Study On The Particle Size Distribution And Chemical Composition Of Ultra-fine Particulate Matter In Diesel Vehicle Exhaust And Roadside Atmospheric Environment

Ultra-fine particulate matter(UPM)has become one of the most important air pollutants,which exert negative impacts on the quality of the atmospheric environment and human health.Nowadays,the contribution of motor vehicles to urban air UPM has received more and more attention as its exhaust is located in the human breathing zone and thus the corresponding risk related to the public health probably is much worsenthan other pollution sources.However,the current knowledge regarding the particle size distribution and chemical composition of UPM in diesel vehicles remain poorly understood.Therefore,we thoroughly explored the emission characteristics and influencing factors of the number concentration and particle size distribution for the UPM emitted from light-duty diesel vehicles based on the actual road driving through a self-built UPM online test platform in this study.The effects of dilution ratio,dilution temperature,length of sampling pipe,and the lower limit of particle size measurement on diesel vehicle UPM emissions were explored.Also,this study investigated the effects of different driving conditions,cold and hot start,and exhaust after-treatment device(DPF)on the UPM emission.In addition,the concentration levels of 21 elements both in the atmospheric UPM and diesel vehicle exhaust UPM were also analyzed.The main results and findings of this paper as follows:The dilution ratio,dilution temperature and length of the sampling pipe can affect the particle number concentration and particle size distribution of light-duty diesel vehicles.In addition,the lower limit of particle size of diesel vehicle exhaust can greatly affect the test results.For example,when 6 nm is selected as the lower limit of particle size,the transient emission rate of particle number(PN)of diesel vehicle exhaust is 2.2 times and 4.2 times that of14 nm and 20 nm,respectively.However,this is still not the actual driving emission situation,and the PN transient emission rate will increase by about 36% if the condensation of volatile organic compounds(VOCs)and the length of the sampling tube are taken into account.The particle size distribution of light-duty diesel vehicle exhaust particles displays a two-mode pattern.The peak size is in the range of 6-14 nm when the distribution is unimodal.However,the peak sizes are in the range of 6-14 nm and 30-97 nm when the distribution show the bimodal pattern.For light-duty diesel vehicles,fuel consumption,exhaust temperature,exhaust difference pressure,engine speed,speed and the exhaust aftertreatment device DPF are the key factors affecting the level of PN emission.The concentration of particulate matter in the exhaust gas of diesel vehicles will increase significantly when the vehicle speed is in the high level and the accelerator is heavily stepped on;the PN emission level at the idling cold start is higher than that at the hot start,and the emission difference is mainly caused by the pretreatment of the exhaust after-treatment device at the cold start.Heat and engine work are caused by the two processes of inflating the brake device;after adding DPF,the emission of particulate matter can be reduced by more than 99.8%,but the concentration of particulate matter emitted in the DPF regeneration stage is much higher than that in the non-regeneration stage.Among the 18 elements measured,the five metal elements with the highest concentrations in diesel vehicle exhaust are Al,Zn,Fe,Ca,and K,and the five elements with the lowest concentrations are Sr,Cd,Rb,Ba,and As.Among them,Ba,Sr,Ni,Cr,and Fe are mainly contributed by the particles with a particle size below 100 nm,while Pb,Al,Rb,Ca,and Zn are mainly contributed by the particles with a particle size between 100 nm and 1 μm and K,Cd is mainly contributed by particles with particle size between 1 ～ 2.5 μm.Principal component analysis shows that industrial source,crustal souce,and mobile source three main pollution sources of elements in the atmospheric particulates of Xingye Avenue,Panyu District,Guangzhou City.At the same time,As,Cu,Ni,Zn,and Cr are mainly distributed in particles with a particle size of less than 100 nm;K,Cs,Cd,Mn,Ga,Pb,V,etc.are mainly distributed in particles with a particle size of 100 nm to 1 μm.Among the particles,the elements Mg,Al,Ca,Ba,Sr,Fe,and Co are mainly found in the particles with a particle size of1.63～2.47μm.