The combining effect of the roof shape, roof-height non-uniformity and source position on the pollutant transport between a street canyon and 3D urban array

Abstract Although most people live in cities, the pollutant transport in so-called street canyons is still not well understood. The main reason is the complexity of the problem, where the roof shape, roof height and position of the pollutant source within a 3D urban array may play a significant role. This paper tries to improve the understanding and investigates the pollutant transport in six different types of street canyons using large-eddy simulations (LES). All street canyons were part of four different 3D urban arrays and differed in the roof shape and height. The results demonstrate that roof shape, roof-height non-uniformity, and the source position have a crucial impact on the advective and turbulent pollutant transport between the studied canyons and urban arrays. Both the flat roofs and roof-height variability enhance the lateral transport between the street canyons and intersections irrespective of the studied cases. This is crucial for the evaluation of the street canyon pollution if one changes the position of the source within the non-uniform urban array. Although the differences in the pollutant transport between the flat and pitched roofs mitigate in the cases of the variable roof heights, they are still appreciable.