Introduction
As an important monoterpene, limonene has a high emission rate from both
biogenic and anthropogenic sources that is second only to pinene (Atkinson
and Arey, 2003; Clausen et al., 2001; Fellin and Otson, 1994; Griffin et al.,
1999; Guenther et al., 1995; Lamb et al., 1993; Seifert et al., 1989;
Sindelarova et al., 2014; Wolkoff et al., 2000). Total monoterpene emissions
are estimated to be 50 Tg C yr-1, and limonene might comprise about
20 % of that (Stroud et al., 2005). In addition to massive emissions from
vegetation, the extensive utilization of limonene in household and industrial
processes makes it non-negligible in the atmosphere. Compared with its
isomers α-pinene and β-pinene, an obvious structural feature
of limonene is that it has two different double bonds (DBs), an endocyclic DB
and an exocyclic DB, which complicates the behavior and the fate of limonene
in the atmosphere. Ozonolysis is an important reaction pathway of limonene
and it serves as a source of free radicals, intermediate products, and
aerosol. The first step in alkene ozonolysis is the addition of O3
to the carbon–carbon double bond, forming an energy-rich primary ozonide
(POZ). The POZ decomposes into two sets of carbonyls plus carbonyl oxides,
which are called excited Criegee intermediates (ECIs) (Criegee, 1975; Fenske
et al., 2000; Gutbrod et al., 1996, 1997; Kroll et al., 2001a, b). ECIs can
isomerize through the hydroperoxide channel followed by OH production,
rearrange to esters with subsequent decomposition, or undergo collisional
stabilization, forming stabilized Criegee intermediates (SCIs) (Cremer et al.,
1993; Presto and Donahue, 2004; Richard et al., 1999; Wegener et al., 2007).
In the last few years, the oxidizing compounds formed in alkene ozonolysis
have received much attention because they are able to remove a series of
trace gases and contribute to atmospheric oxidation capacity
(Möller, 2009; Prinn, 2003; Taatjes et al., 2013). Representatives
of these compounds include OH radicals, SCIs, and peroxides, etc. Investigations
into the oxidation regime of alkene ozonolysis are essential to understanding
of the formation of these critical products and evaluation of the actual impact of
this reaction in the atmosphere. As an important source of OH radicals needed
for OH-initiated reactions occurring in darkness (Kroll et al., 2001a, b), OH
formation pathways in alkene ozonolysis have been extensively studied and the
major pathway is considered to be the unimolecular decomposition of ECIs.
Furthermore, some studies suggest that SCIs could also generate OH radicals
through self-decomposition or reactions with other species (Anglada et al.,
2002; Hasson et al., 2003; Kroll et al., 2001a, b; Tillmann et al., 2010;
Zhang and Zhang, 2005). SCIs generated during the process have been shown to
have a lifetime, usually on the order of sub-seconds (Kroll et al., 2001a, b;
Long et al., 2018; Mauldin et al., 2012), that is sufficiently long to react
with other trace species, such as H2O, SO2, NO2, carbonyls,
alcohols, and carboxylic acids. (Sakamoto et al., 2017; Sipilä et
al., 2014; Yao et al., 2014). A number of studies have focused on the
reactions of SCIs containing 3 or fewer carbon atoms, and their unimolecular
decomposition and reactions with water show strong structure dependence. The
reactions with water monomers and water dimers are the main reaction pathways
for CH2OO and anti-CH3CHOO, while the unimolecular
decomposition would be more important for syn-CH3CHOO and
(CH3)2COO (Huang et al., 2015; Lin et al., 2016; Welz et al., 2012,
2014; Taatjes et al., 2013). However, the reaction mechanisms for larger SCIs
formed from biogenic alkenes ozonolysis are still unclear, since it is
difficult to synthesize and observe them directly. Furthermore, reactions of
alkenes with ozone can also generate a considerable amount of peroxides,
which are considered to be important due to their oxidizability and role as a
radicals' reservoir. H2O2 is the most crucial oxidant to oxidize
S (IV), forming sulfuric acid and sulfate in the aqueous phase (Calvert et
al., 1985; Penkett et al., 1979; Peña et al., 2001). And as a
species accounting for 40 %–50 % of total global organic peroxides
(Crounse et al., 2006; Khan et al., 2015), peroxycarboxylic acids (RC(O)OOH)
play an important role in promoting atmospheric oxidation capacity and
enhancing the acidity of the aqueous phase.
One important reason why limonene chemistry has attracted much attention is
its high secondary organic aerosol (SOA) formation potential, which is higher
than that of α-pinene since limonene is doubly unsaturated
(Andersson-Sköld and Simpson 2001; Kroll and Seinfeld, 2008; Lane et al.,
2008; Lee et al., 2006). Although progress has been made in simulating SOA
formation with the theory of gas–particle partitioning, there are still
large discrepancies between model and experimental results (Cocker et al.,
2001; Griffin et al., 1999; Hoffmann et al., 1997; Odum et al., 1996; Pankow,
1994; Presto et al., 2005; Pye and Seinfeld, 2010). Laboratory studies of SOA
formation in limonene ozonolysis mainly focused on the aerosol yields under
different conditions and the identification of products in the particle phase
(Calogirou et al., 1999; Glasius et al., 2000; Grosjean et al., 1992, 1993;
Leungsakul et al., 2005; Ng et al., 2006; Pathak et al., 2012); however, the
composition of limonene SOA still needs detailed study, especially the effect
of different DBs. As a double-unsaturated terpene, the SOA formation process
of limonene could be more complicated than that for single-unsaturated
terpene, since multi-generation oxidation has a significant influence on SOA.
In this study, two classes of species, peroxides and carbonyls, are studied
for their behaviors in limonene SOA formation. Over the past few years,
analyses of organic peroxides suggest that they are an important component in
aerosol (Docherty et al., 2005; Heaton et al., 2007; Li et al., 2016; Mertes
et al., 2012; Pathak et al., 2012), and particulate peroxides could cause
negative health effects when they penetrate into lungs (Chen et al., 2011;
Fischer and Maier, 2015; Gupta et al., 2014; Verma et al., 2009; Wragg et
al., 2016). The reactive uptake and particle-phase reactions of carbonyls are
believed to be responsible for fractions of aqueous SOA formation (Ervens et
al., 2011; Mcneill et al., 2012), especially for the dicarbonyls glyoxal (GL)
and methylglyoxal (MGL).
Until now, there has been little research into the contribution of peroxides
and carbonyls to limonene SOA and their behaviors in aerosol formation are
only vaguely understood, to the best of our knowledge.
The focus of this study is to investigate the oxidation regime and SOA
composition in limonene ozonolysis, especially for the roles of different
DBs, radicals, and water. We consider the generation of SCIs, OH radicals,
and peroxides in discussions of the oxidation regime in the reaction system.
Furthermore, peroxides and carbonyls are taken as representatives to study
their behaviors in SOA formation.
