Effect of triangular roof angle on dispersion of gaseous pollutants and particulate matter

The Euler–Lagrangian method is adopted to simulate the dispersion of gaseous pollutants and particulate matter (PM) in isolated street canyons, and the influences of the roof angle on the flow structures and distributions of gaseous pollutants and PM are analyzed in detail. Numerical simulation results indicate that gaseous pollutants and PM in the canyons present three typical single main clockwise vortex, transition vortex, and double vortex structures, which are identified at increasing roof slopes. Gaseous pollutants and PM demonstrate the lowest concentration of pollutants when a single vortex structure exists. The concentration of gaseous pollutants and PM reaches the highest value in pedestrian-level areas when the flow field is in a transitional vortex structure. Unlike gaseous pollutants, the concentration of PM does not always decrease with increasing altitude, and higher PM concentrations sometimes occur in the mid-level areas of the canyon. A small roof incline angle is generally recommended for discharging gaseous pollutants and PM.