Influence of Urban Obstacles on Reactive NO2–O3 Transport in a Tropical Street Canyon Using the k-ω SST Model

Abstract

Urban street canyons in tropical regions tend to experience pollutant accumulation due to complex urban geometries and atmospheric chemical transformation processes. This study investigates the influence of urban obstacles, including buildings and simplified roadside obstacles, on the reactive transport of NO$_2$--O$_3$ within a tropical street canyon using Computational Fluid Dynamics (CFD). The CFD framework is based on the Reynolds-Averaged Navier--Stokes (RANS) equations with the $k$-$\omega$ Shear Stress Transport (SST) turbulence model.The case study was conducted along Dr. Ir. H. Soekarno Street, East Surabaya, Indonesia, incorporating NO$_2$ photolysis and subsequent gas-phase reactions leading to the formation of O$_3$ and NO, thereby representing a non-conservative pollutant system. Simulation results indicate that the presence of obstacles alters the airflow structure within the street canyon, characterized by the development of high-pressure zones on the windward side of buildings and weakened airflow in the mid-canyon region.The concentration fields further show that the simplified obstacle configuration influences local ventilation and the spatial distribution of reactive pollutants NO$_2$ and O$_3$. It should be noted that the roadside obstacle elements were represented as simplified solid barriers, and thus the model does not explicitly resolve porous vegetation effects such as canopy drag, turbulence generation, or pollutant deposition. The integration of non-conservative atmospheric chemistry within the CFD model provides insight into NO$_2$--O$_3$ interaction dynamics under the obstacle configuration. Overall, the model achieved numerical convergence and remained stable throughout the simulation, demonstrating its capability to represent the effects of simplified urban obstacle on pollutant transport under the examined conditions.