Experimental
Chemicals
This study used the following: R-(+)-Limonene (Sigma-Aldrich, ≥99.0 %), 2-butanol (Sigma-Aldrich, 99.5 %), cyclohexane (Alfa Aesar,
≥99.9 %), potassium iodide (KI, Alfa Aesar, 99.9 %), hydrogen
peroxide (H2O2, Alfa Aesar, 35 wt %), orthophosphoric acid
(H3PO4, Fluka, 85 %–90 %), hemin (Sigma, ≥98.0 %), 4-hydroxyphenylacetic acid (Alfa Aesar, 99 %), formaldehyde
(FA; Sigma-Aldrich, 37 wt %), acetaldehyde (AA; Amethyst Chemicals, 40 wt %), acetone (ACE; Fluka,
≥99.7 %), hydroxyacetone (HACE; Sigma-Aldrich, 90 %), GL (Sigma-Aldrich, 40 wt %), MGL
(Sigma-Aldrich, 40 wt %), 2-butanone (Alfa Aesar, ≥99 %),
acetonitrile (Alfa Aesar, ≥99.7 %), 2,4-dinitrophenylhydrazine
(DNPH, TCI, 50 wt %), ammonia solution
(NH3⚫H2O, Beijing Tongguang Fine Chemicals
Company, 25.0 %–28.0 %), ammonium chloride (NH4Cl, Beijing
Tongguang Fine Chemicals Company, ≥99.5 %), sulfuric acid
(H2SO4, Xilong Chemical Company, 95.0 %–98.0 %),
ultrapure water (18 MΩ, Millipore), N2 (≥99.999 %,
Beijing Haikeyuanchuang Practical Gas Company Limited, Beijing, China),
O2 (≥99.999 %, Beijing Haikeyuanchuang Practical Gas
Company Limited, Beijing, China), polytetrafluoroethylene (PTFE) filter
membrane (Whatman Inc., 47 mm in diameter), and quartz microfiber filters
(Whatman Inc.).
Apparatus and procedures
A flow tube reactor (2 m length, 70 mm inner diameter, quartz wall), equipped
with a water jacket for controlling the temperature, was used to investigate
the ozonolysis of limonene. All the experiments were conducted at 298±0.5 K in darkness. O3 was generated by O2 photolysis in a 2 L quartz tube with a low-pressure Hg lamp, and the detailed quantification
method of O3 was described in our previous study (Chen et al.,
2008). H2O2 produced by UV irradiation of O2 and trace
water was measured in control experiments and deducted from the results.
Limonene gas was generated by passing N2 flow over liquid limonene in a
diffusion tube at the selected temperature, and OH scavenger (2-butanol or
cyclohexane) gas was generated with a bubbler. The concentrations of limonene
and the OH scavenger were determined using a gas chromatograph with a flame
ionization detector (GC-FID, Agilent 7890A, USA). Water vapor was generated
by passing N2 through a water bubbler, which contained a carborundum
disc submerged in ultrapure water (18 MΩ). The mixing gas, including
limonene, OH scavenger, ozone, and dry or wet synthetic air (80 % N2
and 20 % O2), was successively introduced into the reactor. With a
total flow rate of 2 standard L min-1, the residence time was estimated
to be 240 s.
To explore the reaction mechanism of endocyclic and exocyclic DBs' ozonolysis
and the effect of multi-generation oxidation in limonene ozonolysis, we
conducted two sets of experiments with different ratios of ozone to limonene
concentration. In the following, [O3] denotes the concentration of
ozone, [limonene] denotes the concentration of limonene, and
[O3] / [limonene] denotes the concentration ratio of ozone to
limonene. In the low [O3] / [limonene] experiments, the initial
concentrations of limonene and ozone were ∼280 and
∼500 ppbv, respectively, whereas in the high
[O3] / [limonene] experiments, the initial concentrations of
limonene and ozone were ∼183 ppbv and ∼19 ppmv,
respectively. In both sets of experiments, sufficient quantities of 2-butanol
and cyclohexane were added to scavenge OH radicals in the RH range of
0 %–90 %. In the tables and figures, the low-ratio and high-ratio
experiments are denoted by L and H, respectively, and the conditions in the
absence of the scavenger, in the presence of 2-butanol, and in the presence of
cyclohexane are denoted by No-sca, 2-But, and C-hex, respectively. The
experimental conditions are listed in Table 1.
Experimental conditions.
Exp.
[Limonene]
[O3]
OH
[OH
RH
(ppbv)
(ppbv)
scavenger
scavenger]
(%)
(ppmv)
L(No-sca)
280
500
–
–
0–90
L(2-But)
280
500
2-butanol
350
0–90
L(C-hex)
280
500
cyclohexane
420
0–90
H(No-sca)
183
19 000
–
–
0–90
H(2-But)
183
19 000
2-butanol
350
0–90
H(C-hex)
183
19 000
cyclohexane
420
0–90
Note: L, low [O3] / [limonene] ratio; H, high
[O3] / [limonene] ratio; No-sca,no scavenger; 2-But, 2-butanol; C-hex, cyclohexane; RH, relative humidity.
According to previous studies, the rate constant of the endocyclic DB
reaction with ozone in limonene ozonolysis is 2×10-16 cm3 molecule-1 s-1 (Atkinson, 1990; Shu and Atkinson, 1994),
while the exocyclic DB reaction with ozone is about 30 times slower than the
endocyclic DB reaction (Zhang et al., 2006). Based on those rate constants,
we estimate that at a low [O3] / [limonene] ratio, less than 1 %
exocyclic DB is ozonated, so this situation mainly represents
first-generation oxidation. In this circumstance, because the ozone
concentration is low, the OH reaction would impact the amount of limonene
consumed by O3. In the presence of the OH scavenger, ∼42 % endocyclic DB reacted with O3, while in the absence of
the scavenger, ∼38 % endocyclic DB reacted with O3. At a high [O3] / [limonene] ratio, more than 99 % endocyclic DB and
about 51 % exocyclic DB reacted with ozone, and since the ozone
concentration in this situation was high, the OH effect on the ozonolysis was
presumed to be unimportant. The latter condition, which contained
multi-generation oxidation processes, was more likely to occur since the
ratio of [O3] to [limonene] was similar to the ratio in the real
atmosphere.
It should be noted that one advantage of the flow tube reactor is that the wall
would be in equilibrium with the gas phase after a stationary period.
According to our observations, this process usually requires about 2 h.
Thus, to stabilize the system and diminish the wall effect to the greatest extent
possible, we ran the reactor for 2 h prior to taking measurements. And after
the experiments, the reactor was rinsed with ultrapure water and blown dry
with N2.
Products' analysis
To better investigate the gas–particle partitioning of products formed in
limonene ozonolysis, we analyzed the gas-phase and particle-phase products
simultaneously. We measured total peroxides and a series of
low-molecule-weight (LMW) peroxides. In the following discussion we regard
those peroxides that could be detected by high performance liquid
chromatography (HPLC) to be LMW peroxides, while we take the peroxides
undetermined by HPLC to be high-molecule-weight (HMW) peroxides. For
particle-phase peroxides' detection, a PTFE filter was used for the SOA collection, and the mass of SOA
was measured by a semi-micro balance (Sartorius, Germany). Since the control
experiment showed that a long-duration collection led to the loss of some
peroxides in particles, the collection time was controlled to be 3 h for
each filter, and the accuracy of the particulate products' analysis was
discussed in the Supplement. Each loaded PTFE filter was extracted with
20 mL H3PO4 solution (pH 3.5) using a shaker (Shanghai Zhicheng
ZWY 103D, China) at 180 rpm and 4 ∘C for 15 min, and the
extraction efficiency was confirmed in our previous work (Li et al., 2016).
For gas-phase peroxides' detection, the air samples through the filter were
collected in a glass coil collector at a temperature of 4 ∘C with
H3PO4 solution (pH 3.5) serving as the rinsing solution. The
coil collector is around 30 cm long and its effective length is about
100 cm. The coil is similar with that used in earlier studies (Grossmann et
al., 2003; Sauer et al., 1996, 1997), and the diagram of the thermostatic
coil collector is provided in the Supplement.
