## 3 Results
### Comparison of the PM\({}_{2.5}\) Concentration by Wall- and Tree-Type Fences  
We analyzed the PM\({}_{2.5}\) concentration vertically in the school by averaging the PM\({}_{2.5}\) concentrations simulated for 16 inflow directions using the inflow wind direction frequencies as the weighting (Table 2). Figure 13 shows the vertical profiles of the PM\({}_{2.5}\) concentration change rates for each case (W2M, W4M, T2M, and T4M) compared to the CNTL case averaged for the 16 inflow directions in the school ground and school boundary. In all cases, the PM\({}_{2.5}\) concentrations decreased compared to the CNTL case. The PM\({}_{2.5}\)  
Figure 11: For the east–northeasterly inflow, the difference in the wind speed (upper panels) and vertical velocity (lower panels) of the T4M case compared to the CNTL case at 1 m (**a**,**d**), 5 m (**b**,**e**), and 9 m (**c**,**f**) above the ground. Figure 10: Wind vectors and shading of the vertical velocity distribution at 1 m above the ground for the east–northeasterly inflow in (**a**) the CNTL case, and (**b**) the T4M case. concentration decreased with increasing height of walls and trees under both the wall- and tree-type fence cases. The change rates of the PM\({}_{2.5}\) concentration in the school ground were lower than the change rates in the school boundary. This occurred because the PM\({}_{2.5}\) concentration changes were offset where increasing and decreasing PM\({}_{2.5}\) concentrations existed simultaneously within the school ground. At the lower layer (below a height of 5 m above the ground), the tree effects (T2M and T4M) of reducing the PM\({}_{2.5}\) concentration were more significant than the wall-type fence effects (W2M and W4M) in the school ground (Figure 13a). This indicated that the prevention (by the tree-type fence) of pollutants incoming from the background due to increased vertical speed was greater than the prevention (by the wall-type fence) of pollutants emitted at the road. The PM\({}_{2.5}\) change rates on the first and second floors of the school building in the T4M case were \(-1.10\%\) and \(-0.68\%\), respectively (Table 5). However, with a higher wall-type fence height, the PM\({}_{2.5}\) concentration was reduced even at the upper floors. The PM\({}_{2.5}\) change rates on the third and fourth floors of the school building in the W4M case were \(-0.47\%\) and \(-0.34\%\), respectively. In the T2M and T4M cases, the decrease rates in PM\({}_{2.5}\) concentration in the school boundary were similar to the rates in the school ground (Figure 13b). However, the change rates in PM\({}_{2.5}\)  
Figure 12: For the east–northeasterly inflow, (**a**) PM\({}_{2.5}\) concentration and (**b**) difference in PM\({}_{2.5}\) concentration between the T4M case and the CNTL case at 1 m above the ground. Figure 13: Vertical profiles of the PM\({}_{2.5}\) concentration change rates for each case compared to the CNTL case averaged for the 16 inflow directions in the (**a**) school ground and (**b**) school boundary. concentration attributable to wall-type fence were significantly large because the wall-type fence prevented the transport of PM\({}_{2.5}\) from the road. Specifically, the PM\({}_{2.5}\) change rates on the first, second, third, and fourth floors of the school building in the W4M case were \(-9.47\%\), \(-3.30\%\), \(-1.44\%\), and \(-0.71\%\), respectively (Table 5). The standard deviations of the PM\({}_{2.5}\) concentration calculated for 16 inflow directions also decreased with height as in the vertical variation of the change rate of the PM\({}_{2.5}\) concentration (Table 5). This indicates that the wall-type and tree-type fences were more effective at reducing the PM\({}_{2.5}\) concentrations in a lower layer rather than a higher layer. However, at least in the lower layer, the reduction rate of the PM\({}_{2.5}\) concentration was not proportional to the background PM\({}_{2.5}\) concentration.