Based on a numerical model validated by wind tunnel experiments, the influence of atmospheric stability, characterized by the bulk Richardson number (Ri_b) in the range of -0.21 to 0.79, on the diffusion of pollutants from roof chimneys on step-down buildings was investigated. The results indicate that the flow field and pollutant distribution within the street canyon are significantly altered by atmospheric stability. Under stable conditions (Ri_b > 0), vertical convection within the street canyon was weak. At Ri_b = 0.43, the dimensionless velocity difference (0.25) on the vertical center plane (y/H = 0) was reduced by 13.80% compared to that under neutral conditions. At Ri_b = 0.79, the maximum dimensionless concentration (4.76) at the pedestrian breathing level was increased by 18.70%. Under unstable conditions (Ri_b < 0), vertical convection was enhanced, with increased wind speed near the ground, leading to a notable decrease in pollutant concentration. At Ri_b = –0.19, the dimensionless velocity difference (0.32) on the vertical center plane was increased by 10.34%. At Ri_b = –0.21, the maximum dimensionless concentration (0.88) at the pedestrian breathing level was decreased by 78.19%. The patterns of local urban flow fields and pollutant dispersion under different atmospheric stability conditions are revealed, providing a theoretical basis for targeted air quality improvement.