Study On Variation Characteristics Of Fine Particles In Diesel Vehicles Exhaust Plume

Secondary Organic Aerosol(SOA) is an important component of fine-particle(PM2.5) mass throughout the atmosphere, SOA simulated by model may underestimate more than 10 times compared with the actually measured value. There is a quick reaction process in the area near the exhaust gas discharge outlet when the vehicle emissions exhaust into the air. A large number of secondary particles were producted in this process, but has not included in the existing air quality model. As the diesel vehicles contribute more than 90% PM2.5 emissions of motor vehicles, so it is of great significance to study formation process of secondary particles for reducing the difference of SOA between simulated values and observed values. A new multipoint sampling system were established, which is suit to study the exhaust plume near the exhaust gas discharge outlet. The three kinds of diesel vehicles were studied using this multipoint sampling system. The variation characteristics of PM2.5 mass concentration and its component in diesel exhaust plume were analyzed and compared with the on-board test results. The cause of underestimating SOA was carried on the qualitative analysis and quantitative estimation.The results show that:(1) motor vehicle exhaust multipoint sampling system is reliable, can accurately capture nearly exhaust plume discharge outlet area.(2) The PM2.5 emissions gradually increase with the distance, when the motor vehicle exhaust emissions into the air, explained the process of production of secondary particles existed in the plume. Elemental Carbon(EC) emissions were basically unchanged, Orangic Carbon(OC) and Total carbon(TC) increase with the increase of distance, explained the generation process Secondary Organic Carbon(SOC) existed in the motor vehicle exhaust plume. The farther distance exhaust nozzle, the more SOC generated. The NH4+,NO3-and SO42- emissions increased with the increasing of distance, illustrated the formation process of SNA(Sulfate, Nitrate, Ammounium) existed in the motor vehicle exhaust plume. The NH4+ mainly unites with NO3- in the plume of test area, the proportion of NH4+ uniting with NO3- is decreasing with the increasing of dilution ratio, on the contrary, the proportion of NH4+ uniting with SO42- is increasing with the increasing of dilution ratio.(3) The averaged contribution of SOC to the known secondary PM2.5 is 76%, far more than the contribution of SNA(24%), explained the generate process of SOC plays a leading role in generate process of secondary PM2.5. The generation process of secondary particles will not always continue.(4) The results of on-road test were above the results of exhaust nozzle in plume test, the results of on-road test overestimated the primary particulate. The more overestimating, the higher dilution ratio in on-road test. The on-road tesults tested by PEMS was slightly lower than results for plume test with the similar dilution ratio, the first reason of this phenomenon was that the dilution ratio in on-road test was always changing; The second was that the on-road test dilution air was clean air, but atmosphere for plume test, the particle in atmosphere may promoted new particle generated and provide medium surface for condensation.(5) There are two reasons to the underestimate of SOA, first is that part of the SOA will be treated as POA(about 32-53% POA which was determined by on-road test). Second, vehicle on-road test results contains only part of the secondary particles in the plume, the other part of SOA formed in the plume are lack(about 39% POA which was determined by on-road test).