Introduction
The emission of biogenic volatile organic compounds (BVOCs) from vegetation
to the troposphere and their oxidation in the gas phase is the subject of intense
research (Calvert et al., 2000; Guenther et al., 2012; Ziemann and Atkinson,
2012).
Sesquiterpenes (SQTs, C15H24) with an annual emission of
18–24 million metric tons of carbon (Messina et al., 2015; Sindelarova et al.,
2014) contribute up to 3 % of the annual global BVOC emission of
720–1150 million metric tons of carbon (Guenther et al., 1995, 2012;
Lathière et al., 2005; Sindelarova et al., 2014). They are emitted by a
large variety of plants and fungi and their emission pattern depends strongly
on the region and the season (Ciccioli et al., 1999; Duhl et al., 2008; Geron and
Arnts, 2010; Horváth et al., 2011; Jardine et al., 2011). Biotic stress
can drastically increase SQT emissions (Mentel et al., 2013). β-Caryophyllene emissions were calculated to account for 25 % of global
SQT emissions (Guenther et al., 2012) and can contribute 70 % to the
regional BVOC emissions, e.g. in orange orchards (Ciccioli et al., 1999;
Duhl et al., 2008). The oxidation products are expected to have a very low vapour
pressure making them important for the process of secondary organic aerosol
(SOA) formation (Jaoui et al., 2013; Zhao et al., 2016).
β-Caryophyllene is mainly oxidized by ozone under atmospheric
conditions having a lifetime τ(O3)=2 min for an average
ozone concentration of [O3] = 7×1011 molecules cm-3 (Finlayson-Pitts and Pitts, 1986) and a rate
coefficient k(296K)=1.1×10-14 cm3 molecule-1 s-1 (Richters et al., 2015; Shu
and Atkinson, 1994). Gas-phase product formation from the ozonolysis of
β-caryophyllene was already studied in a series of laboratory
investigations (Calogirou et al., 1997; Grosjean et al., 1993; Jaoui et al.,
2003; Lee et al., 2006; Winterhalter et al., 2009) and by means of
theoretical calculations (Nguyen et al., 2009). A large variety of carbonyl,
epoxide and carboxyl compounds containing up to five oxygen atoms were
experimentally observed using different detection techniques. The total
carbon yield, comprising gas- and particle-phase products, accounts for up to
64 % (Jaoui et al., 2003). A summary of available data in the literature
is given by Winterhalter et al. (2009). DFT (density functional theory) quantum chemical calculations
were conducted accompanying the experimental work by Winterhalter et
al. (2009) with special attention to the first oxidation steps. The fraction
of stabilized Criegee intermediates at atmospheric pressure was calculated to
be 74 %, slightly higher than the experimental value of 60 %.
Furthermore, the calculations support the proposed uni- and bimolecular
reaction pathways of the Criegee intermediates as proposed from the
experimental work. The main reaction product was stated to be the secondary
ozonide with a yield of 64 %. The formation of acids should account for
8 %, dominated by the formation of caryophyllonic acid (Nguyen et al.,
2009). This value is slightly lower than the overall gas- and aerosol-phase
yield of 13.5 % for caryophyllonic acid measured by Jaoui et al. (2003).
Recently, Ehn et al. (2012, 2014) detected highly oxidized multifunctional
organic compounds (HOMs) from the oxidation of α-pinene in field and
laboratory studies. These HOMs contain up to 12 oxygen atoms and are
supposed to have a very low vapour pressure, which led to their classification
as extremely low-volatility organic compounds (ELVOCs) and as important
precursors for SOA formation (Ehn et al., 2014).
Other experimental work on HOM formation from the ozonolysis of monoterpenes
(Jokinen et al., 2014; Mentel et al., 2015) and model substances, such as
cyclohexene (Berndt et al., 2015b; Mentel et al., 2015; Rissanen et al.,
2014), led to the development of an autoxidation mechanism based on RO2
radical chemistry. In this process, an RO2 radical internally abstracts
an H atom, forming an alkyl radical with a hydroperoxide moiety
(RO2 → QOOH). Subsequent oxygen addition forms the next R′O2
radical (QOOH + O2 → R′O2) (Berndt et al., 2015b;
Crounse et al., 2013; Ehn et al., 2014; Jokinen et al., 2014; Rissanen et
al., 2014), which can repeat this reaction sequence. The overall process
results in a repetitive oxygen insertion into the molecules on a timescale
of seconds (Jokinen et al., 2014). The principle of autoxidation is well
known from the liquid phase for more than 100 years (Berezin et al.,
1996; Jazukowitsch, 1875) and was recently extended to atmospheric gas-phase
reactions (Crounse et al., 2013).
For alkenes with multiple double bonds, such as β-caryophyllene, this
mechanism can become more complex caused by the variety of possible reaction
pathways of unsaturated RO2 radicals formed as the intermediates. A
recent study from this laboratory showed that the HOM formation from the
ozonolysis of α-cedrene (a SQT that contains only a single double
bond) was completely explainable by the autoxidation mechanism initiated by
the ozone attack on the double bond (Richters et al., 2016). On the other
hand, in the case of the analogous reaction of β-caryophyllene
(containing two double bonds), the product spectrum was more complex and not
fully in line with an autoxidation mechanism (RO2 → QOOH,
QOOH + O2 → R′O2). This fact points to additional
reaction pathways for HOM generation most likely caused by the presence of a
second double bond.
The scope of the present work is the mechanistic elucidation of possible
new reaction pathways of HOM formation starting from the ozonolysis of
β-caryophyllene. Experiments with heavy water (D2O) and
isotopically labelled ozone (18O3) were conducted in order to
obtain additional information on elementary reaction pathways needed to
explain the observed products. This approach allowed for developing an extended
mechanism for the HOM formation from the ozonolysis of β-caryophyllene.
Experimentation
The gas-phase ozonolysis of β-caryophyllene was investigated in a
free-jet flow system at a temperature of 295 ± 2 K and a pressure of
1 bar purified air. The experimental approach is described in detail in the
literature (Berndt et al., 2015a, b; Richters et al., 2016) and only a brief
summary will be given here.
