We recently demonstrated that the heterogeneous hydroxyl radical (OH) oxidation is an important aging process for isoprene epoxydiol-derived secondary organic aerosol (IEPOX-SOA) that alters its chemical composition, and thus, aerosol physicochemical properties. Notably, dimeric species in IEPOX-SOA were found to heterogeneously react with OH at a much faster rate than monomers, suggesting that the initial oligomeric content of freshly-generated IEPOX-SOA particles may affect its subsequent atmospheric oxidation. Aerosol acidity could in principle influence this aging process by enhancing the formation of sulfated and non-sulfated oligomers in freshly-generated IEPOX-SOA. Many multifunctional organosulfate (OS) products derived from heterogeneous OH oxidation of sulfur-containing IEPOX-SOA have been observed in cloud water residues and ice nucleating particles and could affect the ability of aged IEPOX-SOA particles to act as cloud condensation nuclei. Hence, this study systematically investigated the effect of aerosol acidity on the kinetics and products resulting from heterogeneous OH oxidation of IEPOX-SOA particles. We reacted gas-phase IEPOX with inorganic sulfate particles of varying pH (0.5 to 2.5) in an indoor smog chamber operated under dark, steady-state conditions to form freshly-generated IEPOX-SOA particles. These particles were aged at a relative humidity of 65% in an oxidation flow reactor (OFR) for 0-21 days of equivalent atmospheric OH exposure. Through molecular-level chemical analyses by hydrophilic interaction liquid chromatography method interfaced to electrospray ionization high-resolution quadrupole time-of-flight mass spectrometry (HILIC/ESI-HR-QTOFMS), we observed that highly acidic aerosol has higher oligomer ratio and exhibit much slower mass decay with OH oxidation (pH=0.5, lifetime = 56 days) as compared to less acidic aerosols (pH=2.5, lifetime=17 days). Based on atomic force microscopy (AFM) analysis, aerosol acidity could also affect the morphology and viscosity of IEPOX-SOA during OH oxidation process.
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Stability of Monoterpene-Derived α-Hydroxyalkyl-Hydroperoxides in Aqueous Organic Media: Relevance to the Fate of Hydroperoxides in Aerosol Particle Phases
The alpha-hydroxyalkyl-hydroperoxides [R-(H)C(-OH)(-OOH), alpha-HH] produced in the ozonolysis of unsaturated organic compounds may contribute to secondary organic aerosol (SOA) aging. alpha-HHs' inherent instability, however, hampers their detection and a positive assessment of their actual role. Here we report, for the first time, the rates and products of the decomposition of the alpha-HHs generated in the ozonolysis of atmospherically important monoterpenes alpha-pinene (alpha-P), d-limonene (d-L), gamma-terpinene (gamma-Tn), and alpha-terpineol (alpha-Tp) in water/acetonitrile (W/AN) mixtures. We detect alpha-HHs and multifunctional decomposition products as chloride adducts by online electrospray ionization mass spectrometry. Experiments involving D2O and (H2O)-O-18, instead of (H2O)-O-16, and an OH-radical scavenger show that alpha-HHs decompose into gem-diols + H2O2 rather than free radicals. alpha-HHs decay mono- or biexponentially depending on molecular structure and solvent composition. e-Fold times, tau(1)(/e), in water-rich solvent mixtures range from tau(1)(/e), = 15-45 min for monoterpene-derived alpha-HHs to tau(1)(/e) > 10(3) min for the alpha-Tp-derived alpha-HH. All tau(1)(/e)'s dramatically increase in <20% (v/v) water. Decay rates of the alpha-Tp-derived alpha-HH in pure water increase at lower pH (2.3 <= pH <= 3.3). The hydroperoxides detected in day-old SOA samples may reflect their increased stability in water-poor media and/or the slow decomposition of alpha-HHs from functionalized terpenes.
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- Award ID(s):
- 1744353
- NSF-PAR ID:
- 10208096
- Date Published:
- Journal Name:
- Environmental science and technology
- Volume:
- 54
- Issue:
- 7
- ISSN:
- 0194-0287
- Page Range / eLocation ID:
- 3890–3899
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1021/acs.est.9b07497
