Oxidation state and metal interactions influence aerosol oxidative potential in acellular DTT and OH assays

Oxidative potential (OP) metrics are increasingly used as a relatively low-cost method to help assess potential for particle-induced health effects. Metals are major drivers of OP, yet the roles of metal oxidation state and interactions among metals remain poorly constrained; even reported responses to individual metals are scattered. Here we quantify the response of two widely used OP assays, OH formation and DTT consumption, to a broad suite of elements, including copper(I) and (II), iron(II) and (III) and selected binary mixtures. We find that common non-exhaust emission tracers (e.g., barium (Ba) and antimony (Sb)) are inactive in both assays, suggesting previously reported ambient associations likely reflect source collinearity. Oxidation states strongly modulate OP: Fe(II) generates three times more OH than Fe(III), providing a new chemical explanation for contrasting OP activities among sources with similar redox-active metal profiles. We also uncover strong antagonism between copper(II) and lead(II) in the OH assay. These results show that composition-based OP models should account for oxidation state and inhibitory interactions, and provide a hypothesis for why DTT may not be as effective as a predictor of health impacts.