Experiments in the free-jet flow system (outer tube length: 200 cm, 15 cm
inner diameter, and a moveable inner tube: 9.5 mm outer diameter with a
nozzle) were conducted under conditions of negligible wall loss of products
and with a reaction time of 3.0–7.9 s (Berndt et al., 2015a). The inner
flow of 5 L min-1 (STP, standard temperature and pressure), containing varying ozone concentrations, was
injected through a nozzle to the outer airflow of 95 L min-1 (STP),
which contained β-caryophyllene and CH3COOH if needed. The
addition of CH3COOH was used to scavenge stabilized Criegee
intermediates from the ozonolysis of β-caryophyllene (Beck et al.,
2011; Neeb et al., 1996). Turbulent gas mixing downstream of the nozzle rapidly
generates a homogeneously mixed reactant gas.
Ozone was produced by passing air or 18O2, premixed in N2,
through an ozone generator (UVP OG-2) and was measured at the outflow of the
reactor by a gas monitor (Thermo Environmental Instruments 49C). All gas
flows were set by calibrated gas flow controllers (MKS 1259/1179). β-Caryophyllene was stored in flasks maintained at 278 K, carried along with
38–48 mL min-1 (STP) nitrogen, and diluted with the air stream
just before entering the flow system. Gas chromatography with a flame –
ionization detector (GC–FID; Agilent 6890) as well as proton transfer
reaction – mass spectrometry (PTR–MS; HS PTR-QMS 500, Ionicon) served as the
analytical techniques for β-caryophyllene detection.
The absolute β-caryophyllene concentrations were determined using the
“effective carbon-number approach” from GC–FID analysis using a series of
reference substances with known concentrations (Scanlon and Willis, 1985).
The reference substances were α-pinene, β-pinene and limonene.
The ratio of the effective carbon numbers (equal to the signal ratio for
identical sample concentrations) of β-caryophyllene with respect to
these monoterpenes is 1.5 (Helmig et al., 2003; Scanlon and Willis, 1985).
Before each measurement series, the concentration was determined using GC–FID
analysis measuring the β-caryophyllene signal as well as the signals
of the reference substances with known concentrations simultaneously. The
β-caryophyllene concentration in the flow system was continuously
monitored throughout the experiments by PTR–MS measurements following the ion
traces at 205, 147 and 137 amu.
The β-caryophyllene conversion was varied by changing the initial
ozone concentration for otherwise constant reaction conditions. The needed
gas mixture of CH3COOH was prepared in a gas-mixing unit.
The reactant gases used had the following purities: β-caryophyllene
(98.5 %; Aldrich), CH3COOH (Aldrich; 99.5 %), N2 (Air
Products; 99.9992 %), 18O2 (euriso-top, isotopic enrichment
96 %). Air was taken from a PSA (pressure swing adsorption) unit with
further purification by activated charcoal and 4Å molecular sieve. If
needed, humidified air was produced by passing a part of the airflow through
water saturators filled with D2O (Aldrich, 99.9 atom %).
Reaction products were detected and quantified by means of chemical
ionization – atmospheric pressure interface – time-of-flight (CI-APi-TOF)
mass spectrometry (Airmodus, Tofwerk) using nitrate ions and acetate ions for
chemical ionization. The mass spectrometer settings (applied voltages and
flow rates) as well as the approach applied for the determination of HOM
concentrations are equal to those described in detail by Berndt et
al. (2015b). All stated concentrations represent lower limits (Berndt et al.,
2015b). The calculation of HOM concentrations and information about detection
limitations and the mass axis calibration are given in the Supplement.
The initial concentrations were (unit: molecules cm-3) [β-caryophyllene] = (8.3–8.6) × 1010;
[O3] = (4.7–102) × 1010 and
[CH3COOH] = (0–1.4) × 1014.
Results and discussion
A series of different experiments was conducted in order to investigate the
product formation from the ozonolysis of β-caryophyllene in more
detail. In Sect. 3.1, three different groups of products are proposed as a
result of the identified signals from mass spectra recorded from runs with
nitrate and acetate ionization. The experimental findings utilized for the
signal assignment to the different product groups are described in the
following sections. Section 3.2 discusses results from experiments with
normal (16O3) or isotopically labelled ozone (18O3) which
allows for distinguishing between the origin of the O-atoms in the reaction
products arising either from attacking ozone or from air O2. Experiments
with D2O added to the carrier gas provide information about the total
number of acidic H atoms in each reaction product, being equal to the number
of OH and OOH groups; see Sect. 3.3.
Three groups of highly oxidized products
Figure 1 shows two product mass spectra from β-caryophyllene
ozonolysis in the mass-to-charge range 345–505 Th, which were recorded (a) with
acetate ionization and (b) with nitrate ionization. The products appear as
adducts with the reagent ion (Ehn et al., 2014). Here, a signal of the same
product shows a shift by three nominal mass units comparing acetate ion
adducts (+59 nominal mass units) with nitrate ion adducts (+62 nominal
mass units). Mainly RO2 radicals were detected as reaction products
because the RO2 radical concentrations did not exceed 9×106 molecules cm-3 and bimolecular reactions of the formed
RO2 radicals were less efficient for a reaction time of 3.0–7.9 s in
these experiments. Therefore, the discussion is mainly focused on RO2
radicals.
Highly oxidized RO2 radicals of the three product groups,
simple AutOx., O,O–C15H23-x(OOH)xO2, with x=1–5
(in red), ext. AutOx., O,O–C15H23-y(OO)(OOH)yO2 with
y=1–4 (in blue) and ext. AutOx. -CO2, O–C14H23-α(O)(OOH)αO2 with α=1–3 (in green), and
corresponding closed-shell products (rectangular lines) appearing at -17
nominal mass units regarding the corresponding RO2 radical. The products
were detected by means of (a) acetate ionization and (b) nitrate ionization.
The same molecule gives a signal shifted by three nominal mass units,
comparing the acetate ion adducts (+59 nominal mass units) with the nitrate
ion adducts (+62 nominal mass units). The mass spectra were normalized by
their reagent ion counts. Signals from the simple AutOx. group and the
ext. AutOx. group were detected at the same mass-to-charge ratio. The
green spectrum lines (O3 only) shows the background experiments in which
only ozone (no SQT) was present. The data of (b) were taken from Richters
et al. (2016). [β-caryophyllene] = 8.6×1010 (acetate
ionization), [β-caryophyllene] = 8.3×1010 (nitrate
ionization), [O3] = 1.02×1012 molecules cm-3, reaction
time is 7.9 s.
