Oxidative Potential and Elemental Fingerprints of Urban and Suburban Particulate Matter in Turin

Abstract

The toxicity of atmospheric particulate matter (PM) depends not only on its burden in air, but also on its chemical composition, which affects its ability to induce oxidative stress at the cellular level through reactive oxygen species (ROS) formation. The oxidative potential (OP) of PM has been proposed as an integrative metric for PM toxicity; however, standardized analytical protocols are still lacking. In this study, the performance of the ascorbic acid (AA) assay was evaluated for the determination of PM oxidative potential. A total of 32 filters were analysed, corresponding to 16 filters per site (urban traffic and rural background) and eight filters per year (2020 and 2021). For each selected month (January, May, August, and November), the daily filters corresponding to the minimum and maximum PM10 mass concentrations within that month were chosen. Ascorbic acid depletion was monitored spectrophotometrically at 265 nm under physiological conditions (phosphate buffer, PB, 37 °C). The method was tested using aqueous solutions containing individual metals considered to play a major role in the oxidative behaviours of PM (Cu, Cr, Fe, Mn, Ni and Zn) and four standard reference materials (SRMs). This acellular test was then applied to atmospheric PM10 collected in 2020–2021 at the two monitoring sites in the Turin metropolitan area (Italy). Among the individual metals, Cu showed the highest oxidative activity, consistent with the literature data; BCR 176 showed the highest oxidative activity. Urban PM10 samples exhibited higher oxidative potential (0.12–0.60 nmol AA min−1 m−3) than suburban samples (0.10–0.38 nmol AA min−1 m−3). Metal concentrations in the PM10 samples were determined by ICP-OES and ICP-MS in both acid-digested samples and PB extracts used for OP determination, allowing for comparison between the total and bioaccessible metal fractions. To the best of our knowledge, this is the first study to investigate the relationship between PM10 oxidative potential and its total and bioaccessible elemental composition in the Turin metropolitan area. Site characterization was expanded by quantifying PM10 carbon fractions (EC/OC and brown carbon) through multiwavelength thermal/optical carbon analysis following the EUSAAR_2 protocol. Because of the sampling strategy adopted and the limited number of processed samples, the seasonal and between-site comparisons reported in this study refer specifically to the analysed samples. The combined evaluation of OP and ICP-based elemental composition provides insights into the role of transition metals in PM-induced oxidative stress and supports the integration of OP measurements into air quality assessment frameworks.