<p>Air pollution in street canyons is a serious environmental problem in megacities. This problem is exacerbated in cities with complex terrain. This study analyzed the distribution of pollutants from a street canyon’s chemical reactions. To describe this process, the Reynolds-averaged Navier–Stokes equations were used, along with equations for concentration transport. To numerically solve this system of equations, an approximation procedure was performed using the finite volume method. The SIMPLE method was used to relate the velocity component and pressure value in the Navier–Stokes equations. A sensitivity analysis of the computational grid was performed. The effect of different heights of grass barriers with porosity properties on the distribution of concentrations and the product from the chemical reaction was also considered. In the presence of grass barriers, the wind velocity decreases and vortex flows are formed inside the cavity. To determine the effective height that provides maximum protection for buildings and houses in street canyons, various barrier heights (0.1H, 0.2H, 0.3H, 0.4H) were considered. Among the options considered, the height of 0.4H presented the best results, since with increasing height, less concentration is distributed towards residential buildings. The chemical reaction of carbon monoxide with oxygen was considered as a chemical reaction. As a result of the study, it was found that the best and most reliable grass barrier height is 0.4H. Thus, the mean concentration values for the product from the chemical reaction of CO₂ in the windward and leeward sides of the pre-home area for a grass barrier height of 0.4H compared to a grass barrier with a height of 0.2H can be observed to be more than 4 times and 1.7 times decrease in concentration, respectively.</p>