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Abstract. We report improved synthetic routes to the isomericisoprene-derived β-epoxydiols (β-IEPOX) in high yield(57 %–69 %) from inexpensive, readily available starting compounds. Thesyntheses do not require the protection/deprotection steps or time-consumingpurification of intermediates and can readily be scaled up to yield thetarget IEPOX isomers in gram quantities. Emissions of isoprene(2-methyl-1,3-butadiene, C5H8), primarily from deciduousvegetation, constitute the largest source of nonmethane atmospherichydrocarbons. In the gas phase under low-nitric-oxide (NO) conditions,addition of the atmospheric hydroxyl radical (OH) followed by rapid addition ofO2 yields isoprene-derived hydroxyperoxyl radicals. The major sink(>90 %) for the peroxyl radicals is a sequential reaction withthe hydroperoxyl radical (HO2), OH, and O2, which is then followed bythe elimination of OH to yield a ∼2:1 mixture ofcis- and trans-(2-methyloxirane-2,3-diyl)dimethanol (cis- and trans-β-IEPOX). The IEPOXisomers account for about 80 % of closed-shell hydroxyperoxylproducts and are rapidly taken up into acidic aerosols to form secondaryorganic aerosol (SOA). IEPOX-derived SOA makes a significant masscontribution to fine particulate matter (PM2.5), which is known to be amajor factor in climate forcing as well as adversely affecting respiratory andcardiovascular systems of exposed populations. Prediction of ambientPM2.5 composition and distribution, both in regional- and global-scaleatmospheric chemistry models, crucially depends on the accuracy ofidentification and quantitation of uptake product formation. Accessibilityof authentic cis- and trans-β-IEPOX in high purity and in large quantity forlaboratory studies underpins progress in developing models as well asidentification and quantitation of PM2.5 components.

 

Hydroxyl radical (·OH)-initiated oxidation of isoprene, the most abundant nonmethane hydrocarbon in the atmosphere, is responsible for substantial amounts of secondary organic aerosol (SOA) within ambient fine particles. Fine particulate 2-methyltetrol sulfate diastereoisomers (2-MTSs) are abundant SOA products formed via acid-catalyzed multiphase chemistry of isoprene-derived epoxydiols with inorganic sulfate aerosols under low-nitric oxide conditions. We recently demonstrated that heterogeneous ·OH oxidation of particulate 2-MTSs leads to the particle-phase formation of multifunctional organosulfates (OSs). However, it remains uncertain if atmospheric chemical aging of particulate 2-MTSs induces toxic effects within human lung cells. We show that inhibitory concentration-50 (IC50) values decreased from exposure to fine particulate 2-MTSs that were heterogeneously aged for 0 to 22 days by ·OH, indicating increased particulate toxicity in BEAS-2B lung cells. Lung cells further exhibited concentration-dependent modulation of oxidative stress- and inflammatory-related gene expression. Principal component analysis was carried out on the chemical mixtures and revealed positive correlations between exposure to aged multifunctional OSs and altered expression of targeted genes. Exposure to particulate 2-MTSs alone was associated with an altered expression of antireactive oxygen species (ROS)-related genes (NQO-1, SOD-2, and CAT) indicative of a response to ROS in the cells. Increased aging of particulate 2-MTSs by ·OH exposure was associated with an increased expression of glutathione pathway related genes (GCLM and GCLC) and an anti-inflammatory gene (IL-10). 

Heterogeneous hydroxyl radical (•OH) oxidation is an important aging process for isoprene epoxydiol-derived secondary organic aerosol (IEPOX-SOA) that alters its chemical composition. It was recently demonstrated that heterogeneous •OH oxidation can age single-component particulate methyltetrol sulfates (MTSs), causing ∼55% of the SOA mass loss. However, our most recent study of freshly generated IEPOX-SOA particulate mixtures suggests that the lifetime of the complete IEPOX-SOA mixture against heterogeneous •OH oxidation can be prolonged through the fragmentation of higher-order oligomers. Published studies suggest that the heterogeneous •OH oxidation of IEPOX SOA could affect the organic atmospheric aerosol budget at varying rates, depending on aerosol chemical composition. However, heterogeneous •OH oxidation kinetics for the full IEPOX-SOA particulate mixture have not been reported. Here, we exposed freshly generated IEPOX-SOA particles to heterogeneous oxidation by •OH under humid conditions (relative humidity ∼57%) for 0−15 atmospheric-equivalent days of aging and derived an effective heterogeneous •OH rate coefficient (kOH) of 2.64 ± 0.4 × 10−13 cm^3 molecules−1 s−1. While ∼44% of particulate organic mass of nonoxidized IEPOX-SOA was consumed over the entire 15 day aging period, only <7% was consumed during the initial 10 aging days. By molecular-level chemical analysis, we determined oligomers were consumed at a faster rate (by a factor of 2−4) than monomers. Analysis of aerosol physicochemical properties shows that IEPOX-SOA has a core−shell morphology, and the shell becomes thinner with •OH oxidation. In summary, this study demonstrates that heterogeneous •OH oxidation of IEPOX-SOA particles is a dynamic process in which aerosol chemical composition and physicochemical properties play important roles. 

