Acid-catalyzed multiphase chemistry of epoxydiols formed from isoprene
oxidation yields the most abundant organosulfates (i.e., methyltetrol
sulfates) detected in atmospheric fine aerosols in the boundary layer. This
potentially determines the physicochemical properties of fine aerosols in
isoprene-rich regions. However, chemical stability of these organosulfates
remains unclear. As a result, we investigate the heterogeneous oxidation of
aerosols consisting of potassium 3-methyltetrol sulfate ester
(C5H11SO7K) by gas-phase hydroxyl (OH) radicals at a relative
humidity (RH) of 70.8 %. Real-time molecular composition of the aerosols
is obtained by using a Direct Analysis in Real Time (DART) ionization source
coupled to a high-resolution mass spectrometer. Aerosol mass spectra reveal
that 3-methyltetrol sulfate ester can be detected as its anionic form
(C5H11SO7-) via direct ionization in the negative
ionization mode. Kinetic measurements reveal that the effective heterogeneous
OH rate constant is measured to be 4.74±0.2×10-13 cm3 molecule-1 s-1 with a chemical lifetime against OH
oxidation of 16.2±0.3 days, assuming an OH radical concentration of
1.5×106 molecules cm-3. Comparison of this lifetime with
those against other aerosol removal processes, such as dry and wet
deposition, suggests that 3-methyltetrol sulfate ester is likely to be
chemically stable over atmospheric timescales. Aerosol mass spectra only show
an increase in the intensity of bisulfate ion (HSO4-) after
oxidation, suggesting the importance of fragmentation processes. Overall,
potassium 3-methyltetrol sulfate ester likely decomposes to form volatile
fragmentation products and aqueous-phase sulfate radial anion
(SO4⚫-). SO4⚫- subsequently undergoes
intermolecular hydrogen abstraction to form HSO4-. These processes
appear to explain the compositional evolution of 3-methyltetrol sulfate ester
during heterogeneous OH oxidation.
Introduction
Isoprene (2-methyl-1,3-butadiene, C5H8), emitted from terrestrial
vegetation to the atmosphere, is the largest atmospheric source of
non-methane volatile organic compounds. Apart from enhancing urban ozone
levels via photochemical oxidation initiated by gas-phase hydroxyl (OH)
radicals (Chameides et al., 1988), isoprene-derived oxidation products can
also significantly contribute to the formation of secondary organic aerosol
(SOA) (Carlton et al., 2009). Gas-phase photochemical oxidation of isoprene
by OH radicals can produce isoprene-derived hydroxyhydroperoxides (ISOPOOH)
in yields greater than 70 % under low nitrogen oxide (NOX) conditions
(Paulot et al., 2009). Further reactions of ISOPOOH with OH radicals yield
large quantities of isomeric isoprene epoxydiols (IEPOX), which partition
into aqueous sulfate aerosols through acid-catalyzed ring-opening reactions.
This multiphase chemical pathway is key for the substantial production of
isoprene-derived SOA constituents (e.g., 2-methyltetrols, organosulfates,
3-methyltetrahydrofuran-3,4-diols and oligomers) within atmospheric fine
particulate matter (PM2.5) (Carlton et al., 2009; Froyd et al., 2010;
Surratt et al., 2010; Lin et al., 2012).
Among these SOA constituents, IEPOX-derived organosulfates (e.g.,
methyltetrol sulfates) have been widely detected in atmospheric aerosols and
are estimated to account for 0.3 %–1.7 % of the total organic carbon (Chan
et al., 2010; Froyd et al., 2010; Hatch et al., 2011; Lin et al., 2012;
Stone et al., 2012; He et al., 2014; Budisulistiorini et al., 2015;
Rattanavaraha et al., 2016; Meade et al., 2016; Hettiyadura et al., 2017).
While the formation mechanisms of organosulfates have been extensively
studied (Surratt et al., 2007, 2008; Minerath et al., 2009; Cole-Filipiak et
al., 2010; Nozière et al., 2010; Lin et al., 2012; Nguyen et al., 2014),
their chemical transformations and stability remain unclear. These
low-volatility organosulfates are preferentially present in particle phase
and can be oxidized by gas-phase oxidants (e.g., OH radicals, ozone and
nitrate radicals) at or near the aerosol surface throughout their
atmospheric lifetimes. The heterogeneous oxidative processes can change the
size, composition and physicochemical properties (e.g., light scattering and
absorption, water uptake and cloud condensation nuclei activity) of both
laboratory-generated and atmospheric organic aerosols (Rudich et al., 2007;
George and Abbatt, 2010; Kroll et al., 2015). However, the extent of
heterogeneous oxidation of organosulfates has not been clearly examined to
date. Therefore, a better understanding of particle-phase transformations of
isoprene-derived organosulfates can provide more insights on their potential
impacts on human health, air quality and climate.
