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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. 

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. 

ABSTRACT: At fixed aerosol acidity, we recently demonstrated that dimers in isoprene epoxydiol-derived secondary organic aerosol (IEPOX-SOA) can heterogeneously react with hydroxyl radical (·OH) at faster rates than monomers. Aerosol acidity influences this aging process by enhancing the formation of oligomers in freshly generated IEPOX-SOA. Therefore, we systematically examined the role of aerosol acidity on kinetics and products resulting from heterogeneous ·OH oxidation of freshly generated IEPOX-SOA. IEPOX reacted with inorganic sulfate aerosol of varying initial pH (0.5, 1.5, and 2.5) in a steady state smog chamber to yield a constant source of freshly generated IEPOX-SOA, which was aged in an oxidation flow reactor for 0−22 equiv days of atmospheric ·OH exposure. Molecular-level chemical analyses revealed that the most acidic sulfate aerosol (pH 0.5) formed the largest oligomeric mass fraction, causing the slowest IEPOX-SOA mass decay with aging. Reactive uptake coefficients of ·OH (γOH) were 0.24 ± 0.06, 0.40 ± 0.05, and 0.49 ± 0.20 for IEPOX-SOA generated at pH 0.5, 1.5, and 2.5, respectively. IEPOXSOA became more liquid-like for pH 1.5 and 2.5, while exhibiting an irregular pattern for pH 0.5 with aging. Using kinetic and physicochemical data derived for a single aerosol pH in atmospheric models could inaccurately predict the fate of the IEPOX-SOA. 

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. 

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.