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Abstract. We assessed the oxidative potential (OP) of both water-soluble andmethanol-soluble fractions of ambient fine particulate matter (PM2.5)in the Midwestern United States. A large set of PM2.5 samples (N=241) was collected from five sites set up in different environments, i.e.,urban, rural, and roadside, in Illinois, Indiana, and Missouri during May 2018–May 2019. Five acellular OP endpoints, including the consumption rate ofascorbic acid and glutathione in a surrogate lung fluid (SLF) (OPAA andOPGSH, respectively), dithiothreitol (DTT) depletion rate (OPDTT),and ⚫OH generation rate in SLF and DTT (OPOH−SLF andOPOH−DTT, respectively), were measured for all PM2.5 samples.PM2.5 mass concentrations in the Midwestern US as obtained from thesesamples were spatially homogeneously distributed, while most OP endpointsshowed significant spatiotemporal heterogeneity. Seasonally, higheractivities occurred in summer for most OP endpoints for both water- andmethanol-soluble extracts. Spatially, the roadside site showed the highestactivities for most OP endpoints in the water-soluble extracts, while onlyoccasional peaks were observed at urban sites in the methanol-soluble OP.Most OP endpoints showed similar spatiotemporal trends between mass- andvolume-normalized activities across different sites and seasons. Comparisonsbetween two solvents (i.e., water and methanol) showed that methanol-solubleOP generally had higher activity levels than corresponding water-soluble OP.Site-to-site comparisons of OP showed stronger correlations formethanol-soluble OP compared to water-soluble OP, indicating a betterextraction of water-insoluble redox-active compounds from various emissionsources into methanol. We found a weak correlation and inconsistent slopevalues between PM2.5 mass and most OP endpoints. Moreover, thepoor to moderate intercorrelations among different OP endpoints indicatedifferent mechanisms of OP represented by these endpoints and thusdemonstrate the rationale for analyzing multiple acellular endpoints for abetter and more comprehensive assessment of OP.