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An Investigation of Traffic Related Pollutants Dispersion in Heterogeneous Street Canyon

Karra, S; (2012) An Investigation of Traffic Related Pollutants Dispersion in Heterogeneous Street Canyon. Doctoral thesis , UCL (University College London).

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In urban areas, traffic is a major source of air pollution. Concentrations of pollutants are observed to be particularly high near ground level in street canyons, where tall buildings and different obstacles limit the ventilation. This impedes the dispersion of pollutants, contributing to build-up of harmful substances, which have an adverse effect on human health and comfort. Consequently, there is a desire to understand the mechanisms that contribute to street level ventilation especially in exploring the effect that heterogeneous layout of a street canyon has on the ventilation level. It has been established that extent of ventilation is dependent upon the background meteorological conditions, and how they interact with the building geometry. There is further interest in investigating the effect of the ambient conditions in heterogeneous street canyons. In the existent urban areas it is necessary to improve the ventilation for better comfort of the people who live in these areas and the surrounding environment. Elements that might exist in the street such as traffic lane position, parked cars or other obstacles may also have an effect on the distribution of pollutants or flow regimes. This thesis focuses on exploring the flow features and distribution of traffic-related pollutants in heterogeneous street canyons. Comparisons are made between symmetric and non-symmetric geometrical street layouts. In addition, this thesis examines the effect of local street parameters, such as the position of traffic lanes and the placement of parked cars, on the distribution of pollutants in heterogeneous, symmetrical and non-symmetrical canyons. A heterogeneous street canyon in Cyprus is used as a case study to examine the ventilation in these types of street canyons. This street was modelled in the lab, in a water channel, applying simultaneously the advanced imaging techniques of Particle Image Velocimetry (PIV) and Planar Laser Induced Fluorescence (PLIF). The same techniques were also applied to models of symmetrical and non-symmetrical canyons under constant perpendicular ambient conditions. A field measurement campaign was undertaken in the real heterogeneous street canyon in Nicosia to examine the flow fields and pollution distribution under variable ambient conditions and traffic flow rate. A comparison was made between the results obtained under constant and variable ambient conditions. The heterogeneity of the canyon was found to influence the distribution of pollutants at street level. The overall pollution levels are lower in the heterogeneous 4 street canyon than in symmetrical and non-symmetrical canyons. Flow fields are found to vary along the length of the heterogeneous street canyon and each cross section behaves individually according to its own geometric layout, leading to a different distribution of pollutants in each cross section. In addition, the existence of gaps (either only at the upstream side or both upstream and downstream side) is found to produce higher level of turbulence resulting in lower pollution levels near the ground level than can be expected for a symmetrical canyon. However, the existence of a gap only at the downstream side did not enhance the ventilation near the ground level at this side of the canyon. In this case the pollution levels were slightly higher compared to the locations along the length of the canyon where the gap was at the upstream or both sides. The local street parameters influence the distribution of pollutants near the ground level at all the examined street geometries. It was found that moving the traffic lane near the downstream area and the placement of parked cars on the upstream area produces major reductions of pollutants. The variability of the ambient wind was found in the field to produce different flow regimes compared to the case when the ambient flow was constant and perpendicular. At all the examined locations the flow was three dimensional. Despite this difference in the flow regimes the pollutants have similar distribution to those observed in the laboratory study. One possible explanation is that the local street parameters (such as the location of the traffic lane and the placement of parked cars in the street) had a greater impact at street level compared to the variability of the wind. Finally, results indicate that the recurrent, frequent variations in building heights along the length of the street contribute to significantly increase the dilution of pollutants near ground level, where the implications to pedestrian health and comfort are greatest.

Type: Thesis (Doctoral)
Title: An Investigation of Traffic Related Pollutants Dispersion in Heterogeneous Street Canyon
Language: English
UCL classification: UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Engineering Science
URI: http://discovery.ucl.ac.uk/id/eprint/1389332
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