The observed product signals were classified in three product groups. The
position of the dominant signals in each product group differs by 32 nominal
mass units each due to the stepwise insertion of oxygen molecules.
Signals of the first group, the so-called simple autoxidation group, “simple
AutOx.”, appear at the same positions in the mass spectrum as observed from
the HOM formation of α-cedrene ozonolysis (an SQT with only one
double bond, but with the same chemical formula C15H24 like β-caryophyllene) (Richters et al., 2016). The RO2 radicals from this
group were summarized by the general formula
O,O–C15H23-x(OOH)xO2 with x=1–5 (Jokinen et al.,
2014; Richters et al., 2016). Here, x stands for the number of
hydroperoxide moieties in the molecule, the two oxygen atoms O,O- arise
from the initial ozone attack and the final O2 stands for the RO2
radical functional group (Jokinen et al., 2014). The carbon skeleton of 15
carbon atoms is retained and up to 14 oxygen atoms are inserted into the
products. The number of oxygen atoms arising from the initial ozone attack
was confirmed in experiments with isotopically labelled ozone (18O3)
(Fig. 2).
Ozonolysis of β-caryophyllene using 16O3 (lower
part) and 18O3 (upper part) and applying acetate ionization in the
analysis. Highly oxidized RO2 radicals of the three product groups,
simple AutOx., O,O–C15H23-x(OOH)xO2, with x=1 and
2 (in red), ext. AutOx., O,O–C15H23-y(OO)(OOH)yO2
with y=1 (black signal with blue label) and ext. AutOx. -CO2,
O–C14H23-α(O)(OOH)αO2 with α=1
and 2 (in green) were detected. The black-coloured signals at nominal 390 Th
(16O3) and nominal 394 Th (18O3) stand for the sum of the
signal from the simple AutOx. RO2 radical
O,O–C15H23-x(OOH)xO2 with x=2 and from the ext. AutOx. RO2 radical O,O–C15H23-y(OO)(OOH)yO2 with y=1. Only the arrows and inscriptions (y=1; x=2) indicate the
colours of the product groups. When exchanging 16O3 by
18O3, the signals were shifted by two nominal mass units for each
oxygen atom arising from the initial ozone attack. [β-caryophyllene] = 8.6×1010, [O3] = 8.8×1011 molecules cm-3,
reaction time is 7.9 s.
The second product group, the extended autoxidation group “ext. AutOx.”,
comprises the signals of RO2 radicals with the general formula
O,O–C15H23-y(OO)(OOH)yO2 with y=1–4. Here, (OO)
stands – most likely – for an endoperoxide group. Reactions leading to this
insertion step are discussed in the reaction mechanisms in Sect. 3.4.
RO2 radicals from the simple AutOx. group with
O,O–C15H23-x(OOH)xO2 have the same chemical composition,
and consequently the same position in the mass spectrum like the RO2
radicals from the ext. AutOx. group. A distinction is possible, measuring
the number of acidic H atoms in the molecules (equal to the number of OOH
groups) and applying hydrogen/deuterium (H/D) exchange experiments with heavy water (Figs. 3, 4)
(Rissanen et al., 2014). Products of the ext. AutOx. group contain one less
acidic H atom than the corresponding product from simple AutOx. with
the same composition, for instance for C15H23O8:
O,O–C15H23-y(OO)(OOH)yO2 with y=1 and
O,O–C15H23-x(OOH)xO2 with x=2. H/D exchange
experiments were successfully conducted in order to elucidate the number of
OOH groups in the highly oxidized reaction product from the ozonolysis of
cyclohexene, which represents a model compound for cyclic monoterpenes (Berndt
et al., 2015b; Rissanen et al., 2014). In the case of cyclohexene ozonolysis,
the formation of HOMs strictly followed the simple autoxidation mechanism, and
the results of H/D exchange experiments confirmed the expected number of
hydroperoxide moieties in the products.
Ozonolysis of β-caryophyllene in the absence (lower part)
and presence (upper part) of D2O applying nitrate ionization in the
analysis. Signals highlighted in black stand for the sum of signals in the
absence of D2O from highly oxidized RO2 radicals of the product
groups simple AutOx., O,O–C15H23-x(OOH)xO2 with
x=2 and 3, and ext. AutOx.,
O,O–C15H23-y(OO)(OOH)yO2, with y=1 and 2, and
the corresponding closed-shell product C15H22O9 of the
RO2 radicals for x=3 or y=2. The addition of D2O leads to
an H/D exchange of all acidic H atoms present in the molecule. Accordingly,
signals from the two product groups are separated by the number of acidic H
atoms and the split-up signals are highlighted in red for the simple AutOx. group and in blue for the ext. AutOx. group. [β-caryophyllene] = 8.3×1010, [O3] = 1.02×1012 molecules cm-3, reaction time is 7.9 s.
The third product group (extended autoxidation with CO2 elimination)
named “ext. AutOx. -CO2”, includes the signals of HOMs with a C14
skeleton formed by CO2 elimination in the course of their formation.
Based on experiments with isotopically labelled ozone (18O3)
(Fig. 2) and heavy water (Fig. 5), highly oxidized RO2 radicals of this
product group were assigned to the general formula O–C14H23-α(O)(OOH)αO2 with α=1–3. Here, only one oxygen
atom from the ozone attack, O-, is retained in the HOM. An additional
oxygen atom, (O), is inserted into the molecule arising from air O2.
It is assumed that this (O) exists in an epoxide ring. A possible
reaction sequence leading to epoxide formation is discussed in Sect. 3.4.
Ozonolysis of β-caryophyllene, in the absence (lower part)
and presence (upper part) of D2O applying nitrate ionization in the
analysis. Signals highlighted in black stand for the sum of signals in the
absence of D2O from highly oxidized RO2 radicals of the product
groups simple AutOx., O,O–C15H23-x(OOH)xO2 with
x=2-5, and ext. AutOx.,
O,O–C15H23-y(OO)(OOH)yO2, with y=1–4, and
the corresponding closed-shell product C15H22O7,
C15H22O9, C15H22O11, C15H22O15.
The addition of D2O leads to an H/D exchange of all acidic H atoms
present in the molecule. Accordingly, signals from the two product groups are
separated by their number of acidic H atoms and the split-up signals are
highlighted in red for the simple AutOx. group and in blue for the ext. AutOx. group. The signal at nominal 361 Th can be completely assigned to
the RO2 radical O,O–C15H23-x(OOH)xO2 with x=1
and is highlighted in red. [β-caryophyllene] = 8.3×1010, [O3] = 1.02×1012 molecules cm-3, reaction
time is 7.9 s.
