In case of terror attacks by biochemical material or accidental leakage of inflammable gas on an urban area, the time dependent aspect of transport of scalar becomes problem. However, there has been little speculation on unsteady emission directly related to time history. One of the way which count the problem is the concept of the ability of the ventilation (Purging Flow Rate) , which includes the unit of time. This paper report the time spent for arrival of the concentration after the

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... on of tracer start and the time spent for decaying of the concentration after the generation of tracer end. The experiment was done in the wind tunnel whose wind profile is similar to the urban wind environment. Under the condition of dispersion in an atmosphere, the realization of each measurement shows different results because of the turbulence. The measurements were conducted for several times and the results will be shown with an ensemble averaged manner. The dependence of these time history to the inflow angle is discussed. The relation between these time history and the virtually calculated PFR is shown. As for the concentration fluctuation, the time development of the root mean square of concentration is also investigated.