Chemical structure, properties, effective heterogeneous OH rate
constant and atmospheric lifetime against the OH radical of potassium
3-methyltetrol sulfate ester.
* Using a 24 h average OH concentration of 1.5×106 molecules cm-3.
In this work, we investigate the heterogeneous OH oxidation of potassium
3-methyltetrol sulfate ester (C5H11SO7K, Table 1) as a
single-component aerosol system by using an aerosol flow tube reactor at
70.8 % RH in order to gain a more fundamental understanding of the
kinetics and chemistry. The molecular composition of the aerosols before and
after oxidation is characterized in real time using a soft atmospheric
pressure ionization source (Direct Analysis in Real Time, DART) coupled to a
high-resolution mass spectrometer. The 3-methyltetrol sulfate ester
investigated in this study is one of the isomers of the methyltetrol
sulfates found in atmospheric aerosols, which are collectively the most
abundant particulate organosulfates (Budisulistiorini et al., 2015). On the
basis of aerosol mass spectra and previously reported reaction pathways,
oxidative kinetics and reaction products resulting from the heterogeneous OH
oxidation of 3-methyltetrol sulfate ester are discussed. We acknowledge that
although 3-methyltetrol sulfate ester derived from the reactive uptake of
gas-phase δ-IEPOX onto sulfate seed aerosols is not the sole
contributor to IEPOX-derived organosulfates (Cui et al., 2019), the findings
of this work provide a basis for understanding the heterogeneous OH
reactivity of other IEPOX-derived organosulfates (e.g., 2-methyltetrol
sulfate esters) that predominate in atmospheric aerosols better.
Experimental methods
The heterogeneous OH oxidation experiments were carried out in an aerosol
flow tube reactor at 70.8 % RH. The synthesis of potassium 3-methyltetrol
sulfate ester has been described in the literature (Bondy et al., 2018). The
experimental details of the oxidation experiment have been explained
elsewhere (Chim et al., 2017a, b). Briefly, 3-methyltetrol sulfate
ester aerosols were generated by an atomizer (TSI, model 3076) and mixed
with nitrogen, oxygen, ozone and hexane before entering the reactor. Inside
the reactor, the aerosols were oxidized heterogeneously by gas-phase OH
radicals, which were generated by the photolysis of ozone under ultraviolet
light at 254 nm in the presence of water vapor. The RH within the reactor
was controlled by varying the dry / wet gas ratio. A water jacket was used to
maintain a stable temperature of 20 ∘C inside the reactor. The OH
concentration was controlled by varying the ozone concentration. By
measuring the decay of hexane using a gas chromatograph coupled with a flame
ionization detector, the OH exposure, which is an integral of gas-phase OH
radical concentration and reaction time, can be calculated (Smith et al.,
2009; Davies and Wilson, 2015):
OH exposure=∫otOHdt=lnHexHexokHex,
where [Hex]o and [Hex] are the hexane concentration before and after OH
oxidation, respectively, t is the reaction time (or aerosol residence time),
which was measured to be 1.3 min, kHex is the rate constant for
gas-phase OH reaction with hexane and [OH] is the time-averaged OH radical
concentration. The OH exposure varied from 0 to ∼1.4×1012 molecules cm-3 s. An annular Carulite catalyst
denuder and an activated charcoal denuder were used to remove ozone and
gas-phase species from the aerosol stream leaving the reactor, respectively.
As a result, only particle-phase products are detected. A fraction of the
aerosol stream was sampled by a scanning mobility particle sizer (SMPS, TSI,
CPC Model 3775, Classifier Model 3081) to measure the aerosol size and
number distribution. The surface-weighted diameter of the aerosols was
measured to be 225.9±1.4 nm before oxidation. The remaining flow was
then directed into a stainless steel tube heater at 380–400 ∘C,
where the temperature and aerosol residence time in the heater were
sufficient to completely vaporize the aerosols. The gas-phase species were
then directed into the ionization region, an open narrow space between a
DART ionization source (IonSense: DART SVP) and an atmospheric inlet of a
high-resolution mass spectrometer (ThermoFisher, Q Exactive Orbitrap) for
real-time chemical characterization (Chan et al., 2013). The DART ionization
source was operated in a negative ion mode, with helium as the ionizing gas
(Cody et al., 2005). Metastable helium atoms generated were responsible for
ionizing the gas-phase species in the ionization region. 3-methyltetrol
sulfate ester can be ionized via direct ionization (Block et al., 2010;
Hajslova et al., 2011). Most recently, Kwong et al. (2018) have detected the
ionic form of two organosulfates (sodium salts of methyl sulfate
(CH3SO4Na) and ethyl sulfate (C2H5SO4Na)) using the
DART ionization source in negative ionization mode. Mass spectra were
scanned over a range of m/z 70–700. Each mass spectrum was averaged over a
2–3 min sampling time, with a mass resolution of about 140 000. The mass
spectra were analyzed using Xcalibar software (Xcalibar Software, Inc.,
Herndon, VA, USA).
