A wind tunnel study on the effect of thermal stability on flow and dispersion of rooftop stack emissions in the near wake of a building Mohamed F. Yassin a, b, * a Department of Environmental Technology Management, Kuwait University, P.O. Box 5969, Safat 13060, Kuwait b Faculty of Engineering, Assiut University, Assiut 71516, Egypt highlights < Study on flow and dispersion of rooftop stack emissions in the near wake region. < Varying atmospheric stratifications were stable, neutral and unstable conditions. < Vertical velocity increases under stable conditions. < Longitudinal turbulence intensity increases under unstable conditions. < Concentration increases with stable condition and decreases with unstable condition. article info Article history: Received 10 July 2012 Received in revised form 6 October 2012 Accepted 8 October 2012 Keywords: Atmospheric diffusion Dispersion emissions Rooftop stack Thermal stability Wake region abstract The effect of atmospheric thermal stability on wind flow and dispersion of rooftop stack emissions in the near wake of a building in an urban environment is investigated using stratified wind tunnel experi- ments. Experiments with varying stratifications (stable: Ri B and Re b ¼ 0.023 & 5.1  10 3 , neutral: Ri B and Re b ¼ 0.0 & 5.8  10 3 , and unstable: Ri B and Re b ¼0.016 & 6.6  10 3 ) were conducted using an isolated cubical building model. Tracer gas (C 2 H 4 ) was released from a stack on the building roof. Flow, temperature and dispersion fields were analyzed and measured using a split film, a cold-wire ther- mometer, and high-response flame ionization detector. Thermal diffusion was examined at different distances downwind of the leeward side of the building model. Flow and dispersion characteristics studied included mean velocity, turbulence intensity, mean and fluctuation concentrations in the near wake of the building. The results indicate that when atmospheric stratification is unstable, flow sepa- ration decreases and longitudinal turbulence intensity increases in the near wake region. When this stratification is stable, vertical velocity and vertical turbulence intensity increase in the near wake region. There is an increase in concentrations with stable conditions, and a decrease in concentrations with unstable conditions in the near wake region. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction A large number of industrial, institutional, university and hospital laboratories, as well as manufacturing facilities, emit a wide range of potentially harmful pollutants (e.g., toxic and odorous chemicals) from rooftop stacks in the urban environment. Therefore, the dispersion of potential hazardous exhaust from these stacks is of great concern when addressing possible conse- quences of such releases on human health and safety, as well as the environment in the stack vicinity. Many factors affect this disper- sion, such as wind velocity and behavior, topographic features around the site, stack height, buildings near the stack, and thermal stability (Wilson, 1979; White and Stein, 1990; Schulman and Scire, 1991). Regarding the last factor, it is known that thermal stability is most important in the transport and dispersion of rooftop stack pollutants. The near-ground atmosphere typically exhibits a stable stratification during nighttime and unstable stratification during daytime. For moderate to higher wind speeds, however, sufficient atmospheric turbulence is generated such that near-neutral stability prevails. A number of relevant wind tunnel experiments have been per- formed to investigate various influences, such as stack height and * Department of Environmental Technology Management, Kuwait University, P.O. Box 5969, Safat 13060, Kuwait. E-mail addresses: mohamed_f_yassin@hotmail.com, mfy_64@yahoo.com. Contents lists available at SciVerse ScienceDirect Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv 1352-2310/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.atmosenv.2012.10.013 Atmospheric Environment 65 (2013) 89e100