Reactive oxygen species build-up in photochemically aged iron-and copper-doped secondary organic aerosol proxy

Abstract. The toxicity of particulate matter (PM) is highly related to the concentration of particle-bound reactive oxygen species (ROS). Chemical properties, including metal dissolution and the sources of PM, influence ROS production and its oxidative potential. Here, the photochemical aging of a secondary organic aerosol proxy (citric acid, CA) with metal complexes (iron-citrate, Fe^IIICit) is assessed toward the production of particle-bound ROS with an online instrument (OPROSI). We studied the photochemically induced redox chemistry in iron/copper-citrate particles experimentally mimicked with an aerosol flow tube (AFT) in which UV-aging was probed. Different atmospheric conditions were tested, influencing the physicochemical properties of the particles. We found that UV-aged CA aerosol containing 10 mole% Fe^IIICit generated ROS concentrations on the order of 0.1 nmol H₂O₂ eq μg⁻¹, indicating the photochemically driven formation of peroxides. An increase in relative humidity (RH) leads to only a slight but overall lower concentration of ROS, possibly due to a loss of volatile HO₂ and H₂O₂ in the gas phase in the less viscous particles. The RH effect is enhanced in nitrogen sheath flow, but in air and compared to the Fe^IIICit/CA particles, the iron/copper-citrate samples show a uniformly decreased ROS level. Interestingly, in the high humid nitrogen experiment with copper, we found a much more pronounced decline of the ROS concentration down to 2×10⁻² nmol H₂O₂ eq μg⁻¹ compared to all other irradiation experiments. We suggest that copper may suppress radical redox reactions and therefore consume ROS in an anoxic regime.