Introduction
Organic peroxides are important trace components in the atmosphere, serving
as reservoirs of HOx and ROx radicals, participating in the
formation of secondary organic aerosol (SOA), and causing adverse health
effects as reactive oxygen species. Recently, peroxides were found to
play a key role in the aging of SOA. The particle-bound organic peroxides
undergo atmospheric photolysis with a lifetime of about 6 days (Epstein et
al., 2014), and decline significantly within the mean SOA age of 4–7 days
(Rudich et al., 2007). A laboratory experiment on the photolysis of SOA shows
a high yield of hydroxyl radicals (OH), which are considered to form from the
decomposition of peroxides (Badali et al., 2015). This OH may cause the
in-particle oxidation of SOA.
Model studies have tried to simulate the SOA formation in chamber
experiments, but great discrepancies still exist between predicted and
observed results (Camredon et al., 2010; Hoffmann et al., 1997; Griffin et
al., 1999; Cocker III et al., 2001; Saathoff et al., 2009; Presto et al.,
2005; Pye and Seinfeld, 2010; Farina et al., 2010). Jenkin (2004) added the
formation of dimers and improved the simulation, especially at the beginning
of SOA formation. Organic peroxides were found to be highly abundant in SOA
(Ziemann, 2005; Docherty et al., 2005; Surratt et al., 2006; Nguyen et al.,
2010; Bateman et al., 2011; Mertes et al., 2012; Epstein et al., 2014),
possibly in the form of oligomers, which are even more important than
carboxylic acids (Bonn et al., 2004). In order to improve the simulation of
the production of SOA mass within the chamber, explicit parameters for
gas-particle partitioning of organic peroxides are urgently needed.
The reactions and processes that generate or remove peroxides have been
studied for many years. Cross-reactions of organic peroxy radicals (RO2)
and the hydroperoxy radical (HO2) and self-reactions of HO2 are
thought to be major sources of organic peroxides and hydrogen peroxide
(H2O2), respectively, in the atmosphere. Ozonolysis of biogenic
volatile organic compounds (VOCs) also produces H2O2 in high yields
although its mechanism is unknown (Zhang et al., 2009; Huang et al., 2013).
Hydrolysis and reaction with OH are the main removal pathways for both
organic peroxides and H2O2, and dry/wet deposition removes only a
small portion of peroxides (Khan et al., 2015). However, existing theories
about sources and removal of peroxides cannot account for the field
observation results. Model simulations showed an overestimation on total
peroxides (TPOs) and a underestimation on H2O2 as compared with
field records in the airborne GABRIEL (Guyanas Atmosphere-Biosphere exchange
and Radicals Intensive Experiment with the Learjet) field campaign (Kubistin
et al., 2010), indicating the existence of possible underestimated or new
removal paths for organic peroxides and overestimated or new formation paths
for H2O2. Field observations and laboratory experiments showed that
particulate components, possibly particle-bound organic peroxides, could be
transformed to H2O2. Arellanes et al. (2006) found that
H2O2 in ambient SOA solution was 200–1000 times greater than
expected levels based on the gas–liquid partitioning, implying that almost
all H2O2 is generated from SOA solution. Wang et al. (2011)
investigated several kinds of SOA derived from the oxidation of α-pinene, β-pinene, and toluene, and came to the similar conclusion
that more than 97.5 % H2O2 arose from SOA formation rather than
from gas–liquid partitioning. However, this process happens in SOA solution
and the amount of H2O2 produced by such a pathway is too small to
account for the large discrepancy between observations and simulations for
the gas-phase H2O2.
The effect of water on peroxides is complex. Laboratory experiments suggested
that yields of particle-phase total peroxides in the ozonolysis of alkenes
are not influenced by water vapor (Docherty et al., 2005). Unlike the total
peroxides, yields of individual peroxides depend on relative humidity (RH).
Yield of H2O2 increases under wet conditions (Becker et al., 1990;
Hewitt and Kok, 1991; Simonaitis et al., 1991; Gäb et al., 1995; Huang et
al., 2013), while the yields of bis-hydroxymethyl hydroperoxide and three
unknown organic peroxides decrease under wet conditions (Huang et al., 2013).
Theoretical studies suggest that water helps both the formation and
decomposition of organic peroxides. Water can react with stabilized Criegee
intermediates (SCIs) and generate hydroxyalkyl hydroperoxides (HAHPs). It has
been proposed that not only isolated water molecules, but also water dimers
react with SCIs, and the latter path could even be more important (Ryzhkov and
Ariya, 2004). Numerous laboratory experiments support this proposal (Chao et
al., 2015; Lewis et al., 2015; Berndt et al., 2014). As a result, the
reaction with water dimers will be the largest sink for CH2OO. However,
the quantum chemical calculations predict that the larger SCIs react more
slowly with water, both the water monomer and dimer (Vereecken et al., 2014).
Water also helps gas-phase decomposition of HAHPs, although the decomposition
rate constant is small according to the theoretical calculations (Crehuet et
al., 2001; Aplincourt and Anglada, 2003).
This study investigates the ozonolysis of α-pinene, which is
considered as one of the largest contributors to SOA and a dominant source of
organic peroxides on a global scale (Khan et al., 2015), focusing on the
formation of peroxides in both the gas and the particle phase. Gas-particle
partitioning and water effect are examined carefully.
