Size-Dependent Characteristics, Exposure Risk, and Formation Mechanisms of Environmentally Persistent Free Radicals in Atmospheric Particulate Matter

Environmentally persistent free radicals (EPFRs) in atmospheric particulate matter (PM) are of increasing concern because of their potential health risks. This study systematically investigated the occurrence and characteristics of EPFRs in PM2.5 (particulate matter with an aerodynamic diameter ≤ 2.5 μm), PM10 (particulate matter with an aerodynamic diameter ≤ 10 μm), and total suspended particles (TSP). Mass-normalized EPFRs concentrations and g factors were determined, human exposure was evaluated using an equivalent-cigarette model, and size-dependent formation mechanisms were examined through laboratory simulations. In the simulation system, SiO2, catechol, and ZnO were used as the surrogate particle, organic precursor, and transition-metal component, respectively, while scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDS), Brunauer-Emmett-Teller (BET) analysis, Fourier transform infrared (FTIR) spectroscopy, and X-ray photoelectron spectroscopy (XPS) were applied to characterize particle morphology, elemental composition, and surface properties. The EPFRs in atmospheric PM were identified mainly as carbon-centered radicals with adjacent oxygen-containing functional groups. PM2.5 showed the highest EPFRs concentration (8.05 × 1016 spins·g-1) and g factor (2.00337), indicating that finer particles contained more abundant radicals and a stronger contribution from oxygen-related species. The estimated daily equivalent-cigarette exposure was 0.326 cigarettes day-1 for PM2.5 and 0.346 cigarettes day-1 for PM10. Laboratory simulations showed that smaller particles generated more EPFRs and exhibited higher g factors because of their larger specific surface area and more accessible reactive sites. The EPFRs concentration on 2 μm particles was 1.63 and 16.3 times higher than that on 10 μm and 150-380 μm particles, respectively. Zn also played a key role in promoting EPFRs formation. Integrating field observations and laboratory simulations, this study assesses size-segregated EPFRs exposure and Zn's role in EPFRs formation in PM2.5. These findings underscore the need for coordinated control of heavy metals and organic species and support toxicity-oriented PM control.