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

   MOLLY FRAUENHEIM, Jason Surratt (October 2023, 41st Meeting American Association for Aerosol Research)

   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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2. 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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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. Observation-constrained kinetic modeling of isoprene SOA formation in the atmosphere

   https://doi.org/10.5194/acp-24-6153-2024  

   Shen, Chuanyang; Yang, Xiaoyan; Thornton, Joel; Shilling, John; Bi, Chenyang; Isaacman-VanWertz, Gabriel; Zhang, Haofei (January 2024, Atmospheric Chemistry and Physics)

   Abstract. Isoprene has the largest global non-methane hydrocarbon emission, and the oxidation of isoprene plays a crucial role in the formation of secondary organic aerosol (SOA). Two primary processes are known to contribute to SOA formation from isoprene oxidation: (1) the reactive uptake of isoprene-derived epoxides on acidic or aqueous particle surfaces and (2) the absorptive gas–particle partitioning of low-volatility oxidation products. In this study, we developed a new multiphase condensed isoprene oxidation mechanism that includes these processes with key molecular intermediates and products. The new mechanism was applied to simulate isoprene gas-phase oxidation products and SOA formation from previously published chamber experiments under a variety of conditions and atmospheric observations during the Southern Oxidant and Aerosol Studies (SOAS) field campaign. Our results show that SOA formation from most of the chamber experiments is reasonably reproduced using our mechanism, except when the concentration ratios of initial nitric oxide to isoprene exceed ∼ 2, the formed SOA is significantly underpredicted. The SOAS simulations also reasonably agree with the measurements regarding the diurnal pattern and concentrations of different product categories, while the total isoprene SOA remains underestimated. The molecular compositions of the modeled SOA indicate that multifunctional low-volatility products contribute to isoprene SOA more significantly than previously thought, with a median mass contribution of ∼ 57 % to the total modeled isoprene SOA. However, this contribution is intricately intertwined with IEPOX-derived SOA (IEPOX: isoprene-derived epoxydiols), posing challenges for their differentiation using bulk aerosol composition analysis (e.g., the aerosol mass spectrometer with positive matrix factorization). Furthermore, the SOA from these pathways may vary greatly, mainly dependent on the volatility estimation and treatment of particle-phase processes (i.e., photolysis and hydrolysis). Our findings emphasize that the various pathways to produce these low-volatility species should be considered in models to more accurately predict isoprene SOA formation. The new condensed isoprene chemical mechanism can be further incorporated into regional-scale air quality models, such as the Community Multiscale Air Quality Modelling System (CMAQ), to assess isoprene SOA formation on a larger scale. 

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5. Insights into the Alkene Triol Conundrum: Characterization and Quantitation of Isoprene-Derived C5H10O3 Reactive Uptake Products

   FRAUENHEIM, MOLLY; Beaver, Melinda; Offenberg, John; Zhang, Zhenfa; Surratt, Jason; Gold, Avram (October 2022, AAAR 40th Annual Conference)

   2-Methyltetrols and a group of C5H10O3 isomers referred to as “alkene triols,” are chemical tracers used to estimate the contribution of isoprene oxidation to atmospheric PM2.5. The molecular structures and the mass contribution of alkene triols are uncertain, and their origin as analytical artifacts is contentious. Here, we report that the alkene triols are uptake products and present evidence of partitioning into the gas phase. Based on the hypothesis that rearrangement of IEPOX yields C5H10O3 isomers on reactive uptake, we synthesized “alkene triol” candidates and investigated their behavior under conventional derivatization gas chromatography/electron impact mass spectrometry (GC/EI-MS) and, in parallel, by non-destructive hydrophilic interaction liquid chromatography coupled with high-resolution quadrupole time-of-flight electrospray mass spectrometry (HILIC/ESI-HR-QTOFMS). Synthetic targets were 3-methyltetrahydrofuran-2,4-diol (1) and 3-methylene-1,2,4-trihydroxybutane (2). Using the standards, we confirmed 1 and 2 in chamber-generated cis- and trans-β-IEPOX SOA both by HILIC/ESI-HR-QTOFMS and derivatization GC/EI-MS. In ambient SOA collected in Research Triangle Park, NC, 1 and 2 were confirmed and quantitatively estimated by GC-EI/MS. Trimethylsilyl derivatization of 1 is problematic, yielding predominantly bis- but also a small amount (<10%) of tris-trimethylsilyl forms. Our findings are consistent with reports that the tris-trimethylsilyl derivatives 1 and 2 represent largely thermal decomposition of 2-methyltetrol sulfate esters; however, based on HILIC/ESI-HR-QTOFMS analysis of chamber-generated SOA, we estimate up to 10% and 50% of 1 and 2, respectively are not artifact-derived, and may arise from isomerization of IEPOX upon reactive uptake. Significant quantities of 1 and 2 were detected in impinger samples downstream from a denuder in series with a filter indicating partitioning into the gas phase. Results suggest that isoprene-derived “alkene triols” do form and are preferentially in the gas phase rather than particle phase, warranting studies on partitioning and gas-phase oxidation pathways. 

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