Physicochemical Characteristics and Formation of PM2.5 in Yongin, Korea—A Suburban Metropolitan Area

This study investigated the physicochemical properties of PM_2.5, 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 PM_2.5 concentrations in February and March were higher than those in May and June, with NO₃⁻ being particularly predominant during the colder months when PM_2.5 levels were elevated. During this period, the high levels of gaseous precursors such as NO_X, 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 PM_2.5 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 PM_2.5 in the colder seasons.