The Critical Role Of Synthetic Chemistry In Identifying Isoprene-Oxidation Products β-HPALD In Isoprene-Derived Secondary Organic Aerosol
REBECCA L. RICE, Zhenfa Zhang, Avram Gold, University of North Carolina at Chapel Hill
Abstract Number: 173
Working Group: Aerosol Chemistry
Abstract
Isoprene, the largest biogenic volatile organic compound emitted into the atmosphere, undergoes a cascade of atmospheric oxidations initiated by rapid reaction with hydroxyl radical to form secondary organic aerosol (SOA). Despite intensive studies, the routes of formation are incompletely resolved, particularly the characterization of early generation gas phase oxidation products. Synthetic organic chemistry is integral in providing comprehensive elucidation of SOA formation pathways by enabling confirmation and direct investigation of reactive gas-phase intermediates by using authentic standards. Importantly, first-generation isoprene gas-phase oxidation products formed under low-NOx conditions impact aerosol composition and physicochemical properties. The initial detectable transient in isoprene oxidation is a manifold of hydroxyperoxyl radicals which branches between the well-studied bimolecular reaction with HO2 and understudied H-shift pathways. C5H8O3, species, the earliest closed-shell products of the H-shift branch, have been categorized as δ- and ß-hydroperoxyaldehydes (δ- and β-HPALDs) and may account for as much for 12% of isoprene oxidation products. We have reported that the major products, δ-HPALDs, are cyclic peroxyhemiacetals which can be further oxidized to low-volatility products and form SOA. We now report synthesis of the second, and minor class of H-shift products, 3,4-ß-HPALD and 2,1-ß-HPALD. Characterization methods include proton NMR, UV spectroscopy, and hydrophilic liquid interaction liquid chromatography coupled to an electrospray ionization HR-ToF-MS. The ß-HPALDs do not cyclize or retain the photolabile conjugated carbonyl chromophore but do have the carbonyl functionality with a UV-vis absorbance tail into the ambient UV range. Targeted chamber studies reveal the influence of their structure on oxidation and acid-catalyzed particle-phase reactions. Synthetic chemistry is a powerful tool for rectifying the divergent atmospheric observations and incompletely understood molecular-level processes, by authentic products for modeling isoprene-derived SOA and enhancing our understanding of SOA impacts on climate and air quality.