Air intake contamination by building exhausts: tracer gas investigation of atmospheric dispersion models.

The reintroduction of toxic gases emitted from roof stacks can significantly affect the quality of the air inside a building. The determination of a safe distance between the sources of pollution and the fresh air intakes is based on a complex exercise that must take into account several wind, physical and topographical factors. Estimates of maximum concentrations as a function of downwind distance from a stack can be obtained using empirical models provided by the American Society of Heating, Refrigerating and Air Conditioning Engineers [ASHRAE, 1997](1). However, this modeling approach has not been completely validated using field data. ASHRAE's formulas have mainly been evaluated by comparing their resulting dilution estimates to the wind tunnel data originating from some case studies, and the results of these comparisons have proven contradictory (2,3). Most previous field studies(4,5) were conducted using a limited number of samplers placed near the fresh air intakes. Consequently, the data obtained cannot be used for model validation since samplers were usually not on the plume center-line. In a comparative study conducted by Stathopoulos, Lazure and Saathoff(6), tracer gas experiments were conducted using a building in an urban environment. Air samples were collected at several locations on the roof of this building, and the emission concentration data were correlated with the meteorological data collected on the roof.