###### Abstract  
Due to the rapid advancement of urbanization, traffic-related pollutants in street canyons have emerged as the primary source of PM\({}_{2.5}\), adversely impacting residents' health. Therefore, it is necessary to reduce PM\({}_{2.5}\) concentrations. In this study, a three-dimensional steady-state simulation was conducted using Computational Fluid Dynamics (CFD). Three representative wind directions (\(\theta\) = 0\({}^{\circ}\), 45\({}^{\circ}\), and 90\({}^{\circ}\), corresponding to parallel, oblique, and perpendicular winds) and five different building height ratios (BHR = 0.25, 0.5, 1, 2, and 4) were used to explore the effect of building height variations on PM\({}_{2.5}\) dispersion within street canyons. The results indicated that wind direction significantly influenced PM\({}_{2.5}\) dispersion (\(p\) < 0.001). As \(\theta\) increased (\(\theta\) = 0\({}^{\circ}\), 45\({}^{\circ}\), and 90\({}^{\circ}\)), PM\({}_{2.5}\) concentration in the canyon increased, reaching the most severe pollution under perpendicular wind. Building height variations had a minor impact compared to wind direction, but differences in PM\({}_{2.5}\) concentration were still observed among various BHRs. Specifically, under parallel wind, the influence of BHR on PM\({}_{2.5}\) dispersion was relatively small as compared to oblique and perpendicular winds. For oblique wind, PM\({}_{2.5}\) concentrations varied based on BHR. Street canyons composed of low-rise or multi-story buildings (BHR = 0.25 or 4) slightly increased PM\({}_{2.5}\) concentrations within the canyon, while the lowest PM\({}_{2.5}\) concentration was observed at a BHR of 0.5. Under perpendicular wind, symmetrical (BHR = 1) and step-down canyons (BHR = 2 and 4) exhibited comparable peak concentrations of PM\({}_{2.5}\), whereas step-up canyons (BHR = 0.25 and 0.5) showed relatively lower concentrations. CFD simulation; building height variations; asymmetrical street canyons; wind directions; PM\({}_{2.5}\) dispersion +
Footnote †: journal: Applied Sciences  
a reduction in the annual target value of PM\({}_{2.5}\) from 10 to 5 \(\upmu\)g/m\({}^{3}\)[10]. As a result, it is crucial to manage PM\({}_{2.5}\) distribution and decrease concentrations in street canyons. In May 2021, a joint initiative involving 15 departments, including the Chinese Ministry of Housing and Urban-Rural Development, released a pivotal document titled \"Opinions on Strengthening Green and Low-Carbon Construction in County Towns\". The main focus of the document was to strictly regulate the vertical growth of residential structures. It underscored the importance of residential buildings and explicitly mandated that at least 70% of such structures should not exceed six floors. This implies that the majority of buildings in these areas should be low-rise or multi-story buildings. However, whether street canyons composed of low-rise or multi-story buildings can to some extent reduce the concentration of PM\({}_{2.5}\) remains a question. Generally, it is assumed that variations in building height can either improve or reduce ventilation in street canyons [11]. In most studies, the representation of building height variance commonly involves the ratio between the height of the upwind building (H\({}_{0}\)) and the height of the downwind building (H\({}_{d}\)), referred to as the building height ratio (BHR, [12]). Street canyons are categorized as step-down street canyons when the building height ratio (BHR) is greater than 1, signifying that the upwind building is higher than the downwind building. Conversely, they are termed step-up canyons when BHR is less than 1, indicating that the upwind building is lower than the downwind building. In most studies on the impact of building height variations on the dispersion of pollutants in street canyons [13; 14; 15; 16; 17; 18], the variability of building height is limited (0.33! BHR! 2).