Sulfur dioxide (SO2) can affect aerosol formation in
the atmosphere, but the underlying mechanisms remain unclear. Here, we
investigate aerosol formation and composition from the ozonolysis of
cyclooctene with and without SO2 addition in a smog chamber. Liquid
chromatography equipped with high-resolution tandem mass spectrometry
measurements indicates that monomer carboxylic acids and corresponding dimers
with acid anhydride and aldol structures are important components in
particles formed in the absence of SO2. A 9.4–12.6-times increase in
particle maximum number concentration is observed in the presence of 14–192 ppb SO2. This increase is largely attributed to sulfuric acid
(H2SO4) formation from the reactions of stabilized Criegee
intermediates with SO2. In addition, a number of organosulfates (OSs)
are detected in the presence of SO2, which are likely products formed
from the heterogeneous reactions of oxygenated species with H2SO4.
The molecular structures of OSs are also identified based on tandem mass
spectrometry analysis. It should be noted that some of these OSs have been
found in previous field studies but were classified as compounds from
unknown sources or of unknown structures. The observed OSs are less volatile
than their precursors and are therefore more effective contributors to
particle formation and growth, partially leading to the increase in particle
volume concentration under SO2-presence conditions. Our results provide
an in-depth molecular-level insight into how SO2 alters particle
formation and composition.
Introduction
Secondary organic aerosol (SOA) accounts for a large fraction of the organic
aerosol mass. The atmospheric oxidation of anthropogenic volatile organic
compounds (AVOCs) can produce low-volatility organic products that
contribute to SOA formation and growth (Kelly et al., 2018; Fan et al.,
2020). The oxidation of AVOCs can dominate SOA formation under severe haze
episodes (Nie et al., 2022; He et al., 2020; Huang et al., 2019; Qiu et
al., 2020). Thus, AVOCs have been commonly considered as significant SOA
precursors. SOA can negatively impact air quality, global climate, and
public health (Nault et al., 2021; Zhu et al., 2017). To better
understand air pollution and develop effective particle control strategies,
it is necessary to investigate the formation mechanism and molecular
composition of anthropogenic SOA.
Recently, the impacts of inorganic gases on aerosol chemistry have received
significant attention (Deng et al., 2022). In particular,
there is increasing evidence that sulfur dioxide (SO2) can
modulate SOA formation and composition (Ye et al., 2018; Stangl et al.,
2019; Liu et al., 2017). Liu et al. (2017) reported that SOA
formation from cyclohexene photooxidation was inhibited by atmospherically
relevant concentrations of SO2, as a result of the reaction of hydroxyl
radical (⚫OH) with SO2 (to form sulfuric acid
(H2SO4)) competing with the ⚫OH reaction with
cyclohexene. They demonstrated that H2SO4-catalyzed SOA
enhancement was not sufficient to compensate for the loss of
⚫OH reactivity towards cyclohexene, leading to the
suppression in cyclohexene SOA formation. On the other hand, SO2 can
enhance SOA formation and alter SOA composition by interacting with organic
peroxides or stabilized Criegee intermediate (sCI) during the ozonolysis of
alkenes (Stangl et al., 2019; Ye et al., 2018). For instance, under humid
conditions, the reactive uptake of SO2 into organic aerosols was obvious
and reactions of SO2 with organic peroxides could contribute to
organosulfate (OS) formation (S. Wang et al., 2021; Ye et al., 2018).
H2SO4 originating from sCI-induced oxidation of SO2 is also
linked to OS production (Stangl et al., 2019). OSs have been
detected in different SO2-alkene interaction areas (Hettiyadura et
al., 2019; Wang et al., 2018; Bruggemann et al., 2020). Ubiquitous OSs may
be used as tracers of SOA influenced by SO2 emissions
(Bruggemann et al., 2020). To gain further
mechanistic insights into the complex roles of SO2 in SOA
formation, it is important to explore the chemical nature and formation
mechanism of OSs.
Cycloalkenes emitted from diesel vehicles and industrial processes are a
crucial class of AVOCs in the atmosphere. They can be used to explore key
chemical processes involved in atmospheric oxidation and SOA formation
(Räty et al., 2021). However, SOA formation chemistry from
cycloalkenes has received less attention than that from linear or branched
alkenes, leading to significant uncertainties in our understanding of SOA.
Recent studies have reported that ozonolysis of cycloalkenes could form
highly oxidized products and have considerable SOA yield (Räty et
al., 2021; Rissanen, 2018). Among the most common cycloalkenes (with 5 to 8
carbon atoms), cyclooctene has the largest potential for SOA formation
(Keywood et al., 2004). Ozonolysis is the dominant oxidation
pathway of cyclooctene, with a reaction rate constant of 4.51 × 10-16 cm3 molec.-1 s-1 (298 K). Urban atmosphere is
highly complex and may contain various concentrations of cycloalkenes and
SO2, which complicates SOA formation and composition. While most
previous studies have identified compounds containing carbon, hydrogen, and
oxygen atoms (CHO compounds) as important contributors to cycloalkene SOA
(Hamilton et al., 2006; Gao et al., 2004; Räty et al., 2021), the
potential of OS formation from the ozonolysis of cyclooctene in the presence
of SO2 and the chemical processes behind OS formation remain unclear.