Material and methods
Chemicals
α-Pinene (Aldrich, 99 %), cyclohexane (Sigma-Aldrich, ≥ 99.7 %), potassium iodide (Alfa Aesar, 99.9 %), hydrogen peroxide
(Alfa Aesar, 35 wt %), orthophosphoric acid (Fluka,
85–90 %), hemin (Sigma, ≥ 98.0 %), 4-hydroxyphenylacetic acid
(Alfa Aesar, 99 %), ammonia solution (NH3 ⚫ H2O,
Beijing Tongguang Fine Chemicals Company, 25.0–28.0 %), ammonium
chloride (NH4Cl, Beijing Chemical Works, ≥ 99.5 %), ultrapure
water (18 MΩ, Millipore), N2 (≥ 99.999 %, Beijing
Haikeyuanchang Practical Gas Company Limited, Beijing, China), O2 (≥ 99.999 %, Beijing Haikeyuanchang Practical Gas Company Limited,
Beijing, China), and a polytetrafluoroethylene (PTFE) filter membrane (Whatman
Inc., 47 mm in diameter) were used in this study.
Apparatus and procedures
A flow tube reactor (2 m length, 70 mm inner diameter, quartz wall)
equipped with a water jacket for controlling temperature was used to
investigate the ozonolysis of α-pinene. All the experiments were
conducted at 298 ± 0.5 K and in the dark. O3 was generated by the
photolysis of O2 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). O3 (∼ 25 ppmv) was used in the
experiments. α-Pinene gas was generated by passing a flow of N2
over liquid α-pinene in a diffusion tube at the selected controlled
temperature. The initial concentration of α-pinene, determined by a
gas chromatography flame ionization detector (GC-FID, Agilent 7890A, USA),
was ∼ 273 ppbv in the experiments. Water vapor was generated by
passing N2 through a water bubbler. The mixing gases, including α-pinene, O3, and dry or wet synthetic air (80 % N2 and
20 % O2), were continuously introduced into the reactor with a
total flow rate of 4 standard L min-1 (standard liters per minute)
and a residence time of 120 s. The relative humidity (RH) was controlled at
two levels: < 0.5 % RH (dry conditions) and 60 % RH (wet
conditions). Gas from the reactor (2 standard L min-1) was directed
into a coil collector and scrubbed by H3PO4 stripping solution
(5 × 10-3 M, pH 3.5) for hydroperoxides analysis. SOA
produced from the ozonolysis of α-pinene was collected onto a PTFE
filter for 4 h at a flow rate of 4 standard L min-1, and the mass of
SOA on the filter was immediately measured by a semi-micro balance
(Sartorius, Germany). After that, each loaded filter was extracted with
20 mL H3PO4 solution (5 × 10-3 M, pH 3.5) using a
shaker (Shanghai Zhicheng ZWY 103D, China) at 180 rpm and 4 ∘C for
15 min, and then the SOA solution was immediately analyzed to determine the
particle-phase peroxides. Each SOA solution was analyzed seven times at different
times to investigate the evolution of SOA solution.
To explore the effect of water vapor on the formation of peroxides in the
ozonolysis, two-stage reaction experiments were designed and carried out. In
the first stage, dry synthetic air (2 standard L min-1) with α-pinene (∼ 275 ppbv) and O3 (∼ 42 ppmv) entered the first
2 L flow tube reactor; in the second stage, the gas passed through the
second 2 L flow tube reactor but with the addition of dry or wet synthetic
air (2 standard L min-1). The residence time was 68 s in the first
reactor and 34 s in the second reactor. The concentration of α-pinene at the outlet of the first reactor was found to be below the GC-FID
detection limit (< 5 ppbv), meaning that α-pinene was almost
completely consumed before the gas entered the second reactor. Thus, water
vapor appearing in the second reactor only affected the products from the
first reactor. A filter was placed at the outlet of the first reactor or
second reactor to collect SOA when necessary.
Peroxides' analysis
The low-weight molecular peroxides were measured using high-performance
liquid chromatography (HPLC, Agilent 1100, USA) coupled with a post-column
derivatization module and fluorescence detection, and the concentration of
total peroxides was determined by an iodometric spectrophotometer method.
Details about the HPLC fluorescence method were reported in our previous
study (Hua et al., 2008). Briefly, this method is based on the reaction of
p-hydroxyphenylacetic acid (POPHA) with organic hydroperoxides or hydrogen
peroxide in the catalysis of the hemin, forming POPHA dimer
(2,2′-bisphenol-5,5′-diacetic acid), which is a
fluorescent substance, and then is quantified by a fluorescence detector. The
separation of peroxides was implemented by column chromatography before the
peroxides were reacted with POPHA. The synthetic method for organic peroxides
standards is described in our previous study (Huang et al., 2013).
The iodometric spectrophotometric method is used to quantify all classes of
peroxides (ROOR′, ROOH, and H2O2), with the exception of
tertiary dialkyl peroxides, in the aqueous phase without distinction
(Banerjee and Budke, 1964). Peroxyhemiacetals formed in α-pinene
ozonolysis can be measured using this method. Excess potassium iodide reacts
with peroxides, producing I3- ions (Reaction R1), which can be
quantified by UV/VIS spectrophotometry.
