[A-AS11] Atmospheric Chemistry

[AAS11-P02] Ozonolysis Mechanism of unsaturated organic compounds: Relevance to the multiphase oxidation processes of atmospheric aerosol and human skin lipid

*Tamura Yuya¹, Naoki Numadate¹, Shinichi Enami¹ (1.University of Tsukuba)

Keywords:atmospheric chemistry , aerosol, skin, radical, terpene, laser

Unsaturated organic compounds found in atmospheric aerosols can react with gaseous ozone and form various compounds such as ozonides. This process is important in atmospheric chemistry because it potentially affects the HO_x/NO_x cycle in the atmosphere and contributes to the aging of aerosols. In addition, unsaturated organic compounds such as squalene on the skin surface can react with ozone and release volatile organic compounds into the gas phase, this is being highlighted on the perspective of indoor air pollution. In this study, we investigated ozone oxidations of squalene (C₃₀H₅₀, having 6 C=C bond), an unsaturated hydrocarbon existing in human skin lipid in a large amount, and nerolidol (C₁₅H₂₅OH, having 3 C=C bond), a terpenoid originating from plants, as representative unsaturated organic compounds. 1 mM solution of squalene and nerolidol were prepared using a solvent mixture of acetonitrile and water in the ratio of 4:1 by volume. To detect neutral hydroperoxides and other products, sodium chloride was added to become 0.2 mM. The solution was bubbled with ozone gas and then analyzed by mass spectrometry (Agilent 6130 quadrupole mass spectrometer). Comparing experimental results obtained by H₂O vs H₂¹⁸O solvents, the presence of aldehydes as a key product was inferred. In addition, ozone gas was directly bubbled into neat squalene and nerolidol, and the solution was analyzed by infrared spectroscopy (ATR-FT-IR, Shimadzu IRAffinity-1). We confirmed the formation of C=O bond, and C-O single bond. The latter may be attributable to the formation of ozonides. The results are consistent with previous studies that showed that ozone oxidation of aqueous squalene solutions produces aldehydes via a Criegee intermediate. In addition, the results obtained from the ozone oxidation of aqueous nerolidol solution revealed the formation of multiple hydroperoxides. On the other hand, it was found that the products were different in the ozone reactions of unsaturated organic compounds between in neat condition (i.e., in the absence of water) and in aqueous solution. This difference suggests that the presence or absence of water molecules in the aerosol or on the skin surface has a significant effect on the reaction mechanism. Using a new experimental technique, we plan to execute experiments to directly detect OH radicals, which are predicted to be released into the gas phase in the reaction of unsaturated organic compounds with ozone. This research may reveal the mechanism of the multiphase reaction between unsaturated organic compounds and ozone at the molecular level.