Copper accelerates photochemically induced radical chemistry of iron-containing secondary organic aerosol (SOA)

Photochemical aging in secondary organic aerosol (SOA) particles alters their chemical composition and affects their adverse health effects. However, there is limited mechanistic insight on the role of transition metals in photochemical SOA aging and the evolution of the oxidative potential through their effect on radical chemistry. Here, we investigated the influence of copper (Cu) on the photochemical aging of iron (Fe) containing SOA in single particles using scanning transmission X-ray microscope measurements and chemical box modeling. The SOA proxy included citric acid (CA), iron(III) citrate (Fe III (Cit)), and copper(II) citrate (Cu II (HCit)), which were exposed to UV light (λ = 365 nm) in a humidified environmental cell. We modeled known catalytic radical destruction mechanisms resulting from cross-redox reactions between copper and iron. Simulating anoxic Fe III (Cit)/Cu II (HCit)/CA aging experiments showed a lower initial iron(III) reduction compared to Fe III (Cit)/CA particles, indicating a reduced iron(II) quantum yield than from the photolysis of the Fe III (Cit) alone. We hypothesize that this effect may be due to copper replacing an iron center in a polynuclear complex. At higher relative humidity up to 60 %, a lower iron(II) quantum yield could not account for our observations of iron reoxidation in the dark. Instead, reoxidation appears to be highly sensitive to a potential copper(II)-induced reoxidation reaction. We provide a comprehensive discussion and evaluation of the poorly understood role of copper in modifying redox and radical chemistry, which is relevant for reactions involving transition metals mixed with SOA in the atmosphere.