## 3 Results  
### Effects of the Wall-Type Fence  
We analyzed the effects of the wall-type fence on airflow and PM\({}_{2.5}\) concentration from the perspective of aerodynamic mechanisms. The boxplots in Figure 5 summarizing the results for the 16 inflow directions in the W2M and W4M cases show the percentage changes in the PM\({}_{2.5}\) concentrations compared with the CNTL case in the school grounds and in the school boundary. The school boundary is the area within 4 m of the fences, and the school ground is the area excluding the school boundary. Regarding wall height, the magnitude of the PM\({}_{2.5}\) concentration change was more significant differences for the 4 m height than for the 2 m height. The PM\({}_{2.5}\) concentrations decreased significantly overall in the school boundary. In the school ground, the PM\({}_{2.5}\) concentrations decreased for most inflow directions, except for north-northwesterly to east-southesterly directions. These results were explained through a detailed analysis of the flow and PM\({}_{2.5}\) concentration fields. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{3}{c}{**Daytime**} & \multicolumn{3}{c}{**Early Morning and Late Night**} \\ \hline  & **RF** & **R1** & **R2** & **RF** & **R1** & **R2** \\ \hline RMSE & 3.85 & 3.81 & 5.14 & 4.62 & 5.24 & 5.05 \\ NRMSE & 0.24 & 0.24 & 0.30 & 0.36 & 0.42 & 0.42 \\ VG & 1.07 & 1.05 & 1.09 & 1.18 & 1.23 & 1.24 \\ MG & 1.09 & 0.97 & 0.99 & 1.35 & 1.43 & 1.48 \\ FB & 0.06 & \(-\)0.05 & \(-\)0.05 & 0.26 & 0.33 & 0.35 \\ R & 0.44 & 0.37 & 0.41 & \(-\)0.10 & 0.03 & 0.49 \\ FAC2 & 1.00 & 1.00 & 1.00 & 0.92 & 0.92 & 0.92 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Statistical values of the PM\({}_{2.5}\) concentrations simulated at RF, R1, and R2 (green and yellow dots in Figure 1a). Figure 5: Box plots for the percentage changes of the PM\({}_{2.5}\) concentration (z = 1–11 m heights above the ground) for the W2M (**a**,**c**) and W4M (**b**,**d**) compared to the CNTL case in the school ground (left panels) and school boundary (right panels). The bars above and below the boxes indicate the upper and lower extremes, respectively. The upper, middle, and lower segments of the boxes indicate the upper quartiles, medians, and lower quartiles, respectively. The red and green lines indicate the mean value and the zero line, respectively. We analyzed the flows and PM\({}_{2.5}\) concentrations for the east-southeasterly inflow in detail because the decrease in the PM\({}_{2.5}\) concentration was significant, and there were no obstacles between the road and the school ground, allowing effective analysis of the wall-type fence effects. Figure 6 shows the wind vectors and the distributions of vertical wind components and PM\({}_{2.5}\) concentrations at the surface (z = 1 m) in the CNTL case. The flows along the roads around the school came into the school ground from the east school boundary, and complicated flows occurred within the school ground due to the influence of the surrounding buildings (Figure 6a). The flows coming in from the east school boundary formed a vortex near the northeast school building (1). In the school, the PM\({}_{2.5}\) concentration was lower than the background value because air pollutants transported from the background did not come to the surface due to the shielding effect of the topography and buildings. The PM\({}_{2.5}\) concentrations were high along the road and in high-density areas of buildings (Figure 6b). At the intersection of the north road (2) of the school, the PM\({}_{2.5}\) concentration was high because the pollutants were stagnant where the flow rotated. The relatively high PM\({}_{2.5}\) concentration on the road was transported into the school ground and the vortex region (1) from the east school boundary (Figures 6b and 7a). In the W4M case, the wall-type fence on the east school boundary prevented the incoming flow into the school from the east road; therefore, a southerly wind occurred along the east road (Figure 8a).