*4.3. Factors A*ff*ecting the PM Concentration by Season*

This study showed a high concentration of PM in May. Recently simulated PM concentrations showed a strong negative correlation with regional wind speed, implying that reduced regional ventilation is likely associated with more stagnant conditions that cause severe pollutant episodes in South Korea [35]. Furthermore, temperature and wind speed had a negative correlation with PM, which is related to active atmospheric diffusion with increasing temperature and wind speed [36], and the high temperature influences convective air currents, resulting in rapid dispersion of PM in the atmosphere [29]. It is known that humidity contributes to the increase in PM concentration by acting as the condensation nucleus of air pollutants [37].

PM concentration was positively correlated with NO2, CO, and SO2. This is related to the PM formation, as air pollutants are precursors of PM [38,39]. PM concentration in summer had especially high correlation with SO2, while NO<sup>2</sup> and CO were highly correlated with PM concentration in spring and fall. In summer, as the atmospheric diffusion becomes more active, the SO<sup>2</sup> diffusion produced by the operation of the industrial complex could affect the overall PM concentrations at the measuring points [3,40]. In spring and fall, as NO<sup>2</sup> and CO produced from the vehicles spread in all directions around the roadside, they could have a greater impact on PM concentration when compared to SO<sup>2</sup> from the industrial complex [3,34]. Thus, the analysis of the PM composition is needed to identify the exact path of air pollutants.

O<sup>3</sup> only showed a negative correlation with PM, which might be related to the organic PM generation process through oxidation of volatile organic compounds (VOCs) by O<sup>3</sup> [41]. However, Bell et al. [42] observed a positive correlation between PM and O3, as PM and O<sup>3</sup> are produced by photochemical reactions. These contradictory results indicate that PM and O<sup>3</sup> levels are driven by a combination of the chemical reactions of their precursors and meteorological factors. Therefore, it is necessary to analyze the correlation between PM and O<sup>3</sup> by identifying the environmental factors affecting these occurrences. In addition, measuring O<sup>3</sup> in urban green areas and analyzing PM formation and extinction data should be conducted to understand the real correlation between O<sup>3</sup> and PM in urban green areas.
