Impact of the chirality on the formation of organic condensable vapors and particle formation from monoterpenes oxidation

The majority of atmospheric fine particulate matter (PM₂.₅) by mass is typically organic, predominantly composed of secondary organic aerosol (SOA). SOA forms through the gas-phase oxidation of volatile organic compounds (VOCs), followed by the gas-to-particle conversion of oxidized products. Among these precursors, biogenic VOCs (BVOCs), such as isoprene and monoterpenes, are the most abundant, particularly in regions with dense vegetation. For instance, in boreal forests, α-pinene significantly contributes to SOA formation, primarily through the generation of highly oxygenated molecules (HOMs). These low-volatility compounds play a critical role in atmospheric new particle formation. Chirality, a fundamental molecular property, holds profound implications across chemistry, biology, and environmental sciences. While enantiomers exhibit identical physical and chemical properties under most conditions, they can interact distinctly with biological systems—a phenomenon known as enantioselectivity. In atmospheric chemistry, chirality introduces an additional layer of complexity, influencing VOC emissions, oxidation pathways, and the formation and composition of SOA. Despite the ubiquity of chiral VOCs in the atmosphere, their role in aerosol formation and potential health impacts remains poorly understood. This gap is partly due to the analytical challenges of distinguishing enantiomers in both gas and particle phases. Most monoterpenes have been studied without considering the impact of their specific enantiomeric structures. However, certain sources, such as anthropogenic emissions (e.g., limonene) or drought-stressed vegetation (e.g., α-pinene), release specific enantiomers into the atmosphere. Consequently, the formation of SOA from the oxidation of (+)- and (–)-enantiomers has been largely overlooked in experimental studies and atmospheric models. In this study, we investigated the O₃/OH-initiated oxidation of two common chiral monoterpenes ((+)- and (–)-limonene and (+)- and (–)-α-pinene) using a flow tube reactor and an atmospheric simulation chamber. We characterized gaseous and particle-phase products using online chemical ionization mass spectrometry. Our findings reveal that the chirality of the precursors (+)- vs. (–)-enantiomers significantly influences HOM formation, particle formation, and subsequent SOA aging. Overall, this work highlights the distinct particle formation potentials arising from the oxidation of chiral monoterpenes, offering novel insights into the formation of biogenic SOA.