Evidence for Coupled Autoxidation of Coexisting Lipids in Aerosols Driven by Criegee Intermediates and OH Radicals

Abstract

Lipid oxidation driven by intermolecular interactions plays a pivotal role in human aging and indoor environmental quality, yet the underlying interaction mechanisms remain largely elusive. Here, we uncover a coupled autoxidation network in the heterogeneous co-ozonolysis of model skin lipids (cholesterol and fatty acids), using online atmospheric pressure photoionization coupled with an ultrahigh-resolution mass spectrometer. Individual cholesterol exhibited extremely low heterogeneous reactivity (determined as effective uptake coefficient, γeff, Chol = 2.94 × 10–6). However, upon co-ozonolysis with fatty acids (oleic or linoleic acid), its reactivity was enhanced by 31-fold (γeff, Chol = 9.11 × 10–5 when mixed with linoleic acid), and elevated relative humidity further amplified this effect by an additional 3-fold. The identification of α-alkoxyalkyl hydroperoxides, α-acyloxyalkyl hydroperoxides, hydroxyalkyl hydroperoxides, and secondary ozonides provides direct molecular evidence that Criegee intermediates (CIs) drive the co-ozonolysis interaction network. Furthermore, the detection of highly oxygenated molecules reveals that •OH reactions are triggered by CIs and hydroperoxide decomposition, establishing a free-radical chain propagation pathway that further accelerates autoxidation. We propose a novel interactive lipid autoxidation mechanism: initiated by lipids’ co-ozonolysis, propagated via CI-driven interactions, and amplified by •OH-mediated radical chemistry. These findings unveil a coupling mechanism of CIs and •OH pathways in governing the heterogeneous reactivity of naturally coexisting lipids, providing fundamental insights into their impacts on biological aging and environmental evolution.