The C5-Alkene Triol Conundrum: Structural Characterization and Quantitation of Isoprene-Derived C5H10O3 Reactive Uptake Products

2-Methyltetrols and C5H10O3 compounds, referred to as C-5-alkene triols, are chemical tracers used to estimate isoprene-derived epoxydiol (IEPOX) contributions to atmospheric PM2.5. For nearly two decades, C-5-alkene triol molecular structures and PM2.5 mass contributions have remained uncertain, and their origin as analytical artifacts is unclear. We synthesized C5H10O3 reactive uptake product candidates (3-methyltetrahydrofuran-2,4-diol and 3-methylenebutane-1,2,4-triol) and investigated their behavior under conventional gas chromatography/electron impact-mass spectrometry (GC/EI-MS) with prior trimethylsilylation and, in parallel, by nondestructive hydrophilic-interaction liquid chromatography coupled with electrospray ionization interfaced to high-resolution quadrupole-time-of-flight mass spectrometry (HILIC/ESI-HR-QTOFMS). Using the synthetic standards, we confirmed their presence in laboratory-generated IEPOX SOA. In atmospheric SOA, both synthetic targets were confirmed and quantified by GC/EI-MS. Based on HILIC/ESI-HR-QTOFMS analysis of chamber-generated SOA, we estimate that similar to 10% of GC/EI-MS measured 3-methylenebutane-1,2,4-triol and similar to 50% of 3-methyltetrahydrofuran-2,4-diols are not analytical artifacts but arise from acid-driven particle-phase IEPOX isomerization. Significant quantities were also detected in impingers downstream from filters, demonstrating that C-5-alkene triols are semivolatile. Using chamber-derived yields, we tentatively estimate that atmospheric 3-methyltetrahydrofuran-2,4-diols and 3-methylenebutane-1,2,4-triol could contribute 8.7 Tg C yr(-1). To resolve their significance on air quality and climate, future studies should examine their gas-to-particle partitioning, yields, and atmospheric oxidation chemistry under varying environmental conditions.

Automated summary

This study investigates the molecular structures and atmospheric contributions of C-5-alkene triols, finding that they are not analytical artifacts but result from acid-driven particle-phase IEPOX isomerization. The study also identifies the semivolatile nature of C-5-alkene triols and provides an initial estimate of their potential contribution to air quality and climate.