Impact of source shape on pollutant dispersion in a street canyon in different thermal stabilities

In previous studies of pollutant dispersion around and inside urban street canyons, several kinds source shape assumptions were employed. However, the impact of source shape on pollutant transport in urban street canyon is barely concerned. As a determinant of pollutant distribution, the shape effects of pollutant source should be analyzed carefully. In this study, source shape effects on street canyon pollutant dispersion are investigated using computational fluid dynamics (CFD) approach based on an idealized street canyon model in different thermal stabilities. The line shape source, band shape source and surface shape source assumptions are adopted to represent the widely used source shape patterns. The prediction accuracy of this numerical model is validated against wind tunnel measurement and LES results and is proved to be acceptable. Numerical simulations are performed under neutral and unstable thermal stratifications. Pollutant distribution inside the street canyon and its variation with thermal stability are examined. The results demonstrate that pollutant source shape can significantly alter the distribution of pollutant concentration in the whole street canyon. The most uneven distribution occurs under the surface shape source condition, where more than 60% pollutant is located in the leeward wall zone, but only 19% pollutant is found in the windward wall zone. The surface source assumption contributes to more dispersed distribution. Moreover, different source shape assumptions lead to distinct characteristics of pollutant residue in the street canyon, especially in the near-ground-zone. Also, the variation of pollutant residue with the thermal stability is greatly influenced by the source shape configuration. The source shape effect should not be neglected in future street canyon environment studies. The new findings present in this study can improve the understanding of pollutant dispersion characteristics in urban street canyons, and benefit architects, city-planners and policy makers to achieve optimized solution for urban air pollution.