Bulk PM2.5 Masks Oxidative Potential and Reactive Oxygen Species Responses by Metal–Organic Complexation in PM1 and PM1–2.5 Fractions

Abstract Bulk PM2.5 measurements are widely used to assess the oxidative toxicity of particulate matter, but they integrate size fractions that differ in composition and respiratory deposition. We show that mixing PM1 and PM1–2.5 from a roadside site produces pronounced antagonistic effects on oxidative potential (OP) and reactive oxygen species (ROS) generation, with interaction factors (IFs) below 1 across all end points: OP by the ascorbic acid assay (IF = 0.65–0.81), OP by the dithiothreitol assay (IF = 0.80–0.93), ROS (IF = 0.10–0.88), and mitochondrial superoxide (IF = 0.43–0.78). This antagonism requires metal–organic interactions as metal-free biomass burning PM exhibits strictly additive behaviors. Cu2+–humic acid experiments reproduce the antagonistic effects seen in ambient PM (IFs = 0.22–0.95). Electron paramagnetic resonance spectroscopy confirms the formation of complexed Cu and reveals that the conversion of free Cu to complexed Cu is nonlinear with respect to the HA:Cu ratio, providing a mechanistic explanation for the divergent interaction effects reported across studies. These results indicate that bulk PM2.5 measurements can mask the oxidative toxicity of metal–organic interactions, highlighting the need for size-resolved OP and ROS characterization.