Particulate matter (PM) is a major atmospheric pollution problem in South Korea. In particular, secondary PM accounts for more than 70% of total PM2.5 concentrations. However, studies investigating secondary particle formation at the environmental conditions in South Korea has not been conducted well. Therefore, this study presents a fundamental methodology for investigating secondary particle formation, also known as new particle formation (NPF). The methodology focuses on the construction of wide-range particle number size distributions (PNSDs), which are essential for NPF studies, and on the derivation of proxy nucleation rates that quantify newly formed secondary particles in the absence of PNSD data for freshly generated particles. By using the newly developed method, the nucleation rates of NPF in Seoul and Seosan, South Korea were determined first in South Korea.