3I-+H2O2+2H+→I3-+2H2O
α-Pinene SOA is freely soluble in polar solvents, e.g., water,
acetonitrile, and methanol, but it is poorly soluble in nonpolar solvents,
e.g., chloroform and toluene (Nguyen et al., 2010). Hence, a H3PO4
solution, as a kind of polar solvent, could entirely extract SOA from
filters. The HPLC fluorescence method uses H3PO4 solution as a
solvent for peroxides such as H2O2, hydromethyl hydroperoxide
(HMHP), performic acid (PFA), and peracetic acid (PAA) which are more stable
in acidic solution than in pure water (Zhou and Lee, 1992). In order to be
comparable with the HPLC fluorescence method, SOA loaded filters were also
extracted by H3PO4 solution. The influence of pH on extraction
efficiency is discussed in the Supplement. In this study, SOA solution
(2.5 mL) was added into a 10 mL airtight Micro-Reaction Vessel (Supelco,
USA). Each solution was then purged of oxygen by bubbling with N2 for
5 min. After purging, an aqueous solution of KI (250 µL, 0.75 M)
was added into the vessel. The vessel was then capped tightly, covered with
aluminium foil, and allowed to stand in the dark for 12–24 h. The solution
absorbance was then measured at 420 nm by an UV/VIS spectrophotometer
(SHIMADZU UV-1800, Japan). The efficiency of peroxide measurements is
discussed in the Supplement.
Peroxide content in the gas phase and particle phase of α-pinene ozonolysis as affected by the OH radical scavenger
cyclohexanea.
OH scavenger
None
Cyclohexane
RH
< 0.5 %
60 %
< 0.5 %
60 %
Gas phase
YH2O2b
0.048 ± 0.012
0.16 ± 0.01
0.048 ± 0.010
0.14 ± 0.02
YHMHPb
0.0030 ± 0.0003
0.0062 ± 0.0005
0.0024 ± 0.0005
0.0037 ± 0.0006
YPFAb
0.0057 ± 0.0020
0.012 ± 0.002
0.0020 ± 0.0003
0.005 ± 0.001
YPAAb
0.0067 ± 0.0006
0.009 ± 0.001
0.0022 ± 0.0002
0.0024 ± 0.0001
YTPOb
0.18 ± 0.01
0.20 ± 0.01
0.27 ± 0.02
0.25 ± 0.02
H2O2(g) / TPO(g) (ppbv ppbv-1)
0.26 ± 0.02
0.78 ± 0.04
0.18 ± 0.02
0.50 ± 0.04
Particle phase
YSOAc
0.41 ± 0.01
0.39 ± 0.02
0.28 ± 0.02
0.27 ± 0.01
FH2O2d (ng µg-1)
5.09 ± 0.99
2.67 ± 0.17
1.61 ± 0.11
1.41 ± 0.21
FPFAd (ng µg-1)
0.35 ± 0.06
0.27 ± 0.02
0.14 ± 0.01
0.16 ± 0.04
FPAAd (ng µg-1)
0.11 ± 0.04
–
0.09 ± 0.01
–
FTPOd (µg µg-1)
0.23 ± 0.01
0.25 ± 0.01
0.16 ± 0.01
0.20 ± 0.01
H2O2(p) / TPO(p) (µM µM-1)
0.20 ± 0.03
0.10 ± 0.01
0.09 ± 0.01
0.06 ± 0.01
TPO(p) / TPO(g+p)e
0.19 ± 0.02
0.18 ± 0.03
0.07 ± 0.01
0.09 ± 0.01
a ∼ 275 ppbv α-pinene,
∼ 1300 ppmv cyclohexane, and ∼ 42 ppmv O3 were used in
these experiments; the data represent the mean ±SD of three
observations; b molar yield of peroxides; c mass yield of
SOA; d contribution of peroxides to SOA mass; e fraction
of particulate TPO in gaseous and particulate TPO. Note: – indicates below detection limit (0.01 ng µg-1).
Results and discussion
Gas-particle partitioning of peroxides
Particle-phase peroxides
We measured the different mass values of SOA produced from the ozonolysis of
α-pinene at different RHs in the presence or absence of the OH
scavenger cyclohexane, and found that the typical in-reactor SOA
concentration was 450–650 µg m-3. A comparison of the
aerosol mass yields (YSOA), defined as the ratio of the formed
aerosol mass to the consumed α-pinene mass, (Table 1) indicated that
while the SOA yields were independent of the presence of water vapor, they
decreased in the presence of OH scavenger.
Organic peroxides are considered to be one of the major constituents in SOA
(Docherty et al., 2005; Ziemann, 2005; Surratt et al., 2006; Nguyen et al.,
2010; Mertes et al., 2012; Kidd et al., 2014; Badali et al., 2015) (Table S1
in the Supplement). The sensitivity of the iodometric method to ROOR is
critical to obtain an accurate concentration of total peroxides since
peroxyhemiacetals are a significant component (Docherty et al., 2005). In the
present study, we determined the total molar concentration of peroxides in
SOA using the iodometric method. Stability of peroxides in SOA stored
on-filter was also tested, and the results show that peroxides' concentration
decrease with increasing sitting time (Fig. S4 in the Supplement). Hence, the
peroxides in SOA were determined immediately after collection. Here, the mass
fraction of peroxides in SOA (Fperoxides) is defined as the ratio
of mass of particle-bound peroxides to SOA mass, which is defined as follows:
Fperoxides=mperoxidesmSOA,
where mperoxides is the mass of particle-bound peroxides, such as
PFA, PAA, and TPO, and mSOA is the mass of SOA. Assuming that the
average molecular weight of peroxides is 300, we obtained the mass fraction
of total peroxides in SOA (FTPO) as ∼ 0.21 (Table 1), which
is consistent with 0.22 reported by Epstein et al. (2014), but less than 0.47
reported by Docherty et al. (2005) and 0.34 reported by Mertes et al. (2012).
