Carbon dioxide is a stable constituent of the atmosphere that has no major terrestrial sinks other than atmospheric transport in the absence of photosynthetic activity by plants. In urban atmospheres, CO2 mixing ratios are often elevated above ambient by large local sources from combustion. We measured CO2 mixing ratios and the isotopic composition of CO2 at four locations in the Salt Lake Valley, Utah, during a persistent cold pool event in the winter of 2004. The results showed a strong influence of atmospheric stability and the height of the capping inversion on CO2 mixing ratios and suggested that during persistent cold pool events the air mass beneath the capping inversion can be relatively well mixed. Spatial and temporal patterns in the isotopic composition of CO2 and the relationship between particulate concentrations and CO2 mixing ratio support this interpretation. These results suggest that CO2 mixing ratio, which is abundant and relatively easily measured in urban atmospheres, can provide information about complex wintertime atmospheric transport and mixing as well as carbon cycling in urban mountain basins.