Atmospheric aging suppresses the formation of limonene-derived organic peroxides

Organic peroxides (POs) are an important component of secondary organic aerosol (SOA) and contribute to the generation of reactive oxygen species associated with adverse health effects. However, their molecular-level evolution upon atmospheric aging, particularly in SOA from typical terpenes like limonene, remains poorly characterized. In this study, limonene SOA was generated in an oxidation flow reactor under controlled OH exposure (equivalent photochemical age, PCA = 0.6-13.2 days). The SOA yield exhibited a nonlinear peak at PCA = 7.4 days, followed by a decline at 13.2 days, a trend that was accompanied by a shift in RO₂ reaction pathways from dimerization to reactions with excess oxidants. POs were quantified using iodometry-assisted high-resolution mass spectrometry (HRMS), and their yields decreased progressively with aging. It was found that 100% of highly oxygenated organic molecules (HOMs) were POs at low OH exposure (0.6 days), whereas only 27% of HOMs were POs at high OH exposure (13.2 days), indicating rapid conversion to non-peroxide, highly oxidized molecules (e.g., acids or esters). Key monomeric (C₈-C₁₀) and dimeric (C₁₅-C₁₉) species were identified at the molecular level. This work elucidates the fate of POs within aged SOA, highlighting the limitations of using HOMs as proxies for POs, and provides critical information for assessing the health risks of SOA.