The C-Alkene Triol Conundrum: Structural Characterization and Quantitation of Isoprene-Derived CHO Reactive Uptake Products.

2-Methyltetrols and CHO compounds, referred to as "C-alkene triols," are chemical tracers used to estimate isoprene-derived epoxydiol (IEPOX) contributions to atmospheric PM. For nearly two decades, "C-alkene triol" molecular structures and PM mass contributions remain uncertain, and their origin as analytical artifacts is unclear. Thus, we synthesized CHO 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 non-destructive hydrophilic-interaction liquid chromatography coupled with electrospray ionization interfaced to high-resolution quadrupole-time-of-flight mass spectrometry (HILIC/ESI-HR-QTOFMS). Using novel 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 ~10% and ~50% of GC/EI-MS measured 3-methylenebutane-1,2,4-triol and 3-methyltetrahydrofuran-2,4-diols, respectively, are not analytical artifacts, but arise from acid-driven particle-phase IEPOX isomerization. Significant quantities were also detected in impingers downstream from filters, demonstrating "C-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. Therefore, 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.