PROCESSES AND PARAMETERS CONTROLLING MOTOR VEHICLE ULTRAFINE PARTICLE EMISSIONS: NUMERICAL SIMULATIONS AND COMPARISONS WITH OBSERVATIONS

摘要

The adverse health effect associated with ultrafine particles (UPs) appears to become an emerging environmental problem, and it is important to understand the processes and parameters controlling motor vehicle UP emissions. Measurements of UPs in motor engine exhaust have been made both in the laboratory and in the atmosphere under various conditions. Most of UPs are formed during exhaust dilution from low volatile precursor gases and the measured concentrations are very sensitive to dilution and sampling conditions. In this study, we investigate the key processes and parameters controlling formation and evolution of UPs in vehicle exhaust through model simulations and comparisons with field measurements. The detailed aerosol dynamics (nucleation, condensation, and coagulation) are simulated with an advanced multi-type, multi-component, size-resolved microphysics model. We find that chemiions generated in engine combustor may play an important role in the formation of UPs in vehicle exhaust. The predicted UP properties based on our ion-mediated nucleation of H_2SO_4-H_2O closely match measurements in terms of total UP concentrations, and their sensitivity to fuel sulfur contents and second stage dilution. Besides chemiion concentration and fuel sulfur content, other parameters affecting UP emissions include dilution conditions (temperature and humidity of diluting air, dilution ratio), soot particle concentration, and vapor pressure and concentration of condensable organic species. Our study indicates that the number of UPs at measurement point is very sensitive to initial chemiion concentrations, and we propose a potentially effective technique to control vehicle UP emissions by imposing an electrical field on a section of the tailpipe to remove ions in the rare exhaust. The voltage needed to remove chemiions is estimated to be ～ 100-250 volts if the Chemiion Collector composed of two collecting plates and to be ～ 11-28 volts if the Chemiion Collector composed of four parallel collecting plates. The reductions in fuel sulfur content, while can reduce the growth rate (and size) of initial charged clusters, are not likely to be very effective in reducing the total number of volatile particles formed since the nucleation rate is limited by the CI abundance. While no direct measurements of ions in vehicle engine exhaust are available (to our knowledge), we believe that the CIs generated during the combustion process should be present in the exhaust. The chemiion concentration needed to explain the observations seems to lie within the range of values we estimate. Obviously, direct measurements of CIs in vehicle exhaust are needed to confirm the proposed CI mechanism. Actually, based on our simulation, most of the fresh volatile UPs in diesel exhaust are formed on CIs and a significant fraction of these particles are still charged at plume age of ～ 1 s. Since SMPS is commonly used to measure the size distribution of UPs, a convenient way to test the chemiion theory is to measure the particles with ion neutralizer in SMPS turned off. On the other hand, the UPs measured with SMPS may have to be reassessed if a significant fraction of these particles are originally charged.