By means of numerical simulations the aims of this study are as follows: (1) to investigate the dispersion and mixing of ultrafine particles (UFP) with pre-existing size resolved UFP in a street canyon and its vicinity with the ENVI-met 3D microscale model; (2) to show the effects of boundary conditions, like wind direction and traffic emissions, on the UFP concentration in the near vicinity; and (3) to evaluate the importance of deposition and coagulation at the street scale. The decrease in UFP concentration in nucleation mode particles (diameter < 30 nm) and Aitken mode particles (diameter between 30–100 nm) downwind of the street canyon is caused by the large differences in the size distributions of the emissions and the background. Based on the wind direction and traffic emissions, the UFP concentration over the rooftop increases by 23% for the South-West wind up to 165% for the Northern wind. The background distribution for the South-West wind direction remains mono-modal, with a peak in the Aitken mode as a result of the relatively low contribution of advected particles from the street canyon. For the Northern wind the background size distribution transforms to a bi-modal distribution, with peaks in the nucleation and the Aitken modes as a result of the high UFP concentration advected from the boulevard. The overall effect of deposition and coagulation on the UFP is negligible for the cases considered in this study. Overall, deposition is more efficient and faster than coagulation in removing particles, and especially those in the nucleation mode.