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 FeCl_2 solutions simultaneously exposed to H_2O_2(g). In our experiments, ISO(g) and/or H_2O_2(g) streams intersect aqueous pH 2 FeCl_2 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%. MS^2 analysis reveals that the positive ions derived from the protonation of neutral products split H_2O and O neutrals, whereas the less abundant negative carboxylate ion products undergo CO, H_2O, and CO_2 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 (Fe^(2+)(aq) + H_2O_2(g)) to (ISO)nH^+, followed by fast reactions involving dissolved H_2O_2, HO_2·, and O_2 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