*4.1. Factors A*ff*ecting the Monthly PM Reduction in Urban Green Areas*

Overall, the PM values were higher at IC than at UG and RA. This result could be attributed to the PM reduction effects of UG. As PM generated from IC passed through UG, trees could absorb PM via leaf stomata [8,23] and remove PM by deposition onto the leaves and branches [24,25]. However, we also considered the spatial distance effect on PM reduction according to the distance from pollutant emission areas. PM<sup>10</sup> and PM2.5, which have a long atmospheric lifetime [26], decreased, so we can speculate that the influence of UG on PM reduction was greater than that of distance.

The difference between the PM reduction rates of IC and RA was especially high in August and October. The high PM reduction rate in August seemed to be related not only to PM reduction mechanism of UG but also to an increase of atmospheric mixing height during the summer [27–29]. The dispersion of PM occurred rapidly due to active mixing by air turbulence in the summer [28], leading to a minimum PM concentration. Therefore, it is likely that PM could quickly spread due to the effects of intensive air mixing and prevailing western wind systems, resulting in increasing PM reduction through UG. High PM reduction rates in October were also related to the active atmospheric dispersion affecting the PM reduction, similar to the results observed in August. Moreover, the cleaning effect due to high precipitation in August during the monsoon rainy season should be considered. However, we excluded the effect of PM reduction caused by precipitation because we sampled PM in the dry period, not on rainy days. June, the sharing the summer season with August, had the lowest PM reduction rate. This phenomenon appears to be related to high humidity (remaining in the range of 60% to 70%) in June, whereas other months had lower humidity. The increase of humidity in the atmosphere could reduce solar radiation reaching the earth surface, resulting in adjusting the atmospheric temperature to be similar to that of the earth. As the atmosphere nearer to the surface becomes colder than the upper layers, the air currents seem to decrease and air pollutants become stagnant [30,31]. Therefore, the high humidity interferes with upward-moving air currents, leading to the increase of air pollutant concentrations.
