Dicarboxylic acids (diacids) and related species act as critical molecular tracers for secondary organic aerosols (SOA), which significantly influence atmospheric chemical processes and climate forcing. Nevertheless, the origins and formation pathways of these compounds remain inadequately characterized, introducing considerable uncertainties in evaluating the climate effects of SOA. This study investigates the seasonal variability in molecular distribution and stable carbon isotopic compositions (δ¹³C) of diacid homologs in PM_2.5 to elucidate their origins and formation mechanisms in an urban Liaocheng, China. Elevated concentrations of diacid homologs were observed in winter (1469 ± 1325 ng m⁻³) than in summer (884 ± 343 ng m⁻³), predominantly attributed to significant contributions from fossil fuel combustion and biomass burning for heating during cold months. In contrast, summertime aerosols exhibited stronger photochemical aging, evidenced by enriched δ¹³C values of oxalic acid (C₂) and increased ratios of C₂/succinic acid (C₄), malonic acid (C₃)/C₄, and C₂/diacids. Correlation analyses and positive matrix factorization identified gaseous photochemical oxidation (52.4 %) as the dominant formation pathway driven by O₃ and solar radiation in summer, while aqueous oxidation (57.6 %) modulated by relative humidity, aerosol acidity, and liquid water content prevailed in winter. A consistent decrease in δ¹³C values with increasing carbon chain length aligns with kinetic isotope effects (KIEs) during oxidative processes. Inter-site comparisons reveal that urban sites exhibit the most depleted δ¹³C values in C₂ due to dominant anthropogenic emissions, whereas less depleted values at rural and background sites reflect greater biogenic contributions and extensive photochemical aging. Strong correlations between δ¹³C values of C₂ and the C₂/diacids ratio (R² ≥ 0.55) further support the use of δ¹³C as an effective proxy for organic aerosol aging. These findings underscore the necessity of adopting a balanced and coordinated approach to air pollution control, which must address both primary emissions and secondary formation processes, while considering the interplay with meteorological factors.