A procedure to quantify the impact of mitigation techniques on the urban ventilation Parham A. Mirzaei, Fariborz Haghighat * Department of Building, Civil and Environmental Engineering, Concordia University, Montreal, Canada H3G 1M8 article info Article history: Received 3 February 2011 Received in revised form 1 June 2011 Accepted 2 June 2011 Keywords: Heat island Outdoor environment CFD Exposure Thermal comfort abstract Urban heat island (UHI) and urban pollution island (UPI) are recent phenomena caused by human interference in an environment. This interference has an impact on the pedestrian comfort and pollution exposure (PCE). Even though studies have revealed a mutual relation between the UHI and UPI, their interaction is rarely considered in the development of mitigation techniques. That means UHI mitigation technique does not necessarily have similar impact on the pollution exposure and vice versa. For example, the effect of tree planting in a high-rise street canyon can be different from a low-rise one since solar radiation absorption and airflow regime is remarkably different in these street canyons. This implies that street canyon-scale treatment is necessary in order to first clarify the significance of each interaction and, then to apply a proper mitigation strategy to improve the PCE. The existing urban planning guidelines are mainly qualitative and not quantitative, and no procedure has been developed to evaluate the impact of different mitigation technologies on the PCE. This paper proposes a systematic approach to quantify the level of environmental condition inside a street canyon. This approach is also capable of evaluating the possible advantages of passive and active mitigation strategies using a frequency of occurrence concept. For this purpose, a computational fluid dynamics model is developed to be used to investigate the impact of contributing parameters on the PCE. A case study of a street canyon, located in Montreal, is also considered to investigate the performance of the proposed approach. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction There are certainly some similarities between ventilating a building in order to provide the required environmental condi- tions for occupants’ thermal comfortable and health, and venti- lating an urban canyon to provide the required environmental for the pedestrians’ thermal and health. Traditionally, the buildings were designed and constructed to be naturally ventilated as the urban canyon is ventilated these days. As the city planning changed, natural ventilation technique may not be able to provide the required conditions in certain condition and in some buildings thus the building must be ventilated mechanically. Superstructures and high-rise buildings are becoming part of new cities’ landscapes where their large and high exterior surface area impact the air temperature distribution, and this temperature variation induces significant upward or downward thermal convection flow which impact the air pollution in the urban canyon. This phenomenon profoundly influences the pedestrian thermal comfort and health. Pedestrian thermal comfort strongly depends on the outdoor air temperature, wind velocity, relative humidity, and solar radiation. For example, lack of evapotranspiration and vegetation reduces moisture and consequently the level of pedestrian comfort. In addition, construction of street canyons results in blocking of the prevailing wind breakthrough. This alteration of airflow regime can inversely increase the pollution concentration level and produce a vulnerable air quality for pedestrians. The airflow regime itself is affected by street canyon geometry [16,17]. Moreover, the street canyon geometry has a direct impact on the shading factor which influences human comfort. An extensive review paper by [12] summarized efforts to investigate and mitigate UHI. Many cities recently have provided urban design guidelines and suggested passive mitigation techniques to improve the pedestrian comfort and health. These techniques include: increasing tree planting and vegetation inside urban areas [14], design street canyon and building layout to naturally ventilate urban areas [15], and application of higher-albedo materials [18]. The existing urban planning guidelines are mainly qualitative and not quantitative, * Corresponding author. Tel.: þ1514 848 2424 x 3192; fax: þ1 514 848 7965. E-mail address: haghi@bcee.concordia.ca (F. Haghighat). Contents lists available at ScienceDirect Building and Environment journal homepage: www.elsevier.com/locate/buildenv 0360-1323/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2011.06.007 Building and Environment 47 (2012) 410e420