The method for detecting the peroxides was reported in our previous studies
(Hua et al., 2008; Li et al., 2016) and is only briefly described here. LMW
peroxides were analyzed using HPLC (Agilent 1100, USA) coupled with
post-column derivatization and fluorescence detection online. Peroxides separated by column
chromatography reacted with p-hydroxyphenylacetic acid (POPHA) to form POPHA dimers under the
catalysis of hemin, and then the dimers were quantified
using a fluorescence detector. The standard solution of peroxides was
prepared and used for calibration in each measurement. The detection limit
for the gas-phase LMW peroxides was about 22 pptv, and in the particle phase
the detection limit was around 0.068 ng µg-1 for
H2O2. The accuracy of this method was estimated to be around
7 %, with the precision of the measurement results usually within
20 %. The concentration of total peroxides (H2O2, ROOH, and
ROOR′) was determined using the iodometric spectrophotometric method,
which is based on the reaction of peroxides and iodide ions (Docherty et al.,
2005; Mutzel et al., 2013). Briefly, excessive KI solution was added to
samples purged of O2. After remaining in darkness 12–24 h for the
derivatization, the produced I3- ions were quantified at 420 nm
using a UV–VIS spectrophotometer (SHIMADZU UV-1800, Japan). The efficiency
of the total peroxides' measurement was discussed in our previous work (Li et
al., 2016), and the detection limit of the total peroxides was about
0.92 ppbv in the gas phase and 0.025 µg µg-1 in the
particle phase. The accuracy of this method was estimated to be 10 %,
with the precision of the measurement results usually within 20 %.
To measure the particle-phase carbonyls, SOA was collected onto a quartz
microfiber filter for 3 h. After collection, the filter was placed upside
down in a conical flask, and then 5 mL acetonitrile, 1 mL DNPH saturated
solution, and 50 µL H2SO4 solution (0.25 M) were added into
the flask in sequence. The flask was shaken at 180 rpm and 4 ∘C for 3 h and kept in darkness for 12–24 h prior to detection. For the
gas-phase carbonyls' measurement, gas through the filter was directly
introduced into a Horibe tube, which was placed in a cold trap (Beijing
Tiandijingyi TH-95-15-G, China) at about -98 ∘C to freeze the
products in the tube. The Horibe tube was composed of an inlet tube (25 cm length, 4 cm O.D.), a coil (7 laps, 1 cm O.D.), and an
outlet tube with a carborundum disc. The diagram of the Horibe tube is shown in the Supplement.
After collection, 10 mL acetonitrile was added to rinse the inside of the
Horibe tube to dissolve carbonyl compounds, and then the solution was mixed
with DNPH saturated solution and H2SO4 solution to derivatize for
12–24 h in darkness. The derived samples were analyzed using HPLC with UV
detection (Agilent 1100, USA). Details of this process are provided in our
previous report (Wang et al., 2009). The detection limit of carbonyl
compounds was about 0.3 ppbv in the gas phase, and in the particle phase it
was 0.15 ng µg-1 in terms of FA. The accuracy of this
method was estimated to be 8 % and the precision of the measurement
results was within 30 %. Since the yields of both ACE and
MGL were low, especially under some conditions, their
uncertainty might be high and the precision was within 80 %.
Wall loss experiments
To increase the accuracy of the results, we designed and conducted a series
of control experiments to quantify the wall loss effect, including gaseous
products and aerosol wall loss evaluation. Gas-containing peroxide
constituents were generated by passing N2 through a diffusion tube, at a
controlled temperature of 4 ∘C, which contained a certain peroxide
solution. The synthetic method of multiple organic peroxides was described in
our previous work (Huang et al., 2013). Gas-containing carbonyl constituents
were prepared by injecting a liquid substance into an evacuated steel canister
(15 L, Entech Instruments). Then N2 was added constantly until the
pressure in the canister reached 30 psi. The outlet of the canister was
linked with a mass flow controller that regulated the gas flow rate. The gas-containing peroxides or carbonyls were mixed with synthetic air at different
RH levels and introduced into the reactor at a rate of 2 standard L min-1, and the concentrations of peroxides and carbonyls were controlled
to be similar to the concentrations of products observed in limonene
ozonolysis. After the gas mixture was introduced into the flow tube, a period
of approximately 2 h was needed for the gas and wall to become balanced, and
then the measurements began. The aerosol wall loss experiment used
two-stage reaction equipment that contained two flow tube reactors. In order
to explore the wall loss effect on pure SOA, the first reactor was used to
generate aerosol, so that limonene and O3 had sufficient time to react
completely. The gas-containing aerosol from the first reactor was mixed
with synthetic air at different RH levels and introduced into the second
reactor at a rate of 2 standard L min-1. The particles at the inlet
and outlet of the second reactor were collected on PTFE filters and measured
by a balance to calculate the SOA concentration. The wall loss fraction of the
gas-phase constituent or SOA was determined as the difference between the
inlet and outlet concentrations divided by the inlet concentration as Eq. (S1) in the Supplement. The wall loss experiments were conducted in the RH
range of 0 %–90 %, and the profiles of loss fractions as a function of
RH could be used to correct products and SOA yields to diminish the wall loss
effect.
Results and discussion
Wall loss correction
The wall loss fractions of LMW peroxides increased with increasing RH, and
this tendency was especially obvious for performic acid (PFA) and
peracetic acid (PAA), whose loss fractions increased successively with RH.
For H2O2 and hydroxymethyl hydroperoxide (HMHP), their wall loss
fractions went up quickly above 50 % RH, yet they did not have a large
change below 50 % RH. Generally speaking, HMHP had the highest wall loss
fraction, reaching ∼0.25 at 90 % RH, while PFA had the
lowest wall loss fraction. The average of these LMW peroxides loss fractions
was used to correct the wall loss effect for gas-phase HMW peroxides.
As for carbonyl compounds, the variation of their wall loss fractions with RH
was not very obvious. The loss curves of FA, AA, and ACE were
somewhat irregular. The wall loss fractions of HACE, GL, and MGL were lowest
at 10 % RH, and then they gradually arose with increasing water vapor
concentration. The higher loss fractions of GL and MGL compared with other
carbonyls could be attributed to their inclination towards hydration. FA had
the lowest wall loss fraction around 0.03, while MGL had the highest loss
fraction around 0.12.
The wall loss effect on SOA was also investigated from 0 % to 90 %
RH. The SOA wall loss fraction was ∼0.06 in dry conditions, then it
increased slightly with water vapor concentration. The SOA loss fraction was
∼0.11 and ∼0.17 at 50 % and 90 % RH, respectively. The
dependencies of the peroxides, carbonyls, and SOA wall loss fractions on RH
are provided in the Supplement.
SCIs generation
The reaction between SCIs and water, which produces α-hydroxyalkyl
hydroperoxides (HAHPs) decomposing to H2O2, carbonyls, and
carboxylic acids, is thought to be an essential source of these compounds and
serves as a principle source of H2O2 formation without light
(Becker et al., 1990, 1993; Gäb et al., 1985; Sauer et al., 1999). Figure 1
shows the dependence of H2O2 yield and HMHP yield on RH, and the
six profiles in each subgraph represent conditions at a low or high
[O3] / [limonene] ratio in the presence or absence of the OH
scavenger (2-butanol or cyclohexane). The molar yield used here is defined as
the ratio of the product's number of moles to the number of moles of limonene
consumed, as given by Eq. (S2). It was obvious that, even in different cases,
the variations of H2O2 yield and HMHP yield with RH had similar
tendencies, both of which increased significantly from 0 % RH to 70 %
RH, and then they approached the limiting values. There was little effect of
the OH scavenger. At a low [O3] / [limonene] ratio, the maximum
H2O2 yield was ∼24.00 % without the OH scavenger,
∼24.60 % with 2-butanol, and ∼22.95 % with cyclohexane.