Closed-shell products in all three product groups were detected at -17
nominal mass units compared with the position of the respective RO2
radical in the mass spectrum. The formation of closed-shell products as a
result of consecutive, uni- or bimolecular reactions of the RO2 radicals
can be explained by a formal loss of one oxygen and one hydrogen atom from
the RO2 radical; see proposed reaction pathways as given by Jokinen et
al. (2014).
Highly oxidized reaction products from the ozonolysis of β-caryophyllene detected as nitrate ion adducts and acetate ion adducts using
CI-APi-TOF mass spectrometry. Products were categorized into three product
groups, i.e. simple AutOx., ext. AutOx. and ext.
AutOx. -CO2. Signals from the simple AutOx. and ext. AutOx.
groups were detected at the same mass-to-charge ratio. The percentages
indicate the contribution of a signal to the different product groups,
simple AutOx. and ext. AutOx., as elucidated by H/D exchange
experiments using nitrate ionization.
Nominal mass-to-
Molecular
Product group
RO2 radical
Closed-shell product
charge ratio
formula
(contribution to the
total signal) (%)
Nitrate ion
Acetate ion
adducts
adducts
349
346
C14H23O6
ext. AutOx. -CO2
O–C14H22(O)(OOH)O2
361
358
C15H23O6
simple AutOx.
O,O–C15H22(OOH)O2
364
361
C14H22O7
ext. AutOx. -CO2
O–C14H20O(O)(OOH)2
376
373
C15H22O7
simple AutOx.
(56 %)
O,O–C15H20O(OOH)2
C15H22O7
ext. AutOx.
(44 %)
O,O–C15H21O(OO)(OOH)
381
378
C14H23O8
ext. AutOx. -CO2
O–C14H21(O)(OOH)2O2
393
390
C15H23O8
simple AutOx.
(31 %)
O,O–C15H21(OOH)2O2
ext. AutOx.
(69 %)
O,O–C15H22(OO)(OOH)O2
396
393
C14H22O9
ext. AutOx. -CO2
O–C14H19O(O)(OOH)3
408
405
C15H22O9
simple AutOx.
(30 %)
O,O–C15H19O(OOH)3
C15H22O9
ext. AutOx.
(70 %)
O,O–C15H20O(OO)(OOH)2
413
410
C14H23O10
ext. AutOx. -CO2
O–C14H20(O)(OOH)3O2
425
422
C15H23O10
simple AutOx.
(29 %)
O,O–C15H20(OOH)3O2
C15H22O9
ext. AutOx.
(71 %)
O,O–C15H21(OO)(OOH)2O2
440
437
C15H22O11
simple AutOx.
(29 %)
O,O–C15H18O(OOH)4
C15H22O11
ext. AutOx.
(71 %)
O,O–C15H19O(OO)(OOH)3
457
454
C15H23O12
simple AutOx.
(25 %)
O,O–C15H19(OOH)4O2
C15H23O12
ext. AutOx.
(75 %)
O,O–C15H20(OO)(OOH)3O2
472
469
C15H22O13
simple AutOx.
(22 %)
O,O–C15H17O(OOH)5
C15H22O13
ext. AutOx.
(78 %)
O,O–C15H18O(OO)(OOH)4
489
486
C15H23O14
simple AutOx.
(22 %)
O,O–C15H18(OOH)5O2
C15H23O14
ext. AutOx.
(78 %)
O,O–C15H19(OO)(OOH)4O2
Ozonolysis of β-caryophyllene in the absence (lower part) and
presence (upper part) of D2O applying nitrate ionization in the
analysis. Highly oxidized RO2 radicals of the product group ext.
AutOx. -CO2, O–C14H23-α(O)(OOH)αO2
with α=1 and 2, and the corresponding closed-shell product
(C14H22O7) of the RO2 radical with α=2 are
highlighted in green. The addition of D2O leads to an H/D exchange of
the acidic H atoms being equal to the number of hydroperoxide groups in the
molecules, i.e. a shift by one nominal mass unit for α=1 or a shift
by two nominal mass units for α=2 (including the corresponding
closed-shell product). [β-caryophyllene] = 8.3×1010,
[O3] = 1.02×1012 molecules cm-3, reaction time is
7.9 s.
The same reaction products (RO2 radicals and closed-shell products) were
detected by means of both ionization methods and all signal assignments were
supported by the exact mass-to-charge ratio of the signals (resolving power
at 393 Th: 4100 Th / Th). The detected signal intensity (normalized by the
reagent ion intensity) of the same HOM measured by both ionization techniques
is not necessarily identical and is caused by possible differences of the cluster
ion stability (Berndt et al., 2015b; Hyttinen et al., 2015). As a result of
our analysis, acetate ionization is more sensitive, especially for the
detection of HOMs that contain only one hydroperoxide moiety,
O,O–C15H23-x(OOH)xO2 with x=1 and
O–C14H23-α(O)(OOH)αO2 with α=1. A
similar observation has already been carried out for reaction products from the
ozonolysis of cyclohexene (Berndt et al., 2015b). The signals of the HOMs
with only one hydroperoxide moiety dominate the spectrum recorded with
acetate ionization (Fig. 1a) but are of minor importance in the case of
nitrate ionization (Fig. 1b). Table 1 summarizes the nominal mass-to-charge
ratios of the detected signals and their assignments.
The analysis of the signal intensities points to an important role of
reaction products from the ext. AutOx. and ext. AutOx. -CO2
groups for the total HOM formation from the ozonolysis of β-caryophyllene. The relative contribution of reaction products from the
ext. AutOx. group to the estimated total molar HOM yield, investigated in
the presence of D2O using nitrate ionization, was determined to be
49 %. The simple AutOx. group contribute 29 % and the ext.
