Molecular composition of organic peroxides in secondary organic aerosols revealed by peroxide-iodide reactivity

Organic peroxides are health-relevant organic components in secondary organic aerosols (SOA), which is also a major compound class substantially contributing to SOA mass. However, their molecular identification in SOA is highly challenging and uncertain. Iodide is known to selectively react with peroxides, and their kinetics are fundamentally determined by the structures of individual peroxides. Here we extrapolate this knowledge and develop a novel analytical strategy for molecular characterization of organic peroxides in α-pinene SOA via iodometry kinetic experiments, using liquid chromatography–high resolution mass spectrometry. Through non-targeted analysis, more than 300 organic peroxides are identified in α-pinene SOA with unprecedented accuracy of their chemical formula. Their reactivity with iodide is highly compound-dependent and can vary four orders of magnitude, within the range observed for some commercial organic peroxides with known structures. In particular, more than 65% of organic peroxides in α-pinene SOA exhibit a slow reactivity with iodide, with an e-folding lifetime exceeding one day. The structures of 12 organic peroxides derived from stabilized Criegee intermediates are further proposed and discussed based on different experimental evidences. Our study improves the molecular-level identification and understanding of organic peroxides in SOA, offering numerous opportunities for further investigation into their formation chemistry, atmospheric transformation, and health impact. Additionally, the peroxide-iodide reactivity is proposed as a new metric that can provide both information on the oxidizing capability and structure of individual organic peroxide in SOA.