Abstract: Water-soluble organic matter (WSOC) and metals in atmospheric particulate matter are components that contribute to free radicals significantly, but their formation mechanism is not well understood. In this study, electron paramagnetic resonance spectroscopy (EPR) combined with free radical capture (BMPO/DMPO) technology was used to explore the interaction between WSOC and metal on radical formation. The results showed that the free radicals generated by the reaction of four typical WSOCs (salicylic acid, malonic acid, phthalic acid, catechol) with Fe² /Fe³ were consistent with the free radicals generated by BMPO extract of PM_2.5 and Fe² /Fe³ . Furthermore, through liquid phase reaction of WSOC with Fe² /Fe³ , it was found that alkyl or aromatic radicals were formed from Fe² with WSOC, while Fe³ promoted the formation of hydroxyl radicals, alkoxy radicals, hydroperoxide radicals, alkyl radicals, indicating that the free radical’s formation mechanism by Fe² and Fe³ was different. Subsequently, liquid-solid reaction of four catechols substances with the Fe-loaded mineral particles was conducted and environmentally persistent free radicals (EPFRs) were obtained and no transient radical formation was detected, which indicates that non-free metals and WSOCs can generate EPFRs, while free metals can generate transient free radicals.