AutOx. -CO2 with 22 % to the estimated total molar HOM yield. The
change of the detection sensitivity for different HOMs (especially for those
containing a single hydroperoxide moiety) leads to a different contribution
of the individual product groups to the total HOM signal intensities when
changing from nitrate ionization to acetate ionization. For acetate
ionization, the ext. AutOx. -CO2 group contributes 50 %,
the simple AutOx. group 35 % and the ext. AutOx. group only
15 % to the estimated total molar HOM yield. Thus, the simple AutOx. group contributes 29 % to the estimated total molar HOM
yield when detecting with nitrate ionization and with 35 % when detecting
with acetate ionization. Further detail regarding the HOM concentration
calculations can be found in the Supplement. The values are based on the
lower-limit concentration calculations and on the different detection
sensitivities of the different reagent ions, which depend, e.g. on the
number of hydroperoxide moieties in the molecule of interest. Hence, a
quantitative statement concerning the contributions of the three reaction
product groups is difficult. However, the two new product groups ext. AutOx. and ext. AutOx. -CO2 are crucial for the explanation of HOM
formation from the ozonolysis of β-caryophyllene.
First reaction steps of the ozonolysis of β-caryophyllene.
The attack of the more reactive endocyclic double bond (highlighted in
orange) is exclusively demonstrated. Oxygen atoms arising from the attacking
ozone are highlighted in blue, the alkyl radical functional groups with a
shaded oval.
Experiments with isotopically labelled ozone (18O3)
The signal assignment of the three reaction product groups was supported by
experiments using isotopically labelled ozone, 18O3. When changing
from 16O3 to 18O3 in the ozonolysis, the product signals
in the mass spectra were shifted by two nominal mass units for each oxygen
arising from the initial ozone attack (Jokinen et al., 2014). The
concentration of remaining 18O2 in the carrier gas was about
0.05 % of the total O2 concentration. Hence, 18O2 cannot
compete with 16O2 in the autoxidation steps. Thus, the isotopically
labelled 18O atoms will stem from the ozone attack at the double bond.
For example, Fig. 2 shows a comparison of results from an experiment using
either 18O3 or 16O3 in the ozonolysis reaction for
otherwise constant reaction conditions. The spectra in the range 340–400 Th
are dominated by four signals of RO2 radicals at the nominal
mass-to-charge ratio of 346, 358, 378 and 390 Th, representing signals of all
three product groups. The signals at nominal 358 and 390 Th were shifted by
four nominal mass units when changing from 16O3 to 18O3.
This shift indicates the presence of two oxygen atoms in these reaction
products from the initial ozone reaction. The signal at nominal 358 Th is
attributed to a RO2 radical from the simple AutOx. group, the signal
at nominal 390 Th contains contributions from products of the simple AutOx. as well as the ext. AutOx. group (a further differentiation by
means of H/D exchange experiments is described later). The signal shift by
four nominal mass units shows that reaction products from both product groups
contain two oxygen atoms from the initial ozone attack O,O- as stated in
the general formulas O,O–C15H23-x(OOH)xO2 with x=1–5
(simple AutOx.) and O,O–C15H23-y(OO)(OOH)yO2 with y=1–4 (ext. AutOx.). The third oxygen atom from the attacking ozone is
the oxygen atom of the OH radical that was split off from the Criegee
intermediate forming the alkyl radicals 4a–4c as shown in the first steps of
the ozonolysis mechanism in Fig. 6. Further possible reaction pathways of
species 4b forming simple AutOx. and ext. AutOx. reaction products are
proposed in Fig. 7.
The signals at nominal 346 and 378 Th were shifted by two nominal mass units
applying either 16O3 or 18O3. Consequently, only one
oxygen atom from the initial ozone attack remains in these reaction products
and a second oxygen atom from the initial ozone attack must have been
abstracted in the course of the product formation. The position and the exact
mass-to-charge ratio of these RO2 signals in the mass spectra suggest
that the RO2 radicals contain only 14 carbon atoms. The loss of one
carbon atom and one more oxygen atom from the initial ozone attack points to
an elimination of CO or CO2 in these molecules. The elimination of CO
from highly oxidized RO2 radicals was proposed for reaction products
from the ozonolysis of cyclohexene (Berndt et al., 2015b). The corresponding
reaction products from the ozonolysis of β-caryophyllene including CO
elimination were detected in small yields at nominal 365, 397 and 429 Th
using nitrate ionization and were not further investigated here.
On the other hand, the formation of reaction products from the third product
group is supposed to involve CO2 elimination starting from species 7 in
Fig. 8. Species 7 contains an acyl peroxy radical functional group which might
react with the double bond under formation of an acyl alkoxy radical 15. From
this acyl alkoxy radical, CO2 can easily be released (Jaoui et al., 2003;
Winterhalter et al., 2009). Therefore, the reaction product at nominal
346 Th can be explained by an elimination of CO2 (-44 nominal mass
units) and a subsequent O2 addition (+32 nominal mass units). Reaction
products with signals at nominal 378 and 420 Th can be formed by further
O2 insertion via autoxidation starting from 17; see Fig. 8. Based on
these results, CO2 elimination was proposed for reaction products from
the third product group, named “ext. AutOx. -CO2”. Products of this
group can be explained by the general formula O–C14H23-α(O)(OOH)αO2 with α=1–3. Here, O- stands
for the remaining oxygen atom from the reacting ozone. The proposed reaction
mechanism for the formation of the first member of the ext.
AutOx. -CO2 group with α=1 is given in Fig. 8, 7′ → 15 → 16 → 17. It tentatively includes the formation of an
epoxide ring. The corresponding oxygen atom is marked as (O) in the
general formula O–C14H23-α(O)(OOH)αO2. The
marked oxygen atom, (O), could also belong to an aldehyde or a ketone.
However, it was not possible to explain the formation of a carbonyl
functional group together with the CO2 elimination using known reaction
mechanisms in the literature (Jaoui et al., 2003; Winterhalter et al., 2009).
On the other hand, epoxide formation was already postulated for the OH-radical-initiated oxidation of aromatic compounds (Andino et al., 1996;
Bartolotti and Edney, 1995; Berndt and Böge, 2006; Ghigo and Tonachini,
1999; Suh et al., 2003). The explanation of the oxygen atom, (O), by a
hydroxy moiety can be excluded, because this would imply the presence of two
more hydrogen atoms in the product and hence an increase of two nominal mass
units in the mass spectrum. Furthermore, the possible presence of a hydroxy
moiety would provide an additional acidic H atom in the molecule, which was
not detected in H/D exchange experiments with heavy water (see Sect. 3.3).
