IRSST - Institut de recherche Robert-Sauvé en santé et en sécurité du travail

Analytical evaluation of the dispersion of roof stack emissions on buildings


Dispersion is used here to describe the way in which effluent from stacks or other building exhaust is transported and diluted by the wind as it passes across the proposed development and immediate neighbors. Extreme air pollution occurs when air pollution reaches excessively high levels for several hours or several days and can cause severe discomfort, diseases and even deaths among the most vulnerable people. Extreme pollution has a high probability of occurrence when there are persistent thermal inversions and weak or stagnant winds due to which effluents cannot be dispersed. Atmospheric dispersion modeling is the mathematical simulation of how air pollutants disperse in the ambient atmosphere. Such modeling is performed with computer programs that solve the mathematical equations and algorithms which simulate the pollutant dispersion. The dispersion models are used to estimate or predict the downwind concentration of air pollutants emitted from sources such as industrial plants and vehicular traffic. Such models are important to governmental agencies tasked with protecting and managing the ambient air quality. In particular, the health of workers in laboratory and hospital buildings is at great risk when pollutants generated by the activities in these buildings are reingested through air-conditioning inlets or open windows. The models are typically employed to determine whether existing or proposed new industrial facilities are or will be in compliance with the National Ambient Air Quality Standards (NAAQS) in the United States and other countries. The models also serve to assist in the design of effective control strategies to reduce emissions of harmful air pollutants.

This report investigates the use of the various air-dispersion models, which have been approved by the Environmental Protection Agency (EPA), in modeling dispersion of effluents from stacks located on roof tops to determine their concentrations at various areas of the roofs with these stacks. In this context the effects of roof top structures and the varying directions of wind have been taken into account. Comparisons of the wind tunnel and field data with the results obtained from various dispersion models were made. It was observed that the EPA models, which mostly use the Gaussian equations, are more appropriate for longer rather than shorter distances within the vicinity of the building under consideration. In such cases of proximity of the stack with the points of interest on the roof, the ASHRAE model and wind tunnel data can be more reliable, to predict dispersion or concentration of pollutants.

Additional Information

Category: Research Report
Research Project: 0099-6120
Online since: September 23, 2008
Format: Text