Several factors, such as the presence of OH scavengers, reactor type,
extraction method, SOA mass measurements, and SOA density assumptions, may
cause these discrepancies. In addition to the concentration of total
peroxides, we measured the concentration of two small organic peroxides
peroxyformic acid (PFA) and peroxyacetic acid (PAA) in SOA and calculated the
contribution of PFA (FPFA) and PAA (FPAA) to SOA mass
(Table 1). Under dry conditions, the FPFA and FPAA were
0.35 ± 0.06 and 0.11 ± 0.04 ng µg-1,
respectively, without the OH scavengers and 0.14 ± 0.01 and
0.09 ± 0.01 ng µg-1, respectively, with cyclohexane.
After adding water vapor, FPFA did not significantly change, but
FPAA approached 0.
Gas-phase peroxides
In addition to the particle-phase peroxides, we measured the gas-phase
peroxides generated in the ozonolysis of α-pinene. Here, the molar
yield of gaseous peroxides (Yperoxides) is defined in Eq. (2):
Yperoxides=ΔperoxidesΔα-pinene,
where Δ peroxides are moles of gaseous peroxides formed, such as
HMHP, PFA, PAA, and TPO, and Δα-pinene are moles of consumed
α-pinene. The molar yield of total peroxides (YTPO) was
estimated to be nearly the same under both dry conditions and wet conditions
in the absence of OH scavengers (Table 1), indicating that total yield of
peroxides was unaffected by water vapor. Moreover, when we employed the
Master Chemical Mechanism (MCM) v3.1 mechanism to simulate the present reaction system, the modeled yield of
total peroxides was about 0.25, consistent with our experimental result. The
model results also suggested that hydroperoxides account for more than
99 % of total peroxides. The yields of HMHP (YHMHP), PFA
(YPFA), and PAA (YPAA) are shown in Table 1. Compared with
dry conditions, YHMHP and YPFA doubled under wet
conditions, while YPAA increased only slightly. However, yields of
these three organic peroxides were all lower in the presence of OH
scavengers, indicating the importance of OH in the formation of small organic
peroxides.
Considering that all the peroxides originally existed in the gas phase at the
beginning of the ozonolysis of α-pinene, we estimated the fraction of
peroxides that entered the particulate phase from the gas phase through
gas-particle partition based on measured peroxides in the particle and gas
phases. The fraction of particulate TPO in gaseous and particulate
TPOs (TPO(p) / TPO(g + p)) was essentially the same (Table 1) under
both wet and dry conditions. To the best of our knowledge, for the ozonolysis
of α-pinene, this is the first report of the yield of gas-phase total
peroxides (including hydrogen peroxide and organic peroxides) and the
gas-particle partitioning fraction.
Comparison of observed and theoretical gas-particle partitioning
coefficients (Kp) of PFA, PAA, and TPO at different scenarios
(298 K).
Kp (observed)
Kp (theoretical)
(m3 µg-1)a
(m3 µg-1)
Sc1
Sc2
Sc3
Sc4
PFA
8.06 × 10-5
2.95 × 10-5
9.19 × 10-5
4.20 × 10-5
2 × 10-9
PAA
1.76 × 10-5
–
4.38 × 10-5
–
4 × 10-9
TPO
3.47 × 10-4
3.39 × 10-4
1.61 × 10-4
2.17 × 10-4
–
a The four scenarios represent four reaction conditions: Sc1
(< 0.5 % RH, no OH scavenger), Sc2 (60 % RH, no OH scavenger), Sc3
(< 0.5 % RH, with cyclohexane), and Sc4 (60 % RH, with
cyclohexane). Cyclohexane used here was ∼ 1300 ppmv.
The gas-particle partitioning coefficient (Kp) describes the
partitioning ability of a given species, calculated as follows (Odum et al.,
1996):
Kp=CaCgCom,
where Ca is the concentration of this species in the aerosol phase,
µg m-3; Cg is the concentration of this species in
the gas phase, µg m-3; and Com is the total
concentration of condensed organic matter, µg m-3. Based on
the gas-phase peroxides' concentration, particle-phase peroxides'
concentration, and the aerosol yields summarized in Table 1, we can obtain
the observed Kp (Table 2).
The Pankow absorption model (Pankow, 1994) is the most widely accepted
mechanism to explain the gas-particle partitioning, and has been used to
predict aerosol yields in chamber experiments (Cocker III et al., 2001;
Jenkin, 2004; Yu et al., 1999). Theoretical Kp can be calculated by
the following equation:
Kp=7.501×10-9RTMWomςpL∘,
where R is the ideal gas constant, J K-1 mol-1; T is the
temperature, K; MWom is the mean molecular weight of the condensed
organic material, g mol-1. In the present study, MWom is
estimated to be 130 g mol-1; ς is the activity coefficient
of the given species in the condensed organic phase, and here, is assumed to
be unity; pL∘ is the liquid vapor pressure of this
species, Torr. The theoretical pL∘ can be calculated by an
expended, semiempirical form of the Clausius–Clapeyron equation (Baum, 1997).