Experiments with heavy water (D2O)
A next set of experiments was conducted in the presence of heavy water
(D2O), applying nitrate ionization; see Figs. 3, 4, and 5. The addition
of D2O leads to an H/D exchange of all acidic H atoms present in the
molecule (Rissanen et al., 2014) and thus, to a signal shift in the mass
spectrum by a certain number of nominal mass units being equal to the number
of acidic H atoms in the molecule. For HOMs following the simple autoxidation
process, all oxygen molecules inserted into the molecule, except the RO2
radical functional group, are present as hydroperoxide moieties. The
resulting signal shift in the presence of D2O corresponds to the number
of hydroperoxide moieties as shown for the HOMs from the ozonolysis of
cyclohexene (Berndt et al., 2015b; Rissanen et al., 2014) and α-cedrene (Richters et al., 2016).
Further reaction steps of the alkyl radical 4b. Oxygen atoms arising
from the attacking ozone are highlighted in blue, alkyl radical functional
groups with a shaded oval and RO2 radical functional groups with a
shaded rectangle. Detected species are surrounded by a solid rectangle. The
stated position, where the internal H-transfer takes place 5 → 6,
represents an example only.
Figure 3 shows mass spectra in the presence and absence of D2O focusing
on the signals at nominal 393, 408 and 425 Th which were assigned to
reaction products of the simple AutOx. and ext. AutOx. groups. The
full spectra in the nominal mass-to-charge range 360–495 Th are shown in
Fig. 4. In the presence of D2O, all three signals were split up into two
signals according to their numbers of acidic H atoms in the molecules. This
behaviour indicates that two different reaction products contribute to each
signal. The signal at nominal 393 Th corresponds to the RO2 radical
C15H23O8 and was shifted by one or two nominal mass units when
adding D2O. Two of the eight oxygen atoms arise from the initial ozone
attack (see Sect. 3.2) and two oxygen atoms represent the RO2 radical
functional group. Consequently, two oxygen molecules (four oxygen atoms) at
the maximum can exist in hydroperoxide groups indicated by a signal shift of
two nominal mass units. The corresponding product belongs to the simple AutOx. group, O,O–C15H23-x(OOH)xO2 with x=2,
species 11 in Fig. 8. The signal intensity of the signal, shifted by two
nominal mass units, accounts for 31 % of the total signal intensity; see
the red peak at nominal 395 Th. On the other hand, the signal shift by one
nominal mass unit less (blue peak at nominal 394 Th) can only be explained
by an oxygen molecule insertion without forming a hydroperoxide group. This
insertion is tentatively explained by an endoperoxide formation from the
internal reaction of a RO2 radical with the second, still intact, double
bond in the molecule; see reaction sequence 5′→8→9 in Fig. 7 and
7′→13→14 in Fig. 8. The signal intensity of this reaction product
from the extended autoxidation mechanism ext. AutOx.,
O,O–C15H23-y(OO)(OOH)yO2 with y=1, accounts for
69 % of the total intensity of the shifted peaks. The group, (OO), in
the formula stands for the inserted oxygen molecule appearing as the
postulated endoperoxide; see species 14 in Fig. 8.
The signal of the RO2 radical at nominal 425 Th was shifted by three and
two nominal mass units accounting for 29 and 71 % of the total signal
intensity respectively. Here, compared to the reaction products appearing at
nominal 393 Th, a next oxygen molecule was inserted in the products
resulting in a third hydroperoxide group in simple AutOx.,
O,O–C15H23-x(OOH)xO2 with x=3 and a second
hydroperoxide group in ext. AutOx.,
O,O–C15H23-y(OO)(OOH)yO2 with y=1. The signal of the
corresponding closed-shell product to the RO2 radical at nominal 425 Th
is visible at nominal 408 Th. It shows the same signal shift as its
corresponding RO2 radical by three or two nominal mass units. The signal
intensity of the closed-shell product from the simple AutOx. group,
O,O–C15H22-xO(OOH)x with x=3 accounting for 30 % of the
total signal intensity of the shifted peaks (red peak at nominal 411 Th),
the signal intensity of the reaction product from the ext. AutOx. group,
O,O–C15H22-yO(OO)(OOH)yO2 with y=2 (blue peak at 410 Th) accounts for 70 %.
The relative contributions of the two product groups to the total signal
intensity for all signals are summarized in Table 1. With the exception of
the signal at nominal 376 Th, the ratio of the contributions of the two
product groups is simple AutOx. / ext. AutOx. = 3 / 7–2 / 8. This
ratio shows that the extended autoxidation mechanism is more important than
the simple autoxidation mechanism for reaction products from the ozonolysis
of β-caryophyllene.
It is to be noted that a similar signal splitting-up was detected in H/D
exchange experiments from the ozonolysis of α-pinene by Rissanen et
al. (2015). Here, the authors proposed amongst others an endoperoxide
formation as well based on the literature data from the OH-radical-initiated
oxidation of aromatic compounds and pinenes (Andino et al., 1996; Bartolotti
and Edney, 1995; Vereecken and Peeters, 2004). Figure 5 shows a comparison of
spectra in the nominal mass-to-charge range 345–385 Th recorded in the
presence and absence of D2O. The detected signals at nominal 349, 364
and 381 Th are assigned to the third product group ext. AutOx -CO2.
The signal at nominal 349 Th was shifted by one nominal mass unit when
adding D2O, which indicates the presence of one hydroperoxide moiety in
this reaction product. This signal has the molecular formula
C14H23O6 and one of the six oxygen atoms arises from the
initial ozone attack as observed from the experiments with isotopically
labelled ozone; see Sect. 3.2. Two oxygen atoms are assigned to the RO2
radical functional group. The signal shift by one nominal mass unit from the
H/D exchange experiment indicates that two of the three remaining oxygen
atoms form a hydroperoxide moiety. The third residual oxygen atom must be
inserted into the molecule without generating an additional acidic H atom,
illustrated by (O) in the general formula O–C14H23-α(O)(OOH)αO2. The chemical nature of this (O) in the
product is still uncertain and was tentatively attributed to an epoxide
formation at the second double bond; see 7′ → 15 in Fig. 8 and the
discussion in the Section before (Sect. 3.2). The position of the RO2
radical signal of O–C14H23-α(O)(OOH)αO2 with
α=2 at nominal 381 Th and its corresponding closed-shell product
C14H22O7 at nominal 364 Th were shifted by two nominal mass
units in the presence of D2O. The insertion of the next oxygen molecule leads
to the formation of the RO2 radical O–C14H23-α(O)(OOH)αO2 with α=3 detected at nominal 413 Th,
and its closed-shell product at nominal 396 Th. Both signals were shifted by
three nominal mass units in the presence of D2O. Signals of reaction
products from the ext. AutOx. -CO2 group with more than 10 oxygen
atoms and more than three hydroperoxide moieties were not detected.
