Effects of thermal stability on point-/line-source dispersion in uniform and non-uniform building arrays

Understanding how atmospheric thermal stability affects pollutant dispersion in urban building arrays is critical for improving air quality. This study integrates wind-tunnel experiments and computational fluid dynamics numerical simulations using the standard k-ε Reynolds-averaged Navier–Stokes model to investigate the dispersion characteristics of point/line sources within two typical building arrays: uniform-height (Uni) and non-uniform-height (Non-uni) arrays. Seven atmospheric stability conditions are considered, defined by the bulk Richardson number (Rib). Key new insights are revealed. (i) Two stability regimes control urban dispersion: when Rib ≤ −0.12, flow and concentration fields become effectively independent of Rib, with high mean velocity (approximately 40% of the free-stream velocity) and low pedestrian-level concentrations (average normalized concentration Kave ≈ 0.198). When Rib ≥ 0.43, cold-air subsidence suppresses vertical exchange, triggering reversed near-surface flows and upwind pollutant migration; Kave peaks at Rib = 0.43 (up to 8.045 for point sources). (ii) Building height heterogeneity redistributes exchange pathways: the Non-uni array enhances lateral and vertical dispersion of line sources under unstable conditions (normalized net escape velocity NEV* = 26% higher than Uni), but the taller buildings weaken vertical diffusion of point sources (NEV* = 17.3% lower than Uni). These findings quantify how stability and morphology jointly govern urban ventilation and pollutant removal, providing a mechanistic basis for climate-responsive urban design and emission-control strategies.