Isoprene Epoxydiols as Precursors to Secondary Organic Aerosol Formation: Acid-Catalyzed Reactive Uptake Studies with Authentic Compounds

Isoprene epoxydiols (IEPDX), formed from the photooiddation of isoprene under low-NOx conditions, have recently been proposed as precursors of secondary organic aerosol (SOA) on the basis of mass spectrometric evidence. In the present study, IEPDX isomers were synthesized in high purity (>99%) to investigate their potential to form SOA via reactive uptake in a series of controlled dark chamber studies followed by reaction product analyses. IEPOX-derived SOA was substantially observed only in the presence of acidic aerosols, with conservative lower-bound yields of 4.7-6.4% for beta-IEPOX and 3.4-5.5% for delta-IEPOX, providing direct evidence for IEPDX isomers as precursors to isoprene SOA. These chamber studies demonstrate that IEPDX uptake explains the formation of known isoprene SOA tracers found in ambient aerosols, including 2-methyltetrols, C-5-alkene triols, dimers, and IEPOX-derived organosulfates. Additionally, we show reactive uptake on the acidified sulfate aerosols supports a previously unreported acid-catalyzed intramolecular rearrangement of IEPDX to cis- and trans-3-methyltetrahydrofuran-3,4-diols (3-MeTHF-3,4-diols) in the particle phase. Analysis of these novel tracer compounds by aerosol mass spectrometry (AMS) suggests that they contribute to a unique factor resolved from positive matrix factorization (PMF) of ANIS organic aerosol spectra collected from low-NOx, isoprene-dominated regions influenced by the presence of acidic aerosols.