Molecular Composition of Organic Peroxides in Secondary Organic Aerosols Revealed by Peroxide-Iodide Reactivity.

Organic peroxides are recognized as major contributors to the toxicity and adverse health effects of secondary organic aerosols (SOA). However, their molecular composition and chemical properties in SOA remain largely unexplored. Here we develop a novel analytical strategy for the molecular characterization of organic peroxides in α-pinene SOA, combining iodometry kinetic experiments with liquid chromatography-high-resolution mass spectrometry. Using non-targeted analysis, we identify over 300 organic peroxides in α-pinene SOA, showing a wide range of reactivities with iodide, spanning 4 orders of magnitude. The structures of 12 organic peroxides derived from stabilized Criegee intermediates are further proposed and discussed. Our findings present a reliable, standard-free methodology for identifying previously uncharacterized organic peroxides in SOA at the molecular level, offering new avenues for investigating formation pathways and health impacts of this large, diverse, yet rarely explored compound class. Additionally, we propose peroxide-iodide reactivity as a novel metric for future predictions of oxidative potential─a key indicator of toxicity─and for elucidating the structure of individual organic peroxides in SOA.