At a high [O3] / [limonene] ratio, the maximum
H2O2 yield reached ∼41.20 % without the OH scavenger,
∼41.80 % with 2-butanol, and ∼40.50 % with cyclohexane.
As for HMHP, its yield was much higher at a high
[O3] / [limonene] ratio (∼5.43 %) than at a low
[O3] / [limonene] ratio (∼0.62 %), with the specific
information found in Fig. 1. It is usually believed that H2O2 has two generation pathways:
HO2 self-reaction and HAHPs' decomposition. The former pathway is
considered as a main contributor to H2O2 during daytime, while
the latter is regarded to occur without photochemistry. In the reaction
system discussed here, when we applied a box model coupled with the limonene
reaction mechanism extracted from Master Chemical Mechanism (MCM) v3.3
(http://mcm.leeds.ac.uk/MCMv3.3.1, last access: 15 March 2018) to
simulate the reaction, the yield of H2O2 formed from the
HO2 self-reaction was estimated to be less than 0.1 % under
both dry and wet conditions. When the 2-butanol or cyclohexane reaction
mechanism was taken into consideration, the contribution of this pathway to
H2O2 formation was still very limited; hence, it was assumed
that the HO2 self-reaction was not important for H2O2
generation in this reaction system. Under dry conditions, a small amount of
water vapor might desorb from the flow tube wall and participate in
reactions, resulting in a small amount of H2O2 formation. Thus,
the high H2O2 yield observed here might be mainly attributed to
SCIs' reaction with water.
Dependence of (a) H2O2 yield and (b) HMHP
yield on relative humidity (RH) at a low or high
[O3] / [limonene] ratio in the presence or absence of the OH
scavenger (2-butanol or cyclohexane). H2O2, hydrogen peroxide;
HMHP, hydroxymethyl hydroperoxide; L, low [O3] / [limonene] ratio; H,
high [O3] / [limonene] ratio; No-sca, no scavenger; 2-But,
2-butanol; C-hex, cyclohexane.
![]()
From 0 % to 70 % RH, both the H2O2 yield and HMHP yield were
promoted significantly by increasing RH, indicating that the reaction with
water gradually became dominant for limonene SCIs. Jiang et al. (2013)
suggested that the formation of HAHPs was the most favorable pathway for
limonene SCIs' reaction with H2O, and the subsequent decomposition of
HAHPs was thought to be prior to generation of aldehyde and H2O2
(Chen, 2016; Kumar, 2014). Although theoretical calculation results indicate
that HAHPs' decomposition is slow, some studies proved that water and acid
molecules could greatly promote the decomposition process (Anglada et al.,
2002, 2011; Aplincourt and Anglada, 2003; Crehuet et al.,
2001), and H2O2 formation from HAHPs' decomposition occurs very
rapidly (Chen et al., 2016; Winterhalter et al., 2000). The fact that few
HAHPs larger than HMHP were identified in alkene ozonolysis also provided
evidence for the rapid decomposition of large HAHPs. Above 70 % RH, the
appearance of the limiting values of H2O2 yield and HMHP yield
suggested that the water vapor concentration was high enough that the
bimolecular reaction of SCIs and H2O suppressed the unimolecular
decomposition of SCIs and the reactions of SCIs with other products. In
previous studies, the unimolecular decomposition of SCIs and the reactions of
SCIs with water showed strong structure dependence. For CH2OO and
anti-CH3CHOO, the atmospheric lifetimes of their bimolecular
reactions with H2O and (H2O)2 were less than 1 ms, while their
lifetimes of unimolecular reaction were much longer (Lin et al., 2016; Sheps
et al., 2014; Taatjes et al., 2013; Welz et al., 2012). For
syn-CH3CHOO and (CH3)2COO, the atmospheric lifetimes of
their bimolecular reactions with H2O and (H2O)2 were more than
100 ms, while their lifetimes of unimolecular reaction were just a few
milliseconds at 298 K (Drozd et al., 2017; Huang et al., 2015; Long et al.,
2018; Sheps et al., 2014; Taatjes et al., 2013; Welz et al., 2014). For
larger SCIs their atmospheric lifetimes of unimolecular reaction and
bimolecular reactions with H2O and (H2O)2 were not quantified.
Tillmann et al. (2010) inferred that about 46 % of ECIs formed from α-pinene ozonolysis would stabilize and at 44 % RH about 65 % of SCIs
formed H2O2 after reacting with water and 28 % of SCIs
decomposed. Yao et al. (2014) showed that a large fraction of ECIs formed
from α-cedrene were stabilized and the unimolecular decomposition of
SCIs was suppressed by their bimolecular reactions. Although we could not
estimate the rates of SCIs' decomposition and their reaction with water, the
results here prove that the reaction with water was an essential route for
limonene SCIs, and the rapid decomposition of HAHPs made an important
contribution to H2O2 formation.
According to the experimental conditions elaborated in Sect. 2.2, we tried to
calculate the contribution of endocyclic DB and exocyclic DB ozonolysis to
H2O2 and HMHP formation, and furthermore, infer the SCIs
generated in the ozonolysis of different DBs. The predicted SCI yield was
derived by combining the limiting yields of H2O2 and HMHP
together, as in Eq. (S3), based on the assumption that
the limiting yield of H2O2 was equal to the large SCI yield
(Hasson, 2001a, b). Considering that a portion of SCIs might undergo
unimolecular decomposition, a lower limit yield of SCIs formed in limonene
ozonolysis was estimated here. Additionally, as described in Sect. 3.3, the
results demonstrated that the OH yield was not obviously affected by RH, and
we speculated that the fraction of SCIs that decomposed and formed OH
radicals was small. It was observed that the OH scavenger did not have a
large impact on the SCIs' measurement results, while a big difference existed
between the ozonolysis of two DBs. The SCI yield of the endocyclic DB
ozonolysis was around 24.45 %, yet the exocyclic DB ozonolysis had a larger
stabilization fraction of ECIs, which was about 42.90 %. It should be noted
that, since our experiments just isolated the ozonolysis of different DBs
according to their discrepancy in reaction rates, the SCI yield of the
exocyclic DB ozonolysis estimated here was not for a specific product formed
from endocyclic DB ozonolysis but was the sum of first-generation products
with a remaining double bond, as shown in Fig. 2. The results meant that,
even though exocyclic DB ozonolysis is much slower than endocyclic DB, it
plays a non-negligible role in generating SCIs in limonene ozonolysis.
The reaction scheme of endocyclic and exocyclic double
bonds in limonene ozonolysis.