Theoretical gas-particle partitioning coefficients of PFA and PAA are shown
in Table 2.
The observed gas-particle partitioning coefficients of PFA, PAA, and TPO were
(3–9) × 10-5, (2–4) × 10-5, and
(2–3) × 10-4 m3 µg-1, respectively
(Table 2), which, to the best of our knowledge, are reported here for the
first time. However, the long time collection for SOA does have effects on gas-
and particle-phase constituents, possibly due to repartitioning of species
between the two phases. Collected SOA mass and peroxide amount per unit time
decreases slightly with increasing collection time, and peroxide amount
decreases faster than SOA mass (Supplement). Hence, the gas-particle
partitioning coefficients of peroxides given here are underestimated by about
21 %. Compared with the observed Kp values, theoretical
Kp values of PFA and PAA, 2 × 10-9 and
4 × 10-9 m3 µg-1, respectively, were lower
by a factor of 104. This large difference between observed and
theoretical Kp values has also been reported previously (Cocker III
et al., 2001; Jenkin, 2004; Kamens and Jaoui, 2001). Jenkin (2004) considered
the existence of a significant systematic error, which is independent of key
parameters involved in prediction of Kp values, or the inability to
interpret the Pankow absorption model. After inducing a species-independent
scaling factor of ca. 120 for all partitioning species, Jenkin obtained a
reasonable simulation of the final experimental aerosol concentration, but
was still unable to interpret the early stages of aerosol accumulation. In
addition to the absorptive partitioning mechanism, the participation of bi-
and multifunctional acid dimers in the aerosol formation process was also
considered, resulting in the presentation of simulated results. Organic
peroxides are also important compounds in dimer formation; for instance,
hydroperoxides can react with aldehydes, subsequently producing
peroxyhemiacetals (Tobias and Ziemann, 2000, 2001; Ziemann, 2005). The vapor
pressures of hydroperoxides decreased in the formation of peroxyhemiacetals by
an additional factor of ∼ 102–105 (Tobias and Ziemann,
2000), which could partially explain the large discrepancy between the
observed and theoretical gas-particle partitioning coefficient. However,
related thermodynamic and kinetic parameters need further study to resolve
the problem.
Evolution of SOA in the aqueous phase
We investigated the evolution of SOA in the aqueous phase, focusing on the
change of H2O2 in SOA. The initial concentration of H2O2
was low, but it increased rapidly in the first 3.5 h, approaching the peak
at ∼ 7.5 h, and then decreased slowly, meaning the existence of a
sustained release of H2O2 in the SOA solution at room temperature
(298 K) (Fig. 1). The molar fraction of peak H2O2 to total
peroxides in SOA (H2O2(p) / TPO(p)) under dry conditions
was twice as high as that observed under wet conditions in the absence of OH
scavengers (Table 1). The contribution of H2O2 to the SOA mass
(FH2O2) was 1.9 times higher under dry conditions (5.09 ± 0.99 ng µg-1) than under wet conditions
(2.67 ± 0.17 ng µg-1) (Table 1). In the presence of
cyclohexane, H2O2(p) / TPO(p) was slightly higher under dry
conditions than wet conditions (0.09 ± 0.01 and 0.06 ± 0.01,
respectively), while FH2O2 values were not significantly
affected by RH (Table 1). Our observations are consistent with those of Wang et
al. (2011) who studied α-pinene-derived SOA, and measured the
FH2O2 as 2.01 ± 0.76 ng µg-1, which was
unchanged by a variety of oxidants (NO/light, O3, and
O3/cyclohexane) over the range of 14–43 % RH.
Evolution of total peroxides and H2O2 contents in SOA
produced under dry (< 0.5 % RH) and wet (60 % RH) conditions at
298 K. Circles and squares represent total peroxides and H2O2
contents in SOA, respectively; and solid lines and dashed lines represent that
obtained under dry and wet conditions, respectively. The data represent the
mean ±SD of three observations.
![]()
The sustained release of H2O2 coupled with the attenuation of total
peroxides provided experimental evidence for the hypothesis that the
decomposition/hydrolysis of organic peroxides generates H2O2. The
decay of H2O2 in SOA solution after 18 h is a comprehensive
phenomenon including formation and decomposition, and the rate was estimated
to be 0.06 and 0.03 µM h-1 for SOA produced under dry and wet
conditions, respectively. To assess the formation, we determined the
decomposition rates of pure H2O2 at different concentrations. When
the H2O2 concentration was ∼ 5 µM
(∼ 2.5 µM), the rate of decomposition was
0.11 µM h-1 (0.05 µM h-1). The H2O2
concentrations were equivalent with those of H2O2 in SOA solution.
Thus, we can estimate H2O2 formation in the SOA solution after
18 h to have a rate of 0.05 µM h-1 under dry conditions and
0.02 µM h-1 under wet conditions. As shown in Fig. 2, the
peroxycarboxylic acids (PCAs) (PFA and PAA), decayed quickly while the HAHP
(HMHP) decayed slowly. Hence, the formation of H2O2 after 18 h
could be attributed to the decomposition of HAHPs. However, in the first
7.5 h period, H2O2 increased rapidly which is more consistent with
the decay of PCAs rather than HAHPs. Not all the organic peroxides decayed
during the observation time, since the attenuation of TPO almost stopped
after 40 h. The residual peroxides were more stable, possibly due to the
formation of ROOR by oligomerization.