Given the significance of cycloalkene and SO2 emissions in aerosol
formation, we investigated the effects of SO2 on the formation and
chemical composition of cyclooctene SOA. Aerosol particles were formed from
the ozonolysis of cyclooctene in the absence and presence of SO2 in a
smog chamber. Structural identifications of the observed products were
reported and corresponding formation mechanisms were proposed. We report the
mechanism, showing how SO2 impacts particle formation and growth based
on the observation of sulfuring-containing compounds. Our results provide a
more comprehensive mechanistic understanding of the roles of SO2 in
modulating SOA formation and composition.
Experimental methodsParticle production
Particle formation from the ozonolysis of cyclooctene (k298K=4.51× 10-16 cm3 molec.-1 s-1) was carried out
under dark conditions in a 1.2 m3 Teflon chamber housed in a
temperature-controlled room. A summary of experimental conditions and
results is listed in Table 1. Detailed experimental equipment and methods
have been described in our previous studies (Yang et al., 2022, 2021). Particle formation experiments were operated in a batch mode.
Briefly, cyclooctene was introduced into the chamber by passing zero air
through a tube containing a known volume of cyclooctene (95 %, Alfa).
Then, cyclohexane (99.5 %, Aladdin) was injected in excess
(∼ 130 ppm) into the chamber so that more than 98 % of ⚫OH
generated during the ozonolysis of cyclooctene was scavenged. Control
experiments showed that the presence of cyclohexane could lead to a
significant decrease in particle volume concentration (Fig. S1 in the Supplement). When
desired, SO2 was added to the chamber from an SO2 calibration
cylinder. Initial concentration ratios of SO2 to cyclooctene were in
the range of ∼ 0.07–1 ppb ppb-1 to simulate different
polluted atmospheric conditions. The reactor was stabilized for 20 min under
dark conditions to allow for mixing of species. Finally, ozonolysis of
cyclooctene was initiated by introducing ozone (O3) produced via a commercial
ozone generator (WH-H-Y5Y, Wohuan, China). All experiments were performed at
room temperature (∼ 295 K) and atmospheric pressure
(∼ 1 atm) without seed particles. Temperature and relative
humidity (RH) inside the chamber were measured with a hygrometer (Model 645,
Testo AG, Germany). O3 and SO2 concentrations over the course of
ozonolysis were monitored by a Thermo Scientific model 49i O3 analyzer
and a Thermo Scientific model 43i-TLE SO2 analyzer, respectively. The
detection limits of O3 analyzer and SO2 analyzer were 0.5 and
0.05 ppb, respectively. Size distributions and volume concentrations of
particles were continuously recorded using a scanning mobility particle
sizer (SMPS), which consisted of a differential mobility analyzer (Model 3082,
TSI, USA) and an ultrafine condensation particle counter (Model 3776, TSI,
USA). The particle volume concentration was measured continuously until we
observed a decrease. The particle formation experiments proceeded for 300
min before the collection of aerosol particles.
Experimental conditions and results for particle-formation
experiments.
aΔSO2 represents the consumed SO2 concentration during the
ozonolysis of cyclooctene.
b The volume concentration of particle-phase H2SO4 assuming a
full conversion of SO2 to H2SO4 with a density of 1.58 g cm-3 under moderate humidity conditions (Wyche et al., 2009; Ye et
al., 2018).
cNmax denotes the maximum number concentration of aerosol
particles during the ozonolysis of cyclooctene.
dVparticle is the volume concentration of aerosol particles, which
has been corrected for wall loss of particles. Errors represent standard
deviation for particle-formation experiments.
Particle collection and chemical characterization
Aerosol particles were collected on aluminum foils using a 14-stage
low-pressure impactor (DLPI+, Dekati Ltd, Finland). All samples were
stored in a -20∘C freezer until analysis. Offline functional group
measurements of aerosol particles were performed using an attenuated total
reflectance-Fourier transform infrared spectrometer (ATR-FTIR, Vertex 70,
Bruker, Germany). Before each measurement, the diamond crystal was
thoroughly cleaned with ethanol and ultrapure water to eliminate the
interference of ambient contaminants on functional group measurements of
aerosol particles. ATR-FTIR spectra of blank aluminum foils and aerosol
samples were recorded in the range of 4000–600 cm-1 at a resolution of
4 cm-1 with 64 scans. The data of ATR-FTIR spectra were recorded with
the OPUS software.
Aerosol particles were also collected on polytetrafluoroethylene (PTFE)
filters (0.22 µm pore size, 47 mm diameter, TJMF50, Jinteng, China). The
whole sample filters were extracted twice into 5 mL of methanol
(Optima® LC-MS grade, Fisher Scientific) by ice
sonication (KQ5200E, Kunshan Ultrasonic Instruments, China) for 20 min.
