Aspects of particulate dry deposition in the urban environment.

Micro-scale deposition models, typically used for pipes, were adapted to outdoor situations and combined with computational fluid dynamics (CFD) calculations of flow conditions in order to study the fine structure of the deposition velocity on ground, walls, and roofs in an urban environment. Several deposition modeling techniques taken from the literature were used for the predictions. The urban geometry was represented by two different blocks of houses, which together with two wind directions gave four different cases to study. The calculations show large local variations of the deposition velocity resulting in a pattern similar to the variation of the friction velocity. This demonstrates the strong dependence of the deposition velocity on the friction velocity. Further alteration of the deposition velocity is caused by the variation of the micro-scale roughness and different surface temperatures. The results presented provide some guidance for where to look for hotspots of deposited material and also show that a representation of the deposition velocity in a city by only one or just a few values is a great simplification locally and could lead to serious mistakes.