Acid-driven multiphase chemistry of isoprene epoxydiols (IEPOX) with inorganic sulfate aerosols contributes substantially to secondary organic aerosol (SOA) formation, which constitutes a large mass fraction of atmospheric fine particulate matter (PM2.5). However, the atmospheric chemical sinks of freshly generated IEPOX-SOA particles remain unclear. We examined the role of heterogeneous oxidation of freshly generated IEPOX-SOA particles by gas-phase hydroxyl radical (•OH) under dark conditions as one potential atmospheric sink. After 4 h of gas-phase •OH exposure (∼3 × 108 molecules cm–3), chemical changes in smog chamber-generated IEPOX-SOA particles were assessed by hydrophilic interaction liquid chromatography coupled with electrospray ionization high-resolution quadrupole time-of-flight mass spectrometry (HILIC/ESI-HR-QTOFMS). A comparison of the molecular-level compositional changes in IEPOX-SOA particles during aging with or without •OH revealed that decomposition of oligomers by heterogeneous •OH oxidation acts as a sink for •OH and maintains a reservoir of low-volatility compounds, including monomeric sulfate esters and oligomer fragments. We propose tentative structures and formation mechanisms for previously uncharacterized SOA constituents in PM2.5. Our results suggest that this •OH-driven renewal of low-volatility products may extend the atmospheric lifetimes of particle-phase IEPOX-SOA by slowing the production of low-molecular weight, high-volatility organic fragments and likely contributes to the large quantities of 2-methyltetrols and methyltetrol sulfates reported in PM2.5.

Oxidation of isoprene, the biogenic volatile organic compound (BVOC) with the highest emissions globally, is a large source of secondary organic aerosol (SOA) in the atmosphere. Particulate organosulfates formed from acid-driven reactions of the oxidation products isoprene epoxydiol (IEPOX) isomers are important contributors to SOA mass. Most studies have focused on organosulfate formation on ammonium sulfate particles, often at low pH. However, recent work has shown that sea spray aerosol (SSA) in the accumulation mode (∼100 nm) is quite acidic (pH ∼2) and undergoes further heterogeneous reactions with H 2 SO 4 to form Na 2 SO 4 . Herein, we demonstrate that substantial SOA, including organosulfates, are formed on acidic sodium sulfate particles (pH = 1.4 ± 0.1) via controlled laboratory experiments. Comparable organosulfate formation was observed for acidic sodium and ammonium sulfate particles even though acidic particles with sodium versus ammonium as the primary cation formed less SOA volume. Both exhibited core-shell morphology after the reactive uptake of IEPOX; however, organosulfates were identified with Raman microspectroscopy in the core and shell of ammonium sulfate SOA particles, but only in the core for sodium sulfate SOA. Key organosulfates were also identified in ambient samples from the Galaṕagos Island. Our results suggest that isoprene-derived SOA formed on aged SSA is potentially an important, but underappreciated, source of SOA and organosulfates in marine and coastal regions that could modify SOA budgets.

The phase states and glass transition temperatures (T g ) of secondary organic aerosol (SOA) particles are important to resolve for understanding the formation, growth, and fate of SOA as well as their cloud formation properties. Currently, there is a limited understanding of how T g changes with the composition of organic and inorganic components of atmospheric aerosol. Using broadband dielectric spectroscopy, we measured the T g of organic mixtures containing isoprene epoxydiol (IEPOX)-derived SOA components, including 2-methyltetrols (2-MT), 2-methyltetrolsulfate (2-MTS), and 3-methyltetrol-sulfate (3-MTS). The results demonstrate that the T g of mixtures depends on their composition. The Kwei equation, a modified Gordon−Taylor equation with an added quadratic term and a fitting parameter representing strong intermolecular interactions, provides a good fit for the T g -composition relationship of complex mixtures. By combining Raman spectroscopy with geometry optimization simulations obtained using density functional theory, we demonstrate that the non-linear deviation of T g as a function of composition may be caused by changes in the extent of hydrogen bonding in the mixture.