4.1.1. BD and BED

The relationship between BD, BED and PM2.5 and wind speed all show a trend of sine and cosine curves (Figure 8a,c). As BD or BED rises, PM2.5 shows a trend of decreasing, while wind speeds show a decrease followed by an increase, then increasing and then decreasing. In particular, PM2.5 is lowest when BD is around 10% and continues to rise above 10%, reaching a maximum of PM2.5 at around 20%. PM2.5 is lowest at a BED of around 3 and highest at 5. When BD = 20% or BED = 5, the corresponding wind speed is at its minimum. The effect of BD and BED on the wind environment can be further seen by comparing the local area wind speed vectors for Neighborhood A and Neighborhood B (Figure 9): BD and BED of Neighborhood A with the relatively smooth internal wind environment is higher than Neighborhood B. This is because when the BD and BED are within a specific range, the transport of pollutants from outside is considerably restricted. However, when the BD and BED rise to a certain level, the neighborhood's wind environment tends to stabilize, which is not conducive to the migration and dispersion of atmospheric pollutants and creates a cumulative effect. Therefore, keeping the building density and block envelope degree of neighborhoods within a reasonable range can assist in enhancing air quality.

**Figure 9.** Vector diagram of local wind speed in: (**a**) Neighborhood A; (**b**) Neighborhood B.
