Characteristics And Aging Mechanism Of Individual Particles Emitted By The Vehicles

With rapid development of the economy,the amount of motor vehicles in China has increased rapidly.On the one hand,it has caused an increase in demand for oil in our country,which poses obvious challenges to national energy security.On the other hand,it has caused serious air pollution problems,which affects social development and people's normal life.Vehicle emission is one of the main sources of PM_2.5 in urban atmosphere environment,and vehicle emission control has become one of the most important areas of work for air pollution prevention in big cities in China.The physical and chemical characteristics of particulate matter?PM?emitted from vehicles have different impacts on climate,meteorology and human health,so it is very important to study the physicochemical characteristics and aging mechanism of particulate matter emitted from vehicles.This paper experimentally studied the particle emissions from the vehicles on the chassis dynamometer and the engine test bench.One kind of Port Fuel Injection?PFI?powered gasoline vehicle,one kind of the Gasoline Direct Injection?GDI?engine powered gasoline vehicle and one kind of diesel powered gasoline vehicles from the Chinese market were tested on the chassis dynamometer.The particle number?PN?emission,the morphology,size distribution,chemical composition,mixing state and aging state of particles were measured under the Beijing driving cycle?BDC?.One kind of GDI engine and one kind of PFI engine were tested on the engine test bench.The effects of the engine type?GDI engine and PFI engine?,fuel components?fuels 1-5?,engine loads?25,50,and 75%?,and three-way catalyst?TWC?were tested to understand how different engine operating conditions influence particulate emissions.The morphology,size distribution,chemical compositions,mixing state and aging state of particles were analysed using the transmission electron microscopy with an energy-dispersive X-ray spectrometry?TEM-EDX?,inductively coupled plasma mass spectrometry?ICP-MS?and The nano-scale secondary ion mass spectrometry?NanoSIMS?.The chassis dynamometer test results showed that,under transient conditions,both GDI and PFI vehicles emitted a large amount of particulate matter,especially under the cold start running state,however the PN emission from GDI gasoline vehicles was higher than the emissions of PFI vehicles.Under the cold start running state,particles in the accumulation mode have a certain contribution to the particle emission,and the contribution of particles in the accumulation mode from the GDI vehicle was higher than that from PFI emissions.The PM emission varied under different running conditions?cold start,hot start,hot stabilized running,idle and acceleration running state?.For the GDI vehicle,the emissions were highest under the hot stabilised running state,followed by the hot start,cold start,acceleration,and idle running state.The amount of particles emitted from the vehicles were 2.3x10¹⁰,1.2x10¹⁰,7.1x10⁹,2.9x10⁹ and 7.4x10⁸ particles/?kg fuel?,respectively.For the PFI vehicle,the emissions were highest under the cold start running state,followed by the hot start,hot stabilised,acceleration,and idle running states.The amount of particles emitted from the vehicle were 5.3x10⁹,3.1x10⁹,1.6x10⁹,4.4x10⁸,and 1.0x10⁸ particles/?kg fuel?,respectively.The results of TEM-EDX showed that the particles were mainly composed of organic,soot,and Ca-rich particles,with a small amount of S-rich and metal-containing particles.The particles from the PFI vehicle displayed a unimodal size distribution,with the peak at 0.6?m.While the particles from the GDI vehicle displayed a bimodal size distribution,with the main peak at 0.8-0.9?m and a subsidiary peak at 0.14-0.24?m.The EDX and ICP-MS results showed that the particles contained a variety of elements such as Ca,Mg,P,Fe,Mn,Zn,Ti,Si,Sr,Ba,Mo,Li,Sb,Ni,Pb,V,and Cr,beside C and O.These metal components can enhance and catalyse the formation of sulphate,which is usually the largest secondary species in haze pollution.Organic particles under the hot start and hot stabilised running states were higher than those under other running states.Soot particles were highest under the cold start running state.Under the idle running state,the relative number fraction of Ca-rich particles was high although their absolute number was low.For diesel vehicles,the particle emissions were mainly soot particles,S-rich particles and organic particles.Soot particles and S-rich particles were mainly derived from the combustion of fuel.The organic particle emission was related to the combustion of lubricating oil.The EDX and ICP-MS results showed that the particle emitted from diesel vehicles also contain various metal elements such as Ca,Mg,Fe,Zn,Cr,Ti,Ni,Ba,Mo,Mn,V and Sr.The elements in diesel engine exhaust particles were basically the same as those emitted from GDI gasoline vehicles,but the contents of elements were different,which may be related to the properties of lubricating oil and the degree of cylinder wear.The particles emitted by diesel vehicles were mainly soot particles,accounting for more than 50%,under all the running conditions,including cold start,hot start,hot stabilized running,idle and acceleration state.In addition to soot particles,we also found a certain amount of organic particles and metal particles,especially under the idle running state,the relative percentages of organic particles was higher than that under other running states.As the soot particles occupied a large proportion in the exhaust from diesel vehicles and the morphology has fractal characteristics.We calculated the fractal dimension of soot particles emitted from diesel vehicles under the cold start running state.The fractal dimension was 1.75,which indicated that the soot particles were mostly chain-like shape under the cold start running state.The engine