Fine particulate matter plays a central role in adverse health effects of air pollution. Inhalation and deposition of aerosol particles in the respiratory tract can lead to the release of reactive oxygen species (ROS), which may cause oxidative stress. In this study, we have detected and quantified a wide range of particle-associated radicals using electron paramagnetic resonance (EPR) spectroscopy. Ambient particle samples were collected using a cascade impactor at a semi-urban site in central Europe, Mainz, Germany in May&ndash;June 2015. Concentrations of environmentally persistent free radicals (EPFR), most likely semiquinone radicals, were found to be in the range of (1&ndash;7) × 1011&thinsp;spins&thinsp;µg<sup>&minus;1</sup> for particles in the accumulation mode, whereas coarse particles with a diameter larger than 1&thinsp;µm did not contain substantial amounts of EPFR. Using a spin trapping technique followed by deconvolution of EPR spectra, we have also characterized and quantified ROS including OH, superoxide (O<sub>2</sub><sup>&minus;</sup>) and carbon- and oxygen-centred organic radicals, which were released upon extraction of the particle samples in water. Total ROS amounts of (0.1&ndash;3) × 1011&thinsp;spins&thinsp;µg<sup>&minus;1</sup> were released by submicron particle samples and the relative contributions of OH, O<sub>2</sub><sup>&minus;</sup>, C-centred and O-centred organic radicals were ~11&ndash;31&thinsp;%, ~2&ndash;8&thinsp;%, ~41&ndash;72&thinsp;% and ~0&ndash;25&thinsp;%, respectively, depending on particle sizes. OH was the dominant species for coarse particles. Based on comparisons of the EPR spectra of ambient particulate matter with those of mixtures of organic hydroperoxides, quinones and iron ions followed by chemical analysis using liquid chromatography mass spectrometry (LC-MS), we suggest that the particle-associated ROS were formed by decomposition of organic hydroperoxides interacting with transition metal ions and quinones contained in atmospheric humic-like substances (HULIS).
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