Vehicular exhaust gas-to-nanoparticle conversion and concentration distribution in the vehicle wake region

In the present study, the interaction effects of different sulfur contents, relative humidities, driving modes and vehicular exhaust tailpipe exit conditions on the three-dimensional exhaust gas-to-nanoparticle conversion and concentration distribution in the wake region of a typical bus-shaped vehicle in urban road microenvironments were comprehensively simulated using large eddy simulation (LES) coupled with the aerosol dynamics and turbulent dispersion model. Time-scale analysis of coagulation, nucleation and turbulent dispersion processes in the near-wake region of the studied vehicle show that the coagulation process is slowest and generally can be neglected. On the other hand, nucleation process is taken place much faster (i.e., nanoseconds) which makes the normal time integration method impractical. A time filtering (averaging) method was developed to tackle the extreme fast nucleation problem. Numerical results show that more than 95% (by number concentration) nanoparticles are generated in the region with temperature of 310-320K. The higher vehicle speed will also decrease the number concentration of nanoparticles. However, lower exhaust gas temperature will produce more nanoparticles. A reduction of sulfur content in diesel fuel from 500 ppm to 50 ppm will decrease the number concentration of nanoparticles more than 1000 times.