## 3 Results
### Effects of the Tree-Type Fence  
We then analyzed the effects of the tree-type fence on the flow and PM\({}_{2.5}\) concentrations. The box plots (Figure 9) summarizing the results for the T2M and T4M cases for the 16 inflow directions show the percentage changes in the PM\({}_{2.5}\) concentrations compared to the CNTL in the school ground and school boundary. In the T2M and T4M cases, the horizontal wind speeds decreased largely because of the drag in the school ground, which had been planted with many trees (not shown). For each of the 16 inflow directions, the PM\({}_{2.5}\) concentration decreased in the school ground, where the wind speeds decreased. Figure 6: (a) Wind vectors and shading of the vertical velocity distribution, (b) PM\({}_{2.5}\) concentrations at 1 m above the ground for the east–southeasterly inflow in the CNTL case. The effects of tree-type fence on the PM\({}_{2.5}\) concentration in the school ground were similar to the effects in the school boundary. The magnitude of the PM\({}_{2.5}\) concentration change in the school boundary was similar in each inflow direction. However, the PM\({}_{2.5}\) concentrations decreased significantly in the school ground when easterly (from northeasterly to southasterly) flows existed. This occurred because the drag effect was significant for the easterly inflow directions due to the absence of obstacles. Figure 8: For the east–southeasterly inflow, (**a**) wind vectors and shading of the vertical velocity distribution in the W4M case and (**b**) the difference in PM\({}_{2.5}\) concentrations of the W4M case compared to the CNTL case at 1 m above the ground. Figure 7: Streamlines and PM\({}_{2.5}\) concentrations in the vertical plane of y = 230 m for the east–southeasterly inflow direction in the (**a**) CNTL, (**b**) W2M, and (**c**) W4M cases. We analyzed the flows and PM\({}_{2.5}\) concentrations for the east-northeasterly inflow, which showed a significant change in the PM\({}_{2.5}\) concentration. Figure 10 shows the wind vectors and the distributions of vertical wind components at the surface (z = 1 m) in the CNTL and T4M cases. In the CNTL case, the ambient wind was descended by the northwest school buildings (3), resulting in westerly flows around the northwest school ground (Figure 10a). Southerly flows were dominant in the south school ground. Upward flows appeared by convergence of opposite flows (westerly and southerly) in the school ground (4). In the T4M case, the 4-m-tall trees in the school ground greatly decreased the horizontal wind speeds up to a height of 5 m above the ground due to drag (Figure 11a,b), resulting in a shift of the flow convergence region to the southeast region (5) (Figure 10b). Additionally, the dominant westerly flows pushed the incoming flows from the east roads. Therefore, vertical velocities decreased at the vanished flow convergence region (4) and increased in the south school ground newly formed flow convergence region (5) (Figure 11d-f). The change in vertical velocity was relatively large above 5 m above the ground. The flow characteristics in the T4M case were similar to those in the T2M case, except for the magnitudes of the wind speed and the vertical velocity. The PM\({}_{2.5}\) concentrations were high along the north and east roads, where the flows converged, in the CNTL case (Figures 10a and 12a). The PM\({}_{2.5}\) concentrations were relatively high near the south school ground, where downward flows were dominant, because of the high PM\({}_{2.5}\) levels transported by the descending flows. The PM\({}_{2.5}\) concentrations were relatively low in the upward flow regions (4). In the T4M case, the PM\({}_{2.5}\) concentration in the north school ground increased as the upward flow weakened (Figures 10b and 12b). The PM\({}_{2.5}\) concentration in the south school ground decreased in the upward flow region because the number of pollutants transported from the background to the surface decreased due to the upward flows.