Evaluation of numerical simulations of CO2 transport in a city block with field measurements

Studying urban air-transport phenomena is highly complex, because of the heterogenous flow patterns thatcan arise. The main reason for these is the variable topology of urban areas, however, there are a large numberof influencing variables such as meteorological conditions (e.g., wind situation, temperature) andanthropogenic factors such as traffic emissions. During a one-year CO 2  measurement campaign in the city ofBasel, Switzerland, steep CO 2  gradients were measured around a large building. The concentration differencesshowed a strong dependency on the local flow regimes. Analysis of the field data alone did not provide acomplete explanation for the mechanisms underlying the observed phenomena. The key numerical parameterswere defined and the influence of turbulent kinetic energy dependency on the time interval for the Reynoldsdecomposition 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 significantmeteorological situations and compared to the measured data. Two flow regimes dependent on the windsituation, 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 thebackyard- and street-sides of a building forming the one wall of the canyon. The specific characteristics ofthe flow patterns led to the identification of the processes determining the observed differences in CO 2concentrations. The combined analysis of measurement and modeling showed the importance of reliable fieldmeasurements and CFD simulations with a high spatial resolution to assess transport mechanisms in urbanareas.