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1. HPALD2 is a Peroxyhemiacetal and a Source of SOA

   RICE, REBECCA; Surratt, Jason; Zhang, Zhenfa; Gold, Avram (October 2023, 2023 AAAR Meeting)

   Abstract Number: 51 Working Group: Aerosol Chemistry Abstract Isoprene, a volatile organic compound (VOC) is emitted largely by vegetation at a rate of 512 Tg/yr. Based on theoretical calculations and mass spectrometric evidence, Z-δ-hydroperoxyalkenal structures (HPALD1 and HPALD2) have been assigned to C5H8O3 gas-phase compounds accounting for up to 12% of the total first-generation isoprene oxidation products. The putative HPALDs are conjugated carbonyls expected to have a significant absorption cross section at ambient UV wavelengths (> 315 nm). Fast internal energy transfer from the excited alkenal to the O-OH bond is predicted to cause rapid bond dissociation degradation into volatile fragments, with little or no formation of SOA. We undertook synthesis of HPALD2 to verify the structure assigned solely from mass spectrometry. By proton NMR, HPALD2 exists exclusively as the peroxyhemiacetal tautomer, with no carbonyl detected, even in D2O. Tautomerization to the cyclic peroxyhemiacetal is strongly favored by the Z geometry of HPALD2. The peroxyhemiacetal structure of the isoprene photochemical oxidation product was confirmed by matching the IMS drift time of the synthetic standard with a major C5H8O3 product from hydroxyl radical oxidation of isoprene. Lacking the conjugated chromophore, the peroxyhemiacetal does not absorb at > 250 nm will persist at ambient UV wavelengths. In chamber experiments, OH oxidation caused rapid nucleation in the absence of seed, and reactive uptake in the presence of both (NH4)2SO4 and (NH4)HSO4 seed. Products at m/z C5H8O5, C5H10O5, C5H10O6 were detected by on-line monitoring of the gas phase by an iodide-CIMS-high resolution time-of-flight mass spectrometer (HR-ToF-MS). Analysis of filter extracts by hydrophilic interaction liquid chromatography coupled to an electrospray ionization HR-ToF-MS detector operated in the negative mode showed major products with compositions C5H10O5 in all experiments, and major sulfated products with compositions C5H10O8S and C3H6O6S in seeded experiments. 

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2. Identifying a Missing Link: Confirmation of the Structure and Origin of 4-hydroperoxy-3-methylbut-2-enal (4-HPALD) with an Authentic Standard

   RICE, REBECCA; Yan, Jin; Gerber, Sebastian; Graf, Stephan; Kamrath, Michael; Lopez-Hilfiker, Felipe; Riva, Matthieu; Zhang, Zhenfa; Surratt, Jason; Gold, Avram (October 2022, AAAR 40th Annual Conference)

   Isoprene (C5H8) is the largest non-methane volatile organic compound emitted into the atmosphere. Isoprene reacts rapidly with ambient hydroxyl radicals (OH) and subsequent addition of O2 results in the formation alkyl peroxy (RO2) radicals. The fate of the initially formed RO2 radicals has been the focus of continuing theoretical and experimental research. Under pristine conditions where bimolecular reactions of RO2 are limited, the thermodynamically favored RO2 undergoes an intramolecular H-shift followed by reaction with O2 and elimination of HO2 to yield 4-hydroperoxy aldehyde (4-HPALD, C5H8O3), predicted to account for up to 13% of first-generation isoprene photochemical oxidation products. Mass spectrometric evidence has been reported for 4-HPALD, but lack of an authentic standard has precluded definitive confirmation of both the structure of 4-HPALD and its origin as a first-generation product of OH oxidation of isoprene. We report the synthesis and characterization of 4-HPALD and establish that it is a major product of isoprene oxidation. Synthetic 4-HPALD is isolated as the peroxyhemiacetal. As expected for the 4-hydroperoxy aldehyde, 1H NMR spectra show no evidence for equilibration with the carbonyl form, even in protic solvents, and gas-phase chemical analysis by CIMS also shows only a single form. OH oxidation of isoprene in an oxidation flow reactor coupled to an ion mobility source with an HR-CIMS detector unequivocally demonstrates 4-HPALD (and likely also 1-HPALD) as isoprene oxidation products. Although HPALDs have been discounted as significant contributors to SOA, oxidation of 4-HPALD in a potential aerosol mass (PAM) reactor in the presence of ozone and OH indicates 4-HPALD rapidly undergoes autooxidation reactions forming low-volatility particulate products. We have confirmed highly oxygenated compounds with compositions C5H8O6 and C5H10O6 likely from OH oxidation, and C5H10O7 and C5H10O8 compounds likely products of ozonolysis. The PAM oxidation experiment further demonstrates that the highly oxygenated, low-volatility products efficiently nucleate particles. 

