In the aqueous phase, fine particulate matter can form reactive species (RS) that influence the aging, properties, and health effects of atmospheric aerosols. In this study, we explore the RS yields of aerosol samples from a remote forest (Hyytiälä, Finland) and polluted urban locations (Mainz, Germany; Beijing, China), and we relate the RS yields to different chemical constituents and reaction mechanisms. Ultra-high-resolution mass spectrometry was used to characterize organic aerosol composition, electron paramagnetic resonance (EPR) spectroscopy with a spin-trapping technique was applied to determine the concentrations of .sup.â« OH, O2â«-, and carbon- or oxygen-centered organic radicals, and a fluorometric assay was used to quantify H.sub.2 O.sub.2 . The aqueous H.sub.2 O.sub.2 -forming potential per mass unit of ambient PM.sub.2.5 (particle diameter 2.5 Âµm) was roughly the same for all investigated samples, whereas the mass-specific yields of radicals were lower for sampling sites with higher concentrations of PM.sub.2.5 . The abundances of water-soluble transition metals and aromatics in ambient PM.sub.2.5 were positively correlated with the relative fraction of .sup.â« OH and negatively correlated with the relative fraction of carbon-centered radicals. In contrast, highly oxygenated organic molecules (HOM) were positively correlated with the relative fraction of carbon-centered radicals and negatively correlated with the relative fraction of .sup.â« OH. Moreover, we found that the relative fractions of different types of radicals formed by ambient PM.sub.2.5 were comparable to surrogate mixtures comprising transition metal ions, organic hydroperoxide, H.sub.2 O.sub.2, and humic or fulvic acids. The interplay of transition metal ions (e.g., iron and copper ions), highly oxidized organic molecules (e.g., hydroperoxides), and complexing or scavenging agents (e.g., humic or fulvic acids) leads to nonlinear concentration dependencies in aqueous-phase RS production. A strong dependence on chemical composition was also observed for the aqueous-phase radical yields of laboratory-generated secondary organic aerosols (SOA) from precursor mixtures of naphthalene and Î²-pinene. Our findings show how the composition of PM.sub.2.5 can influence the amount and nature of aqueous-phase RS, which may explain differences in the chemical reactivity and health effects of particulate matter in clean and polluted air.