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    Abstract. In the aqueous phase, fine particulate matter can form reactive species (RS)that influence the aging, properties, and health effects of atmosphericaerosols. In this study, we explore the RS yields of aerosol samples froma remote forest (Hyytiälä, Finland) and polluted urban locations(Mainz, Germany; Beijing, China), and we relate the RS yields to differentchemical constituents and reaction mechanisms. Ultra-high-resolution massspectrometry was used to characterize organic aerosol composition, electronparamagnetic resonance (EPR) spectroscopy with a spin-trapping technique wasapplied to determine the concentrations of ⚫OH,O2⚫-, and carbon- or oxygen-centered organic radicals, anda fluorometric assay was used to quantify H2O2. The aqueousH2O2-forming potential per mass unit of ambient PM2.5(particle diameter < 2.5 µm) was roughly the same for allinvestigated samples, whereas the mass-specific yields of radicals werelower for sampling sites with higher concentrations of PM2.5. Theabundances of water-soluble transition metals and aromatics in ambientPM2.5 were positively correlated with the relative fraction of⚫OH and negatively correlated with the relative fraction ofcarbon-centered radicals. In contrast, highly oxygenated organic molecules(HOM) were positively correlated with the relative fraction ofcarbon-centered radicals and negatively correlated with the relativefraction of ⚫OH. Moreover, we found that the relative fractionsof different types of radicals formed by ambient PM2.5 were comparableto surrogate mixtures comprising transition metal ions, organichydroperoxide, H2O2, and humic or fulvic acids. The interplay oftransition metal ions (e.g., iron and copper ions), highly oxidized organicmolecules (e.g., hydroperoxides), and complexing or scavenging agents (e.g.,humic or fulvic acids) leads to nonlinear concentration dependencies inaqueous-phase RS production. A strong dependence on chemical compositionwas also observed for the aqueous-phase radical yields oflaboratory-generated secondary organic aerosols (SOA) from precursormixtures of naphthalene and β-pinene. Our findings show how thecomposition of PM2.5 can influence the amount and nature ofaqueous-phase RS, which may explain differences in the chemical reactivityand health effects of particulate matter in clean and polluted air. 
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