| Isoprene (ISO), the most abundant non-methane VOC, is the major contributor to secondary organic aerosols (SOA) formation. The mechanisms involved in such transformation, however, are not fully understood. Current mechanisms, which are based on the oxidation of ISO in the gas-phase, underestimate SOA yields. The heightened awareness that ISO is only partially processed in the gas-phase has turned attention to heterogeneous processes as alternative pathways toward SOA.;During my research project, I investigated the photochemical oxidation of isoprene in bulk water. Below, I will report on the photolysis (lambda ≥ 305) of H2O2 in dilute ISO solutions. This process yields C10H15OH species as primary products, whose formation both requires and is inhibited by O2. Several isomers of C 10H15OH were resolved by reverse-phase high-performance liquid chromatography and detected as MH+ (m/z = 153) and MH +-18 (m/z = 135) signals by electrospray ionization mass spectrometry. This finding is consistent with the addition of ·OH radicals to ISO, followed by HO-ISO· reactions with ISO (in competition with O 2) leading to second generation HO(ISO)2· radicals that terminate as C10H15OH via beta-H abstraction by O2.;It is not generally realized that chemistry on the surface of water cannot be deduced, extrapolated or translated to those in bulk gas and liquid phases. The water density drops a thousand-fold within a few Angstroms through the gas-liquid interfacial region and therefore hydrophobic VOCs such as ISO will likely remain in these relatively 'dry' interfacial water layers rather than proceed into bulk water. In previous experiments from our laboratory, it was found that gas-phase olefins can be protonated on the surface of pH ≤ 4 water. This phenomenon increases the residence time of gases at the interface, an event that makes them increasingly susceptible to interaction with gaseous atmospheric oxidants such as ozone and hydroxyl radicals.;In order to test this hypothesis, I carried out experiments in which ISO(g) collides with the surface of aqueous microdroplets of various compositions. Herein I report that ISO(g) is oxidized into soluble species via Fenton chemistry on the surface of aqueous Fe(II)Cl2 solutions simultaneously exposed to H2O2(g). Monomer and oligomeric species (ISO) 1-8H+ were detected via online electrospray ionization mass spectrometry (ESI-MS) on the surface of pH ~ 2 water, and were then oxidized into a suite of products whose combined yields exceed 5% of (ISO)1-8 H+. MS/MS analysis revealed that products mainly consisted of alcohols, ketones, epoxides and acids. Our experiments demonstrated that olefins in ambient air may be oxidized upon impact on the surface of Fe-containing aqueous acidic media, such as those of typical to tropospheric aerosols. |
