4.1.2. GCR

As the GCR increases, PM2.5 concentration and wind speed all showed a decreasing trend (Figure 8b). This is because wind speed is reduced and dust is suppressed by vegetation in the vertical zone beneath the canopy, with a progressive decrease in the zone above the canopy. As an example, Figure 10 illustrates the local vegetation XZ plane wind speed and PM2.5 distribution in Neighborhood A, which suggests that the influence of vegetation on pollutant concentrations and wind speed is related to the height of the vegetation and its dust retention effect is most noticeable in the vertical zone beneath the canopy, with a progressive decrease in the zone above the canopy.

**Figure 10.** XZ plane in the local area of Neighborhood A of: (**a**) velocity vector; (**b**) PM2.5 concentration distribution.

#### 4.1.3. ABF, ABV and SDH

ABF, ABV and SDH demonstrated varying degrees of positive and negative correlations with PM2.5 and wind speed, respectively. When ABV = 40,000 m<sup>3</sup> and ABF = 20F, the lowest PM2.5 concentration was observed. The rise in ABF indirectly increases the building separation and enhances the air circulation within and outside the neighborhoods. When ABF exceeds 20, wind speed is further increased, but the static wind zone is formed on the leeward side of the building, which hinders the diffusion of pollutants (Figure 8d). As the ABV increases, PM2.5 concentration shows a tendency to decrease and then increase (Figure 8e). This is because, within a certain volume range, increasing ABV helps to increase the open space and reduces the weakening effect on wind speed due to a large number of buildings. However, excessive ABV might increase the static wind area in the neighborhood and limit the effect of wind on pollution transmission.

The results reveal that the difference in height between buildings on the windward and leeward sides of the neighborhoods affects wind speed in the direction of incoming airflow differently (Figure 8f). Taking the example of four street valleys in the typical YZ plane of two neighborhoods (Figure 11), rising valley 1 and valley 4 are more effective in diffusing PM2.5 in vertical space than falling valley 2 and valley 3. Therefore, reasonable SDH regulation promotes airflow rising, establishing a pleasant neighborhood wind environment, and boosting pollutant dispersion in vertical space. Moreover, angular flow zones are created with building heights up to a certain level, which enhances the dispersion of pollutants to the leeward side.

**Figure 11.** PM2.5 diffusion distribution of typical Y-Z section in: (**a**) Neighborhood A; (**b**) Neighborhood B.
