also comply with piston effect,and has a positive correlation with the mass concentration of PM2.5 in tunnels.The morphology and water soluble ions of size-segregated particles collected by Anderson was derived by SEM and IC.The results showed that the particles in the larger size ranges?2.1-5.8 ?m?were mainly individal mineral particles,the shapes were irregular,and the mass concentration of water soluble ions was low,Ca2+ and NO3- were relatively higher,indicating that the mineral paricles in larger size ranges were likely changed.The particles in the smaller size ranges?0.43-2.1 ?m?were spherical or approximately spherical.Under strong electron beam,the smaller particles would be rapidly volatilized.The water soluble ions in smaller size ranges were relatively higher,and SNA accounted for a large proportion,this phenomenon indicated that the smaller particles under SEM was secondary particles.SNA accounted for 95.55% in all the water soluble ions in the particles in the smaller size ranges,and mainly concentrated in 0.56-1.1 ?m size ranges.In addition,except for SNA,the concentration of K+ was relatively higher,suggesting that secondary particles contained a little K,this phenomenon verify that the long-striped sulfate particles contained a little K in the former text.In summary,the characteristics of single particle,aging state and chemical composition of PM2.5 in the tunnels and the urban road environments are systematically studied by a variety of analytical methods. |
PDF Full Text Request