The vertical transport of heat and carbon dioxide (CO2) at the top of the urban canopy layer (UCL) is examined over two intersecting street canyons (one wide and one deep) during calm, anticyclonic conditions in Toulouse. Wavelet analysis of data from the Canopy and Aerosol Particle Interaction in Toulouse Urban Layer project (CAPITOUL) shows that in the narrow, deep street canyon venting of CO2 from the UCL is strongly related to the vertical transport of heat away from the surface throughout the diurnal cycle. In contrast the wider street canyon showed increased penetration of turbulent structures from aloft and a weak relationship between the vertical transport of heat and CO2 was observed. Wind direction played an important role in determining the characteristics of vertical transport in the broad street canyon but had little influence in the deep street canyon. Wind speed did not appear to influence the rate of vertical exchange between the UBL and UCL under these conditions. Clear evidence is provided to show that these processes are not only responsible for pollutants venting out of street canyons but may also transport polluted air from aloft downwards into the street canyon network. These results also demonstrate the spatial and temporal variability of vertical mixing processes in the UCL.