Design And Experimental Verification Of High-resolution Measurement System For Particle Size Spectrum Of Ultrafine Particles Emitted By Motor Vehicles

Pollutants emitted by motor vehicles are one of the important sources of compound atmospheric pollution in most cities in China,which have a relatively large negative impact on human health and the national economy.In 2021,China had fully implemented the national six emission standards for heavy-duty diesel vehicles,and measure the particle size with 23 nm as the lower limit.Ultrafine particles account for a high proportion of the total concentration,and the proportion of particles with a particle size of less than 23 nm may be close to half.The EU’s latest ’Euro 7’ regulation proposal further adjusts the lower limit of particle number concentration measurement to 10 nm.Therefore,in order to more accurately study the mechanism and control technology of motor vehicle emission particles and provide a basis for the next stage of standard formulation in China,it is urgent to carry out High-resolution on-road monitoring of particle size spectrum of ultrafine particles emitted by motor vehicles.However,the current on-road monitoring technology for sub-23 nm particles still has problems such as low particle size resolution and low number concentration retrieval accuracy.For this reason,this paper deeply studies the measurement method of particle size spectrum of sub-23 nm particles,develops a prototype of particle size spectrum system of ultrafine particles,and completes the inversion algorithm and test experiment.The main research work and results are as follows:(1)Based on the principle of differential electromigration classification,a high-resolution particle size spectrum measurement method combined with antiturbulence slits and semi-ellipsoid airflow regulators is proposed.The influence model of the anti-turbulence slit on the particle size resolution of differential mobility analyzer(DMA)was established by using COMSOL simulation software,and the evaluation index for quantifying the classify performance of the DMA in COMSOL—broadening voltage was proposed.The smaller the broadening voltage,the higher the resolution of DMA particle size.The simulation results show that under the condition that the sheath gas flow rate does not exceed 20 L/min,the DMA particle size resolution is positively correlated with the depth of the anti-turbulence slit,but negatively correlated with its width;Considering the particle classify speed,the optimal size parameters of the anti-turbulence slit are finally determined to be 9 mm in depth and 1.8 mm in width.(2)According to the simulation results,the electromigration classification module is designed and developed,which can classify the ultrafine particles emitted by motor vehicles in the particle size range of 10 nm～55 nm,and the particle size resolution of the transfer function can reach 7.0～20.0.The experimental results show that the semi-ellipsoid airflow regulator can increase the flow rate of the sheath gas in the module,and the porous ring at the sample gas inlet can improve the axisymmetric mixing of the sample gas and the sheath gas;The anti-turbulence slit can suppress the diffusion loss of particles in the module and improve the particle size resolution of the module.The particle size resolution of the transfer function of the electromigration classification module for 12 nm,15 nm,23 nm,30 nm,and 50 nm particles reaches 7.0,9.1,12.5,13.0,and 20.0 respectively.The main innovations of the electromigration fractionation module are the design of the porous ring,semi-ellipsoidal damper,and anti-turbulence slit.Under the condition of relatively low sheath gas flow rate,the transfer function height of 10 nm～55 nm particles can reach the same level as that of the comparison device(TSI Model 3081),and the particle size resolution of the transfer function is increased by 2.1 times.(3)Developed a particle size spectrum system for ultrafine particles emitted by motor vehicles,completed the calibration test of key modules and the whole machine,used the self-developed system to carry out motor vehicle emission test experiments under actual working conditions.The control system and communication system are designed based on the Micro-Control Unit(STM32F407).The control system includes sheath gas and sample gas flow control,high-pressure source control,charger control,etc.;the communication system includes "sieving device","particle size spectrum","setting".The temperature and zero-point stability of the Faraday cup electrometer,and the stability of the sample gas and sheath gas were tested.The results showed that the relative error of the temperature of the Faraday cup electrometer was less than 0.4%,and the relative error of the zero-point value of the Faraday cup electrometer was less than 4.5%.The relative error of sample gas and sheath gas flow is less than 1.7%and 0.5%,and the calibration of electromigration classification module is completed.The prototype was tested with a soot generator and a NaCl particle generator under laboratory conditions,and the results showed that the system prototype had a relative error of no more than 3.0%in the peak value of the particle size spectrum within the range of 10 nm to 55 nm.The test experiment of particles emitted by motor vehicles have been completed,and the stability of the system and the accuracy of the inversion algorithm have been verified.The results show that the particle size peaks of the motor vehicle emission particles in one measurement period are 18.95 nm and 31.6 nm respectively,and the number concentrations are 6536#/cm3 and 3127#/cm3 respectively.