Comparative study of concentrations and distributions of CO and NO in an urban area: Gaussian plume model and CFD analysis

The dispersion of air pollutants, CO and NO, due to three point sources and a busy traffic road in a certain area located in the central part of Tokyo has been studied using two different approaches. The first approach was based on the well known Gaussian plume model (GPM) while the second approach was a numerical modelling using a CFD code. In the first approach, the domain of study was divided into a fine grid and the line source (road) was treated as a number of continuous adjacent stacks. The number of ground level stacks was taken according to the length of the road and the diameter of each stack was set equal to the width of the road. The GPM of a point source was applied upon each stack and the concentration at any receptor was calculated by applying the principle of superposition. In the second approach, numerical analysis was performed using the CFD code STAR-CD in order to simulate the wind flow over the area considered by applying the standard k-epsilon turbulent model. Steady state analysis was adopted and the second order upwind difference scheme was applied to the spatial difference. Wind velocity in the first approach, was set equal to the velocity at 10 m height for the ground level line source, and equal to the velocity value at the stack height level for the three tall stacks, while in the numerical approach, the wind velocity was obeying the power law relationship. Results of the two approaches were compared together and their results have shown the same trend for the distributions of CO and NO concentrations inside the area of study. The agreement between the concentration values calculated by the two approaches was good in less densely built-up locations inside the domain of study, and it was not accepted in the high densely built-up sites.