The Reynolds number in an Urban Street Canyon is a difficult parameter to match between reduced-scale experiments and full-scale measurements. It is possible to overcome this mismatch in Reynolds numbers by satisfying the Reynolds number independence criterion, which states that above a certain critical Reynolds number, the flow field remains invariant with increasing Reynolds numbers. For an urban canyon with an aspect ratio 1, this critical Reynolds number is often reported to be 12000 for the mean flow quantities. This critical Reynolds number, however, is not applicable for higher-order quantities such as turbulence intensity and Reynolds stresses. In this study, 3D robotic particle tracking velocimetry was used to study the flow in an urban street canyon with an aspect ratio of 1 at several Reynolds numbers to estimate the critical Reynolds number for mean flow quantities and turbulence quantities. It was found that the critical Reynolds number for mean flow quantities was between Re = 13000 and Re = 17000. For the higher-order quantities, it was found that the critical Reynolds number was above 22000.@en