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OH radicals generation
In this study, OH yield was determined by adding sufficient 2-butanol as a
scavenger and measuring how much 2-butanone was generated. Aschmann et al. (2002) suggested that using 2-butanol to measure OH formation would be more
accurate than using cyclohexane. The yield of 2-butanone formed from the
reaction of 2-butanol and OH was detected previously (Aschmann et al., 2002;
Baxley and Wells, 1998; Chew and Atkinson, 1996), and we used 0.66, which is
the average of the reported values (Forester and Wells, 2011), to calculate
OH radicals' yield. The results showed that the yield of OH radicals produced
from endocyclic DB ozonolysis was 0.65±0.21, while for exocyclic DB,
the OH yield was 0.24±0.13. The OH generation did not show a clear
dependence on water vapor concentration. The OH yield of endocyclic DB
ozonolysis was within the range of published values, while the OH yield of
exocyclic DB ozonolysis was determined to be higher than that reported by
Herrmann et al. (2010). The effect of water on OH formation in alkene
ozonolysis is still under debate as there are discrepancies among the
existing publications. Anglada et al. (2002) used quantum mechanical
calculations to indicate that water could increase OH production, and
Tillmann et al. (2010) reported higher OH yield under humid conditions.
Nevertheless, more studies showed that OH formation was independent of water
vapor concentration (Aschmann et al., 2002; Atkinson and Aschmann, 1993;
Atkinson et al., 1992; Berndt et al., 2003; Forester and Wells, 2011; Hasson
et al., 2003). The fact that the OH yields of both unsaturated bonds were
not obviously affected by RH suggests that the major pathway of OH
generation in limonene ozonolysis is the decomposition of ECIs through
hydroperoxide channel. The possibility that other OH formation pathways also
exist cannot be totally excluded, but we speculate that any other pathways
are not significant. Hence, it could be concluded that in limonene
ozonolysis, the endocyclic DB is inclined to generate OH radicals through
ECIs' decomposition, while the exocyclic DB has a higher fraction of
stabilization-forming SCIs.
Peroxycarboxylic acids generation
In our experiments, two kinds of peroxycarboxylic acids, PFA (CH(O)OOH) and
PAA (CH3C(O)OOH), were observed, and the yields of PFA and PAA both
showed large discrepancies between the low-ratio and high-ratio experiments
(Fig. 3). The yields of PFA and PAA at a high [O3] / [limonene]
ratio were about 3 times as much as those at a low
[O3] / [limonene] ratio. When the exocyclic DB was ozonated,
the generation of PFA and PAA was enhanced to a large extent compared with
only endocyclic DB ozonolysis, indicating that exocyclic DB ozonolysis had an
important impact on PFA and PAA. As far as we know, this was the first time
that such high yields of PFA and PAA in alkene ozonolysis have been reported.
For both PFA and PAA, the highest molar yields were observed when no OH
scavenger was used, demonstrating that the OH reaction contributed to part of
their formation. When OH radicals were scavenged, we speculated that PFA and
PAA were mainly formed through ECIs' isomerization and decomposition
following the generation of HC(O)OO and CH3C(O)OO radicals. These
radicals could further react with HO2 and form peroxycarboxylic acids, which
provided a plausible explanation for the higher yields of PFA and PAA
observed in experiments with 2-butanol instead of cyclohexane.
Dependence of (a) PFA yield and (b) PAA
yield on relative humidity (RH) at a low or high
[O3] / [limonene] ratio in the presence or absence of the OH
scavenger (2-butanol or cyclohexane). PFA, performic acid; PAA,
peracetic acid; L, low [O3] / [limonene] ratio; H, high
[O3] / [limonene] ratio; No-sca, no scavenger; 2-But, 2-butanol;
C-hex, cyclohexane.
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The yields of PFA and PAA had a positive relationship with water vapor
concentration in all the cases, reaching the highest level at 90 % RH.
Model simulation results significantly underestimated the PAA formation and
ignored the RH effect, indicating some missing pathways of PAA generation
related to water. Because of the deficiency of PFA mechanism in MCM, we did
not simulate the PFA formation. However, the similar variation tendency of
PFA and PAA yields could provide some evidence for the assumption that they
might have similar formation mechanisms. In previous studies, the dominant
formation pathway of PAA was considered to be the reaction of
CH3C(O)OO radicals with HO2 radicals (Groß et al., 2014; Lightfoot et al.,
1992; Winiberg et al., 2016), while the formation pathway of PFA had not
been identified yet and was speculated to be the reaction of HC(O)OO
radicals with HO2 radicals (Liang et al., 2015). The rate constant of
the RO2 reaction with HO2 was investigated in a series of studies
and was thought to be unaffected by water (Atkinson et al., 1999; Lightfoot
et al., 1992; Tyndall et al., 2001; Wallington et al., 1992). Therefore, the
increase of PFA and PAA yields with RH might be attributed to the promoting
effect of water on HC(O)OO and CH3C(O)OO radical generation. Because
the highly reactive peroxides were easy to lose on the wall, it might be hard for chamber
studies, which usually last for hours, to observe these
peroxycarboxylic acids. In field observations, PAA is widely reported in
remote and urban areas (Lee et al., 2000; Liang et al., 2013; Zhang et al.,
2010), yet there are few reports on PFA existence in the atmosphere (Liang
et al., 2015). The results here demonstrate that limonene ozonolysis could
contribute to PFA and PAA formation, and although its high instability and
reactivity make PFA difficult to observe, it may have a short duration in the
atmosphere.
H2O2 generation per particle mass (ng ug-1) in
SOA formed with different OH scavengers in the relative humidity (RH) range
of 0 %–90 % at a low [O3] / [limonene] ratio.
0 % RH
10 % RH
30 % RH
50 % RH
70 % RH
80 % RH
90 % RH
No-sca
1.13±0.22
1.42±0.40
1.53±0.28
1.88±0.16
2.24±0.42
2.26±0.44
2.45±0.48
2-But
1.33±0.15
1.56±0.18
1.77±0.12
2.02±0.65
2.55±0.43
2.89±0.42
2.66±0.57
C-hex
3.22±0.52
3.95±0.43
4.12±0.40
4.22±0.33
4.63±0.24
4.26±0.33
4.63±0.96
No-sca, no scavenger; 2-But, 2-butanol; C-hex, cyclohexane.
Particulate peroxides and <inline-formula><mml:math id="M239" display="inline"><mml:mrow class="chem"><mml:msub><mml:mi mathvariant="normal">H</mml:mi><mml:mn mathvariant="normal">2</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn mathvariant="normal">2</mml:mn></mml:msub></mml:mrow></mml:math></inline-formula> generation
<inline-formula><mml:math id="M240" display="inline"><mml:mrow class="chem"><mml:msub><mml:mi mathvariant="normal">H</mml:mi><mml:mn mathvariant="normal">2</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn mathvariant="normal">2</mml:mn></mml:msub></mml:mrow></mml:math></inline-formula> evolution at a low [<inline-formula><mml:math id="M241" display="inline"><mml:mrow class="chem"><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn mathvariant="normal">3</mml:mn></mml:msub></mml:mrow></mml:math></inline-formula>] <inline-formula><mml:math id="M242" display="inline"><mml:mo>/</mml:mo></mml:math></inline-formula> [limonene] ratio
H2O2 generation from SOA in the aqueous phase is thought to be a
possible way of producing H2O2 in cloud water or releasing
H2O2 continuously after inhalation. Here, we quantitatively
measured H2O2 generation from SOA produced in limonene
ozonolysis in different cases. Considering that some unstable constituents
would decay rapidly when SOA extract solution was kept at room temperature
(298 K), we chose to keep the SOA solution at 4 ∘C to maintain the
stability of the samples and prolong their storage time. In the low-ratio
experiments, the concentration of total peroxides in solution was nearly
stable in 48 h. The H2O2 concentration went through a
brief period of increase and then became stable. After the aqueous
H2O2 concentration reached a plateau, the amount of particulate
H2O2 per particle mass formed in different cases, from 0 % to
90 % RH, was calculated and shown in Table 2. Regardless of whether the OH
scavenger was used or not, SOA produced at higher RH was inclined to have a
higher capacity of producing H2O2. When no OH scavenger was
used, the lowest H2O2 content per particle mass was
∼1.13 ng µg-1 in dry conditions, and the highest value was
∼2.45 ng µg-1 at 90 % RH. When 2-butanol was used the
changing trend of H2O2 generation resembled the condition
without the scavenger, and the minimum was ∼1.33 ng µg-1 at
0 % RH, while the maximum was ∼2.89 ng µg-1 at 80 % RH,
which was a little higher than the value at 90 % RH. It was interesting to
note that, in the presence of cyclohexane, the trend of H2O2
generation in SOA solution differed greatly from both of the above. Even
under dry conditions, the H2O2 content per particle mass could
reach ∼3.22 ng µg-1, and the maximum was ∼4.63 ng µg-1 at 90 % RH.