Further reaction steps of the RO2 radical (7). Oxygen atoms
arising from the attacking ozone are highlighted in blue, alkyl radical
functions with a shaded oval and RO2 radical functional groups with a
shaded rectangle. Detected species are surrounded by a solid rectangle. The
stated position, where the internal H-transfer takes place 7 → 10a,
represents an example only. The dashed arrows indicate that the stated
reaction pathway remains uncertain.
Mechanism of HOM formation
Figures 6–8 show the proposed initial reaction steps of the ozonolysis of
β-caryophyllene with a focus on the HOM formation. The reaction is
initiated by the ozone attack at the more reactive, endocyclic double bond of
β-caryophyllene 1 marked by the orange oval in Fig. 6. The rate
coefficient of the reaction of ozone with the endocyclic double bond is about
100 times higher than that of the exocyclic double bond (Winterhalter et al.,
2009). Therefore, the reaction of ozone with the exocyclic double bond is
neglected here. The reaction of ozone with a double bond is exothermic and
forms carbonyl oxides, the so-called Criegee intermediates (CIs), 2a and 2b
(Criegee, 1975). Due to the reaction exothermicity, the CIs initially exist
with a large amount of excess energy (chemically activated CIs), which is
stepwise lost by collisions with the bath gas molecules (Kroll et al., 2001).
CIs with an internal energy below a definite threshold energy, needed for
prompt decomposition, are called stabilized CIs (Vereecken and Francisco,
2012). Both stabilized and chemically activated CIs can undergo
unimolecular reactions or can be further collisionally stabilized by the bath
gas (Kroll et al., 2001; Vereecken et al., 2012). Stabilized CIs can also
react in a variety of bimolecular reactions depending on their molecular
structure (Vereecken et al., 2012). An important unimolecular isomerization
step gives the corresponding vinyl hydroperoxide 3a, 3b and 3c (Drozd et al.,
2011; Kroll et al., 2001; Vereecken et al., 2012), which further decomposes
under OH radical release and formation of the alkyl radicals 4a, 4b and 4c.
For simplicity, the reaction scheme does not differentiate between excited
and stabilized molecules.
Figure 7 focuses on further reaction pathways of the alkyl radical 4b. It is
supposed that 4a and 4c are reacting similarly. Molecular oxygen rapidly adds
to 4b forming the first RO2 radical 5. Species 5 can either react via an
intramolecular H-transfer, 5 → 6, followed by O2 addition forming
the RO2 radical 7 from the product group simple AutOx.,
O,O–C15H23-x(OOH)xO2 with x=1 or can internally
attack the remaining double bond, forming an endoperoxide and an alkyl
radical, 5 → 5′ → 8, and after O2 addition, the RO2
radical 9. This cyclization leads to an O2 insertion without forming a
hydroperoxide moiety, indicated by (OO) in the formula
O,O–C15H23-y(OO)(OOH)yO2 of the product group ext. AutOx. (OO) represents the endoperoxide group. The RO2 radical 9
can be further oxidized via the autoxidation mechanism forming RO2
radicals belonging to the product group ext. AutOx.,
O,O–C15H23-y(OO)(OOH)yO2 with y=1–4, not shown here.
A similar endoperoxide formation was already predicted for the OH
radical-initiated oxidation of aromatic compounds (Andino et al., 1996;
Bartolotti and Edney, 1995; Berndt and Böge, 2006; Ghigo and Tonachini,
1999; Suh et al., 2003). Berndt et al. (2015b) validated the formation of
endoperoxide-group containing RO2 radicals from the OH radical-initiated
oxidation of mesitylene (1,3,5-trimethylbenzene) based on the detection of
accretion products of these RO2 radicals. Endoperoxide formation was
also proposed from theoretical investigations for the reaction of OH radicals
with the monoterpenes α- and β-pinene (Vereecken et al., 2007;
Vereecken and Peeters, 2004, 2012) and tentatively
confirmed in chamber experiments (Eddingsaas et al., 2012).
Figure 8 shows the further reaction pathways of the RO2 radical 7 from
the simple AutOx. group, O,O–C15H23-x(OOH)xO2 with x=1. The step 7 → 10a → 11 is an intramolecular H-transfer
with subsequent O2 addition under formation of the RO2 radical 11,
O,O–C15H23-x(OOH)xO2 with x=2 (simple AutOx.).
Furthermore, the closed-shell product 12 can be formed via intramolecular
H-transfer and subsequent OH radical elimination, 7 → 10b → 12.
The formation of HOMs from the ext. AutOx. group can be explained by the
internal RO2 radical reaction with the remaining double bond. This might
lead to the cyclization product 13 that subsequently adds O2 forming the
next RO2 radical 14, O,O–C15H23-y(OO)(OOH)yO2 with
y=1.
The formation of HOMs from the product group ext. AutOx. -CO2 is
uncertain at the moment. A possible reaction sequence starting from the
RO2 radical 7 is shown in Fig. 8, 7 → 7′ → 15 → 16 → 17. In this reaction mechanism an epoxidation step is
proposed, 7′ → 15. Subsequently, CO2 is eliminated from the
acyl alkoxy radical functional group, 15 → 16, resulting in an alkyl
radical 16 that rapidly adds O2 forming the RO2 radical 17. This
new RO2 radical 17, O–C14H23-α(O)(OOH)αO2 with α = 1, can further react via autoxidation, i.e.
intramolecular H-transfer and subsequent O2 addition, forming the next
RO2 radicals of the ext. AutOx. -CO2 group with α=2
and 3.
The epoxide formation cannot be proven and represents only a proposed
reaction pathway in order to explain the experimental results. A similar
epoxide formation step was postulated for the OH radical-initiated oxidation
of aromatic compounds (Bartolotti and Edney, 1995; Glowacki et al., 2009;
Motta et al., 2002; Pan and Wang, 2014; Suh et al., 2003; Yu and Jeffries,
1997). Possible reaction products, e.g. epoxide carbonyls, were detected in
small quantities using GC-MS analysis (Glowacki et al., 2009; Yu and
Jeffries, 1997).
