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Secondary organic aerosol (SOA) formation is important in the field of atmospheric science due to its impacts on public health and climate change. However, the molecular compositions or formation mechanisms of SOA have not been fully characterized in ambient atmospheres. In this study, ambient fine particle (PM 2.5) samples were collected in two urban cities in Beijing, China and Gwangju, Korea during the winter and summer seasons, and the SOA-associated molecular features were characterized by using Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR MS). We found that the secondary organic carbon (SOC) during wintertime in Gwangju exhibit a more aliphatic nature, and those in Beijing have more aromatic structures like humic compounds. The contribution of CHOS to the total identified WSOC associated SOC was the most abundant at Beijing site (∼40%) compared to the Gwangju site (∼30%), and the proportion of CHON species in the WSOC associated SOC accounted for ∼30% in Beijing and 20–30% in Gwangju. Majority of SOC-derived CHOS species (∼99%) likely contained sulfate functional groups at both sites regardless of the sampling season. Organonitrates predominated in CHON species in summer compared to winter, but some fractions appeared to contain other potential nitrogen functional groups such as amide and nitrile (∼20%), suggesting the existence of other formation pathways of organic nitrogen, especially during winter. This work gave an observation insight into the molecular features of SOC at two different sites, and will deepen the understanding on the region-specific SOA formation mechanisms. [Display omitted] • A total of 10,790 chemical formulas assigned from PM 2.5 -drived WSOC samples. • SOC fractions were discriminated from the WSOC compositions. • SOC compounds during winter in Gwangju exhibit a more aliphatic nature than in Beijing. • Gwangju exhibit a more favorable environment for SOA formation process than Beijing. [ABSTRACT FROM AUTHOR]