Particle phase state (e.g., solid or aqueous droplet) is known to play an
important role in governing the heterogeneous kinetics and chemistry of
organic aerosols (McNeil et al., 2008; Renbaum and Smith, 2009; Chan et al.,
2014; Slade and Knopf, 2014; Zhang et al., 2018). The hygroscopicity of
potassium 3-methyltetrol sulfate ester aerosols has not been experimentally
determined. Recently, Estillore et al. (2016) have measured the
hygroscopicity of a diverse set of organosulfates, including potassium salts
of glycolic acid sulfate, hydroxyacetone sulfate, 4-hydroxy-2,3-epoxybutane
sulfate and 2-butenediol sulfate as well as sodium salts of methyl sulfate,
ethyl sulfate, propyl sulfate and benzyl sulfate. According to Estillore et
al. (2016), these organosulfate aerosols did not show a distinct phase
transition but absorbed or desorbed water reversibly when the RH increased
or decreased, suggesting that these organosulfate aerosols were likely to be
aqueous when RH was above 10 %. Based on the literature results, we assume
that potassium 3-methyltetrol sulfate ester exhibits hygroscopicity similar
to the potassium salts of organosulfates reported by Estillore et al. (2016)
(e.g., potassium 4-hydroxy-2,3-epoxybutane sulfate, C4H7SO6K)
and remains aqueous prior to oxidation.
Volatilization of 3-methyltetrol sulfate ester and the impact of ozone and
UV light on the aerosol composition were investigated in the absence of OH.
The intensity of parent ions with aerosols removed from the gas stream was less
than 5 % of that measured in the presence of 3-methyltetrol sulfate ester
aerosols, suggesting that the volatilization of 3-methyltetrol sulfate ester
is insignificant. No reaction product was observed in the presence of ozone
without UV light or in the absence of ozone with the UV light, suggesting
that 3-methyltetrol sulfate ester is not likely to be photolyzed or react
with ozone under our experimental conditions.
Results and discussionsAerosol mass spectra
Figure 1 shows the data measured before and after oxidation at 70.8 % RH.
Before oxidation (Fig. 1a), a dominant ion peak at m/z 215 is observed, which
corresponds to the ionic form of potassium 3-methyltetrol sulfate ester
(C5H11SO7-) (Table 1). After oxidation (Fig. 1b), the
parent ion remains the most dominant ion peak at the maximum OH exposure of
∼1.4×1012 molecules cm-3 s. There is no
significant change in the ion intensity except for bisulfate ion
(HSO4-; m/z 97). Figure 2 shows the evolution of HSO4-
against OH exposure at 70.8 %, which indicates that the intensity of
HSO4- increases with the OH exposure. In the following sections,
the kinetics and chemistry will be discussed based on the aerosol mass
spectra and aerosol-phase reactions previously proposed in the literature.
Aerosol mass spectra of potassium 3-methyltetrol sulfate ester
before (a) and after (b) OH oxidation at 70.8 % RH.
The evolution of the ion intensity of bisulfate ion
(HSO4-) as a function of OH exposure during the heterogeneous OH
oxidation of potassium 3-methyltetrol sulfate ester at 70.8 % RH.