Extracts were then filtered, concentrated to near dryness, and subsequently
reconstituted in 200 µL of 50:50 (v/v) methanol and ultrapure water.
Blank filters were also subjected to the same extraction and preparation
procedure. Obtained extracts of blank and sample filters were analyzed using
a Thermo Scientific ultrahigh-performance liquid chromatograph, which was
coupled with a high-resolution Q Exactive Focus Hybrid Quadrupole-Orbitrap
mass spectrometer equipped with an electrospray ionization (ESI) source
(UHPLC/ESI-HRMS). Samples were first separated on an Atlantis T3 C18 column
(100 Å pores, 3 µm particle size, 2.1 mm × 150 mm, Waters,
USA) at 35 ∘C. The used binary mobile-phase system consisted of
ultrapure water with 0.1 % (v/v) formic acid (A) and methanol with 0.1 %
(v/v) formic acid (B). The LC gradient employed was as follows: 0–3 min at
3 % B, 3–25 min increased linearly to 50 % B, 25–43 min ramped
linearly to 90 % B, 43–48 min returned to 3 % B, and 48–60 min B
held constant at 3 % to re-equilibrate the column. The injected volume of
samples and flow rate were 2 µL and 200 µL min-1,
respectively. The ESI source was operated in both positive (+) and
negative (-) ion modes to ionize analyte components with a scan range of
mass-to-charge (m/z) of 50 to 750. LC/ESI-MS parameter settings were as follows:
3.5 kV spray voltage (+), -3.0 kV spray voltage (-), 50 V S-lens radiofrequency (RF) level (+), 50 V S-lens RF level (-), 320 ∘C
capillary temperature, 2.76 × 105 Pa sheath gas (nitrogen)
pressure, and 3.33 L min-1 auxiliary gas (nitrogen) flow.
Data-dependent tandem mass spectrometry (MS/MS) analysis was also carried
out by high-energy collision-induced dissociation (CID) with stepped
collision energies of 20, 40, and 60 eV. For MS/MS experiments, an isolation
width of 2 m/z units was applied. Other parameters were also selected in MS/MS
experiments as follows: 2 × 105 automatic gain control (AGC)
target, 50 ms maximum IT, 3 loop count, 1 × 105 minimum AGC
target, 2–6 s apex trigger, and 6 s dynamic exclusion. The mass resolution
of MS and MS/MS were 70 000 (full width at half maximum, FWHM, at m/z 200) and
17 500, respectively. Detailed data processes are reported elsewhere (Yang
et al., 2021, 2022).
The double bond equivalent (DBE) value is the combined number of rings and
double bonds in the product CcHhOoNnSs (Here, subscripts c, h, o, n, and s represent the number of carbon, hydrogen, oxygen, nitrogen, and sulfur atom in the product CcHhOoNnSs.) and could be
calculated according to Eq. (1). For organosulfate, the two S = O bonds in the
sulfate group were not considered based on calculations in previous studies
(Wang et al., 2016; Riva et al., 2016b; Kuang et al., 2016). The DBE
value of organosulfate reflects the unsaturation degree of its side carbon
chain.
DBE=1+c+n-h2.
Kendrick mass defect (KMD) analysis could provide chemical insights into the
chemical compositions of complex organic mixtures (Kundu et al., 2017;
Kenseth et al., 2020). The KMD value is the same for homologous species that
differ from each other only by their base units. CH2 and the oxygen
atom (O) are usually chosen as base units for Kendrick analysis of complex
organic mass spectra. Kendrick mass (KM) could be converted into a new mass
scale from the IUPAC mass (Eqs. 2 and 4). KMD is determined as the difference
between the nominal mass of a compound (the rounded integer mass) and KM
(Eqs. 3 and 5).
2KMCH2=m/z×14.0000014.015653KMDCH2=Nominal mass-KMCH24KMO=m/z×16.0000015.994925KMDO=Nominal mass-KMO
The saturation mass concentration (Co, µg m-3) of product i
was also calculated based on its elemental composition using the following
expression (Li et al., 2016):
log10Cio=(nC0-nCi)bC-nOibO-2nCinOinCi+nOibCO-nNibN-nSibS,
where nC0 is the reference carbon number;
nCi,nOi,nNi and
nSi, represent the numbers of carbon, oxygen, nitrogen,
and sulfur atoms, respectively; bC,bO,
bN, and bS denote the contribution of
each carbon, oxygen, nitrogen, and sulfur atom to
log10Cio; and
bCO is the carbon–oxygen nonideality.
Wall loss corrections
The wall loss rates of O3 and SO2 inside the chamber were
determined to be 2.05 × 10-4 and 2.02 × 10-4 min-1 (Fig. S2), respectively, indicating that the losses of
these two gas-phase species to the chamber walls were negligible over the
course of experiments. The wall loss of cyclooctene (5.23 × 10-6 min-1) was also negligible, while its oxidation products may
deposit to the inner walls. However, wall losses of gas-phase products could
be mitigated due to excess O3 concentration (Sect. S1 in the Supplement). The quick
oxidation and nucleation could provide attractive condensation surfaces for
oxidation products, thereby reducing the product wall losses to some extent
(Stirnweis et al., 2017). Although wall losses of organic
vapors may underestimate the particle mass, this work mainly focuses on the
characterization of particle composition rather than the absolute SOA yield.
