Measurements and three-dimensional modeling of air pollutant dispersion in an Urban Street Canyon

The concentrations of gaseous pollutants carbon monoxide, nitrogen oxides (NOx=NO+NO2), and sulfur dioxide were measured, and the vehicle type and traffic flow rates in an urban street canyon with an aspect ratio of 0.8 and a length to width ratio of 3 were recorded. Three-dimensional (3D) airflow and dispersion of pollutants were modeled using the RNG k–ε turbulence model, which was solved numerically using the finite volume method. Vehicle emissions were estimated from the measured traffic flow rates and modeled as banded line sources along the street. Both measurements and simulations reveal that pollutant concentrations typically follow the traffic flow rate; they decline as the height increases, and are higher on the leeward side than on the windward side. 3D simulations reveal that the vortex line, joining the centers of cross-sectional vortices of the street canyon, meanders between street buildings, and the flows leave the canyon through the top. Entrainment of outside air reduces pollutant concentrations near the street outlet, where counter-rotating vortices or secondary flows are present. Numerical predictions agree reasonably well with measurements.