*4.2. Factors A*ff*ecting the Hourly PM Reduction in Urban Green Areas*

High PM levels during the morning rush hour seemed to be related to increased traffic loads. Thus, this result is likely because of the effects of PM generated by vehicles and atmospheric congestion during the early morning. We observed that the PM concentrations in the morning rush hour slightly decreased after noon. This was related to active atmospheric dispersion, as temperature and wind speed slowly increased and humidity decreased after noon. A similar pattern was reported in a metropolitan city in Korea, where high PM concentration and size distribution of PM were affected by increased traffic volumes during the morning rush hour (6–9 h) and then decreased when the wind speed increased [22].

A great reduction in PM concentration during the evening rush hour was related to the strong west wind blowing from IC to RA. It is likely that the PM-reducing effect of UG is greatly increased when PM in IC passes through the UG due to the strong west wind and active atmospheric diffusion after noon. On the other hand, a low PM reduction rate was shown in the morning rush hour in study areas, which was related to the high PM concentration at RA. In the case of RA, with an increase in traffic during the morning rush hour, PM congestion was caused by the east wind blowing and the residential area being surrounded by high apartments [22,32,33]. PM concentration in RA was more affected by high traffic volume and PM congestion than PM from IC during the morning rush hour. The atmospheric diffusion models indicated that air pollutants would not spread actively in the residential area due to the west wind from the coast and the east wind from land coexisting in the study area [34].
