Epidemiological studies have consistently linked urban fine particles (FPs, diameter less than or equal to 2.5 µm) to adverse health effects, including increased morbidity and mortality in people with respiratory and cardiac disease. A major source of fine particles in urban areas is the motor vehicle emissions. Present engine particulate emission standards are based on mass, and the EPA has proposed more stringent standards on ambient fine particles. The concerns about the vehicle emissions of ultrafine particles (UPs, diameter less than or equal to 0.1 µm) are growing, mainly as a result of suggestions that a decrease in the particle sizes increases its toxicity and indications that reductions in mass emissions of FPs may increase number emissions of UPs. A recent comparison of the health effects of UPs with those of FPs found that UPs as well as FPs are associated with mortality, and data also show that FPs cannot be used as indicator for UPs. In view of the strong adverse health effects associated with UPs, future standards might be imposed on UP emission. Effective and least costly means of UP emission reduction must be based on a firm physical understanding of the processes and parameters controlling formation and evolution of ultrafine particles in vehicle exhaust.

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. The classical binary homogeneous nucleation of H2SO4-H2O fails to explain the observed UP properties. 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 H2SO4-H2O closely match measurements in terms of total UP concentrations, and their sensitivity to fuel sulfur contents and dilution conditions. The low volatile hydrocarbon contributes significantly to the growth rate of the newly formed particles. The predicted size distributions of the number and compositions (including sulfuric acid, hydrocarbons, and soot) of UPs in vehicle exhaust will be compared with those measured using scanning mobility particle sizer and nano-DMA/thermal desorption particle beam mass spectrometer. The implications of our new understanding of UP formation mechanism to the UP control strategies will be discussed.