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.
more »
« less