Oxidation kinetics
Oxidation kinetics can be quantified by analyzing the parent decay of
3-methyltetrol sulfate ester against the OH exposure. Figure 3 shows the
normalized decay of 3-methyltetrol sulfate ester against OH exposure. At the
maximum OH exposure (∼1.4×1012 molecules cm-3 s), ∼45 % of 3-methyltetrol sulfate ester is
oxidized. The decay of the 3-methyltetrol sulfate ester can be fitted with
an exponential function to obtain an effective second-order heterogeneous OH
rate constant (k) through Eq. (2) (Smith et al., 2009):
lnIIo=-k⋅OHt,
where I is the ion signal at a given OH exposure, Io is the ion signal
before oxidation, [OH] is the concentration of gas-phase OH radicals and
t is the reaction time. The k is determined to be 4.74±0.2×10-13 cm3 molecule-1 s-1 (Table 1). Based on the fitted
k value, the chemical lifetime of 3-methyltetrol sulfate ester against
heterogeneous OH oxidation (τ) can be estimated by Eq. (3):
τ=1k[OH],
where [OH] is the 24 h averaged OH radical concentration of 1.5×106 molecules cm-3. The chemical lifetime against
oxidation is calculated to be 16.2±0.3 days. The estimated
timescales are longer than those of other important aerosol removal
processes, such as dry and wet deposition (∼7–10 days)
(Seinfeld and Pandis, 2016). In addition to heterogeneous oxidation,
organosulfates can undergo hydrolysis to form polyols and sulfuric acid, with
rates depending on their molecular structure and aerosol acidity (Darer et
al., 2011; Hu et al., 2011). According to Darer et al. (2011), primary
isoprene-derived organosulfates are stable against hydrolysis, even at low
pH, while secondary and tertiary organosulfates are less thermodynamically
stable than primary organosulfates. Since 3-methyltetrol sulfate ester is a
primary organosulfate (Table 1), it is unlikely to hydrolyze. With reference
to the literature results and our new experimental observations,
3-methyltetrol sulfate ester may possibly be considered chemically stable
against heterogeneous OH oxidation and hydrolysis over atmospheric
timescales.
The normalized parent decay as a function of OH exposure during
the heterogeneous OH oxidation of potassium 3-methyltetrol sulfate ester at
70.8 % RH.
Proposed reaction mechanisms
Based on the aerosol mass spectra and well-known aerosol-phase reactions
previously reported in the literature (George and Abbatt, 2010; Kroll et
al., 2015), we tentatively propose reaction mechanisms for the heterogeneous
OH oxidation of 3-methyltetrol sulfate ester. The reaction schemes proposed
can be found in the Supplement (Schemes S1–S5). Briefly,
potassium methyltetrol sulfate ester likely dissociates and exists in its
ionic form in the droplets. In the first oxidation step, the OH radical
abstracts a hydrogen atom to form an alkyl radical, which quickly reacts with
oxygen to form a peroxy radical. We propose that the formation of alkoxy
radical may be favored over the Russell mechanism (Russell, 1957) and
Bennett–Summers reactions (Bennett and Summers, 1974) as functionalization
products were not detected. Alkoxy radicals, once formed, may tend to
undergo fragmentation due to the presence of vicinal hydroxyl groups, which
lower the activation energy required for the decomposition of the alkoxy
radicals (Cheng et al., 2015; Wiegel et al., 2015; Jimenez et al., 2009;
Peeters et al., 2004; Vereecken and Peeters, 2009).
Sulfate radical anion (SO4⚫-) can be formed
through the decomposition of the alkoxy radical and is a strong oxidant in
aqueous phase (Neta et al., 1988; Clifton and Huie, 1989; Padmaja et al.,
1993). SO4⚫- can abstract a hydrogen atom from a
neighboring organic molecule (e.g., unreacted 3-methyltetrol sulfate ester)
to form HSO4- (Reaction R1) or react with particle-phase water to yield a
HSO4- and an OH radical (Reaction R2) (Tang et al., 1988) as illustrated
below. It is noted that SO4⚫- or OH radical recycled from
Reaction (R2) can react with 3-methyltetrol sulfate ester, contributing to the
secondary chain reactions.
C5H11O7S-+SO4⚫-→C5H10O7S⚫-+HSO4-SO4⚫-+H2O⇌OH⚫+HSO4-
Since 3-methyltetrol sulfate ester is unlikely to hydrolyze (Darer et al.,
2011), the formation of the HSO4- upon OH oxidation could be best
explained by the formation and subsequent reactions of SO4⚫-.
Based on the proposed reaction mechanisms, the decomposition of alkoxy
radicals can lead to formation fragmentation products (without sulfate
group) and smaller organosulfates. We acknowledge that the ionization
efficiency and detection limit of the reaction products are not fully
understood. The absence of the potential products might attribute to the
DART ionization and detection issues. More work is needed to investigate the
formation and abundance of the reaction products formed upon oxidation in
order to better understand the reaction pathways. It is also noted that the
formation of second- or higher-generation products is possible due to the
high OH concentrations used in this study. As the potential second- or
higher-generation products have not been detected, possibly due to their low
concentrations and/or ionization issues, for clarity, the formation and
oxidation of these higher-generation products are not discussed further.
