Urban morphology and air quality: microclimatic simulation of PM2.5 and NO₂ dispersion in urban canyons scale

Air pollution in megacities such as São Paulo is one of the major public health challenges, with fine particulate matter (PM2.5) and nitrogen dioxide (NO₂) among the most critical air pollutants due to their respiratory and cardiovascular effects. Intensified land use along the Urban Structuring Axes (EETU), a strategy within transit-oriented development, has promoted densification and verticalization, altering the urban microclimate and pollutant dispersion. This study analyzed how different urban morphological configurations influence the dispersion of PM2.5 and NO₂ in street canyons along the EETU in São Paulo city, considering the interaction between built form and street trees. The ENVI-met microclimatic model, based on Computational Fluid Dynamics (CFD), was used to simulate pollutant concentration and dispersion in five street canyons representative of distinct urban morphologies. Indicators such as urban density, verticality (average building height), H/W ratio, occupation, directionality (building orientation relative to prevailing winds), and Normalized Difference Vegetation Index (NDVI) were applied to characterize each area. Results show that compact and highly occupied urban forms exhibit substantially higher NO₂ concentrations at pedestrian level, with differences of up to ~ 80% between contrasting urban morphologies. In contrast, PM2.5 concentrations vary only modestly between canyons (< 5% variation), remaining close to urban background levels. PM2.5 deposition mass, however, shows pronounced sensitivity to urban form and vegetation, varying by over 60% between canyons, indicating that deposition responds differently from pollutant concentration. Urban form therefore plays a key role in shaping ventilation efficiency and air quality in dense areas of São Paulo. Strategies that combine building height variation, transversal block openings, orientation relative to prevailing winds, and context-sensitive street-tree management are essential to reduce population exposure to air pollution in compact urban environments. Compact and highly occupied urban forms show up to ~ 80% higher NO₂ concentrations compared to more open canyon morphologies. PM2.5 concentrations remain relatively uniform across urban forms (< 5% variation), while PM2.5 deposition mass varies strongly (~ 60% difference) depending on vegetation and ventilation conditions. Dense and continuous tree canopies reduce effective ventilation, increasing local pollutant accumulation, while simultaneously enhancing PM2.5 deposition through increased surface interception. Street canyons with transversal openings and height variation promote more effective cross-canyon airflow, reflected in higher wind speeds and enhanced pollutant dispersion. Integrated ENVI-met simulations across five real transit-oriented development corridors, in São Paulo city, reveal how densification strategies shape local air pollution exposure.