Independent wall-loss experiments of ammonium sulfate
((NH4)2SO4) particles were also performed to determine the
size-dependent wall-loss rate constants of particles inside the chamber. An
aqueous (NH4)2SO4 solution was added to a TSI Model 3076
atomizer to produce droplets. The droplets were passed through a silica gel
diffusion dryer to get dry (NH4)2SO4 particles and then these were
injected into the chamber. The size distributions of
(NH4)2SO4 particles were characterized using the SMPS for 6 h. The relationship between the wall-loss rate (k, h-1) of particles and
their size (dp, nm) can be expressed as k(dp)=1.20× 10-7×dp2.32+20.59×dp-1.39 based on a size-dependent particle wall-loss correction
method.
Results and discussionSO2 effects on aerosol formation
Insights into SO2 effects on particle formation could be gained through
investigating the number and volume concentration as well as the size
distribution of particles under various SO2 level conditions. In the
absence of SO2, the particle number concentration increased in a burst
within the first 20 min of cyclooctene ozonolysis and then decreased because
of their coagulation and wall deposition, while the particle volume
concentration increased gradually and reached its maximum within 240 min
(Fig. S3). An elevating SO2 level can result in significant increases in
the number and volume concentration of particles (Fig. 1a), which is
consistent with observations from previous studies (Ye et al., 2018; Yang
et al., 2021). We observed a 9.4–12.6-times increase in particle maximum
number concentration in the presence of 14–192 ppb SO2 (Table 1). The
promoted effect of SO2 is shown more clearly in Fig. 1b, where SO2
was seen to be consumed on a similar timescale to particle formation.
Specifically, upon initiation of cyclooctene ozonolysis, SO2
concentration decreased and the particle volume concentration increased
simultaneously. After cyclooctene was completely consumed, both SO2
consumption and particle production slowed down. SO2 consumption and
particle formation resumed when more cyclooctene was introduced into the
reactor. This result indicates that SO2 may react with certain highly
reactive species produced from cyclooctene ozonolysis. For instance,
reactions of SO2 with sCI could form H2SO4
(Boy et al., 2013), which is a key species for new
particle formation (Lehtipalo et al., 2018; Yao et al., 2018). Inorganic
sulfate absorption at 617 cm-1 was observed in the ATR-FTIR spectra of
particles formed in cyclooctene–O3–SO2 systems (Fig. 2)
(Hawkins et al., 2010; Coury and Dillner, 2008), indicating the formation
of H2SO4. We assumed that all consumed SO2 was converted to
particle-phase H2SO4, which represents an upper limit of
H2SO4 formation (Wyche et al., 2009; Ye et al., 2018). The
amount of H2SO4 produced could not fully account for the
enhancement of particle volume concentration (Table 1). H2SO4 has
been considered as an important driver of particle acidity
(Tilgner et al., 2021). Acid catalysis induced by
H2SO4 may also promote the formation of additional organic
products, leading to the increase in particle volume concentration
(Deng et al., 2021).
SO2 can also affect the growth of new aerosol particles (Fig. 1c). Once
O3 was introduced into the reactor, aerosol particles were produced
rapidly. After cyclooctene was depleted, the aerosol particle mass increased
slowly. The initial stage of particle formation was then defined as the time
from reaction initiation to the complete consumption of cyclooctene
(∼ 10 min). From Fig. 1c, in the initial stage of ozonolysis
(10 min), particles formed in cyclooctene–O3–SO2 systems had a
smaller size mode than those formed in the cyclooctene–O3 system, which may
be attributed to the following two factors: first, oligomers formed from sCI
reactions with organic species could partition into the condensed phase to
contribute to particle growth (Riva et al., 2017). SO2
presence may lead to a decrease in these oligomers because SO2 can
compete with organic species to react with sCI. Second, counterbalancing the
reduction of oligomers via sCI + SO2 reactions is the production of
H2SO4. The production of more new particles in
cyclooctene–O3–SO2 systems could provide more condensation sinks.
Organic vapors that can condense onto particles are dispersed via new
particles, resulting in small particle size at the initial phase of
cyclooctene–O3–SO2 systems (Stangl et al., 2019).
Interestingly, particles could grow quickly in the presence of SO2. At
300 min reaction time, particles formed in the presence of SO2 even had
slightly larger sizes than those formed in the absence of SO2.
H2SO4-catalyzed heterogenous reactions could produce
lower-volatility organic species from higher-volatility reactants in the aerosol
phase (Yang et al., 2020; Han et al., 2016). Semi-volatile species could
undergo evaporation after partitioning to the aerosol phase while
low-volatility products generally have a negligible evaporation rate from the
aerosol phase. Low-volatility products formed via H2SO4-catalyzed
heterogenous reactions could build particle mass at a rate almost equal to
the condensation rate and, thus, effectively facilitate particle growth in
cyclooctene–O3–SO2 systems (Apsokardu and Johnston,
2018).
