Field study on rooftop air dispersion using SF₆ as a tracer gas

The tracer gas concentrations near building exhaust was investigated in field experiments carried out on the rooftop of laboratory block of the Hong Kong University of Science and Technology (HKUST). Wind data in the experiments covered most local meteorological conditions. Sulfur hexafluoride (SF6), used as a tracer gas in the field experiments, was released in a particular fume hood of the laboratory block. In all experiments, the field dilution rates varied from 1x10[to the power of 3] to 1x10[to the power of 6] at different sampling points of roof plane and fresh air intakes. One of the main goals of this study was to evaluate the Pasquill-Gifford formula for concentration field prediction, moreover, the Wilson type of formulas were adopted for minimum dilution rate estimation. Comparisons of field concentration with that calculated from Pasquill-Gifford formula and that of reported wind tunnel results have been performed. The results demonstrated the validity of the modified Pasquill-Gifford formula for concentration field prediction on large plain roof where the predictions and field data were in the same order of magnitude. The concentrations detected at the fresh air intakes were in the same range. For the area around a bossed building, the field data detected lower readings than Pasquill-Gifford predictions due to some strong turbulent mixing. On the other hand, the Wilson formula provided sufficient conservative prediction for minimum dilution rate. Nevertheless, limitations of all the formulas were highlighted when the wind speed was low. Key Words: Sulfur hexafluoride, Tracer gas, Rooftop air dispersion, Field stud