Bulk PM_2.5 Masks Oxidative Potential and Reactive Oxygen Species Responses by Metal–Organic Complexation in PM₁ and PM_1–2.5 Fractions

We show that mixing PM₁ and PM_1–2.5 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.