To evaluate the applicability of online monitoring for water-soluble ions and inorganic elements in atmospheric PM_2.5, this study conducted a comparative analysis using both online and manual monitoring data collected in Taizhou during autumn and winter (18 September 2023–6 January 2024). The Positive Matrix Factorization (PMF) model was employed for source apportionment. For water-soluble ions, online and manual results for Cl⁻, NO₃⁻, SO₄²⁻, NH₄⁺, and K⁺ showed good agreement (R² > 0.8), while Na⁺ exhibited moderate correlation and Ca²⁺/Mg²⁺ showed poor correlation. For inorganic elements, eight elements (K, Ca, Cr, Mn, Fe, Zn, Pb, Si) agreed well between methods, though online values were consistently lower than manual ones. Cu and Ni showed poor correlation. The PMF source apportionment results from online and manual monitoring data indicate that PM_2.5 in Taizhou during autumn and winter mainly originated from six sources: traffic (25.8%, 28.9%), secondary nitrate (18.6%, 20.9%), dust (16.1%, 14.2%), sea salt (15.8%, 11.5%), secondary (12.1%, 13.8%) and biomass burning (11.6%, 10.7%). Sensitivity analysis confirmed the robustness of PMF results to uncertainty assumptions. Overall, the results suggest that online monitoring data are generally applicable for PM_2.5 chemical characterization and source apportionment in Taizhou.