Conclusions and atmospheric implications
This work investigates the kinetics of oxidation and molecular
transformations of potassium 3-methyltetrol sulfate ester resulting from
heterogeneous OH oxidation. Kinetic measurements reveal that the chemical
lifetime of 3-methyltetrol sulfate ester against heterogeneous OH oxidation
and hydrolysis is longer than those against other aerosol removal
processes, such as dry and wet deposition. 3-methyltetrol sulfate ester is
potentially chemically stable over its atmospheric lifetime. In the
atmosphere where particles may contain compounds with different
surface-active properties, the chemical lifetime could be affected due to
the inhomogeneity in surface concentration. If surface-active compounds are
present, the chemical lifetime will likely be longer due to a lower surface
concentration of the parent molecules. This reduces the collision
probability between gas-phase OH radicals and parent molecules at the
particle surface, leading to a smaller overall oxidation rate.
Aerosol mass spectra reveal that only the intensity of HSO4- increases after oxidation, suggesting the dominance of fragmentation
processes over functionalization processes. During oxidation, alkoxy radicals
are likely to be formed following hydrogen abstraction of 3-methyltetrol
sulfate ester by OH radicals. The alkoxy radicals subsequently fragment into
volatile products and SO4⚫-. SO4⚫- undergoes intermolecular hydrogen abstraction to form HSO4- in
the aerosol phase, while the volatile fragmentation products may tend to
partition into the gas phase. It is also noted that volatile fragmentation
products likely contain polar functional groups. They may partition back to
aerosols, for instance, aqueous droplets because of their high water
solubilities or Henry's law constants. Additionally, they could be
reactively uptaken by aerosols which contain reactive nitrogen or oxygen
species through reactions. Smaller organosulfates have not been observed,
possibly due to the rapid continuous decomposition of the reaction
intermediates proposed in Schemes S1–S5. The absence of smaller
organosulfates suggests that the oxidation of 3-methyltetrol sulfate ester
is not a source of smaller organosulfates detected in atmospheric aerosols.
Further investigations into whether large organosulfates yield smaller
organosulfates upon heterogeneous OH oxidation are desirable. Aerosol mass
spectra have revealed that the OH oxidation of 3-methyltetrol sulfate ester
can lead to the formation of inorganic sulfate (e.g., HSO4-), in
accord with our report that the heterogeneous OH oxidation of sodium methyl
sulfate and sodium ethyl sulfate can lead to the formation of
HSO4- (Kwong et al., 2018). Given the high atmospheric abundance
of organosulfates in atmospheric aerosols, further study of the contribution
and transformation of organosulfates to inorganic sulfate through chemical
reactions (e.g., heterogeneous oxidation, aqueous-phase oxidation and
hydrolysis) is desirable. Methyltetrol sulfates are the most abundant
isoprene-derived organosulfates measured in atmospheric PM2.5 samples
collected from isoprene-rich regions influenced by anthropogenic emissions
(Budisulistiorini et al., 2015; Hettiyadura et al., 2017). Additional studies
are required to better understand the role of the molecular structure (i.e.,
position of the methyl and sulfate group) in the kinetics and chemistry of
methyltetrol sulfates and other organosulfates upon heterogeneous OH
oxidation, in particular the effect on the formation of smaller
organosulfates, volatile fragmentation products and inorganic sulfate, since
the 2-methyltetrol sulfate ester and its isomers, rather than the
3-methyltetrol sulfate ester investigated in this study, predominate in
atmospheric aerosols (Cui et al., 2018). Future investigations on the
transformation of other organosulfates, for instance, glycolic acid sulfate,
which is the most abundant organosulfate in the overall atmosphere, are
also desirable (Liao et al., 2015).
Data availability
The underlying research data are available upon request from the corresponding author (mnchan@cuhk.edu.hk).
The supplement related to this article is available online at: https://doi.org/10.5194/acp-19-2433-2019-supplement.
Author contributions
HKL and MNC designed and ran the experiments.
HKL, MNC and KCK prepared and wrote the manuscript. All
authors provided comments and suggestions for the manuscript.
Competing interests
The authors declare that they have no conflict of
interest.
Acknowledgements
Hoi Ki Lam, Kai Chung Kwong, Hon Yin Poon and Man Nin Chan are supported by the CUHK
direct grant (4053281) and Hong Kong Research Grants Council (HKRGC) Project
ID: 2191111 (Ref 24300516) and 2130626 (Ref 14300118). Synthesis of
potassium methyltetrol sulfate was supported by the National Science
Foundation (NSF) under Atmospheric and Geospace (AGS) grant 1703535. We
would like to thank Kevin Wilson for his insightful comments on the reaction
mechanisms proposed for the OH reaction with 3-methyltetrol sulfate.
Edited by: Frank Keutsch
Reviewed by: three anonymous referees
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