Particle formation from the ozonolysis of cyclooctene
under various SO2 conditions. (a) Maximum particle number concentration
as a function of initial SO2 level. Circle color represents particle
volume concentration. (b) Temporal profiles of SO2 concentration and
particle volume concentration. (c) Size distributions of aerosol particles
formed with various SO2 concentrations at 10, 60, and 300 min after the initiation of cyclooctene ozonolysis.
ATR-FTIR spectra of aerosol particles generated from
cyclooctene ozonolysis in the presence of different SO2 concentration.
Aerosol chemical composition under SO2-free conditions
Figure 3 shows the base peak chromatograms (BPCs) of cyclooctene-derived
particles in the absence of SO2. The chromatograms of blank filter
clearly showed no peaks eluted at retention times (RTs) between 0 and 30 min,
while there were several significant peaks for cyclooctene SOA chromatograms
in both positive and negative ion modes. Each chromatogram peak of
cyclooctene SOA represents at least one ion, and major peaks are only
labeled with the mass of the most abundant single ion. Compared to the
negative chromatogram of cyclooctene SOA, the corresponding label ions in
the positive chromatogram were 24 Da higher in mass. This is consistent with
the fact that many ions produce adducts with sodium ion ([M + Na]+)
in positive ion mode, while negative ion mode leads to the production of
deprotonated ions ([M – H]-) (Mackenzie-Rae et al.,
2018). From Fig. 3, products with molecular weight (MW) <200 Da
eluted from the column at shorter RTs than those with MW >200 Da.
Low-molecular-weight products (MW <200 Da) likely correspond to
small monomer type compounds (hereafter termed monomeric products), which
directly originate from the ozonolysis of cyclooctene. Compounds with
MW >200 Da mainly dominate the later part of the chromatogram,
and they may be homo- or heterodimeric species (hereafter denoted dimeric
products) formed using two monomeric products as building blocks.
Base peak chromatograms of both blank filter and
particles generated from the ozonolysis of cyclooctene in the absence of
SO2. Labels represent the most abundant single ion of each peak. (a)
Positive ion mode. (b) Negative ion mode.
Possible structures of major monomeric products were proposed based on their
accurate m/z, fragmentation mass spectra, and previous mechanistic insights.
Note that the fragmentation of [M + Na]+ is relatively difficult
(Zhao et al., 2016) and, thus, the positive ion mode was not
further analyzed in providing structural insights in the current study. The
negative chromatogram peaks with RT at 11.85 min (N-145), 16.13 min (N-159),
and 20.41 min (N-173) were significant peaks for cyclooctene SOA (Fig. 3b),
and they were assigned neutral formulas of C6H10O4,
C7H12O4, and C8H14O4, respectively. As
shown in Fig. 4, MS/MS spectra of monomer C6H10O4,
C7H12O4, and C8H14O4 were similar.
Taking C8H14O4 as an example (Fig. 4c), its fragmentation mass
spectrum was characterized by a loss of 44 Da (CO2), suggesting the
presence of a carboxyl group. The neutral loss of 18 Da (H2O) upon
fragmentation of the parent ion (C8H13O4-, m/z=173.08209) led to the production of an ion with m/z 155.07143. The loss of
H2O is an unspecific fragmentation mechanism, which likely
originates from a carboxyl or hydroxyl group (Noziere et al., 2015). The
fragment ion (m/z=111.08166) representing the simultaneous neutral losses
of CO2 and H2O was also formed. MS/MS spectra can result from
multiple isomeric structures in many cases
(Wang et al., 2019). Yasmeen et al. (2011)
showed the detailed fragmentation spectrum for the dicarboxylic acid
standard (azelaic acid) and indicated that deprotonated azelaic acid also
showed losses of H2O, CO2, and CO2+ H2O. In addition,
Noziere et al. (2015) showed that the neutral losses of CO2 and
H2O indicates two carboxyl groups. Thus, monomer C8H14O4
was tentatively assigned to suberic acid and the corresponding fragmentation
pathway for C8H13O4- is proposed in Fig. S4. The
fragment ions originating from losses of H2O, CO2, and CO2+ H2O were also observed in MS/MS spectra of C6H10O4 and
C7H12O4, indicative of adipic acid and pimelic acid,
respectively. Carboxylic acids have also been observed in SOA produced from
previous alkene ozonolysis (Hamilton et al., 2006; Kenseth et al., 2020;
Mackenzie-Rae et al., 2018; Zhang et al., 2015). Carboxylic acids represent
a significant class of aerosol components, and they play a significant role
in particle chemistry via their influences on particle acidity and through
direct involvement in certain heterogeneous reactions to produce low-volatility species (Millet et al.,
2015). More experiments using available authentic standards are necessary to
better understand their structures, sources, and formation mechanism. Other
prominent monomer peaks at RTs of 8.30 min (N-175) and 14.18 min (N-189)
corresponded to compounds with neutral formula, namely
C7H12O5 and C8H14O5. The losses of H2O,
CO, and CO2 in the MS/MS spectrum of C7H12O5 indicated
hydroxyl, terminal carbonyl, and carboxyl groups, respectively
(Mackenzie-Rae et al., 2018; Riva et al., 2016a), and
C7H12O5 was identified as hydroxy-containing oxoheptanoic
acid (Fig. S5a and c). Monomer C8H14O5 only showed losses
of H2O and CO (Fig. S5b), and it is difficult to determine the specific
type and positioning of oxygen-containing functionalities within
C8H14O5 with five oxygen atoms based on its MS/MS spectrum.
