OH Radical-Initiated Chemistry of Isoprene in Aqueous Media. Atmospheric Implications

The fate of isoprene (2-methyl-1,3.-butadiene, ISO) emissions into the atmosphere 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. A hydrophobic gas, ISO would preferentially partition to the surface rather than the bulk of aqueous media. Such media normally contain dissolved O-2 and water-soluble unsaturated organics and support center dot OH generation rates (from the solar photolysis of dissolved H2O2) that are several orders of magnitude larger than in the gas-phase. Thus, ISO should be converted therein to heavier products rather than into the C-4-C-5 volatile compounds produced in the gas-phase. Here we substantiate such a scenario by reporting that the lambda > 305 nm photolysis of H2O2 in dilute aqueous ISO solutions yields C10H15OH species as primary products, whose formation both requires and is inhibited by O-2. A minimum of seven C10H15OH isomers are resolved by reverse-phase high-performance liquid chromatography and detected as MH+ (m/z = 153) and MH+-8 (m/z = 135) signals by electrospray ionization mass spectrometry. Our findings are consistent with the addition of center dot OH to ISO, followed by HO-ISO center dot reactions with ISO in competition with O-2, leading to second generation HO(ISO)(2)center dot radicals that terminate as C10H15OH via beta-H abstraction by O-2. We show that a significant fraction of gas-phase olefins should be converted into less volatile species via this process on wet airborne particles.