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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. 

Aerosol acidity is a critical variable that influences the formation of particulate matter in the atmosphere. Particle pH affects the rate of heterogeneous reactive uptake of carbonyl- and epoxide-containing species, aqueous-phase accretion reaction rates, dissolution of transition metal ions, and sulfate oxidation pathways. However, directly measuring aerosol pH is analytically challenging due to the low liquid water content and high ionic strength of aerosol particles. The colorimetric method for bulk aerosol pH determination is low-cost and easily field-deployable, but pH measurements cannot be obtained for highly acidic particles (pH < 0) or particles with low liquid water content that do not saturate the pH indicator paper. We have developed a new approach that extends aerosol pH measurements into the water-limited regime, using a condensational growth tube to induce water uptake onto submicron aerosol prior to impaction onto pH paper. Aerosol pH prior to condensational growth is calculated based on the volume of water accumulated over the growth tube and the shift in relevant acid-base equilibria. Herein, we use this water uptake colorimetric method to explore a range of different conjugate acid-base pairs and measure aerosol pH for particles of increasing chemical complexity, including chamber-generated SOA. Aerosols are generated from bulk solutions at a moderate relative humidity (60%), simulating the conditions of “wet”-seeded chamber experiments and a common atmospheric RH value. Aerosol pH can be more acidic than the original solution by up to 3 pH units, with this difference being highly dependent on the initial pH and conjugate acid-base pair of the bulk solution. These results highlight the importance of accurate aerosol acidity measurements to understand the relationship between aerosol pH and secondary aerosol mass formation, both in complex laboratory studies and ambient locations where aerosol pH remains highly uncertain. 

2-Methyltetrol sulfate diastereomers (2-MTS) form from acid-driven multiphase chemical conversion of inorganic sulfate (Sulfinorg) aerosol by isoprene-derived epoxydiols (IEPOX). Studies have explored 2-MTS sinks within aerosols, but observations of 2-MTS in atmospheric waters indicate a need to understand the fate of 2-MTS in clouds and fogs. In batch reactor experiments, we oxidized 2-MTS in 30, 300, and 3000 μM solutions to understand 2-MTS fate in cloud, fog, and aerosol water. Ion chromatography (IC) revealed production of acetate, oxalate, and Sulfinorg with 2-MTS oxidation. Our 30, 300, and 3000 μM experiments revealed a 5%, 20%, and 32% gap in the sulfur mass balance, respectively, suggesting organosulfates (OSs) formed from 2-MTS oxidation. OSs produced by 2-MTS oxidation were detected by hydrophilic liquid interaction chromatography interfaced to electrospray ionization high-resolution tandem mass spectrometry at mass-to-charge ratios 139 (C2H3O5S−), 169 (C3H5O6S−), 185 (C3H5O7S−), 199 (C4H7O7S−), 211 (C5H7O7S−), 213 (C5H9O7S−), 227 (C5H7O8S−), and 229 (C5H9O8S−). 2-MTS oxidation was then accurately predicted with a rate constant of 1.5 × 109 M−1s−1. Results emphasize the utility of 2-MTS for sulfur cycling in the atmosphere, and the potential for IC to measure 2- MTS in atmospheric waters. 

Isoprene epoxydiols (IEPOX), major atmospheric oxidation products of the abundant biogenic VOC isoprene, contribute extensively to secondary organic aerosol (SOA) formation in Earth’s troposphere in low-NOx regions. IEPOX undergoes acid-catalyzed multiphase chemical reactions with anthropogenically derived acidic sulfate aerosol particles, forming a variety of low-volatility and semivolatile SOA products. One class of low-volatility SOA products is organosulfates (OS), which result from inorganic sulfate covalently bonded with IEPOX upon its epoxide-ring opening reactions. However, the mass balance between inorganic sulfate OS has yet to be constrained by laboratory studies under varying aerosol acidity conditions representative of atmospheric aerosols. In this study, smog chamber experiments were conducted to constrain the mass balance between organic and inorganic sulfur. The SOA formed during these experiments were collected onto filters and subsequently chemically analyzed with ion chromatography (IC) and inductively coupled plasma interfaced to optical emission spectroscopy (ICP-OES) to determine the change in sulfur form before and after reaction with IEPOX. From our initial results, the percentage of inorganic sulfate converted to OS in IEPOX-derived SOA is approximately 47.1 to 50.2%. Methyltetrol sulfates, the most abundant form of OS found in atmospheric aerosols, will be quantified by HILIC/ESI-HR-QTOFMS, allowing for the first determination of the total mass fractions of oligomeric OS. This work will help atmospheric modelers in their predictions of SOA formation and sulfur mass balance in atmospheric aerosols, which has not been well constrained by models thus far. Ongoing work will include further constraining the effect of initial IEPOX: inorganic sulfate mass ratio at a fixed aerosol acidity on the particulate sulfur mass balance in chamber experiments. 

