Synthesis and Characterization of Organic Peroxides from Monoterpene-derived Secondary Organic Aerosol

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 and characterization in SOA is highly challenging and uncertain. Ozonolysis of alkenes is known to produce reactive intermediates ─ stabilized Criegee intermediates, and their subsequent bimolecular reactions with various carboxylic acids can form α-acyloxyalkyl hydroperoxides (AAHPs), which is considered a major class of organic peroxides in SOA. Here we use this knowledge to synthesize a number of atmospherically relevant AAHPs through liquid-phase ozonolysis from either α-pinene or 3-carene in the presence of ten different carboxylic acids. These AAHPs with diverse structures are identified individually by liquid chromatography coupled with high-resolution mass spectrometry (LC-HRMS). AAHPs were previously thought to decompose quickly in aqueous environment such as cloud droplets, but we demonstrate here that AAHPs hydrolysis rates are highly compound-dependent with rate constants differing by 2 orders of magnitude. Some synthesized AAHPs were further identified via targeted analysis in monoterpene SOA samples collected from laboratory flowtube experiments.

Another focus of this study is to expand the molecular identification ability of organic peroxides in SOA, which goes beyond peroxide standards. Iodide is known to selectively react with peroxides, and their kinetics are fundamentally determined by the structures of individual peroxides. We extrapolate this knowledge and develop a novel analytical strategy for molecular characterization of organic peroxides in SOA via iodometry kinetic experiments using LC-HRMS. 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 4 orders of magnitude. 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.