MS/MS spectra of major monomers and dimers. Monomers: (a)
C6H10O4, (b) C7H12O4, and (c)
C8H14O4. Dimers: (d) C15H24O8, (e)
C15H24O7, and (f) C14H24O5.
The labeled dimer peaks in negative ion mode corresponded to [M - H]-
ion masses of 271, 285, and 331 (Fig. 3b), which were assigned neutral
formulas of C14H24O5, C15H26O5, and
C15H24O8, respectively. The number of fragment ions of dimers
are generally limited, and determining the exact structure of dimers is less
certain compared to monomers (Witkowski and Gierczak, 2017).
Therefore, only a decrease in molecular structure possibilities is provided.
For dimer C15H24O8, fragment ions m/z 159.06642
(C7H11O4-) and m/z 189.07722 (C8H13O5-)
were detected in the MS/MS spectrum (Fig. 4d). When dimers are subjected to
CID, fragment ions corresponding to their building blocks are commonly
observed (Witkowski and Gierczak, 2017; Hall and Johnston, 2012). Based
on this rule, it could be concluded that dimer C15H24O8 was
an association product of C7H12O4 and C8H14O5.
Similarly, for dimer C15H24O7, there were two significant
product ions of C15H23O7-, with accurate masses of m/z
159.06651 (C7H11O4-) and 173.08217
(C8H13O4-) (Fig. 4e). Furthermore, fragment ions
corresponding to secondary loss of CO2+ H2O from product ions
C7H11O4- and C8H13O4- were also
observed. The fragmentation spectrum of C15H24O7 was similar
to the MS/MS spectra of C7H12O4 and C8H14O4
(Fig. 4b and c), suggesting again that C7H12O4 and
C8H14O4 may be the building blocks of
C15H24O7. Acid-catalyzed heterogeneous processes can result
in the formation of high-molecular-weight dimers in both biogenic and
anthropogenic systems (Barsanti et al., 2017).
Carboxylic acid monomers formed could be important sources of particle
acidity in the absence of SO2. Dimers C15H24O7 and
C15H24O8 may be produced by heterogeneous reactions involving
the loss of a water molecule, and the linkage between building blocks is an
acid anhydride (Fig. S6) (Hamilton et al., 2006). Another
abundant dimer peak (N-271) in the negative chromatogram was identified as
C14H23O5- with mass accuracy of -0.02492 ppm.
C14H23O5- could dissociate to the product ions of
C14H21O4-, C13H21O2-, and
C7H11O- (Fig. 4f). Both secondary ozonide and aldol
structures shown in Fig. 4f could match the assigned elemental formula of
C14H24O5. However, the neutral losses of H2O and
CO2 were not easily produced by secondary ozonide, but were more likely for
the aldol structure (Hall and Johnston, 2012). Aldol condensation
products were also among the most commonly observed species in previous
ozonolysis of alkenes (Zhao et al., 2016; Kenseth et al., 2018;
Kristensen et al., 2016). Therefore, C14H24O5 as shown in Fig. 4f is likely an aldol condensation product.
To examine the overall composition of particles, average mass spectra (Fig. S7) corresponding to the chromatogram where particle components eluted were
also analyzed. Figure 5 summarizes the oxidation products observed in
particles mapped in O-KMD and van Krevelen plot. The molecular formulas of
identified oxidation products could be largely classified into homologous
series of monomers and dimers (Fig. 5a and b). The elemental composition
distribution of products measured in positive and negative ion modes was
similar, with most monomers and dimers having O/C ratios ranging from 0.2 to
0.8, and H/C ratios ranging from 1.2 to 1.8 (Fig. 5c). Lines with slopes of
0, -0.5, -1, and -2 in Fig. 5c can be used to illustrate the addition of
hydroxyl/peroxide, carboxylic acid (with fragmentation), carboxylic acid
(without fragmentation), and carbonyl groups to a saturated carbon chain,
respectively (Heald et al., 2010). As shown in
Fig. 5c, cyclooctene SOA occupied a relatively wide range in the van
Krevelen diagram, and there are a large number of points scattered in the space
between lines with slopes of -0.5 and -2. This behavior is consistent with
the importance of high-abundance carboxylic acids in the above analysis.