test bench results showed that fuel types had an obvious influence on the relative proportions of individual particles.The fuel with more olefins?F1?produced a higher percentage of organic particles compared to the gasoline characterised by lower olefin content?F5?,the fuel with more aromatics?F2?produced a higher percentage of soot particles,and the fuel containing MMT?F3?generated a higher percentage of soot particles.The E10 gasoline?F4?gave rise to a lower percentage of soot particles compared to the fuel with more olefins?F1?.Compared with PFI engines,the particles emitted by GDI engines had a higher percentage of soot particles and lower percentage of organic particles.The particles emitted by the PFI engines were smaller than those from the GDI engines.Engine loads had an obvious influence on the relative proportions of individual particles.With increasing engine load,the relative percentage of soot increased while the relative percentage of organic particles decreased.The relative percentages of the S-rich,Fe-rich,and Ca-rich particles decreased initially before increasing later.The use of TWC influenced the individual particle compositions.The post-TWC engine emitted a higher percentages of soot particles.The sizes of the particles collected from the post-TWC engine were larger than those from the pre-TWC engine.The smog chamber results showed that the particles in the smog chamber were mainly secondary particles,soot particles,organic particles,and Ca-rich particles.The relative percentage of secondary particles was the highest,accounting for more than 70%.It should be pointed out that these secondary particles contained C and O,and rapidly decomposed under high-energy electron beam irradiation in the TEM.The secondary particles could be divided into sulfur-containing and non-sulfur-containing secondary particles.The non-sulfur-containing particles could be further divided into rectangle secondary particles,irregular secondary particles,spherical secondary particles,and core-shell secondary particles.After being emitted to the ambient environment,the morphology and composition of the particles changed.The soot particles changed from a chain-like shape into compact ones,and some of them became core-shell structured.The organic particles and Ca-rich particles changed from spherical to more irregular shape.Moreover,the content of sulfur in organic particles and Ca-rich particles increased.The timescales to achieve morphology modification were estimated to be 3.5 h in Beijing.The aging process was affected by many factors,including the initial concentrations of pollutants emitted by gasoline vehicles?including VOC_S,NO_x,etc.?,relative humidity,solar radiation,and oxidation levels.We also observed the formation of sulfate,though the concentration of SO₂ was low?6 ppb?in the smog chamber.The results of the aging degrees of diesel exhaust soot particles showed that,in the fresh state,most of soot particles exhibited a chain-like structure,accounting for 78%of the total soot particles.After aged for 30 minutes,some soot particles changed into nearly spherical ones.After aged for 180 minutes,most of the particles were aged into near spherical shape,and the chain-like soot particles accounted for less than 10%of the total soot particles.In order to quantify the degree of aging of soot particles,the circularity factor and the fractal dimension were used.With the increase of aging time,the circularity factor and the fractal dimension of soot particles increased.In the fresh state,the average value of circularity factor was 0.36.After aging for 180 minutes,the average value of the circularity factor reached 0.54.This demonstrated that most of the soot particles were near spherical.In the fresh state,the fractal dimension of soot particles was 1.67.After aging for 180 minutes,the value of fractal dimension was close to 3.0.This showed that the aging process made the soot particles more spherical and compact.Compared with the smog chamber results,the results of the urban road environment showed that the relative percentages of secondary particles were smaller and the relative number percentages of mixed particles were larger.Mixed particles were generally composed of secondary particles,mineral particles,soot particles,organic particles,and metal particles.These mixed particles had an irregular shape and core-shell structure.The secondary particles in the smog chamber mainly contained C and O elements,only a small number of the secondary particles contained S.Most of the secondary particles in road samples contained C,O,and S elements.This suggested that the secondary particles were mainly a mixture of organic matter and sulfate in the urban road environment.The mixing state of sulfate and organic matter was very complicated,mainly in the form of core-shell structure particles with cores of sulfate.These results help to explain the formation mechanism of SOA formation.Compared with the particles emitted by the vehicles,the types of particles in the urban road environment were more complex.The Fe-rich particles and the organic particles with C,O,P,S,and Ca elements in the urban road environment may be come from vehicle emissions.The Ca,P and Zn in organic particles may serve as fingerprints of gasoline vehicle source.The fractal dimensions of the soot particles in the ambient atmosphere were larger than the fractal dimension of the soot particles from the vehicle and biomass source emissions.In summary,the characteristics and chemical compositions of PM emitted from vehicles were analyzed by a variety of analytical methods.A unique classification standard for particles and an inventory of particle categories and amounts emitted from vehicles under different running states were established.The aging mechanism of gasoline vehicle exhaust in the atmospheric environment was established,and the degree of aging of soot particles was quantitatively described.