Abstract 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. Gas-phase IEPOX was reacted with inorganic sulfate particles of varying pH (0.5 to 2.0) in an indoor smog chamber operated under dark, steady-state conditions to form freshly-generated IEPOX-SOA particles. These particles were then aged at a relative humidity of 60% in an oxidation flow reactor (OFR) for 0-15 days of equivalent atmospheric ·OH exposure. Aged IEPOX-SOA particles were sampled by an online aerosol chemical speciation monitor (ACSM) to measure real-time aerosol mass and chemical changes of the SOA particles, and were also collected onto Teflon filters and into PILS vials for molecular-level chemical analyses by hydrophilic liquid interaction chromatography method interfaced to electrospray ionization high-resolution quadrupole time-of-flight mass spectrometry (HILIC/ESI-HR-QTOFMS), ion chromatography, and total OS mass amounts.
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Kinetics and Products of Heterogeneous Hydroxyl Radical Oxidation of Isoprene Epoxydiol‐Derived SOA.
In isoprene‐rich regions, acid‐catalyzed multiphase
reactions of isoprene epoxydiols (IEPOX) with inorganic
sulfate (Sulfinorg) particles form secondary organic aerosol
(IEPOX‐SOA), extensively converting Sulfinorg to lowervolatility
particulate organosulfates (OSs), including 2‐
methyltetrol sulfates (2‐MTSs) and their dimers. Recently,
we showed that heterogeneous hydroxyl radical (OH)
oxidation of particulate 2‐MTSs generated multifunctional
OS products. However, atmospheric models assume that
OS‐rich IEPOX‐SOA particles remain unreactive towards
heterogeneous OH oxidation, and limited laboratory studies
have been conducted to examine the heterogeneous OH
oxidation kinetics of full IEPOX‐SOA mixtures. Hence, this
study investigated the kinetics and products resulting from
heterogeneous OH oxidation of freshly‐generated IEPOXSOA
in order to help derive model‐ready parameterizations.
First, gas‐phase IEPOX was reacted with acidic Sulfinorg
particles under dark conditions in order to form fresh
IEPOX‐SOA particles. These particles were then
subsequently aged at RH of 56% in an oxidation flow
reactor at OH exposures ranging from 0~15 days of
equivalent atmospheric exposure. Aged IEPOX‐SOA
particles were sampled by an online aerosol chemical
speciation monitor (ACSM) and collected onto Teflon filters
for off‐line molecular‐level chemical analyses by hydrophilic
liquid interaction chromatography method interfaced to
electrospray ionization high‐resolution quadrupole time‐offlight
mass spectrometry (HILIC/ESI‐HR‐QTOFMS). Our
results show that heterogeneous OH oxidation only caused
a 7% decay of IEPOX‐SOA by 10 days exposure, likely owing
to the inhibition of reactive uptake of OH as fresh IEPOXSOA
particles have an inorganic core‐organic shell
morphology. A significantly higher fraction of IEPOX‐SOA
(~37%) decayed by 15 days exposure, likely due to the
increasing reactive uptake of OH as IEPOX‐SOA become
more liquid‐like with aging. Freshly‐generated IEPOX‐SOA
constituents exhibited varying degrees of aging with 2‐MTSdimers
being the most reactive, followed by 2‐MTSs and 2‐
methyltetrols (2‐MTs), respectively. Notably, extensive
amounts of previously characterized particle‐phase
products in ambient fine aerosols were detected in our
laboratory‐aged IEPOX‐SOA samples.
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- Award ID(s):
- 2001027
- NSF-PAR ID:
- 10313543
- Date Published:
- Journal Name:
- 2021 AAAR 39th Annual Conference
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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