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This study investigated the physicochemical properties of PM2.5, especially among secondary aerosols, based on the particulate matter and gaseous precursors in Yongin, Korea measured between February and June 2022. A comparative analysis of PM composition across two seasons highlighted the atmospheric characteristics of this suburban area. As observed, the average PM2.5 concentrations in February and March were higher than those in May and June, with NO3− being particularly predominant during the colder months when PM2.5 levels were elevated. During this period, the high levels of gaseous precursors such as NOX, HNO3, and NMHC likely contributed to secondary aerosol formation. The intermediate oxidation states of organic matter in Yongin indicate its suburban characteristic, which is intermediate between urban and rural areas. Inorganic aerosols were enriched with (NH4)2SO4 with sufficient NH3 availability, and then the formation of NH4NO3 was promoted through the reaction of the same phase (gas–gas) HNO3 with NH3. Additionally, the temperature variations influenced the PM2.5 composition, promoting the production of NH4NO3 in February–March. In Yongin, HNO3 acted as a limiting factor in NH4NO3 production. Thus, the management of precursor gases such as HNO3 and NO2 is crucial during periods of high PM2.5 in the colder seasons. [ABSTRACT FROM AUTHOR]