## 4 Discussion
### The Synergy between Wind Direction and Building Height Variation  
Overall, irrespective of the BHR, the total mean concentration of PM\({}_{2.5}\) within the canyon increased as \(\theta\) increased (\(\theta\) = 0\({}^{\circ}\), 45\({}^{\circ}\), and 90\({}^{\circ}\)), indicating that the wind direction played a more significant role than variations in building height (Figure 10). Additionally, the Two-Way ANOVA analysis of wind direction and BHR on PM\({}_{2.5}\) concentration revealed a significant positive correlation (\(p\) < 0.001) between wind direction and PM\({}_{2.5}\) concentration, whereas the correlation between BHR and PM\({}_{2.5}\) concentration was not very significant (\(p\) = 0.348). Figure 10 illustrates the synergistic effect of wind direction and BHRs on PM\({}_{2.5}\) dispersion. Under parallel wind directions, the total mean concentration of PM\({}_{2.5}\) within the canyon remains consistently low in all cases, slightly exceeding 3. Compared to oblique  
Figure 10: Total mean concentration in different canyon configurations under various wind directions. and perpendicular winds, building height variations under parallel wind directions have a lower impact on the dispersion of PM\({}_{2.5}\). Under oblique winds, the BHR affects the total mean concentration of PM\({}_{2.5}\). For street canyons composed of low-rise or multi-story buildings (BHR = 0.25 or 4), the total mean concentration of PM\({}_{2.5}\) within the canyon slightly increases compared to the symmetrical canyon. With BHR values of 2 or 0.5, there is a decrease in total mean concentration of PM\({}_{2.5}\), with BHR = 0.5 exhibiting the lowest concentration in the canyon. This suggests that when the BHR is 2 or 0.5, the atmosphere is more easily mixed in the canyon, promoting the dispersion of PM\({}_{2.5}\). Under perpendicular wind direction, regardless of variations in building height, the total mean concentration of PM\({}_{2.5}\) within the canyon is higher than that of parallel and oblique winds (Figure 10), indicating that perpendicular wind direction is the least favorable for PM\({}_{2.5}\) dispersion, consistent with the research results of [PERSON] et al. (2008). Specifically, the study found that the total mean concentration of PM\({}_{2.5}\) in symmetrical and step-down canyons (BHR = 2 and 4) is similar, with both forming concentration peaks within the canyon, while the total mean concentration of PM\({}_{2.5}\) in step-up street canyons (BHR = 0.25 and 0.5) is significantly lower. This indicates that both symmetrical and step-down canyons are not conducive to PM\({}_{2.5}\) dispersion in the perpendicular wind direction. The research findings underscore the significance of wind direction and building height variations as crucial reference factors in urban planning. When considering wind direction as an influencing factor, the observed lowest PM\({}_{2.5}\) concentration in the canyon occurred under parallel wind directions, an observation that holds valuable insights for urban strategies; that is, in the process of urban planning, we can try to align the orientation of street canyons parallel to the dominant wind direction as much as possible. This approach can enhance ventilation and subsequently improve air quality within the canyon. On the other hand, when considering the impact of building height variations, in cities where wind flows parallel to the street axis, the influence of BHRs on PM\({}_{2.5}\) dispersion is comparatively small compared with oblique and perpendicular wind directions, affording urban planners greater flexibility in determining building height. Under oblique wind directions, canyons with low-rise or multi-story buildings may result in an increase of PM\({}_{2.5}\) concentration compared to symmetrical street canyons. Conversely, street canyons with a BHR of 0.5 help optimize airflow and promote the dispersion of PM\({}_{2.5}\) In perpendicular winds, step-up street canyons demonstrate superior dispersion of PM\({}_{2.5}\) compared to both symmetrical and step-down canyons.