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3. Constraining Gas Phase Yields and Reactive Uptake Coefficients of Oxidation Products from the Hydroxyl Radical-Isoprene Reaction onto Acidic Particles by Vocus Ammonia-Adduct Chemical Ionization Mass Spectrometry (Vocus NH ₄ ⁺ CIMS)

   https://doi.org/10.1021/acsestair.4c00367  

   Zhao, Jiayun; Gagan, Sahir; Frauenheim, Molly P; Niu, Sining; Aridjis-Olivos, Bianca; Surratt, Jason D; Zhang, Zhenfa; Gold, Avram; Zhang, Renyi; Zhang, Yue (April 2025, ACS ES&T Air)

   ABSTRACT: Isoprene, the most abundant nonmethane volatile organic compound in the atmosphere, undergoes photochemical reactions with hydroxyl radical (•OH), a major sink for isoprene, leading to the formation of secondary organic aerosol (SOA). Using a Vocus Chemical Ionization Mass Spectrometer with ammonium-adduct ions (Vocus NH4+ CIMS), this study used the positive ion mode to quantify the yields and time-dependent reactiveuptake of oxidized volatile organic compounds (OVOCs) produced from •OH-initiated oxidation of isoprene under dry conditions. Molar gas-phase yields of key oxidation products were constrained using sensitivities derived from a voltage scan of the front and back end of the Vocus ion−molecule reactor region. Carefully designed chamber experiments measured uptake coefficients (γ) for key isoprene-derived oxidation products onto acidic sulfate particles. The γ values for both C5H10O3 isomers (IEPOX/ISOPOOH) and C5H8O4, another epoxy species from isoprene photo-oxidation, rapidly decreased as the SOA coating thickness increased, demonstrating a self-limiting effect. Despite ISOPOOH/IEPOX contributing around 80% to total reactive uptake, other oxidation products from isoprene photooxidation were estimated to contribute 20% of the total SOA formation. These findings highlight the importance for future models to consider the self-limiting effects of ISOPOOH/IEPOX and SOA formation through non-IEPOX pathways. 

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4. Chemical Composition of Secondary Organic Aerosol Formed from the Oxidation of Semivolatile Isoprene Epoxydiol Isomerization Products

   https://doi.org/10.1021/acs.est.4c06850  

   Frauenheim, Molly; Offenberg, John; Zhang, Zhenfa; Surratt, Jason D; Gold, Avram (December 2024, Environmental Science & Technology)

   ABSTRACT: 3-Methylenebutane-1,2,4-triol and 3-methyltetrahydrofuran-2,4-diols, previously designated “C5-alkene triols”, were recently confirmed as in-particle isomerization products of isoprene-derived β-IEPOX isomers that are formed upon acid driven uptake and partition back into the gas phase. In chamber experiments, we have systematically explored their gas phase oxidation by hydroxyl radical (•OH) as a potential source of secondary organic aerosol (SOA). •OH-initiated oxidation of both compounds in the presence of ammonium bisulfate aerosol resulted in substantial aerosol volume growth. Compositions of low-volatility products in both the gas and particulate phases were established by high-resolution mass spectrometry measurements. Under conditions mimicking the Southeast USA (50% relative humidity, bulk seed aerosol pH 1.4), we estimate the SOA yield from •OH-initiated oxidation of 3-methylenebutane-1,2,4-triol to be 93.1%, equating to 1.95 ± 0.81 Tg C Yr-1, and from 3-methyltetrahydrofuran-2,4-diol oxidation to be 26.7%, equating to 1.76 ± 1.26 Tg C Yr-1. Previously unreported isoprene-derived oxidation products, 2,3-dihydroxy-2-(hydroxymethyl)propanal, 1,3,4-trihydroxybutan-2-one, and four organosulfates have been confirmed in ambient SOA, and aid in understanding isoprene oxidation pathways in HO2• dominated environments as NOx levels continue to decline in the US. This work underlines the need for inclusion of partitioning of in-particle formed semivolatile products and their atmospheric oxidation pathways in atmospheric models. 

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5. Chemical Composition of Secondary Organic Aerosol Formed from the Oxidation of Isoprene-Derived C5H10O3 Reactive Uptake Products

   FRAUENHEIM, MOLLY; Surratt, Jason; Zhang, Zhenfa; Gold, Avram (October 2023, 2023 AAAR Meeting)

   Abstract Number: 99 Working Group: Aerosol Chemistry Abstract Isoprene, the largest non-methane volatile organic species emitted into Earth’s atmosphere, reacts with hydroxyl radicals to initiate formation of secondary organic aerosol (SOA). Under low nitric oxide conditions, the major oxidative pathway proceeds through acid catalyzed reactive uptake of isoprene-epoxydiol isomers (IEPOX). We have recently established the structures of the semivolatile C5H10O3 uptake products (formerly designated “C5-alkene triols) of cis- and trans-β-IEPOX as 3-methylenebutane-1,2,4-triol and isomeric 3-methyltetrahydrofuran-2,4-diols. Importantly, both uptake products showed significant partitioning into the gas phase. Here, we report evidence that the uptake products along with their gas phase oxidation products constitute a hitherto unrecognized source of SOA. We show that partitioning into the gas phase results in further oxidation into low volatility products, including highly oxygenated C5-polyols, organosulfates, and dimers. In the chamber studies, gas phase products were characterized by online by iodide-Chemical Ionization Mass Spectrometry (I-CIMS) and particle phase products by offline analysis of filter extracts by HILIC/(-)ESI-HR-QTOFMS using authentic standards. The chamber studies show the potential for a substantial contribution to SOA from reactive uptake of the second generation gas phase oxidation products onto both acidified and non-acidified ammonium bisulfate seed aerosols. Identification of these previously unrecognized early-generation oxidation products will improve estimates of atmospheric carbon distribution and advance our understanding of the fate of isoprene oxidation products in the atmosphere. 

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