SO2 effects on aerosol chemical composition
To obtain further detailed mechanisms regarding SO2 effects and determine
whether heterogeneous processes occur, aerosol samples were analyzed
using ATR-FTIR and LC/ESI-MS. Both IR and MS analysis of particles revealed
changes in aerosol chemical composition induced by SO2 addition.
Characteristics of functional group in aerosol-phase products
Figure 2 shows ATR-FTIR spectra of aerosol particles. Hydroxy (3600–3200 cm-1), alkyl (2935 and 2864 cm-1), and carbonyl (1702 cm-1)
groups were identified in particles collected from the cyclooctene–O3 system
(Table S1 in the Supplement). These particles also had a broad absorption across the 1500–800 cm-1 region, which may arise from C–H deformation in 1480–1350 cm-1, C–C stretching in 1250–1120 cm-1, and C–O stretching in
different regions for various oxygenated species
(Hung et al., 2013). Three additional absorption
bands at 1413, 1095, and 617 cm-1 were observed in ATR-FTIR spectra of
particles formed with the introduction of SO2 (Tammer, 2004; Lal et
al., 2012). Absorption bands at 1413 and 1095 cm-1 may be associated
with asymmetric and symmetric stretching of -SO2- while inorganic
sulfates could give rise to strong absorption at 617 cm-1. The presence
of absorption bands of sulfur-containing groups suggests that SO2
addition can result in the production of sulfur-containing compounds.
Oxidation products observed in particles produced from
the ozonolysis of cyclooctene in the absence of SO2. Oxygen
(O) Kendrick mass defect plots of (a) monomers and (b) dimers. (c) Van
Krevelen diagram.
Simplified formation schemes for the selected
organosulfates formed from the ozonolysis of cyclooctene.
(a) Two-dimensional volatility–oxidation space
of the identified organosulfurs and their precursors. (b) Carbon atom number
distribution of organosulfurs observed in the current work and in the
studies of Cai et al. (2020), Boris et al. (2016),
and Y. Wang et al. (2021). Detailed formulae of these OSs can be found
in Table S3. Organosulfurs from previous studies are of unknown
origin or unknown structure.
Organosulfate formation in the presence of SO2
In addition to CHO compounds, products with CcHhOoSs
elemental formulas were identified in the presence of SO2 (Fig. S8). OS
could undergo highly efficient ionization to give deprotonated molecular
ions in negative ion mode. Based on MS/MS analysis, unambiguous
identification of OS can be achieved, since OSs could give characteristic
fragment ions at m/z 80 (SO3-), 81 (HSO3-), and/or
97 (HSO4-) in their MS/MS spectra (Figs. S9–S17). Accurate mass
measurements of OSs as well as their retention times and DBE values are
provided in Table S2. The proposed structure and fragmentation scheme of
each OS and the corresponding precursor are presented in Figs. S9–S17. For
instance, OS-209 and OS-223 showed prominent product ions for losses of
HSO4- and SO3- (Figs. S11–S12), confirming the
organosulfate moiety. Neither a hydroxyl nor a carboxyl group fragment ion
(i.e., -H2O or -CO2) was observed in their MS/MS spectra.
C6H10O3 and C7H12O3 were proposed as the
precursors of OS-209 and OS-223, respectively. MS/MS spectra of
C6H10O3 and C7H12O3 were characterized by loss
of CO, indicating a terminal carbonyl group (Figs. S11–S12). Considering
structural features of OS precursor measurements as well as OS-209 and
OS-223 all corresponding to DBE = 2, two carbonyl groups could well explain the observed MS/MS spectra of OS-209 and OS-223. The
organosulfate substituent was expected to attach to a terminal carbon atom.
Although the carbonyl group is more readily observed in positive ion mode,
ESI-MS is also highly sensitive to carbonyl compounds containing sulfate
substituents and thereby gives intense [M - H]- ions in negative ion
mode (Riva et al., 2016b).
A relatively high abundance of OS is helpful for the acquisition of MS/MS
data, and therefore high-abundance [M - H]- ions were chosen as
representative candidates to clarify the precursors and formation pathways
of OSs. A simplified chemical mechanism describing OS production from
the ozonolysis of cyclooctene (C8H14) is proposed in Fig. 6. The
ozonolysis of cyclooctene (C8H14) can be initiated by O3
addition to the endocyclic double bond, forming an energy-rich primary
ozonide (POZ). POZ can decompose rapidly to an excited CI containing both a
terminal carbonyl and carbonyl oxide group. The excited CI could lead to the
formation of sCI, vinylhydroperoxide, and dioxirane, illustrating the
multiplicity and the complexity of cyclooctene ozonolysis. SCI is
mainly capable of involvement in bimolecular reactions to form carboxylic
acids and acid esters. Vinylhydroperoxide rapidly decomposes into an alkyl
radical (C8H13O2⚫) and an
⚫OH. Molecular oxygen could be subsequently added to
C8H13O2⚫ to produce an alkyl peroxy
radical (RO2, C8H13O4⚫). Dioxirane
intermediate may also undergo decomposition and produce
C7H13O3⚫.
