For a comprehensive evaluation of the key factors determining the oxidative potential (OP) of PM_2.5, 75 samples of PM_2.5 were collected in urban Seoul, South Korea, during 2019-2021, and dithiothreitol consumption (DTTv) was measured using a DTT assay, coupled with an analysis of major constituents and stable isotope ratios of PM_2.5. For the entire sample set, the mean DTTv value was 0.58 ± 0.48 nmol m^-3 min^-1 for PM_2.5 of 29.1 ± 12.2. DTTv exhibited a general dependence on of PM_2.5 concentrations and major constituents, including NO₄⁺, SO₄^2-, NH₄⁺, and organic carbon (OC). Specifically, NO₃^- and NH₄⁺ demonstrated the most robust correlation with DTTv during the cold season, whereas only elemental carbon (EC) showed a significant correlation with DTTv in the warm season. The δ¹³C of total carbon (TC) and δ¹⁵N of total nitrogen (TN) displayed an inverse correlation concerning DTT activities, suggesting the significant contribution from solid fossil fuels and biomass burning to the oxidative potential of PM_2.5, particularly during the cold season when PM_2.5 was notably high. In contrast, vehicle emissions were found to influence DTTv even at low PM_2.5 levels in warm seasons. This study provides insights into the intricate dynamics influencing the oxidative potential of PM_2.5.