Understanding Isoprene Photo-oxidation from Continuous-Flow Chamber Experiments: Unexpectedly High SOA Yields and New Insights into Isoprene Oxidation Pathways

Abstract:

Secondary organic aerosol (SOA) accounts for a substantial fraction of tropospheric aerosol and has significant impacts on climate and human health. Results from the CARES (Carbonaceous Aerosol and Radiative Effects Study) field mission suggested that isoprene oxidation moderated by anthropogenic emissions plays a dominant role in SOA formation, but current literature isoprene yields and oxidation mechanisms are unable to explain the CARES observations. In this study, we conducted a series of continuous-flow chamber experiments to investigate the yield and chemical composition of SOA formed from isoprene photo-oxidation as a function of NO_x concentration. Under low-NO_x (< 1ppbv) conditions, we measure SOA mass yields that are significantly larger than previously reported, reaching up to 20%, and the yields are strongly dependent on H₂O₂ concentrations. The higher yields are likely a result of differences between batch mode and continuous-flow experiments and the photochemical fate of the ISOPOOH intermediate under the high HO₂ conditions of the chamber experiments. Online analysis of the SOA using the University of Washington FIGAERO HR-ToF-CIMS instrument shows that a C₅H₁₂O₆ compound can explain a significant fraction of the mass measured by the AMS. We tentatively identify this compound as a dihydroxy dihydroperoxide produced from the oxidation of ISOPOOH. To our knowledge, we believe this represents the most direct confirmation that such dihydroperoxides form during isoprene oxidation and contribute to SOA. A van Krevelen analysis of HR-AMS data is consistent with hydroperoxide species forming the majority of the SOA. As progressively more NO was added to the system, yields initially increase to a maximum at an NO:isoprene ratio of ~1, and then rapidly decrease, to 3.6% at an NO:isoprene ratio of 4. As NO concentrations increased, alkyl nitrates accounts for an increasing portion of the SOA mass, though hydroperoxides remain significant. These observations of increased yields and the elucidation of isoprene oxidation pathways will allow for more accurate predictions of organic aerosol formation from the photochemical oxidation of isoprene in the ambient atmosphere.