Decomposition/hydrolysis of organic peroxides in the aqueous phase.
Ct / C0 is the ratio of peroxides' concentration at
time = t h to peroxides concentration at time = 0 h. Lines are
exponential fits for HMHP, PFA, and PAA. The decay rate constants of HMHP,
PFA, and PAA are 0.09, 1.06, and 0.64 h-1, respectively. The data represent
the mean ±SD of three observations.
![]()
Unexpectedly high levels of H<inline-formula><mml:math display="inline"><mml:msub><mml:mi/><mml:mn mathvariant="normal">2</mml:mn></mml:msub></mml:math></inline-formula>O<inline-formula><mml:math display="inline"><mml:msub><mml:mi/><mml:mn mathvariant="normal">2</mml:mn></mml:msub></mml:math></inline-formula> in the gas phase
Table 1 shows the molar yields of gas-phase H2O2. The
H2O2 yield in the absence and presence of cyclohexane was
essentially the same in dry conditions, but under conditions of high RH
increased to 0.16 ± 0.01 in control studies and to 0.14 ± 0.02 in
the presence of cyclohexane. Thus, the presence of water vapor elevated the
H2O2 yield, while the presence of a radical scavenger had no
effect. Previous studies on gas-phase H2O2 yields of the ozonolysis
of α-pinene are reviewed (Table S2). Becker et al. (1990)
first reported that the presence of water vapor will significantly promote the
H2O2 yield, and our work confirmed this observation. However, our
measured values of H2O2 were 10 times higher than those reported
by others under both dry and wet conditions, except that by Simonaitis et
al. (1991). Differences in reactant concentration, reactor type, and measuring
methods account for these discrepancies. Worth noting is that the reactants'
concentrations used in these previous and our experiments are very high,
therefore, yields of peroxides may not represent actual yields of peroxides
in oxidation of α-pinene in nature (Supplement).
The source of gas-phase H2O2 remains unclear. We suggest that
further ozonolysis and OH oxidation of gaseous products and reactants in the
aqueous phase during and after gas collection are not likely to be the main
sources of H2O2. In this study, the online GC-FID test showed that
α-pinene was completely consumed in the gas phase. Hence, the
contribution of aqueous-phase α-pinene ozonolysis to the measured
H2O2 in the coil collector should be negligible. The main gas-phase
non-peroxy organic products of α-pinene ozonolysis are carbonyls and
organic acids, e.g., pinonaldehyde, formaldehyde, acetone, and pinic acid;
these compounds without carbon–carbon double bonds cannot be oxidized by
O3. The ozonolysis of α-pinene produces the OH radical in high
yield (0.68–0.91) (Berndt et al., 2003), which potentially oxidizes
carbonyls and organics. However, we observed no difference of
YH2O2 in the absence and presence of OH scavengers, indicating
that OH oxidation in the aqueous phase may not be a source of H2O2.
Decomposition/hydrolysis of organic peroxides in the aqueous phase during and
after gas collection is also found to be a minor source of gas-phase
H2O2. HAHPs and PCAs, two kinds of organic peroxides, are the
probable candidates for generating H2O2. HAHPs are the main products
of the reaction of SCI with water molecules and dimers (Ryzhkov and Ariya,
2004), and they can decompose to H2O2 plus the corresponding
aldehyde, or H2O plus the corresponding organic acid (Hellpointner and
Gäb, 1989). The hydrolysis of PCAs, which are generated from the reaction
of RC(O)OO with HO2, is another possible source of H2O2.
Several kinds of PCAs have been qualitatively observed in the ozonolysis of
α-pinene (Venkatachari and Hopke, 2008). In this study, we
quantitatively observed PFA and PAA in the gas phase (Table 1), and simulated
the formation of PCAs using the MCM v3.1 mechanism. Model results showed that the
yield of total PCAs was extremely low, 0.0005, and PAA contributed more than
half of the yield, while the formation pathway of PFA was not included. The
large discrepancy between modeled and experimental results indicates that
PCAs play a more important role than was expected previously. We estimate the
H2O2 generated from organic peroxides in the aqueous phase by
measuring the decomposition/hydrolysis rate of organic peroxides. Considering
the effects of concentration, coexisting components, and ionic strength, we
conducted the measurements with coil collection solutions rather than with
synthesized samples. The decomposition/hydrolysis of organic peroxides is a
pseudo-first-order reaction due to the excess of the other reactant, i.e.,
water. The decay rate constants of HMHP, PFA, and PAA were determined to be
0.09, 1.06, and 0.64 h-1, respectively (Fig. 2). Larger HAHPs were less
active compared with HMHP and should have lower decay rate constants. If all
the TPOs are composed of HAHPs and the production of H2O2 plus
aldehydes is the only decomposition pathway of HAHPs, the upper bound of
H2O2 formed in the aqueous phase within 8 min may be estimated to
be 1.2 % of TPO. However, the observed ratio of gas-phase H2O2
to TPO was 28–78 %, indicating that the aqueous-phase decomposition of
HAHPs is insignificant. Compared with HMHP, the decay rates of PFA and PAA
were quite high. Assuming that all the TPOs, except for PFA, are PCAs and
their decay rates were the same as that of PAA, H2O2 formed in the aqueous
phase within 8 min is estimated to be 13.2 % of TPO, which can partially
explain the observed H2O2 level. However, the yield of PCAs in the
ozonolysis of α-pinene is predicted to be low in the MCM v3.1 model.
