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Abstract Most fine ambient particulate matter (PM_2.5)-based epidemiological models use globalized concentration-response (CR) functions assuming that the toxicity of PM_2.5is solely mass-dependent without considering its chemical composition. Although oxidative potential (OP) has emerged as an alternate metric of PM_2.5toxicity, the association between PM_2.5mass and OP on a large spatial extent has not been investigated. In this study, we evaluate this relationship using 385 PM_2.5samples collected from 14 different sites across 4 different continents and using 5 different OP (and cytotoxicity) endpoints. Our results show that the relationship between PM_2.5mass vs. OP (and cytotoxicity) is largely non-linear due to significant differences in the intrinsic toxicity, resulting from a spatially heterogeneous chemical composition of PM_2.5. These results emphasize the need to develop localized CR functions incorporating other measures of PM_2.5properties (e.g., OP) to better predict the PM_2.5-attributed health burdens. 

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. 

Abstract. Several automated instruments exist to measure the acellularoxidative potential (OP) of ambient particulate matter (PM). However,cellular OP of the ambient PM is still measured manually, which severelylimits the comparison between two types of assays. Cellular assays couldprovide a more comprehensive assessment of the PM-induced oxidative stress,as they incorporate more biological processes involved in the PM-catalyzedreactive oxygen species (ROS) generation. Considering this need, wedeveloped a semi-automated instrument, the first of its kind, for measuring thecellular OP based on a macrophage ROS assay using rat alveolar macrophages.The instrument named SCOPE – semi-automated instrument for cellularoxidative potential evaluation – uses dichlorofluorescein diacetate (DCFH-DA)as a probe to detect the OP of PM samples extracted in water. SCOPE iscapable of analyzing a batch of six samples (including one negative and onepositive control) in 5 h and is equipped to operate continuously for24 h with minimal manual intervention after every batch of analysis,i.e., after every 5 h. SCOPE has a high analytical precision asassessed from both positive controls and ambient PM samples (coefficient of variation (CoV)<17 %). The results obtained from the instrument were in good agreementwith manual measurements using tert-butyl hydroperoxide (t-BOOH) as thepositive control (slope =0.83 for automated vs. manual, R2=0.99)and ambient samples (slope =0.83, R2=0.71). We furtherdemonstrated the ability of SCOPE to analyze a large number of both ambientand laboratory samples and developed a dataset on the intrinsic cellular OPof several compounds, such as metals, quinones, polycyclic aromatichydrocarbons (PAHs) and inorganic salts, commonly known to be present inambient PM. This dataset is potentially useful in future studies toapportion the contribution of key chemical species in the overall cellularOP of ambient PM.