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Isoprene (C5H8) is the largest non-methane volatile organic compound emitted into the atmosphere. Isoprene reacts rapidly with ambient hydroxyl radicals (OH) and subsequent addition of O2 results in the formation alkyl peroxy (RO2) radicals. The fate of the initially formed RO2 radicals has been the focus of continuing theoretical and experimental research. Under pristine conditions where bimolecular reactions of RO2 are limited, the thermodynamically favored RO2 undergoes an intramolecular H-shift followed by reaction with O2 and elimination of HO2 to yield 4-hydroperoxy aldehyde (4-HPALD, C5H8O3), predicted to account for up to 13% of first-generation isoprene photochemical oxidation products. Mass spectrometric evidence has been reported for 4-HPALD, but lack of an authentic standard has precluded definitive confirmation of both the structure of 4-HPALD and its origin as a first-generation product of OH oxidation of isoprene. We report the synthesis and characterization of 4-HPALD and establish that it is a major product of isoprene oxidation. Synthetic 4-HPALD is isolated as the peroxyhemiacetal. As expected for the 4-hydroperoxy aldehyde, 1H NMR spectra show no evidence for equilibration with the carbonyl form, even in protic solvents, and gas-phase chemical analysis by CIMS also shows only a single form. OH oxidation of isoprene in an oxidation flow reactor coupled to an ion mobility source with an HR-CIMS detector unequivocally demonstrates 4-HPALD (and likely also 1-HPALD) as isoprene oxidation products. Although HPALDs have been discounted as significant contributors to SOA, oxidation of 4-HPALD in a potential aerosol mass (PAM) reactor in the presence of ozone and OH indicates 4-HPALD rapidly undergoes autooxidation reactions forming low-volatility particulate products. We have confirmed highly oxygenated compounds with compositions C5H8O6 and C5H10O6 likely from OH oxidation, and C5H10O7 and C5H10O8 compounds likely products of ozonolysis. The PAM oxidation experiment further demonstrates that the highly oxygenated, low-volatility products efficiently nucleate particles.
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This content will become publicly available on February 3, 2027
Hydroxyl Radical-Initiated Oxidation of Isoprene Leads to a Cyclic Peroxyhemiacetal via 1,6-Hydrogen Shift Chemistry That Yields Secondary Organic Aerosol
Isoprene, the largest nonmethane volatile hydrocarbon emitted into Earth’s atmosphere, is rapidly oxidized by ambient hydroxyl radical (•OH) to yield secondary organic aerosol (SOA). Early generation gas-phase products of isoprene oxidation remain to be fully characterized. •OH addition at terminal C1/C4 followed by the addition of O2, yields β- and δ-peroxy (ISOPO2 •) isomers. Under low-nitric oxide conditions, δ-ISOPO2 • isomers branch between bimolecular reaction with HO2 • and intramolecular H-shifts. Initial C5H8O3 products of H-shifts reported in chamber experiments were assigned δ-hydroperoxyalkenal (HPALD) structures and predicted to photolyze rapidly. We observe by ion mobility spectrometry that a putative δ-HPALD isomer from •OH addition at C4 is the cyclic peroxyhemiacetal, 5-methyl dioxinol, and may be the relevant product in atmospheric chemistry. 5-Methyl dioxinol is robust to photolysis at tropospheric UV−vis wavelengths and is thus potentially a significant previously unrecognized source of SOA. In chamber experiments, we found that 5-methyl dioxinol forms SOA and determined that the steady-state ratio of the proximal major unimolecular/bimolecular products downstream of ISOPO2 • is ∼0.26. Using the total steady-state dioxinol level formed in the isoprene oxidation experiment, dioxinols may contribute ∼3.2 × 109 g yr−1, or 4% to isoprene-derived atmospheric carbon.
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- Award ID(s):
- 2304669
- PAR ID:
- 10689750
- Publisher / Repository:
- ACS
- Date Published:
- Journal Name:
- Environmental Science & Technology
- Volume:
- 60
- Issue:
- 4
- ISSN:
- 0013-936X
- Page Range / eLocation ID:
- 3374 to 3383
- Subject(s) / Keyword(s):
- δ-HPALD, peroxyhemiacetal, secondary organic aerosol, branching ratio
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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