<inline-formula><mml:math id="M274" display="inline"><mml:mrow class="chem"><mml:msub><mml:mi mathvariant="normal">H</mml:mi><mml:mn mathvariant="normal">2</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn mathvariant="normal">2</mml:mn></mml:msub></mml:mrow></mml:math></inline-formula> evolution at a high [<inline-formula><mml:math id="M275" display="inline"><mml:mrow class="chem"><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn mathvariant="normal">3</mml:mn></mml:msub></mml:mrow></mml:math></inline-formula>] <inline-formula><mml:math id="M276" display="inline"><mml:mo>/</mml:mo></mml:math></inline-formula> [limonene] ratio
In the experiments with a high [O3] / [limonene] ratio, SOA produced with
different scavengers was found to have different rates of H2O2
generation in solution at 4 ∘C, according to which we determined
appropriate detection frequency and total duration for the three kinds of
SOA. SOA produced without the OH scavenger had the lowest rate of
H2O2 generation, so an 8-day measurement result was reported
here. For SOA produced in the presence of 2-butanol, H2O2
generation was a little faster than the former and reached the limiting value
within 6 days. However, SOA produced with cyclohexane had a much faster
rate than both of the above, and the H2O2 concentration in
solution became stable within 3 days. For these three types of SOA, the
total peroxides' concentration decreased slightly in the analysis duration,
which is clarified in the Supplement.
It is obvious that SOA produced in the high-ratio experiments has a greater
ability of generating H2O2, and Fig. 4 shows the time profiles
of H2O2 evolution of different kinds of SOA. When no OH
scavenger was used, H2O2 concentration in solution rose
constantly for 6 days then became stable. The limiting value of the
H2O2 generation was influenced by RH, which was ∼5.28 ng µg-1 at 0 % RH, then increased gradually with increasing RH
until 80 % RH (∼14.45 ng µg-1). In the experiments with
2-butanol, the H2O2 concentration kept rising in the first 4
days, then it became stable. Under dry conditions, the limiting value of the
H2O2 content was ∼7.60 ng µg-1, then it
increased until 50 % RH. SOA produced above 50 % RH did not show any
obvious difference, and the highest H2O2 content was
∼15.50 ng µg-1. When cyclohexane was added, the
H2O2 concentration in solution increased rapidly on the first
day then tended to become stable. The limiting value of H2O2
generation was also affected by water vapor concentration, yet the promoting
effect was not very significant, and no large difference was observed above
30 % RH. The limiting H2O2 content per particle mass was
∼16.64 ng µg-1 in dry conditions and was ∼30.00 ng µg-1 above 30 % RH.
Time profiles of H2O2 evolution per particle
mass of different SOA formed (a) without the OH scavenger, (b) with 2-butanol,
and (c) with cyclohexane in the relative humidity (RH) range
of 0 %–90 % at a high [O3] / [limonene] ratio.
H2O2, hydrogen peroxide; No-sca, no scavenger; 2-But,
2-butanol; C-hex, cyclohexane.
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Different stabilities of particulate peroxides
According to the measurement results of the total peroxides and
H2O2 in the SOA solution, the particulate peroxides could be
roughly divided into two categories: one is unstable and could decompose or
hydrate to generate H2O2, the other one is stable and could
nearly remain unchanged in several days. Supposing that all the peroxides
contained one peroxy bond, the molar fractions of stable or unstable
peroxides could be calculated as follows:
φi=NiNtotal,
where φi is the molar fraction of stable or unstable peroxides
in particles, Ni (mol) is the number of moles of stable or unstable
peroxides in particles, and Ntotal (mol) is the number of moles of total
peroxides in particles. At a low [O3] / [limonene] ratio, the molar
fraction of unstable peroxides in SOA was around 0.11–0.13 in the case of
adding no scavenger, which was similar to the case of adding 2-butanol. In
the presence of cyclohexane, the molar fraction of unstable peroxides would
increase to 0.20–0.32. At a high [O3] / [limonene] ratio, the molar
fractions of unstable peroxides in the case of adding no scavenger and adding
2-butanol were also similar, with both of them in the range 0.13–0.25, yet
this value would reach ∼0.50 when cyclohexane was used.
Detailed information is provided in the Supplement. Model results showed that
most peroxides produced in the reaction were ROOH and R(O)OOH; in addition,
some studies proposed that peroxyhemiacetals also made an important contribution
to aerosol (Tobias and Ziemann, 2000; Tobias et al., 2000). The acetal
reaction producing peroxyhemiacetals is reversible, so part of the
peroxyhemiacetals in SOA may have hydrolyzed to form some peroxides. To
investigate the stabilities of ROOH and R(O)OOH, we synthesized methyl
hydroperoxide (MHP) and ethyl hydroperoxide (EHP) to represent ROOH. PFA and
PAA were used to represent R(O)OOH. All the synthesis solutions were stored
at 4 ∘C, which was the same as the experimental condition. PFA and
PAA were found to decompose and generate H2O2 within several
days, yet MHP and EHP maintained stable. Hence, we speculate that
peroxycarboxylic acids and peroxyhemiacetals might be the main components of
unstable peroxides in particles, and they contribute to H2O2
generation in the aqueous phase.
The amount of H2O2 generation at a low
[O3] / [limonene] ratio measured here was comparable with the
published value of H2O2 produced from α-pinene SOA in
solution (Li et al., 2016; Wang et al., 2011). However, at a high
[O3] / [limonene] ratio, the H2O2 generation level
increased significantly, which proved that the multi-generation oxidation
improved the formation of peroxycarboxylic acids and peroxyhemiacetals in
particles. These results demonstrated that SOA produced in limonene
ozonolysis could behave as an important source of H2O2 in the
aqueous phase, and they also showed the difference between the SOA formed
from single-unsaturated monoterpene ozonolysis and double-unsaturated
monoterpene ozonolysis.
The effect of the OH scavenger
Prior research has provided evidence for OH radicals' formation in ozonolysis
experiments, and the OH scavenger is often used to avoid the disturbance of the
OH reaction. Both 2-butanol and cyclohexane are commonly used OH scavengers,
and Chew and Atkinson (1996) showed that there was no difference in their
abilities to scavenge OH radicals. However, it should be noted that the OH
scavenger could convert OH radicals into a mixture of hydroperoxy (HO2)
and alkylperoxy (RO2) radicals, and a higher [HO2] / [RO2] ratio is
observed when 2-butanol is used (Docherty and Ziemann, 2003; Jonsson et al.,
2008a). As shown in the Sect. 3.6.1, the fact that both at a low and high
[O3] / [limonene] ratio, the SOA yield was higher with 2-butanol than
cyclohexane suggested that the increase in HO2 concentration promoted
SOA formation. This result is consistent with that suggested by Keywood
et al. (2004) who observed the higher [HO2] / [RO2] ratio, resulting in
a higher SOA yield in cyclohexene ozonolysis, while Docherty and Ziemann (2003)
showed that an increased [HO2] / [RO2] ratio inhibited aerosol formation in
β-pinene ozonolysis.
