The thermal inertia, dryness and roughness of urban areas, especially in densely-built districts, typically results in the development of a weakly convective nocturnal surface boundary layer, often capped by a nocturnal inversion. Although this has significant implications for the dispersion of atmospheric pollutants, particularly from street canyons, few studies have focused on the development and characteristics of turbulence under these conditions. Carbon dioxide is a comparatively inert gas which, in the absence of space heating sources and localized industrial emissions, is primarily emitted into the urban boundary layer at street level from vehicle exhausts. In this study we use a comparatively new analytical tool, wavelet analysis, to examine the hypothesis that carbon dioxide concentrations can be used as a tracer to identify characteristics of pollutant venting from street canyons into the nocturnal urban boundary layer. This is investigated using carbon dioxide, sensible and latent heat flux observations collected using a variable height tower located above the center of Marseille (France) as part of the ESCOMPTE field experiment in the summer of 2001. Preliminary examination of the nocturnal data reveals intermittent bursts of carbon dioxide, superimposed upon a more stable mean background concentration. These bursts, observed above roof-level, may be related to intermittent venting of sensible heat from the warmer urban canopy layer.