Optimization of Air Purification System Placement to Minimize NOx Exposure at Medellín Public Bus Stops

Urban bus stops represent critical micro-environments where pedestrians may be exposed to elevated concentrations of traffic-related air pollutants, particularly under low-wind conditions. This study presents a computational fluid dynamics (CFD) analysis of airflow patterns, nitrogen dioxide (NO2) dispersion, and exposure mitigation strategies within a representative urban bus stop in Medellín, Colombia. A three-dimensional model was developed in COMSOL Multiphysics v5.6, solving the Reynolds-Averaged Navier–Stokes equations coupled with the SST k–ω turbulence model and a passive scalar transport formulation for NO2. Simulated breathing-zone concentrations ranged from 2.25μmol/m3 under low-wind conditions (0.5 m/s) to 1.03μmol/m3 at higher ventilation (4.0 m/s), highlighting the strong dependence on atmospheric dilution. Recirculation zones formed beneath the shelter roof and near confined regions, promoting pollutant accumulation within the waiting area. Optimized ventilation configurations achieved exposure reduction efficiencies (ERE) between 6.8% and 15.6%, with the highest mitigation observed under low-ventilation scenarios. The results indicate that system performance is primarily governed by the interaction between induced airflow and ambient wind conditions, with recirculation and advection patterns driving pollutant dispersion beyond local removal. These findings provide quantitative guidance for the aerodynamic design of mitigation systems in open urban transport infrastructure.