Some studies demonstrated that acid and peroxide products were sensitive to
OH scavengers (Ahmad et al., 2017; Henry and Donahue, 2011; Ma et al.,
2008). Here, through investigating the stabilities of particulate peroxides
formed in different conditions, we showed that the choice of the OH scavenger
would influence the types of particulate peroxides produced in the reaction.
In Sect. 3.5.3, we suggest that peroxycarboxylic acids and
peroxyhemiacetals be the main components of unstable peroxides in particles.
The peroxyhemiacetals formed by heterogeneous reactions of peroxides and
aldehydes could dissociate into these species, yet the formation of
peroxyhemiacetals was not considered to be affected by the OH scavenger
since both the peroxides and carbonyl compounds observed in the gas phase
did not show large differences when the OH scavenger changed. We speculate
that the impact of the OH scavenger on particulate peroxides could be mainly
attributed to the formation of peroxycarboxylic acids under a different
[HO2] / [RO2] ratio. Keywood et al. (2004) indicated that the OH scavenger
impacted the HO2 reaction with acyl peroxy radicals forming acid and
peracid products, which were considered to have low volatility. With higher
HO2 concentration, the SOA yield in the presence of 2-butanol was
higher than that in the presence of cyclohexane, since the
HO2–acyl peroxy reactions contributed to some low-volatility products.
However, the RO2 radicals formed from cyclohexane may also participate
in reactions and help produce more unstable peroxycarboxylic acids which
could partition into the particle phase.
The SOA yield and mass fraction of particulate peroxides at
a low or high [O3] / [limonene] ratio in the presence or absence of the OH scavenger from 0 % to 90 % relative humidity (RH).
L(No-sca)
L(2-But)
L(C-hex)
H(No-sca)
H(2-But)
H(C-hex)
0 % RH
0.065±0.006
0.101±0.009
0.087±0.011
0.401±0.016
0.502±0.008
0.477±0.010
10 % RH
0.091±0.010
0.124±0.013
0.113±0.009
0.436±0.009
0.534±0.009
0.502±0.013
30 % RH
0.125±0.010
0.147±0.011
0.143±0.015
0.458±0.020
0.553±0.015
0.506±0.011
50 % RH
0.149±0.007
0.174±0.011
0.161±0.016
0.466±0.016
0.571±0.009
0.512±0.007
70 % RH
0.155±0.009
0.178±0.009
0.169±0.014
0.486±0.023
0.576±0.010
0.503±0.011
80 % RH
0.169±0.013
0.189±0.010
0.189±0.012
0.492±0.015
0.580±0.013
0.502±0.014
90 % RH
0.156±0.010
0.183±0.013
0.189±0.013
0.492±0.017
0.580±0.018
0.506±0.016
SOA yield
0.379±0.039
0.337±0.048
0.288±0.038
0.511±0.097
0.479±0.044
0.401±0.068
Note: L, low [O3] / [limonene] ratio; H, high
[O3] / [limonene] ratio; No-sca, no scavenger; 2-But, 2-butanol; C-hex, cyclohexane.
Contribution of peroxides to SOA
SOA formation
The yield of SOA produced from limonene ozonolysis in different cases ranging
from 0 % to 90 % RH was measured. The SOA yield is defined as the ratio
of aerosol mass concentration to the mass concentration of limonene consumed,
as given by Eq. (S4). In the six different cases, SOA
yield was determined to be unaffected by water, yet it showed strong
dependence on the reactants' ratio and the use of the OH scavenger, as shown in
Table 3. Whether at a low or high [O3] / [limonene] ratio, the case of
no OH scavenger had the highest SOA yield, which suggested the effect of OH
reaction on aerosol formation, and the order of SOA yield was no scavenger > 2-butanol > cyclohexane.
The case with the highest
SOA yield was at a high [O3] / [limonene] ratio without the OH scavenger
(0.511±0.097), while the lowest SOA yield was detected at a low
[O3] / [limonene] ratio with cyclohexane (0.288±0.038).
Peroxides' mass fraction
In this study, we used the iodometric method to analyze the particle-phase
total peroxides' content in limonene ozonolysis. The iodometric method is
commonly regarded as a standard method for detecting total peroxides and it
could almost quantify all kinds of peroxides (Bonn et al., 2004; Jenkin,
2004). The mass fraction used here is defined as the ratio of particulate
peroxides' mass to SOA mass, as given in Eq. (S5). The
average molecular weight of peroxides in particles is assumed to be 300 g mol-1, which is estimated to be slightly less than the molecular weight of
peroxyhemiacetals, and this value has been used to calculate the mass of
particulate peroxides in some studies (Docherty et al., 2005; Nguyen et al.,
2010; Surratt et al., 2006). The mass fractions of peroxides in the six cases
from 0 % to 90 % RH are summarized in Table 3. In each case, the
peroxides' mass fraction in SOA increased slightly with RH, and there was a
significant difference between the low-ratio and high-ratio experiments,
while there was little effect of the OH scavenger. The mass fraction of
peroxides at a low [O3] / [limonene] ratio was usually below
0.2;
however, at a high [O3] / [limonene] ratio the peroxides' mass fraction
increased to 0.4–0.6. Docherty et al. (2005) reported that the peroxides'
mass fraction in α-pinene SOA was ∼0.47, while for
β-pinene SOA the fraction was ∼0.85. Li et al. (2016)
observed that peroxides could account for ∼0.21 in α-pinene SOA. We are the first to report the mass fraction of peroxides in
SOA derived from limonene ozonolysis, highlighting the important role of
organic peroxides in SOA composition, especially when multi-generation
oxidation happens.
Peroxides' partitioning
The molar yields of HMW peroxides in both the gas phase and the particle phase were
determined in different cases. There was little effect of RH and the OH
scavenger, while at a high [O3] / [limonene] ratio the HMW
peroxides' yield increased by ∼10 % in contrast with the yield at a low
[O3] / [limonene] ratio. Furthermore, the gas-phase HMW peroxides and
particle-phase HMW peroxides were discussed separately, which is shown in
Fig. 5. For gas-phase HMW peroxides, the molar yield showed a decreasing
tendency with increasing RH, which was obvious at a high
[O3] / [limonene] ratio. For particle-phase HMW peroxides, the molar
yield showed a slight increasing dependence on RH and their yield was
promoted significantly by the degree of oxidation. A possible explanation for
the RH effect was that the water content in aerosol would increase when RH
increased, and this promoted the uptake of some compounds into particles.
However, some research proved that the effect of water on the partitioning
of organic products seemed to be small, especially when no inorganic seed
particles were used (Jonsson et al., 2006, 2008a, b). Thus, we propose that
the promoting effect of water on particulate HMW peroxides can be mainly
attributed to the effect of water participating in some gas-phase and
particle-phase reactions, resulting in the formation of more low-volatility
peroxides. The contribution of multi-generation oxidation products to SOA
formation in limonene ozonolysis has been stated by some studies (Hoffmann et
al., 1997; Ng et al., 2006), and the results here indicate that organic
peroxides might account for a considerable proportion of these products.
