Abstract. Binary homogeneous nucleation (BHN) of sulphuric acid and water (H 2 SO 4 /H 2 O) is one of the most important atmospheric nucleation processes, but laboratory observations of this nucleation process are very limited and there are also large discrepancies between different laboratory studies. The difficulties associated with these experiments include wall loss of H 2 SO 4 threshold (10 8 -10 9 cm −3 ) needed to produce the unit J measured from the laboratory studies compared to the atmospheric conditions (10 6 -10 7 cm −3 ), imply that H 2 SO 4 /H 2 O BHN alone is insufficient to explain atmospheric aerosol formation and growth. Particle growth rates estimated from the measured aerosol size distributions, residence times (t r ), and [H 2 SO 4 ] were 100-500 nm h −1 , much higher than those seen from atmospheric field observations, because of the higher [H 2 SO 4 ] used in our study.

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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 2 O 2 (g). In our experiments, ISO(g) and/or H 2 O 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) n H + 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 2 O and O neutrals, whereas the less abundant negative carboxylate ion products undergo CO, H 2 O, 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 2 O 2 (g)) to (ISO) n H + , followed by fast reactions involving dissolved H 2 O 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.