Chemical\nComposition\nof Secondary Organic Aerosol\nFormed from the Oxidation of Semivolatile Isoprene Epoxydiol Isomerization\nProducts

3-Methylenebutane-1,2,4-triol and\n3-methyltetrahydrofuran-2,4-diols,\npreviously designated “C<sub>5</sub>-alkene triols”,\nwere recently confirmed as in-particle isomerization products of isoprene-derived\nβ-IEPOX isomers that are formed upon acid-driven uptake and\npartition back into the gas phase. In chamber experiments, we have\nsystematically explored their gas phase oxidation by hydroxyl radical\n(<sup>•</sup>OH) as a potential source of secondary organic\naerosol (SOA). <sup>•</sup>OH-initiated oxidation of both compounds\nin the presence of ammonium bisulfate aerosol resulted in substantial\naerosol volume growth. Compositions of low-volatility products in\nboth the gas and particulate phases were established by high-resolution\nmass spectrometry measurements. Under conditions mimicking the Southeast\nUSA (50% relative humidity, bulk seed aerosol pH 1.4), we estimate\nthe SOA yield from <sup>•</sup>OH-initiated oxidation of 3-methylenebutane-1,2,4-triol\nto be 93.1%, equating to 1.95 ± 0.81 Tg C Yr<sup>-1</sup>, and\nfrom 3-methyltetrahydrofuran-2,4-diol oxidation to be 26.7%, equating\nto 1.76 ± 1.26 Tg C Yr<sup>-1</sup>. Previously unreported isoprene-derived\noxidation products, 2,3-dihydroxy-2-(hydroxymethyl)propanal, 1,3,4-trihydroxybutan-2-one,\nand four organosulfates have been confirmed in ambient SOA, and aid\nin understanding isoprene oxidation pathways in HO<sub>2</sub><sup>•</sup> dominated environments as NO<sub><i>x</i></sub> levels continue to decline in the US. This work underlines\nthe need for inclusion of partitioning of in-particle formed semivolatile\nproducts and their atmospheric oxidation pathways in atmospheric models.