Three-dimensional pollutant concentration dispersion of a vehicular exhaust plume in the real atmosphere

The pollutant dispersion process from the vehicular exhaust plume has a direct impact on human health, particularly on vehicle drivers and passengers, bicyclists, motorcyclists, pedestrians and people working nearby. A three-dimensional vehicular pollutant dispersion numerical model was developed based on the Reynolds-averaged Navier–Stokes equations coupled with a k–εk–ε turbulence model to simulate the initial pollutant dispersion process of carbon monoxide, CO, from a vehicular exhaust plume in the real atmospheric environment. Since the ambient wind direction and velocity are stochastic and uncontrollable in the real atmospheric environment, a wind-direction–frequency-weighted (WDFW) approach was used to obtain the real pollutant concentration dispersion along with the development of the vehicular exhaust plume. Within the specified sampling period, the ambient windflow conditions are transformed into the corresponding frequencies of wind directions and averaged magnitudes of wind velocities from directions N, E, S or W. Good agreement between the calculated and measured data for two diesel-fuelled vehicles indicates that with the WDFW approach the initial dispersion of pollutant concentration from a vehicular exhaust plume in the real atmospheric environment can be truly reflected. The present study shows that the dispersion process in the near region for the relative concentration of CO, from RC=0.1RC=0.1 (or 10%) to 1 (or 100%), is less influenced by the ambient wind velocity than by the vehicular exhaust velocity, but it is vice versa in the far region from RC=0RC=0 (or 0%) to 0.1 (or 10%). It implies that the effect of vehicular exhaust exit velocity on the dispersion process is more pronounced than that of ambient wind velocity at the vicinity of the exhaust tailpipe exit, while the effect of ambient wind velocity gradually shows a significant role for the dispersion process along with the development of a vehicular exhaust plume.