If one desires to quantitatively measure the dispersal of pollutants in wind tunnel studies, it is necessary to simulate the pollutant source with a tracer gas. Typically used for this purpose is sulfur hexafluoride (SF₆), since this gas is non-poisonous and can be detected and measured in very small concentrations. Sampling sources are built into the model at the desired immissions locations, so that the concentrations can be determined dependent upon the other parameters like wind direction and velocity (Figure 4/13). Since boundary-layer wind tunnels can simulate at present only neutral temperature gradations, statements about dispersal in the presence of a stable temperature gradation (inversion) are not possible. This limitation affects in particular the study of nightly cold air flows.

	Fig. 4/13: Concentration field of an exhaust stack with a given wind direction, measured in a wind tunnel, Source: Office of Dr. A. Lohmeyer
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