Acidity-Driven Shifts in Aerosolized Sodium Sulfate Crystal Structure via Raman Microspectroscopy and Implications for Isoprene-Derived Secondary Organic Aerosol Formation 

Isoprene, the largest nonmethane volatile hydrocarbon emitted into Earth’s atmosphere, is rapidly oxidized by ambient hydroxyl radical (•OH) to yield secondary organic aerosol (SOA). Early generation gas-phase products of isoprene oxidation remain to be fully characterized. •OH addition at terminal C1/C4 followed by the addition of O2, yields β- and δ-peroxy (ISOPO2 •) isomers. Under low-nitric oxide conditions, δ-ISOPO2 • isomers branch between bimolecular reaction with HO2 • and intramolecular H-shifts. Initial C5H8O3 products of H-shifts reported in chamber experiments were assigned δ-hydroperoxyalkenal (HPALD) structures and predicted to photolyze rapidly. We observe by ion mobility spectrometry that a putative δ-HPALD isomer from •OH addition at C4 is the cyclic peroxyhemiacetal, 5-methyl dioxinol, and may be the relevant product in atmospheric chemistry. 5-Methyl dioxinol is robust to photolysis at tropospheric UV−vis wavelengths and is thus potentially a significant previously unrecognized source of SOA. In chamber experiments, we found that 5-methyl dioxinol forms SOA and determined that the steady-state ratio of the proximal major unimolecular/bimolecular products downstream of ISOPO2 • is ∼0.26. Using the total steady-state dioxinol level formed in the isoprene oxidation experiment, dioxinols may contribute ∼3.2 × 109 g yr−1, or 4% to isoprene-derived atmospheric carbon. 

10AC.5 Unexpected Stereochemical Specificity for Organosulfates Formed from Isoprene-Epoxide Ring Opening Reactions as a Function of Aerosol Acidity. ANDREW P. AULT, Madeline Cooke, N. Cazimir Armstrong, Cara Waters, Qiyuan Zhao, Scarlet Aguilar-Martinez, Yuzhi Chen, Ziying Lei, Zhenfa Zhang, Avram Gold, Paul Zimmerman, Jason Surratt, University of Michigan Chirality is a central property of organic molecules that plays a key role in chemical properties and biological impacts of molecules with only a slight difference at a single stereocenter within a molecule. In secondary organic aerosol (SOA) species with different stereoisomers are assumed to be present equally within racemic mixtures, unless there is an initial stereospecificity of the condensing or reacting oxidized VOCs. Unexpectedly, we found that certain stereoisomers of organosulfates are formed preferentially from isoprene epoxydiols without initial stereospecificity. This surprising result is confirmed with computational chemistry showing a significant energy difference in the formation of certain stereoisomers. In addition, we see a pH-dependent shift in the mechanism of reaction from an SN1 dominating the lowest acidities studied to an increase in the concerted SN2 mechanism at higher pH values. These results were enabled through a combination of advanced analytical methodologies, specifically hydrophilic interaction liquid chromatography coupled with mass spectrometry (HILIC/ESI-HR-QTOFMS) that can separate the MTS diastereomers and computational chemistry studies using advanced sampling methods to explore the reaction mechanisms. Identifying this unexpected chirality from racemic precursors and connecting it to mechanisms occurring within atmospheric aerosols has broad importance for SOA chemical composition, physicochemical properties, and potentially human health after inhalation. Further studies are needed to determine the breadth of this chirality within aerosols and its potential implications. 

Isoprene-derived secondary organic aerosol (SOA) constituents, such as the 2-methyltetrols (2-MT) and 2-methyltetrol sulfates (2-MTS), have been readily detected in atmospheric fine aerosols (PM2.5). Isoprene-derived SOA compounds exist within aerosol mixtures containing inorganic salts, such as ammonium sulfate (AS). Despite its atmospheric significance, the water uptake of 2-MT, 2-MTS, and their mixtures are not well understood. In this study, we determine the physicochemical properties of 2-MT, 2-MTS, and their mixtures with AS. 2-MT and 2-MTS have been previously identified as viscous; thus, dynamic surface tension (σs/a) measurements were taken for both compounds to determine their organic diffusion coefficients (Ds). The droplet growth of the synthesized organics and AS mixtures was measured under subsaturated conditions (< 100% RH) using a humidified tandem differential mobility analyzer (H-TDMA) at a relative humidity (RH) of 88.2% ± 1.5%. Aerosol activation and droplet growth was also measured under supersaturated (> 100% RH) conditions using a cloud condensation nuclei counter (CCNC); supersaturation (SS) ranged from 0.3-1.4%. Both subsaturated and supersaturated hygroscopicity were parameterized by the single hygroscopicity parameter κ. Furthermore, aerosol viscosity and phase morphology were analyzed using atomic force microscopy (AFM) measurements. This study demonstrates how diffusion and salting-in effects influence the water uptake of SOA-proxy mixtures. The presence of inorganic compounds, such as AS, can enhance organic diffusion, leading to ideally well-mixed aerosol hygroscopic behavior that can be described by traditional Köhler theory. As a result, this work shows that for atmospherically ubiquitous isoprene-derived SOA like 2-MT and 2-MTS, the impact of its non-ideal thermodynamic properties must be considered in aerosol-cloud interactions. 

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.