The experimental results mentioned above concluded that the aqueous-phase
formation of H2O2 is not important, for the decay rate of HAHPs was
too slow, and the amount of PCAs was too low, although their decay rate was
higher.
Whether the self-reaction of HO2 and decomposition of HAHP in the gas
phase are the main sources of H2O2 is discussed here. The
self-reaction of HO2 is considered to be the main source of ambient
H2O2 (Lee et al., 2000; Reeves and Penkett, 2003) and occurs in the
ozonolysis of α-pinene. When we estimated the contribution of this
pathway to the observed H2O2 from α-pinene ozonolysis using
MCM v3.1 mechanisms, the yield was less than 0.001 under both dry and wet
conditions, meaning that this pathway is negligible. Chamber experiments
showed that SCI mainly reacts with water molecules even under dry conditions
(Jenkin, 2004), and the major product is HAHP. Aplincourt and Anglada (2003)
considered that the unimolecular decomposition of gaseous HAHPs was unlikely
to occur, and only the water-assisted decomposition was efficient in the gas
phase. They estimated the water-assisted decomposition rate constant of
2-propenyl α-hydroxy hydroperoxide to be
1.5 × 10-30 cm3 molecule-1 s-1 by quantum
chemical calculation. Based on their work, the gas-phase decomposition
fraction of HAHP in 2 min can be calculated to be less than 0.01 %,
which is too small to account for the H2O2 observed in our
experiments.
In summary, the high H2O2 yields in the gas phase cannot be
explained by the bias caused by measuring method and the current formation
mechanism of H2O2. An unknown or underestimated pathway producing H2O2 may exist. In Sect. 3.4, we propose that gaseous
organic peroxides can undergo rapid heterogeneous decomposition in the
presence of water and produce H2O2.
Hydrogen peroxide in the coil collector at different scenarios in
the two-stage experiments.
Scenarios
Filter positiona
Water vaporb
H2O2
Species in second reactor
1d
Second reactor
No
100 ± 1 %
Gaseous products, SOA
1w
Second reactor
Yes
165 ± 6 %
Gaseous products, SOA, water vapor
2d
First reactor
No
103 ± 6 %
Gaseous products
2w
First reactor
Yes
87 ± 7 %
Gaseous products, water vapor
3d
No filter
No
164 ± 9 %
Gaseous products, SOAc
3w
No filter
Yes
172 ± 5 %
Gaseous products, SOA, water vapor
a The filters were placed at the outlet of the first
reactor or second reactor. b Water vapor was induced into the
second reactor. c Although gaseous products and SOA did not contact
water vapor in the second reactor, they were in contact with the condensed
water in the coil collector.
Rapid heterogeneous decomposition of gaseous organic peroxides
Our results demonstrate that water vapor has no significant effect on either
the yield of total peroxides (combining gaseous and particulate peroxides) or
the contribution of peroxides to SOA mass. However, water vapor does change
the concentrations of H2O2, PFA, and PAA in the gas phase and
particle phase in an opposite manner (Table 1). In the presence of water
vapor, H2O2 yield increased dramatically by ∼ 300 %, and
gas-phase H2O2 / TPO increased from 0.26 to 0.78. Yields of
HMHP, PFA, and PAA also increased with the presence of water vapor. These
results clearly indicate that water vapor can change the formation and
distribution of peroxides.
We carried out a series of two-stage experiments using two reactors under
various scenarios to further study the effect of water vapor on peroxides
(Table 3). In scenario 1d, no water vapor was added and a filter was used to
intercept SOA entering the coil collector, which is similar to measuring
H2O2 under dry conditions with one reactor. The concentration of
H2O2 observed in the coil collection solution under this condition
was considered to be the baseline value, 100 %. When the filter was
placed at the outlet of the first reactor (scenario 2d) instead of at the
second reactor, the concentration of H2O2 was 103 ± 6 %,
almost the same as the baseline, indicating that the coexistence of gaseous
products and SOA will not lead to the formation of H2O2. In
scenario 2w, a filter was placed at the outlet of the first reactor and water
vapor was added to the second reactor, resulting in the coexistence of
gaseous products and water vapor (50 % RH) in the second reactor. The
concentration of H2O2 observed in this scenario was 87 %,
slightly lower than the baseline, possibly due to loss on the wall of the
reactor under wet conditions, which has been reported to be 5 % for
H2O2 at 50 % RH (Huang et al., 2013). When we maintained the
water vapor and moved the filter to the outlet of the second reactor
(scenario 1w), the H2O2 concentration increased to
165 ± 6 % of baseline. In scenario 3w, with water vapor added to
the second reactor and without a filter in the gas flow, the H2O2
concentration was 172 ± 5 %, almost the same as that in scenario 1w.
In scenario 3d, no water vapor and no filter were used, but a high
H2O2 concentration, 164 ± 9 %, was also observed. For
scenarios 1w, 3w, and 3d, where H2O2 increased by ∼ 67 %,
gaseous products, SOA, and water were all present in the second reactor or
coil collector. The coexistence of gaseous products and water vapor (see
scenario 2d and 2w), the coexistence of gaseous products and SOA (see
scenario 1d and 2d), and the coexistence of SOA and water (see Sect. 3.3) did
not result in a high yield of H2O2. We therefore concluded that the
presence of three components together, the gaseous products, SOA, and water,
was necessary for a high yield of H2O2. Once the gaseous products
and SOA had been in contact with water vapor in the second reactor, the
levels of H2O2 were increased to the same extent, whether or not
these compounds were mixed with condensed water (see scenario 1w and 3w),
indicating that the process producing H2O2 in the gas phase is
quite rapid.
