Summary

The dispersion of pollutants from rooftop emissions within the boundary layer is greatly influenced by buildings and local topography. It is thus very difficult to make accurate assessments of plume concentrations on various building surfaces. Pollutants released from a rooftop stack can cause potential health hazards to building occupants by re-entering the building from which they are released or by entering a neighbouring building through openings on building surfaces. Most studies in the past have focussed on rooftop emissions from isolated buildings, which seldom exist in the urban environment. A majority of dispersion models are incapable of providing reasonable dilution estimates on building surfaces and do not incorporate the effect of adjacent buildings. Therefore, there is a need to develop a new model or modify an existing model to take into account the effects of dispersion of effluents and focus on the impact of buildings that are in close proximity with the source of pollutants. To address this issue, a collaborative research program between Concordia University and IRSST was elaborated relying on both experimental and numerical modeling. This report presents the experimental findings while the numerical findings are published in a companion report  (Bahloul et al. 2014).

The experimental modeling consisted of performing tracer gas studies in the Boundary Layer Wind Tunnel at Concordia University for various adjacent building configurations. These configurations include buildings of different geometries placed upstream (or downstream) of the emitting building (source). Another configuration consisting of a building placed upstream and another building placed downstream of the source was also studied. In this regard, various parameters that include: building dimensions, spacing between buildings, stack height and location; exhaust parameters and wind azimuth were varied. These results were compared to the Gaussian based ASHRAE 2007 and 2011 models.

Results from the wind tunnel data indicate that an emitting building placed within the recirculation length of a taller upstream building produces lower dilutions on the roof of the emitting building. Similarly, taller downstream buildings disallow the plume from spreading thereby increasing plume concentrations on the roof and leeward wall of the emitting building. In general, spacing between buildings and exhaust speed were found to be critical parameters influencing the plume characteristics. Based on the experimental data, design guidelines for the safe placement of stack and intake on various building surfaces are also presented. ASHRAE 2007 predictions are overly conservative, while ASHRAE 2011 predictions compare well with wind tunnel data for the isolated building for low exhaust momentum ratios (M) of less than 3. Rectifications to the ASHRAE 2007 model are also presented to obtain reasonable dilution estimates for the isolated case, besides incorporating the effects of adjacent buildings. The rectified ASHRAE 2007 model was found to perform well for most cases when compared to results obtained from present and previous studies.