OH-Initiated Oxidation of Isoprene in Aqueous Media A. J. Colussi, F. Rifkha Kameel, and M. R. Hoffmann

Secondary organic aerosol (SOA) formation from isoprene (2-methyl-1,3-butadiene, ISO), the most abundant volatile organic component, is not fully understood. Increasing awareness that ISO is only partially processed in the gas-phase has turned attention to its reactive uptake by fog, cloud and aerosol droplets. ISO, being poorly soluble in water, would preferentially partition to the surface of such droplets rather than to the bulk of the liquid. Such media also contain dissolved O₂ and water-soluble unsaturated organics, and support much faster ●OH generation rates (from the solar photolysis of H₂O₂dissolved therein) than in the gas-phase. Herein we report that under such conditions ISO is converted to heavier products than into the C4-C5 volatile compounds formed in the gas-phase. We present the results of the λ > 305 nm photolysis of H₂O₂ in aqueous dilute ISO solutions. We found C₁₀H₁₅OH species as primary products, whose formation both requires and is inhibited by O₂. A minimum of seven C₁₀H₁₅OH isomers are resolved by reverse-phase high-performance liquid chromatography and detected as MH⁺ (m/z = 153) signals by electrospray ionization mass spectrometry. Our findings are consistent with the addition of ●OH to ISO, followed by reactions with ISO (in competition with O₂) leading to the formation of a pool of chain or cyclic C₁₀H₁₅OH mono-terpenols via β-H abstraction by the O₂ present in the solution.