Studying urban air-transport phenomena is highly complex, because of the heterogenous flow patterns that
can arise. The main reason for these is the variable topology of urban areas, however, there are a large number
of influencing variables such as meteorological conditions (e.g., wind situation, temperature) and
anthropogenic factors such as traffic emissions. During a one-year CO 2  measurement campaign in the city of
Basel, Switzerland, steep CO 2  gradients were measured around a large building. The concentration differences
showed a strong dependency on the local flow regimes. Analysis of the field data alone did not provide a
complete explanation for the mechanisms underlying the observed phenomena. The key numerical parameters
were defined and the influence of turbulent kinetic energy dependency on the time interval for the Reynolds
decomposition was studied. A Reynolds-Average Navier-Stokes Computational Fluid Dynamics (CFD)
approach was applied in the study area and the CO 2  concentrations were simulated for six significant
meteorological situations and compared to the measured data. Two flow regimes dependent on the wind
situation, which either enhanced or suppressed the concentration of CO 2  in the street canyon, were identified.
The enhancement of CO 2  in the street canyon led to a large difference in CO 2  concentration between the
backyard- and street-sides of a building forming the one wall of the canyon. The specific characteristics of
the flow patterns led to the identification of the processes determining the observed differences in CO 2
concentrations. The combined analysis of measurement and modeling showed the importance of reliable field
measurements and CFD simulations with a high spatial resolution to assess transport mechanisms in urban
areas.