The gas-phase ozonolysis of β-caryophyllene (C15H24). Part II: A theoretical study

The gas-phase ozonolysis of b-caryophyllene (C15H24).

Part I: an experimental study

Richard Winterhalter,*a Frank Herrmann,a Basem Kanawati,a Thanh Lam Nguyen,b

Jozef Peeters,bLuc Vereecken

band Geert K. Moortgat

a

Received 13th October 2008, Accepted 6th March 2009

First published as an Advance Article on the web 25th March 2009

DOI: 10.1039/b817824k

The gas phase reaction of ozone with b-caryophyllene was investigated in a static glass reactor at

750 Torr and 296 K under various experimental conditions. The reactants and gas phase products

were monitored by FTIR-spectroscopy and proton-transfer-reaction mass spectrometry

(PTR-MS). Aerosol formation was monitored with a scanning mobility particle sizer (SMPS) and

particulate products analysed by liquid chromatography/mass spectrometry (HPLC-MS). The

different reactivity of the two double bonds in b-caryophyllene was probed by experiments with

different ratios of reactants. An average rate coefficient at 295 K for the first-generation products

was determined as 1.1 � 10�16 cm3 molecule�1 s�1. Using cyclohexane as scavenger, an

OH-radical yield of (10.4 � 2.3)% was determined for the ozonolysis of the more reactive internal

double bond, whereas the average OH-radical yield for the ozonolysis of the first-generation

products was found to be (16.4 � 3.6)%. Measured gas phase products are CO, CO2 and HCHO

with average yields of (2.0 � 1.8)%, (3.8 � 2.8)% and (7.7 � 4.0)%, respectively for the more

reactive internal double bond and (5.5 � 4.8)%, (8.2 � 2.8)% and (60 � 6)%, respectively from

ozonolysis of the less reactive double bond of the first-generation products. The residual FTIR

spectra indicate the formation of an internal secondary ozonide of b-caryophyllene. Fromexperiments using HCOOH as a Criegee intermediate (CI) scavenger, it was concluded that at

least 60% of the formed CI are collisionally stabilized. The aerosol yield in the ozonolysis of

b-caryophyllene was estimated from the measured particle size distributions. In the absence of a

CI scavenger the yield ranged between 5 and 24%, depending on the aerosol mass. The yield

increases with addition of water vapour or with higher concentrations of formic acid. In the

presence of HCHO, lower aerosol yields were observed. This suggests that HCOOH adds to a

Criegee intermediate to form a low-volatility compound responsible for aerosol formation. The

underlying reaction mechanisms are discussed and compared with the results from the

accompanying theoretical paper.

Introduction

Biogenic volatile organic compounds (BVOC), whose emissions

largely exceed anthropogenic emissions,1 play an important

role in the atmosphere. Although isoprene and monoterpenes

are the most abundantly emitted biogenic compounds, the

sesquiterpenes (C15H24) are of special importance due to their

high reactivity towards ozone and their large aerosol formation

potential.2 Helmig et al.3 found that sesquiterpene emission

from a variety of pine tree species can account for as much as

29% of the monoterpene emissions. Sesquiterpene emission

rates remain highly uncertain because few quantitative emission

rate measurements have been made and biological pathways

for their formation are not well understood. Kanakidou

et al.4 estimated that the uncertainties in global BVOC

emissions could be as high as a factor of 5 for sesquiterpenes

and other terpenes, and a factor of 3 for isoprene. Recently, a

sensitivity study for mono- and sesquiterpene emissions

was performed for the United States taking into account

the contribution of individual plant functional types,

chemical speciation, light and temperature dependence of the

emissions.5

Sesquiterpenes are emitted from the flowers and foliage of a

variety of coniferous and deciduous plants.3,6,7 Among the

most common sesquiterpenes, b-caryophyllene has been

observed to be emitted by pine trees,3 orange orchards8 and

a variety of agricultural plant species such as potato plants,

leaves of tobacco, sunflower, maize and cotton.9,10

Due to the rapid degradation of b-caryophyllene, and

the low volatility of some of the degradation products,

b-caryophyllene is assumed to have a high particle formation

potential.11 Results from reaction chamber experiments12–14

and ambient observations have confirmed the important role

of b-caryophyllene in the formation of secondary organic

aerosol (SOA). Aerosol particles sampled in a coniferous

forest in Greece were found to consist partly of photooxidation

products of b-caryophyllene.15

aMax Planck Institute for Chemistry, Atmospheric ChemistryDepartment, P.O. Box 3060, D-55020, Mainz, Germany.E-mail: [email protected]

bDepartment of Chemistry, University of Leuven, Celestijnenlaan200F, B-3001, Heverlee-Leuven, Belgium