Multi-generation oxidation helps produce more low-volatility peroxides in the
gas phase that could partition into particles, and moreover, it accelerates
the occurrence of some heterogeneous reactions by providing more reactive
species.
The variation of (a) gas-phase and (b)
particle-phase high-molecular-weight peroxides' molar yields with relative
humidity (RH) at a low or high [O3] / [limonene] ratio in the
presence or absence of the OH scavenger (2-butanol or cyclohexane). L, low
[O3] / [limonene] ratio; H, high [O3] / [limonene] ratio;
No-sca, no scavenger; 2-But, 2-butanol; C-hex, cyclohexane.
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Yields (%) of carbonyls at a low or high
[O3] / [limonene] ratio in the presence or absence of the OH
scavenger.
HACE
FA
AA
ACE
GL
MGL
L(No-sca)
2.04±0.48
7.02±0.90–10.58±0.94
1.32±0.24
0.22±0.15
0.89±0.25
0.56±0.34
L(2-But)
3.94±0.55
4.90±0.86–7.77±0.86
3.98±0.60
0.35±0.18
0.69±0.25
0.55±0.44
L(C-hex)
–
5.21±0.66–8.25±0.55
–
–
0.81±0.24
0.67±0.56
H(No-sca)
4.45±0.52
13.11±0.63–27.00±1.56
2.16±0.84
0.79±0.22
1.33±0.41
1.35±0.61
H(2-But)
10.15±2.11
11.03±0.77–23.33±0.62
7.86±1.32
1.00±0.28
1.25±0.36
1.31±0.21
H(C-hex)
–
10.80±1.28–23.32±1.21
1.89±1.22
0.60±0.47
1.25±0.51
1.23±0.22
Note: –, below detection limit; L, low
[O3] / [limonene] ratio; H, high [O3] / [limonene] ratio;
No-sca, no scavenger; 2-But, 2-butanol; C-hex, cyclohexane; HACE,
hydroxyacetone; FA, formaldehyde; AA, acetaldehyde; ACE, acetone; GL,
glyoxal; MGL, methylglyoxal.
Contribution of carbonyls to SOA
Carbonyls' formation
Carbonyl compounds including HACE, FA, AA, ACE, GL, and MGL were detected in
the reaction. Unlike peroxides, whose generation often showed obvious
increasing dependence on RH, only FA yield would increase with RH and the
production of other carbonyls was not significantly affected by water. The
yields of HACE, AA, ACE, GL, and MGL in different cases are summarized in
Table 4. The fact that HACE formed without the OH scavenger but did not form when
cyclohexane was used indicated that HACE might be generated from OH
reactions. As for AA and ACE, both of their yields in the presence of
cyclohexane were found to be lower than the case without the scavenger,
especially at a low [O3] / [limonene] ratio. This suggested that the OH
reaction contributed to a portion of their formation and the endocyclic DB
ozonolysis did not tend to generate AA and ACE. It was speculated that these
three kinds of carbonyls could also be generated from the reaction of 2-butanol
or that HO2 radicals promoted their formation, so the presence of 2-butanol
increased their yields. As regards to GL and MGL, RH and the OH scavenger did not
have a large influence, and both of their yields at a high
[O3] / [limonene] ratio were higher than the conditions of a low
[O3] / [limonene] ratio. The generation of FA had a positive
dependence on RH, which was illustrated in the Supplement and the total
variation range could be found in Table 4.
Experimental and theoretical partitioning coefficients
A parameter that has been used widely to describe the partitioning feature
of a compound is described as follows (Pankow, 1994; Pankow and Bidleman,
1992):
Kp,i=Fi/TSPAi,
where Kp,i (m3 µg-1) is the partitioning coefficient
of compound i, TSP (µg m-3) is the concentration of total
suspended particulate matter, and Fi (µg m-3) and Ai (µg m-3)
are the particulate and gaseous concentrations of compound i,
respectively. The measured partitioning coefficients of FA, AA, and ACE were
on the magnitude of 10-5 m3 µg-1, and the partitioning
coefficients of HACE, GL, and MGL were on the magnitude of 10-4 m3 µg-1.
Theoretical gas–particle partitioning coefficients of these compounds were
calculated using the absorption equilibrium equation defined by Pankow (1994), which has been used widely to estimate the ability of a substance to
partition into the particulate phase and to predict SOA yield in models
(Griffin et al., 1999; Hohaus et al., 2015; Odum et al., 1996; Yu et al.,
1999):
Kp,i=760RTfOMMWOM106ζipL,i0,
where R is the ideal gas constant (8.206×10-5 m3 atm mol-1 K-1), T is the temperature (K), fOM is the mass
fraction of TSP that is the absorbing organic matter (OM) and is equal to
1 here, MWOM (g mol-1) is the mean molecular weight of the OM
phase, which is estimated to be 130 g mol-1 in this study, ζi
is the activity coefficient of compound i in the OM phase, which is assumed
to be unity, and pL,i0 (Torr) is the vapor pressure of compound i,
which is predicted using the method of Moller et al. (2008). The calculated
gas–particle partitioning coefficients of HACE, FA, AA, ACE, GL, and MGL
were 3.972×10-8 m3 µ g-1, 2.096×10-11 m3 µg-1, 1.624×10-10 m3 µg-1, 6.192×10-10 m3 µg-1,
5.476×10-10 m3 µg-1, and 1.246×10-9 m3 µg-1, respectively, at 298 K. The experimental
Kp value of HACE was about 10 000 times larger than the theoretical
value, and for other carbonyls, the experimental Kp value was about 100 000
times larger than the theoretical value. The gap between the experimental
Kp and predicted Kp of GL and MGL we estimated was comparable with
the results of Healy et al. (2008, 2009), but it was higher than that of
Ortiz et al. (2013). The fact that the gas–particle partitioning
coefficients of carbonyls observed were much higher than the theoretical
values indicated that carbonyl compounds made a more important contribution
to SOA formation than estimated. Although the partitioning coefficients
measured in experiments showed a huge difference with the calculated values, a
relationship between the measured Kp and the vapor pressure of carbonyl
compounds was observed. Figure 6 shows the dependence of the measured
lg(Kp) and predicted lg(Kp) on lg(p0), and their linear fitting curves. The slope of the linear
fitting equation of the predicted lg(Kp) versus lg(p0) was -0.964, and R2 was 0.998. The slope of the linear
fitting equation of measured lg(Kp) versus lg(p0) was -0.484, and R2 was 0.750, which indicated that the
lg(Kp) of carbonyls observed in laboratory also had a
negative correlation with lg(p0). A plausible explanation
for the large difference between the measured and predicted Kp was that
carbonyl compounds were easy to polymerize and react with other species on
particles, resulting in these carbonyls existing in the form of hydrates and
oligomers in SOA (Corrigan et al., 2008; Hastings et al., 2005; Kroll et
al., 2005; Volkamer et al., 2007). The hydrates and oligomers of carbonyls
have much lower vapor pressures than their precursors, and they could
reversibly return to their carbonyl monomers during analysis (Healy et al.,
2008; Ortiz et al., 2013; Toda et al., 2014).
The relationship of measured and predicted partitioning
coefficients (m3 µg-1) versus vapor pressure (atm) of
carbonyls produced in limonene ozonolysis. Kp, partitioning
coefficients; p0, vapor pressure, HACE, hydroxyacetone; FA,
formaldehyde; AA, acetaldehyde; ACE, acetone; GL, glyoxal; MGL,
methylglyoxal.
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