## 4 Discussion
### Changes in PM According to Mist Spraying  
Data collected at YCF and CRP showed that the PM concentration increased rapidly during mist spraying and returned to the previous concentration after spraying (Figures 3 and 4). This could be related to mist spraying, which could affect the concentrations of PMx in surrounding environments, resulting in rapid changes in the PM concentration. We also considered the overestimation of PM concentration, as a PM measuring device using the light-scattering method was highly affected by moisture. However, we used silica gel, which absorbs moisture. The same nozzle generally sprays particles of the same size; thus, only a certain PM concentration may increase sharply when spraying the mist.  
In one case during the lunch hour at YCF, the changes in PM ratio of \(\text{PM}_{2.5}\cdot\text{PM}_{10}{}^{-1}\) significantly decreased, whereas those of \(\text{PM}_{1.0}\cdot\text{PM}_{10}{}^{-1}\) and \(\text{PM}_{1.0}\cdot\text{PM}_{2.5}{}^{-1}\) did not change after mist spraying (Table 2). In four cases during the morning rush and lunch hour at CRP, the changes in the PM ratio of \(\text{PM}_{2.5}\cdot\text{PM}_{10}{}^{-1}\) and during the morning rush hour and lunch, the changes in the PM ratio according to mist spraying decreased and \(\text{PM}_{1.0}\cdot\text{PM}_{10}{}^{-1}\) significantly decreased whereas those of \(\text{PM}_{1.0}\cdot\text{PM}_{2.5}{}^{-1}\) did not change after mist spraying (Table 3). This finding indicates that PM particles of size 2.5-10 \(\upmu\)m were likely to increase faster than those of other sizes when spraying mist. Thus, the combination of mist and PM resulted in condensation growth, which led to an increase in PM size.  
The increased size of PM could facilitate faster deposition than that observed for PM of smaller size [18]. Therefore, reducing PM through mist spraying and mitigating high PM damage to trees will be possible. During the evening rush hour, there was no significant difference in \(\text{PM}_{2.5}\cdot\text{PM}_{10}{}^{-1}\), \(\text{PM}_{1.0}\cdot\text{PM}_{10}{}^{-1}\), and \(\text{PM}_{1.0}\cdot\text{PM}_{2.5}{}^{-1}\), which decreased with mist spraying (Tables 2 and 3). Unlike the growth of large PM observed during the morning rush hour and lunch, PM concentration (1.0-2.5 \(\upmu\)m) increased faster during the evening rush hour. It is difficult to show the statistical relation with the microclimatic conditions and PM concentrations with our data. However, the air stagnation during the evening rushing hour could imply the relatedness between microclimate conditions and PM concentration in micro-scale environments. This seems to be more related to the microclimate conditions in the measuring period than to the effects of the mist.  
During the morning rush hour and lunch, when high wind speed and low relative humidity occurred, the mist moved freely and combined with the surrounding PM [41; 42; 43].  
However, during the evening rush hour, high relative humidity and low wind speed limited mist movement, which might have a greater effect on PM concentration than mist. Therefore, it is necessary to consider the microclimate conditions when using a mist generator in a high PM episode to increase condensation PM growth.  
However, this study has a limitation, which is to apply the mist generator to control the PM concentration in the field condition directly and widely. But this study implies the possibility of the mist generator to reduce the PM concentration on a micro-temporal and spatial scale, but also to be applied to the healthy condition of trees during high episodes of PM.