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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.

 

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.