4152 | Phys. Chem. Chem. Phys., 2009, 11, 4152–4172 This journal is �c the Owner Societies 2009

PAPER www.rsc.org/pccp | Physical Chemistry Chemical Physics

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The ability of b-caryophyllene-ozonolysis to participate in

new particle formation has been described16,17 and the cloud

condensation nuclei (CCN) activity of the newly formed

particles measured.18,19 Products of sesquiterpene ozonolysis

were observed to be less CCN-active than products of mono-

terpene ozonolysis.18 The CCN activity for b-caryophyllenewas found to be a strong function of temperature, suggesting

that the hygroscopic fraction of the SOA is volatile.19

The aerosol yield in the ozonolysis of b-caryophyllene was

measured by Grosjean et al. to be 12%,20 whereas Lee et al.

reported an aerosol yield of 45%.14 The latter authors used

3-fold excess ozone over the sesquiterpene, using cyclohexane

as OH scavenger and ammonium sulfate as seed aerosol

ato10% relative humidity (RH). Jaoui et al.13 reported a yield

of 39.3% without the use of an OH scavenger, but at 80%RH,

working with slight excess of O3 over b-caryophyllene(1.06 : 1). High aerosol yields of 103% and 125% were

reported from photooxidation experiments of b-caryophyllenein the temperature range 316–322 K.12 Aerosol yields ranging

from 17 to 67% were obtained in the photooxidation of

b-caryophyllene for organic mass concentrations between

5 and 40 mg m�3.21 Also Lee et al.22 measured aerosol yields

of (68 � 7)% in a photooxidation study of b-caryophyllene.The kinetics of reactions of BVOC with O3, OH- and

NO3-radicals have been reviewed by Calvert et al.23 The rate

constant for the reaction of b-caryophyllene with O3 was

determined by Shu and Atkinson24 as kO3 =(1.16 � 0.11) �10�14 cm3 molecule�1 s�1 at 296 K. The rate constant for

reaction with OH radicals kOH =(1.97 � 0.25) � 10�10 cm3

molecule�1 s�1 and with NO3 radicals kNO3 =(1.93 � 0.35) �10�11 cm3 molecule�1 s�1 was also measured by Shu and

Atkinson25 and tropospheric lifetimes for b-caryophyllenewith respect to these reactions was estimated as 2 min for

O3 and 53 min and 2 min against OH- and NO3-radicals

respectively.25

Products and mechanism of the tropospheric reactions of

BVOC with O3, OH- and NO3-radicals have been reviewed.26

Product studies of the ozonolysis of b-caryophyllene have beenconducted by various groups.11,13,14,20,27,28 The OH-radical

formation yield from ozonolysis of b-caryophyllene was

measured to be 6%.24

Grosjean et al.,20 determined gas phase carbonyl products

of the reaction between b-caryophyllene and O3 and identified

formaldehyde (80% yield) and an unsaturated C14 ketone with

MW = 206. Calogirou et al.11 investigated b-caryophylleneozonolysis using a combination of GC-MS and HPLC, (with

an ozone: terpene ratio of 1.5 : 1) and identified formaldehyde

(14% yield) and several semi-volatile ketoaldehydes (listed

later in summary product Table 5) based on electron impact

(EI) and chemical ionisation (CI) mass spectra. The unsaturated

C14 ketone with MW = 206 observed by Grosjean et al.20

could not be detected.

Dekermenjian et al.27 used a low pressure impactor (LPI) to

collect aerosols generated from the reaction of ozone with

b-caryophyllene in a Teflon chamber and examined the

chemical composition using FTIR spectroscopy. Aerosol was

observed only in the 0.26–0.075 mm size fraction. Ketones

and aldehydes were the dominant functional groups, with

2.4 groups per average molecule. Also alcohol and carboxylic

acid functional groups were determined with an average per

molecule of 0.59 and 0.19, respectively.

A detailed study of the ozonolysis of b-caryophyllene was

performed by Jaoui et al.13 using a slight excess of ozone. They

identified 16 gas and particulate reaction products using

a combination of HPLC and GC-MS (operated in either

electron impact or chemical ionization mode). On average,

measured gas and particle phase products accounted for about

64% of the reacted b-caryophyllene carbon.

