Highly Oxidized Second-Generation Products from the Gas-Phase Reaction of OH Radicals with Isoprene.

The gas-phase reaction of OH radicals with isoprene has been investigated in an atmospheric pressure flow tube at 293 ± 0.5 K with special attention to the second-generation products. Reaction conditions were optimized to achieve a predominant reaction of RO radicals with HO radicals. Chemical ionization-atmospheric pressure interface-time-of-flight mass spectrometry served as the analytical technique in order to follow the formation of RO radicals and closed-shell products containing at least four O atoms in the molecule. The reaction products were detected as adducts with the reagent ions using acetate, lactate, or nitrate in the ionization process. Observed signals were attributed to a series of C-products with multiple hydroxy, hydroperoxy, and probably carbonyl groups. H/D exchange experiments supported the product identification. The generation of the detected second-generation products can be mechanistically explained starting from the OH radical reaction of hydroxy hydroperoxide isomers, HO-CH-OOH. These isomers represent the dominant products of the initial OH radical attack on isoprene. Dihydroxy dihydroperoxides, (HO)-CH-(OOH), were analyzed as the main second-generation products beside the dihydroxy epoxides. A simple kinetic analysis revealed that the observed second-generation products in total (other than dihydroxy epoxides) were formed with an estimated molar yield of 10.0 % with respect to converted hydroxy hydroperoxides. A formation yield of 5.8 % has been deduced for the main product (HO)-CH-(OOH). The detected, highly oxidized isoprene products represent potential secondary organic aerosol precursors. An annual, global (HO)-CH-(OOH) formation strength of (16-35) × 10 metric tons is estimated based on product measurements of this study and literature data regarding the formation of the dihydroxy epoxide isomers for an annual isoprene emission of 454 × 10 metric tons of carbon.