Influence And Its Simulation Of Vehicular Exhaust On The Air Quality In The Street

The number of motor vehicles in China has recently increased remarkedbly with the rapid urbanization and industrialisation, leading to the prosperity of the economy. However, it also deteriorates the air quality in large cities. The air pollution problem in the proximity of urban street has received more and more attentions in many scientific fields. Traffic emission is nowadays considered as the main air pollution source in the urban environment,and it has many adverse effects on the indoor air quality of the buildings along the street.. Therefore, it is necessary to investigate thoroughly the characteristics of the traffic-related air pollution in the urban street and the relationship between the indoor air quality and the outdoor air pollution for the buildings along the street. With these understandings, scientific suggestions can be made for the control of the vehicular emissions, for the improvement of indoor air quality of the buildings along the street, and for the urban road planning.For the above mentioned purposes, CO concentration and other related parameters were measured in some representative main traffic roads and secondary traffic roads in Wuhan City. Parameters measured included the traffic status, meteorological quantities, and street donfigurations. In addition, the CFD method was adopted to simulate the wind field and the pollution concentration field of the street canyons' models. The simulation result was used to adress how the various factors influence the pollutant dispersion and the pollutant distribution in the urban street. Finally, indoor and outdoor CO concentrations of the buildings along the street were measured in this study to illuminate how the indoor air quality is affected by the traffic emissions.Measurement results show that the various factors have significant influence on the pollutant dispersion, including traffic vehicle flow rate, wind speed, temperature and street canyon aspect ratio, which is the ratio of the building height to the street width. The pollutant concentration is positively correlated with the traffic vehicle flow, and negatively correlated with the wind speed and the temperature. Furthermore, it was found that the aspect ratio is the key factor governing the pollutant dispersion. The larger the aspect ratio is, the more difficultly the pollutant diffuses, the higher the polllutant concentration is.From the CO concentration data collected at the road intersection, conclusion can be drawn that the traffic-generated pollution at the intersection was the most serious, and the pollutant concentraion in the windward was lower than that in the leeward. To reduce the pollution at the intersection, it is better to lay the angle of the intersection properly.For the 2-D case, where the wind direction is perpendicular to the street, results demonstrate that CFD method is suitable for simulating the pollution concentration in the street canyon. In terms of turbulence model, the standard k-εmodel performs much better than the RNG k-εmodel in simulating the street canyon's wind field. For the concentration field simulation, the standard k-εmodel gives also better results than the RNG k-εmodel, though RNG k-εmodel predicts the pollutant concentration at the windward side more accurately in some cases. Depending on the aspect ratio, there may exist only one vortex, or one main vortex plus a seconcary vortex, or more than three vortexes in the street canyon, and the rotation direction of the vortex also relies on the aspect ratio. According to the simulation results, the pollutant concentration reduces with the increase of the wind speed, In addition, the concentration of the pollutant becomes lower when moving upward from the street floor. As the ratio of the building height to the street width is decreased, the air quality in the street proximity can be improved. In all the cases considered in this study, pollutant concentration distribution along the height differs between the leeward and windward side of the street. Except for the relatively high ratio of the street width to the building height, the concentration at the leeward side is generally higher than that at the windward side.Comparsion of the measured indoor and outdoor CO concentration at the buildings along the street indicates that the outdoor's pollution has great influence on the indoor's pollution, and the influence near the traffic main roads is more significant than that far from the main traffic roads. The indoor pollutant concentration has no obvious "time-delay effect" with the outdoor pollutant concentration. Obviously, vehicular exhaust poses health risk to those people working in the buildings along the street.