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. 

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. 

Acid-driven multiphase chemistry of isoprene epoxydiols (IEPOX) with inorganic sulfate aerosols contributes substantially to formation of secondary organic aerosol (SOA), which constitutes a large mass fraction of atmospheric fine particulate matter (PM2.5). However, atmospheric chemical sinks of freshly generated IEPOX-SOA particles remain unclear. We examined the role of heterogeneous oxidation of freshly-generated IEPOX-SOA particles by gas-phase hydroxyl radical (•OH) under dark conditions as one potential atmospheric sink. After 4 h of gas-phase •OH exposure (~3x108 molecules cm-3), chemical changes in smog chamber-generated IEPOX-SOA particles were assessed by hydrophilic interaction liquid chromatography coupled with electrospray ionization high-resolution quadrupole time-of-flight mass spectrometry (HILIC/ESI-HR-QTOFMS). Comparison of molecular-level compositional changes in IEPOX-SOA particles during aging with or without •OH revealed that decomposition of oligomers by heterogeneous •OH oxidation acts as a sink for •OH and maintains a reservoir of low-volatility compounds including monomeric sulfate esters and oligomer fragments. We propose tentative structures and formation mechanisms for previously uncharacterized SOA constituents in PM2.5. Our results suggest that this •OH-driven renewal of low-volatility products may extend atmospheric lifetimes of IEPOX-SOA particles by slowing production of low-molecular weight, high-volatility organic fragments, and likely contributes to large quantities of 2-methyltetrols and methyltetrol sulfates reported in PM2.5. 

Isoprene is one of the most common biogenic volatile organic compounds (BVOC) in the atmosphere, produced by many plants. Isoprene undergoes oxidation to form gaseous isoprene epoxydiols (IEPOX) under low-NOx conditions, which can lead to the formation of secondary organic aerosol (SOA) particles. SOA-containing particles affect climate by scattering and absorbing solar radiation or acting as cloud condensation nuclei (CCN). High concentrations of SOA are also associated with adverse health impacts in people. While in the atmosphere, IEPOX SOA particles continue to undergo reactions with atmospheric oxidants, including hydroxyl radical (OH). To isolate and probe this process, we studied atmospheric chemical processes in an aerosol chamber to better understand the evolution of heterogeneous OH oxidation of IEPOX-derived SOA particles. Since very little is understood about the structural and spectroscopic properties because of the complexity of their many sources and atmospheric processing, individual particle measurements are necessary to provide better understanding of the composition of IEPOX SOA. We injected particles composed of mixtures of ammonium sulfate and sulfuric acid across a range of acidities(PH = 0.5 – 2.5) and gas-phase IEPOX into the chamber to generate SOA. The SOA particles were then sent to an oxidation flow reactor, and exposed to different OH concentrations representative of aging of a number of days. We kept relative humidity (RH) constant at ~65%, the temperature was ~23 °C, and levels of oxidation were controlled by adjusting lamp intensity. After oxidized SOA was impacted on quartz substrates, we used single-particle Raman microspectroscopy to identify their functional group compositions. From the Raman vibrational spectra of submicron particles (~500-1000 nm aerodynamic diameter), we observed a distinct difference in core-shell morphology and composition: an organic outer layer and an aqueous-inorganic core. The core also has significantly more CH-stretch than the shell. Small changes were also observed with increasing oxidation, which are important to consider when predicting SOA particle evolution in the atmosphere. 

Isoprene has a strong effect on the oxidative capacity of the troposphere due to its abundance. Under low-NOx conditions, isoprene oxidizes to form isoprene-derived epoxydiols (IEPOX), contributing significantly to secondary organic aerosol (SOA) through heterogeneous reactions. In particular, organosulfates (OSs) can form from acid-driven reactive uptake of IEPOX onto preexisting particles followed by nucleophilic addition of inorganic sulfate, and they are an important component of SOA mass, primarily in submicron particles with long atmospheric lifetimes. Fundamental understanding of SOA and OS evolution in particles, including the formation of new compounds by oxidation as well as corresponding viscosity changes, is limited, particularly across relative humidity (RH) conditions above and below the deliquescence of typical sulfate aerosol particles. In a 2-m3 indoor chamber held at various RH values (30 – 80%), SOA was generated from reactive uptake of gas-phase IEPOX onto acidic ammonium sulfate aerosols (pH = 0.5 – 2.5) and then aged in an oxidation flow reactor (OFR) for 0 – 24 days of equivalent atmospheric ·OH exposure. We investigated the extent of inorganic sulfate conversion to organosulfate, formation of oligomers, single-particle physicochemical properties, such as viscosity and phase state, and oxidation kinetics. Chemical composition of particle-phase species, as well as aerosol morphological changes, are analyzed as a function of RH, oxidant exposure times, and particle acidity to better understand SOA and OS formation and destruction mechanisms in the ambient atmosphere.