Lee et al.14 used proton transfer reaction mass spectrometry

(PTR-MS) to study the gas-phase products and SOA from the

dark ozonolysis of b-caryophyllene. Gas phase product yields

were determined for formaldehyde (76 � 20)%, acetaldehyde

(0.9 � 0.3)%, formic acid (3.9 � 1)%, acetone (1.1 � 0.3)%,

acetic acid (20 � 5)%, with a total carbon mass of (39 � 2)%.

A SOA yield of (45 � 2)% was obtained for this sesquiterpene.

The components of the organic aerosol compounds formed

in the gas-phase ozonolysis of b-caryophyllene were recently

characterized by Kanawati et al.28 using HPLC coupled to a

triple quadrupole and a time-of-flight analyzer using two

different ionisation sources (ESI� and ACPI+). A large

number of multifunctional oxidation products were detected

in the aerosol samples, and structures have been deduced from

collision-induced dissociation (CID) fragmentation pathways

of pseudo-molecular ions.

There are two double bonds (DB) in b-caryophyllene, eachhaving different reactivity. After oxidation of the first,

very-reactive DB the first-generation products are still reactive

and their further oxidation has not been studied so far.

Experiments varying the ratios of ozone and b-caryophyllenecan help to determine the kinetics of the ozone reaction with

the two double bonds, the OH-radical formation yield and

second-generation products (products of the reaction between

the first-generation products and ozone).

In this study, the ozonolysis of b-caryophyllene was

investigated in a glass reactor under atmospheric conditions.

In addition to the measurement of aerosol yields under various

experimental conditions, yields of the gas-phase products CO,

CO2 and HCHO with respect to the oxidation of the first and

second DB were determined. Furthermore the rate constant

for the first-generation products as well as OH-radical yields

for the first and second DB were measured. Finally the

reaction mechanism of the ozonolysis of b-caryophyllene is

discussed and compared with the accompanying theoretical

study.29

Experimental methods

Experiments were performed in an evacuable 570 l spherical

glass vessel maintained at T = 296 � 2 K and filled with

750 � 3 Torr purified air. For FTIR studies synthetic air

was used as pure air and was prepared by combining pure

oxygen and nitrogen at an appropriate ratio. Most experiments

were performed under dry conditions, however in some cases

water vapour was added after the ozone generation process. The

humidity was adjusted by passing air through a bubbler filled

with 18 MO water (Elgastat) and determined with a humidity

sensor (Panametrics). In order to elucidate the mechanism of

the ozonolysis, HCOOH or HCHO were occasionally added as

This journal is �c the Owner Societies 2009 Phys. Chem. Chem. Phys., 2009, 11, 4152–4172 | 4153

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scavenger for stabilized Criegee intermediates. Ozone was

generated internally, prior to admission of b-caryophyllene,through use of a Hg Pen-Ray lamp mounted inside the

reactor. The initial ozone mixing ratio was determined by

UV-spectroscopy at 254 nm and FTIR-spectroscopy.

Initial mixing ratios of the reactants were varied between 90

and 1050 ppb (1 ppb = 2.4 � 1013 molecule cm�3 at the stated

temperature and pressure) for b-caryophyllene and between

100 and 2000 ppb for O3 (see Tables 1–4). The mixing ratios of

the added Criegee intermediate scavengers ranged from 0.25 to

1.4 ppm HCOOH, 6.8 to 7.8 ppm HCHO, and 12 800 ppm

H2O (corresponding to a relative humidity of 36%). Mono-

meric HCHO was obtained by thermal depolymerization of

paraformaldehyde. Cyclohexane mixing ratios of 10 and

100 ppm were used to scavenge at least 80% or 95% of the

OH radicals, respectively. A pressurised b-caryophyllene–N2

mixture was flushed into the reactor with the help of a 1.38 L

transfer cylinder and N2 carrier gas to reach a final pressure of

750 Torr. During the filling, three Teflon stirrers were acti-

vated to ensure rapid mixing of the reactants. A more detailed

description of the experimental setup was published by Neeb

et al. 199830 and Winterhalter et al. 2000.31

FTIR spectroscopy

Mixing ratios of reactants and products were determined by

long-path FTIR spectroscopy (Bruker IFS 28, 0.5 cm�1

resolution, 43.4 m path-length). For the spectral range bet-

ween 700 and 2000 cm�1 an HgCdTe detector was used while

an InSb detector was used in the range between 2000 and

Table 1 Experimental conditions and measurements for gas phase chemistry investigations