C8H13O4⚫ and
C7H13O3⚫ are considered as the starting
point of the RO2⚫ and alkoxy radical
(RO⚫) chemistry, resulting in termination CHO compounds
with hydroperoxy, carbonyl, or hydroxy groups (Fig. 6). Acid-catalyzed
heterogenous reactions of CHO products have been evidenced to play a major
role in OS formation in the atmosphere (Riva et al., 2016c, b). Although acidic seed particles were not directly injected into the
reactor during cyclooctene ozonolysis, SO2-induced H2SO4 may create acidic conditions for the occurrence of heterogeneous reactions.
In the case of CHO products with a hydroxyl group, H2SO4 could
protonate the hydroxyl group, leading to the formation of OS and water. The
low RH (∼ 20 %) of ozonolysis was helpful for shifting the
reaction equilibrium in favor of OS production.
Detailed information about the volatility of oxidation products is necessary
to evaluate their potential to contribute to aerosol formation. As shown in
Fig. 7a, the products could be categorized into intermediate-volatility OCs
(IVOCs), semi-volatile OCs (SVOCs), low-volatility OCs (LVOCs), and extremely
low-volatility OCs (ELVOC), with Co in the range of 300–3 × 106, 0.3–300, 3 × 10-4–0.3, and <3× 10-4µg m-3, respectively (Donahue et al.,
2011). The saturation mass concentration of OSs spanned more than six orders
of magnitude (Fig. 7a), suggesting their inherent chemical complexity. A large number of OSs are SVOCs and LVOCs, while their precursors are
classified as IVOCs and SVOCs, indicating that the SO2 presence
facilitates the reduction of product volatility (Yang et al., 2020; Han
et al., 2016). Lower-volatility OSs generated from acid-catalyzed heterogenous
reactions may build particle mass at a faster rate compared to their
higher-volatility precursors, and thereby benefit the formation and growth of
particles in the presence of SO2.
Figure 7b displays the DBE–carbon atom number space for organosulfur
compounds. There are some overlaps of organosulfur compounds detected in
this work with previous data from field observations (Y. Wang et al., 2021;
Boris et al., 2016; Cai et al., 2020). For example, Y. Wang et al. (2021) comprehensively analyzed OS in PM2.5 filter samples collected
at an urban site in Shanghai, China, and observed the presence of
C6H10O6S (Fig. 7b, cyan cross). In the absence of
chromatographic data such as retention times, C6H10O6S was
tentatively assigned to diesel vapor-derived OS. Alkenes are important
components of diesel and cyclooctene may be also responsible for
C6H10O6S formation in the atmosphere. The overlaps of
organosulfur compounds indicate that the ozonolysis of cycloalkenes in the
presence of SO2 is likely an important source of organosulfur compounds
in the ambient atmosphere. In addition, our work further suggests that the
sources of OS cannot be determined only based on their elemental formula,
and techniques that enable the identification of molecular structures (e.g.,
MS/MS) are greatly beneficial in field studies. The identified molecular
structures of OSs in this study are also helpful in source apportionment
in field studies.
Conclusion
We have explored O3-initiated oxidation of cyclooctene in the absence
and presence of SO2, with a focus on the mechanism by which SO2
impacts particle formation and composition. Cyclooctene can produce a large
number of particles upon reacting with O3. Higher SO2
concentration led to higher particle number concentration as a result of
H2SO4 formation from the reactions of sCI with SO2.
Cyclooctene SOA mainly consisted of carboxylic acids and corresponding
dimers with acid anhydride and aldol structures when SO2 was not added.
SO2 addition can induce changes in particle chemical composition
through the formation of OSs. Some OSs, classified as compounds of unknown
origin or unknown structure in previous field studies, were also observed in
this work. The OSs found here are less volatile than their precursors,
indicating the stronger ability of OS for particle formation. The formation
of OSs can in part lead to the increase in particle volume concentrations in
the presence of SO2.
The results here suggest that SO2 can influence aerosol particle
formation and composition by producing sulfur-containing compounds (i.e.,
H2SO4 and OSs). Nevertheless, the observed number of OSs may be
amplified by the high SO2 concentration used in the present work. In
order to determine the actual mass yields of OSs and better quantify roles of
SO2 in particle formation, further experiments using ambient
SO2 levels and authentic standards are warranted.
Data availability
Experimental data are available upon request to the corresponding author.
The supplement related to this article is available online at: https://doi.org/10.5194/acp-23-417-2023-supplement.
Author contributions
ZY designed the experiments and carried them out. ZY performed data analysis
with assistance from XL, NTT, KL, and LD. ZY prepared the paper with
contributions from all co-authors. NTT, KL, and LD commented on the paper.
Competing interests
The contact author has declared that none of the authors has any competing interests.
Disclaimer
Publisher’s note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Financial support
This research has been supported by the National Natural Science Foundation of China (grant no. 22076099), the Department of Education of Shandong Province (grant no. 2019KJD007), and the Fundamental Research Fund of Shandong University (grant no. 2020QNQT012).
Review statement
This paper was edited by Dantong Liu and reviewed by two anonymous referees.
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