Linking Transition Metal Concentration and Oxidative Potential in PM2.5 by Ascorbic Acid Assay via Quasi-Michaelis-Menten Mechanism.

Transition metals (TMs) in the ambient fine particulate matter (PM2.5) catalyze the formation of multiple reactive oxidative species (ROS), such as superoxide, hydrogen peroxide, and hydroxyl radicals, in lung lining fluid, contributing to the oxidative potential (OP) of inhaled particles. Complexation with ambient organics, particularly humic-like substances (HULIS), further modulates TM-catalyzed ROS generation by influencing electron transfer processes. While OP measurements are widely reported for both ambient and laboratory samples, the detailed catalytic mechanisms and chemical kinetics underlying TM-induced OP remain under-investigated and poorly understood. Here, we systematically investigated the OP of Fe and Cu using ascorbic acid (AA) assay under varying conditions. Reaction kinetics and studies of OP dependence on TM concentration have led us to propose a quasi-Michaelis-Menten mechanistic framework that involves a TM-AA complex as a key intermediate for OP generation in the AA assay. This mechanism explains the observed nonlinear kinetics and dose-response behavior of AA depletion and hydroxyl radical production. We also explored the mixing effects between TMs and HULIS in generating OP. These findings clarify the mechanistic link between TM concentration and OP in the AA assay and provide a foundation for OP modeling based on PM2.5 chemical composition.