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Oxidation of isoprene, the biogenic volatile organic compound with the highest emissions globally, is a large source of secondary organic aerosol (SOA) in the atmosphere. Organosulfates, particularly methyltetrol sulfates formed from acid-driven reactions of the oxidation product isoprene epoxydiol (IEPOX) onto particulate sulfate, are important contributors to SOA mass. To date, most studies have focused on organosulfate formation on ammonium sulfate particles at low pH. However, recent work has shown that sea spray aerosol (SSA) in the accumulation mode (~100 nm) is often quite acidic (pH ~ 2). Marine biota are well-established sources of isoprene, with annual global oceanic fluxes of isoprene estimated to range from 1-12 Tg, and IEPOX-derived organosulfates have been identified in marine environments. Herein, we demonstrate that substantial SOA, including organosulfates, are formed on acidic sodium sulfate particles, representative of marine aerosol heterogeneously reacting with H2SO4 to form Na2SO4. We compare SOA formed from the reactive uptake of IEPOX onto particulate sulfate and find that the cation (sodium vs. ammonium) impacts the physical properties and chemical composition of the SOA formed. Additionally, we investigate the formation of SOA derived from sodium sulfate based on key properties including particle acidity and the extent of exposure to oxidation via OH radicals. 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 and could modify SOA budgets and composition in these environments.
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This content will become publicly available on April 11, 2026
Constraining Gas Phase Yields and Reactive Uptake Coefficients of Oxidation Products from the Hydroxyl Radical-Isoprene Reaction onto Acidic Particles by Vocus Ammonia-Adduct Chemical Ionization Mass Spectrometry (Vocus NH 4 + CIMS)
ABSTRACT: Isoprene, the most abundant nonmethane volatile organic compound in the atmosphere, undergoes photochemical reactions with hydroxyl radical (•OH), a major sink for isoprene, leading to the formation of secondary organic aerosol (SOA). Using a Vocus Chemical Ionization Mass Spectrometer with ammonium-adduct ions (Vocus NH4+ CIMS), this study used the positive ion mode to quantify the yields and time-dependent reactiveuptake of oxidized volatile organic compounds (OVOCs) produced from •OH-initiated oxidation of isoprene under dry conditions. Molar gas-phase yields of key oxidation products were constrained using sensitivities derived from a voltage scan of the front and back end of the Vocus ion−molecule reactor region. Carefully designed chamber experiments measured uptake coefficients (γ) for key isoprene-derived oxidation products onto acidic sulfate particles. The γ values for both C5H10O3 isomers (IEPOX/ISOPOOH) and C5H8O4, another epoxy species from isoprene photo-oxidation, rapidly decreased as the SOA coating thickness increased, demonstrating a self-limiting effect. Despite ISOPOOH/IEPOX contributing around 80% to total reactive uptake, other oxidation products from isoprene photooxidation were estimated to contribute 20% of the total SOA formation. These findings highlight the importance for future models to consider the self-limiting effects of ISOPOOH/IEPOX and SOA formation through non-IEPOX pathways.
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- Award ID(s):
- 2304669
- PAR ID:
- 10594817
- Publisher / Repository:
- ACS
- Date Published:
- Journal Name:
- ACS ES&T Air
- Volume:
- 2
- Issue:
- 4
- ISSN:
- 2837-1402
- Page Range / eLocation ID:
- 665 to 676
- Subject(s) / Keyword(s):
- Isoprene Photooxidation Vocus NH4 + Chemical Ionization Mass Spectrometry Yields Reactive Uptake Secondary Organic Aerosol
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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