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A comprehensive year-round field campaign was conducted in the coastal city of Ningbo, China to examine the driving factors of oxidative potential (OP) of PM 2.5 and hence its potential health impacts. Using dithiothreitol (DTT) cell-free assay, OP of water-soluble (OPws) and methanol-soluble components (OPmeth) of PM 2.5 -based samples were measured. Volume-normalised OP (OP v ws and OP v meth) were higher in heating seasons, and mass-normalised OP (OP m ws and OP m meth) peaked in summer. Five sources were identified to be DTT active in OP v ws, with road dust (RD) being dominant, while four sources contributed to OP v meth, with industrial emission (IE) as the largest contributor. Water extracted more toxic components from RD while toxic components in IE and vehicle emission (VE) were more soluble in methanol. We further compared OP values between PM 2.5 of similar concentrations and observed that OP v ws prevailed in PM 2.5 toxicity during clean days (PM 2.5 concentration < 38.5 µg m−3), while OP v meth became dominant when PM 2.5 concentration was larger than 69.1 µg m−3, implying the need to extract whole PM 2.5 using both solvents to develop the mitigation strategy for reducing the overall PM 2.5 toxicity. Efforts should be devoted to controlling the emissions from dominant sources of both OP indicators in specific episode. [Display omitted] • Road dust dominated OP v ws, while industry and vehicle emissions governed OP v meth. • OP v ws and OP v meth prevailed PM 2.5 toxicity in respective clean and polluted days. • O 3 , selected metals and ions drove OP v ws under high Temp, RH and sun radiation. • OP v meth was affected by high-molecular-weight PAHs, Ni, Co, and Cl-. [ABSTRACT FROM AUTHOR]