Atmospheric Environment 37 (2003) 4037–4049 A numerical study of atmospheric pollutant dispersion in different two-dimensional street canyon configurations V.D. Assimakopoulos a , H.M. ApSimon a , N. Moussiopoulos b, * a Environmental Modelling, Management and Assessment Group, Department of Environmental Science and Technology, Imperial College of Science Technology and Medicine, UK b Laboratory of Heat Transfer and Environmental Engineering, Mechanical Engineering Department, Aristotle University of Thessaloniki, Box 486, Thessaloniki 5, Greece Received 20 December 2002; received in revised form 13 June 2003; accepted 20 June 2003 Abstract The scope of this paper is to study numerically the dispersion of atmospheric pollutants within different street canyon geometrical configurations and building height scenarios. The microscale model MIMO, designed to take into account pollution dispersion in the vicinity of buildings, is validated against a two-dimensional wind-tunnel experiment for a square (W =H ¼ 1) and a deep canyon (W =H ¼ 1=2) configuration. It was found that the model performance is satisfactory. Having established this, the study involved the alteration of the building heights bounding the canyon for both square and deep configurations. Furthermore, the effects of different aspect ratios on the pollution dispersion characteristics are investigated. It is demonstrated that, under certain conditions, pollutants disperse more easily, while under others pollutant levels increase, with implications for pedestrian exposure. Furthermore, interaction of air in and above the canyons is investigated under different street geometries. It is found that both street geometry and building height influence the interaction between air inside and above the canyons. r 2003 Elsevier Ltd. All rights reserved. Keywords: Microscale model; Street canyon; Dispersion 1. Introduction The dispersion of pollutants in urban environments is dominated by the wind flow around complex building structures. Down-wash phenomena and increased local turbulence, strongly influence not only the mean flow field but also the diffusion parameters. A typical configuration is the so-called street canyon, formed along a street in a densely built urban area with buildings lining on both sides, where vehicles emit gaseous pollutants. Under certain conditions, e.g. for non-zero wind components perpendicular to the street, higher concentration levels are observed on the upwind than on the downwind side of the street canyon. Since traffic is accepted to be a major emission source of air pollutants in urban areas, and further increase of city traffic is expected, investigations of dispersion processes in street canyons have become a focal point in environmental research. Due to their rather coarse resolution, mesoscale models cannot accurately predict these influences. Therefore, in recent years microscale models were developed taking into account the building structure and providing a detailed insight into urban dispersion. One of the earliest street pollution models is the STREET model by Johnson and Hunter (1995) and Johnson and Hunter (1998). A quite different approach was introduced by Yamartino and Wiegand (1986) in their Canyon Plume-Box Model. An empirical approach was also used to determine traffic pollution in Dutch city streets, the CAR (Calculation of Air pollution from Road traffic) model (Boeft et al., 1996). The Operational ARTICLE IN PRESS *Corresponding author. E-mail address: moussio@vergina.eng.auth.gr (N. Moussiopoulos). 1352-2310/$-see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S1352-2310(03)00533-8