A tracer gas measurement method was developed to investigate the single-side removal process of pollutants through the roof level, conducted in a wind tunnel utilizing a deep box model with various openings at its top. A homogeneous emission was modelled by filling the box with well-mixed nitrous oxide released from a line source inside. The removal efficiency from the openings with box at different orientations was evaluated by normalized purging velocity (PV) and spatial-mean age of air (). The PIV system was adopted to visualize the flow field above the opening. Varying interactions between the perturbations from the openings and the deflected flow over them resulted in diverse PVs at different orientations. There are also clues showing that the Reynolds-number independent conditions for cavity flows, such as inside a street canyon, are also related to the scale of the open area at the roof height.