Figures 6–8 show the proposed reaction paths leading to the first RO2
radicals of all three product groups. Consecutive oxidation processes lead to
the next RO2 radicals in competition to bimolecular reactions like
RO2 + R′O2, or RO2 + NO. The formation of
first-generation closed-shell products from highly oxidized RO2 radicals
is discussed by Jokinen et al. (2014) and is not included here.
Experiment with addition of the sCI scavenger CH3COOH
A measurement series in the presence of acetic acid (CH3COOH) has been
conducted in order to get an indication of whether the HOM formation starts from
the chemically activated CI or from the collisionally stabilized CI (sCI),
species 2a and 2b in Fig. 9. Small organic acid were found to efficiently
react with sCIs (Beck et al., 2011; Neeb et al., 1996) while chemically
activated CIs exclusively react via unimolecular reactions, and bimolecular
reactions with other species (such as acids) can be neglected (Vereecken and
Francisco, 2012); see also Sect. 3.4.
Highly oxidized RO2 radicals from the ozonolysis of β-caryophyllene from the three product groups, simple AutOx. with
O,O–C15H23-x(OOH)xO2, x=2 and 3, ext. AutOx. with
O,O–C15H23-y(OO)(OOH)yO2, y=1 and 2, and ext. AutOx. -CO2 with O–C14H23-α(O)(OOH)αO2, α=2 as a function of the CH3COOH
concentration. CH3COOH acts as a sCI scavenger. The black-coloured data
points stand for the RO2 radicals from the simple AutOx. group and
from the ext. AutOx. group with x=2 and y=1 (circle) as well as
with x=3 and y=2 (triangle). The adduct (CH3COOH)NO3-
was detected with lower-limit concentrations which are a factor of 2×107 lower than the acetic acid concentration in the tube. [β-caryophyllene] = 8.3×1010, [O3] = 4.7×1010,
[CH3COOH] = (0–1.4) × 1014 molecules cm-3; reaction
time is 7.9 s.
Figure 9 shows the concentrations of three highly oxidized RO2 radicals
from the three product groups as a function of the acetic acid (CH3COOH)
concentration in the reaction gas. These measurements were performed by applying
nitrate ionization. Additionally, also acetic acid was detectable by the
(CH3COOH)NO3- adduct. The stated (lower limit) adduct
concentrations are by a factor of 2×107 smaller than the acetic
acid concentration in the reaction gas. Even for the highest CH3COOH
concentrations of 1.4×1014 molecules cm-3, no influence of
the HOM concentrations on the acid concentration was detected (Fig. 9).
The absolute rate coefficient of the reaction of acetic acid with sCIs
(CH2OO or CH3CHOO) was measured at 4 torr and 298 K to
(1.2–2.5) × 10-10 cm3 molecule-1 s-1 (Welz
et al., 2014). Assuming a value of 2×10-10 cm3 molecule-1 s-1 for the rate coefficient of
the reaction of acetic acid with the sCIs from β-caryophyllene
ozonolysis, a sCI lifetime with respect to this reaction of 3.6×10-5 s using [CH3COOH] = 1.4×1014 molecules cm-3 follows. The sCI lifetime with respect to the
unimolecular reactions, 2a → 3a and 2b → 3b/c, is substantially
longer with 4×10-3 s assuming the kinetic data for the largest
sCI ((CH3)2COO) available in the literature (Olzmann et al., 1997).
That means that for [CH3COOH] > 1013 molecules cm-3, the
fate of the sCIs is dominated by the reaction with CH3COOH and the
formation of 3a–3c, the expected precursors species of the HOMs, is
suppressed. The absence of any effect of the HOM concentrations on the acetic
acid concentration is taken as an indicator that the sCIs are not involved
in the HOM formation. Consequently, the HOM formation is tentatively
attributed to reactions starting from the chemically activated Criegee
intermediates.
Time dependence of RO2 radical formation
All previous experiments were conducted with a reaction time of 7.9 s. A
variation of the reaction time allows a examination of the possible time
dependence of the reaction processes.
Therefore, the reaction time was varied for constant initial conditions in
the time range of 3.0–7.9 s using acetate ionization and the concentration
changes of RO2 radical from all three product groups were investigated;
see Fig. 10. All RO2 radical concentrations increased proportionally
with time. That shows, firstly, that no significant RO2 radical consumption
occurred at these reaction conditions. Secondly, the interconversion of all
RO2 radicals, including the RO2 radicals from the ext. AutOx.
group with a proposed endoperoxide formation and those from the ext. AutOx. -CO2 group with the proposed CO2 elimination, proceeds at
a timescale of seconds, i.e. with a rate coefficient ≥ 1 s-1.
The RO2 concentrations increased by a factor of 2.3–2.7 from the
shortest to the longest reaction time, which is almost identical to the
increase of the reaction time by a factor of 2.6. This finding differs from
the results of an investigation of cyclohexene ozonolysis using the same
experimental set-up where a concentration increase by a factor of 20–35 was
detected when extending the reaction time from 1.5 to 7.9 s (Berndt et al.,
2015b). This strong increase was explained by the presence of a rate-limited
entrance channel for the highly oxidized RO2 radicals detected from the
cyclohexene ozonolysis. A similar behaviour was not observed for the formation
of highly oxidized RO2 radicals from β-caryophyllene ozonolysis.
Time dependence of highly oxidized RO2 radical formation from
the ozonolysis of β-caryophyllene using acetate ionization, data from
the simple AutOx. group with O,O–C15H23-x(OOH)xO2
with x=1 and 2 (in red), from ext. AutOx. with
O,O–C15H23-y(OO)(OOH)yO2 with y=1 (in blue) and from
ext. AutOx. -CO2 with O–C14H23-α(O)(OOH)αO2 with α=1 and 2 (in green). The black-coloured data
points (open circles) stand for the sum of the RO2 radical from the
simple AutOx. group, O,O–C15H23-x(OOH)xO2 with x=2
and from the ext. AutOx. group,
O,O–C15H23-y(OO)(OOH)yO2 with y=1. [β-caryophyllene] = 8.6×1010, [O3] = 3.1×1011 molecules cm-3, reaction time is 3.0–7.9 s.