Fenton oxidation of gaseous isoprene on aqueous surfaces

We report that gaseous isoprene ISO(g) is oxidized into soluble species on the surface of aqueous acidic FeCl2 solutions simultaneously exposed to H2O2(g). In our experiments, ISO(g) and/or H2O2(g) streams intersect aqueous pH ∼ 2 FeCl2 microjets for ∼10 μs. The products formed in these reactive encounters are identified in situ via online electrospray ionization mass spectrometry. We found that the (ISO)nH+ oligomers generated from ISO(g) on the surface of pH < 4 water are oxidized into myriad products whose combined yields exceed 5%. MS2 analysis reveals that the positive ions derived from the protonation of neutral products split H2O and O neutrals, whereas the less abundant negative carboxylate ion products undergo CO, H2O, and CO2 losses. Significantly, all products are fully quenched by ·OH scavenger tert-butyl alcohol. These results are consistent with an oxidation process initiated by the addition of ·OH from (Fe2+(aq) + H2O2(g)) to (ISO)nH+, followed by fast reactions involving dissolved H2O2, HO2·, and O2 that lead to polyols; carbonyls; and, to a lesser extent, carboxylic acids. Our experiments demonstrate that gas-phase olefins are oxidized upon colliding on the surface of Fe-containing acidic aqueous media under typical tropospheric conditions.