Abstract
Air quality within urban canopies is strongly influenced by the interaction between atmospheric stability and urban geometry. This study examines how stable (SBL), neutral (NBL), and unstable (UBL) stratification, combined with wind–street direction (0° and 45°), affects pollutant dispersion and lower-canopy respirability in a regular urban array using the DIPLOS/EnFlo wind tunnel dataset. A set of complementary indices was evaluated, including dimensionless concentration (
C
∗
), net escape velocity (NEV), dimensionless mean pollutant age (
τ
p
∗
), exposure improvement index (IEI), and worst-exposure risk index (WRI). Atmospheric stability exerted a strong control on the ventilation–exposure balance. Stable conditions increased pollutant accumulation, residence time, and exposure risk, while reducing pollutant-removal capacity. Relative to neutral conditions, stable stratification increased
C
∗
,
τ
p
∗
and WRI by approximately 51%, 69%, and 63%, respectively, while decreasing NEV by 54%. In contrast, unstable conditions reduced
C
∗
,
τ
p
∗
and WRI by approximately 43%, 35%, and 29%, respectively, and increased NEV by 76%, indicating enhanced pollutant removal and reduced occurrence of high-exposure zones. Wind direction further modulated these effects, with oblique flow promoting greater lateral dispersion than aligned flow. The study introduces a stability-resolved diagnostic framework that integrates concentration, ventilation, residence-time, and exposure metrics into a unified assessment of lower-canopy respirability. By combining
C
∗
, NEV,
τ
p
∗
, IEI and WRI, the framework distinguishes average pollutant removal from localized exposure hotspots. A Ventilation–Exposure Assessment Matrix (VEAM) is proposed to classify corridor performance and support stability-aware assessment of urban ventilation pathways and exposure mitigation strategies.