# Seasonal Characteristics of Particulate Matter by Pollution Source Type and Urban Forest Type
## 4 Discussion  
Urban forests have been studied and evaluated for their effectiveness in mitigating or purifying pollutants and reducing their negative impacts from increasingly diverse pollution sources. Although it is challenging to quantify the various forms and structures of urban forests using a consistent standard, efforts to identify common characteristics of urban forests to consistently reproduce the benefits of green spaces are essential. When examining the characteristics of PM\({}_{10}\) concentrations at the study sites, the concentration-based proportions of each wind direction varied by period. The proportion of \"good\" air quality was relatively lower in the first half of the year, with early spring showing the lowest proportion of \"good\" levels and the highest proportion of \"bad\" levels. Seasonal differences in PM\({}_{10}\) concentrations (Figure 4) indicated that the median concentrationswere the highest at the beginning of the year and early spring, whereas short-term, extreme events, such as high PM\({}_{10}\) occurrences, were more frequent in the spring. When comparing the average wind speeds of the most frequently observed wind directions by season and region with the Beaufort wind scale, summer wind speeds generally fell into category 1 (0.3-1.5 m/s, \"light air\"), whereas winter wind speeds were classified as category 2 (1.6-3.3 m/s, \"light breeze\"), indicating relatively higher wind speeds during winter compared to summer. However, calm wind speeds were more prominent in summer and autumn, yet PM\({}_{10}\) concentrations were higher in winter (Figure 3, Table S1). These results contrast with previous studies, which suggest that low wind or calm wind conditions promote the accumulation of particulate matter [35; 36]. Further detailed studies incorporating various forest environments, wind directions, and speed conditions are needed to better understand the impact of wind conditions on PM\({}_{10}\) concentrations. To investigate the median differences between plots to understand the PM\({}_{10}\) reduction effect of urban forests, the Kruskal-Wallis (KW) test was performed. For road pollution sources, during the early year to early spring period and for industrial pollution sources during all periods except early year, the median PM\({}_{10}\) concentration was lowest in forest interior 1 plots (road source) and forest plots (industrial source). However, when examining the differences in the probability of PM\({}_{10}\) values exceeding \"bad\" levels (Table 3) and the distribution at lower levels (Figure 5), it was found that, even during periods of high PM\({}_{10}\) levels in the first half of the year, the PM\({}_{10}\) concentrations observed in urban forests were predominantly clustered in the lower range. Additionally, even during seasons with high PM\({}_{10}\) concentrations, the characteristics of PM\({}_{10}\) occurrences varied by period. For road pollution sources, unlike early year winter and early spring, during spring, there were frequent high PM\({}_{10}\) events, yet PM\({}_{10}\) concentrations were relatively low during normal conditions, even at the pollution sources, resulting in smaller differences between plots. In contrast, for industrial pollution sources, no significant differences in trends were found between plots within the area across all three seasons in the first half of the year. The differences in the interaction between surrounding environments and forests could be attributed to the differing PM emission sources, compositions, and particle sizes for road and industrial pollution sources [37; 38]. [PERSON] et al. (2024) analyzed seasonal and regional PM concentration differences across urban traffic, urban background, industrial, and rural areas, classifying particulate matter as fine (0.2-2.5 \(\upmu\)g), coarse (2.5-10 \(\upmu\)g), and large (>10 \(\upmu\)g).