Impact of atmospheric water-soluble iron on α-pinene-derived SOA formation and transformation in the presence of aqueous droplets†

Abstract

The impact of water-soluble atmospheric iron on formation, growth and aging of secondary organic aerosol (SOA) is a controversial subject in the literature. Iron chemistry drives Fenton reactions in the aqueous phase which is dependent on pH. Flow reactor experiments in the dark and under humid conditions were conducted to investigate systematically the influence of ferrous iron in the aqueous phase on α-pinene SOA by online physical analysis and offline high-resolution mass spectrometry. During the experiments increased SOA formation under conditions favorable for dark Fenton chemistry in the aqueous phase was observed. Furthermore, samples with an acidified and iron-containing aqueous phase showed a degradation of pinyl-diaterpenyl (C₁₇H₂₆O₈) ester which ages through oxidation via OH radicals and can thus be evidence for ongoing degradation processes of high molecular weight molecules by iron chemistry. Moreover, higher abundance of dimer MW338 (C₁₉H₃₀O₅) in the acidic sample affected by Fenton's chemistry was detected which is suggested to be formed via acid catalysis indicating competing acidity-driven reactions influencing SOA formation. Therefore, this study provides insight into the impact of aqueous phase iron on SOA formation and transformation under simulated natural conditions.