In this paper, the correlation between the ventilation and pollutant removal behaviour over idealized two-dimensional urban street canyons is studied by means of wind tunnel experiments in the Departmental of Mechanical Engineering, The University of Hong Kong. Movable rectangular aluminium blocks are placed in the test section of wind tunnel, constructing street canyon models of different buildingheight-to-street-width (aspect) ratios. Throughout the experiments, water evaporation methods,

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... h are implemented by applying soaked filter papers on the model, are used to simulate a uniform mass concentrations on the building facades and ground surface. The local mass transfer coefficients on various surfaces of street canyons of aspect ratios 0.25 to 2 are determined that are compared with those available in literature. The variations of the average mass transfer coefficients over a wider range of aspect ratios are also examined. Scaling effect, however, is needed to account for. Hence, a further in-depth data analysis is required for a more accurate comparison with our CFD findings.