When we measured the total peroxides formed from gaseous products and SOA in
scenarios 1d and 1w, the results showed that for these two scenarios, the
levels of the total peroxides in both gaseous products and SOA were not
significantly different, indicating that SOA does not change in the presence
of water vapor and no new peroxides are formed in the gas phase. This outcome
supports the idea that the increment of H2O2 comes from the
redistribution of gaseous peroxides, which is induced by the heterogeneous
decomposition of gaseous products in the presence of both SOA and water.
Based on the measured increment of H2O2 and concentration of
gaseous total peroxides, we concluded that 18 % of the gaseous total
peroxides undergo rapid heterogeneous decomposition.
Heterogeneous reactions of trace gases on the surface of particles relevant
to the atmosphere have been studied for many years. The investigated trace
gases, including nitrogen oxides (e.g., HNO3, NO2, and
N2O5), SO2, O3, H2O2, and oxygenated VOCs
(Liggio et al., 2005; Kroll et al., 2005; Prince et al., 2007; Zhao et al.,
2010, 2011, 2014; Huang et al., 2015), could react with the active sites on
the surfaces of mineral dust (Goodman et al., 2001; Fu et al., 2007). Unlike
mineral dust, however, SOA has no such active sites. The elucidation of the
mechanism of the rapid heterogeneous decomposition of organic peroxides on
SOA particles remains a great challenge and needs urgent study.
Conclusions and atmospheric implications
Our laboratory study has provided more evidence that organic peroxides are
important components of SOA derived from the ozonolysis of alkenes. In the
case of α-pinene, organic peroxides account for ∼ 21 % of
the SOA mass and this fraction is not affected by RH and the presence of OH
scavengers. More interestingly, the gas-particle partitioning coefficients of
organic peroxides have been estimated for the first time based on the
measurements of both gaseous and particulate peroxides. Due to the
long time collection for SOA, these coefficients reported here are lower bounds in this
study. For PFA and PAA, the observed values were 104 times higher than
that of the theoretical value calculated by the Pankow absorption model. This
discrepancy indicates a more important role of peroxides in SOA formation
than expected previously and the existence of mechanisms in addition to the
absorption that are not yet defined. The reaction of organic hydroperoxides with
carbonyls forming peroxyhemiacetals may explain part of the enhancement of
the partitioning of peroxides. However, the kinetic parameters of
peroxyhemiacetal formation are lacking. The explicit mechanisms of
gas-particle partitioning and the determination of gas-particle partitioning
coefficients of larger organic peroxides deserve further study to improve the
simulation of SOA mass.
We also examined gas-phase peroxides. The yield of gaseous total peroxides
was ∼ 0.22, which was independent of RH and OH scavengers. The MCM v3.1
mechanism predicted this yield but failed to explain the yields of individual
peroxides, i.e., H2O2, HMHP, PFA, and PAA, indicating that our
previous understanding of α-pinene ozonolysis was insufficient. For
H2O2 with a yield of 0.048 under dry conditions and 0.16 under wet
conditions, the known pathways, including dissolution of SOA, aqueous
oxidation of gaseous compounds, and decomposition/hydrolysis of organic
peroxides in the aqueous phase, cannot explain such an unexpectedly high
yield of H2O2. The presence of both water and SOA leads to the
rapid transformation of gaseous organic peroxides into H2O2. This
heterogeneous process increases the H2O2 yield by ∼ 67 %.
Our results also show that water vapor affects the distribution of gaseous
peroxides, although it cannot change the yield of total peroxides.
The rapid heterogeneous transformation of organic peroxides to H2O2
helps to explain the differences between modeled and observed levels of
peroxides and OH in the forest area. In the airborne GABRIEL field campaign
in equatorial South America (Surinam) in October 2005 (Kubistin et al.,
2010), two issues arose. (1) Organic peroxides were overestimated while
H2O2 was underestimated, and (2) OH and HO2 were also
underestimated, especially when concentrations of VOCs were high. These
investigators suggested the occurrence of additional recycling from HO2
to OH or the contributions of additional direct OH sources. Our finding that
organic peroxides can transform to H2O2 by rapid heterogeneous
reactions can address the first discrepancy directly and the second
indirectly. Peroxides influence OH through the removal pathways as follows:
ROOH+hv→RO+OHROOH+OH→RO2+H2OH2O2+hv→2OHH2O2+OH→H2O+HO2.
Predominant removal paths for organic peroxides in the atmosphere are the
reaction with OH (95 %) and photolysis (4.4 %) (Khan et al., 2015),
while for H2O2, these two paths are almost equally important. The
OH oxidation process consumes OH, while the photolysis process produces OH.
Obviously, H2O2 plays a different role in the OH cycling compared
with organic peroxides. One molecule of organic peroxides, transformed into
H2O2, yields ∼ 1.4 molecules of OH. Thus, the rapid
transformation of organic peroxides to H2O2 by the heterogeneous
process would increase OH levels. However, not all the organic peroxides
could be transformed to H2O2 by the heterogeneous process. Further
studies are needed to clarify this process in the atmosphere and unveil the
features of the peroxides undergoing heterogeneous transformation.