Experiment Ozone (ppb) BC (ppb) CI-Scavenger/OH scavenger ppm Method Determination of:

BC1008 810 240 — — FTIR Rate constantBC2908 810 260 — — FTIR Rate constantBC0609 1656 463 — — FTIR Rate constantBC0619 1656 530 — — FTIR Rate constantBC0305 700 100 Cyclohexane 100 PTR-MS OH-radical yieldBC0805 240 100 Cyclohexane 10 PTR-MS OH-radical yieldBC0212 400 100 HCOOH 1.4 FTIR Gas phase productsBC0222 300 90 — — FTIR Gas phase productsBC2K29 300 100 — — FTIR Gas phase productsBC0307 2000 1050 — — FTIR Gas phase productsBC1107 1600 1050 — — FTIR Gas phase productsBC1607 2000 800 — — FTIR Gas phase products

Table 2 OH-Radical yields from b-caryophyllene and a-pinene ozonolysis

AlkeneTerpene0(ppb)

Ozone0(ppb)

Cyclohexane0(ppm)

Cyclohexanonemax

(ppb)OH-yield (%)1. DB

OH-yield (%)2. DB

literature yield1. DB (%)

b-Caryophyllene 100 700 100 11 9.6 � 2.1 16.9 � 3.7 6a

b-Caryophyllene 100 240 10 10 11.3 � 2.5 15.9 � 3.5 6a

a-Pinene 110 275 10 26 84 � 25 — 85,b 76,c 70,d

83,e 91,f 77,g 68h

a Shu and Atkinson (1994).21 b Atkinson et al. (1992).39 c Chew and Atkinson (1996).40 d Paulson et al. (1997).41 e Rickard et al. (1999).42

f Siese et al. (2001).43 g Aschmann et al. (2002).44 h Berndt et al. (2003).38

Table 3 Yields (in %) of CO, CO2 and HCHO versus reacted b-caryophyllene (1. DB) and additional reacted ozone (2. DB)

ExperimentOzone(ppb)

b-Caryophyllene(ppb) CI-Scavenger

CO CO CO2 CO2 HCHO HCHO1. DB 2. DB 1. DB 2. DB 1. DB 2. DB

BC1408 200 300 — 1 a N.m.b a 15 a

BC1608 180 560 — N.d.b a N.m.b a 8 a

BC1618 180 565 — N.d.b a N.m.b a 9 a

BC0212 400 100 1.4 ppmHCOOHc

6 13 5 7 2 63

BC0222 300 90 — 2 2 2 10 7 50BC2K29 300 100 — 2 10 7 11 4 66BC0307 2000 1050 — 1 4 1 5 12 57BC1107 1600d 1050 — 1 2 N.m.b N.m.b 5 65BC1607 2000d 800 — 1 2 N.m.b N.m.b 7 58Average yield � 1s 2.0 � 1.8 5.5 � 4.8 3.8 � 2.8 8.2 � 2.8 7.7 � 4.0 60 � 6

a Only first DB ozonolysed (excess b-caryophyllene). b N.d.: not detected. N.m: not measured due to high CO2-absorptions in background

spectrum or additional CO2 after background spectrum (caused by external addition of ozone). c The observed consumption of HCOOHwas 60%

of the ozone consumption. d First addition of b-caryophyllene and then slow production of ozone inside the reactor (oxidation of 1. DB), then

addition of excess ozone for 2. DB oxidation.


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4000 cm�1. For each spectrum, either 32 scans at the beginning of

the reaction or 128 scans at later reaction times, were averaged

resulting in a time resolution for the data acquisition of 1 min

and 4 min, respectively. b-Caryophyllene, CO and CO2 were

calibrated by standard volumetric methods. The calibrations

of HCOOH and HCHO have been described earlier.32 For O3,

the absorption cross section at 254 nm (s = 1.14 � 10�17 cm2

molecule�1)33 was used for the UV-absorption calibration.

Scanning mobility particle sizer (SMPS)

Aerosol concentration and size distribution were monitored

with a scanning mobility particle sizer (SMPS, TSI 3936). The

SMPS consists of an electrostatic classifier (TSI 3080) with a

long differential mobility analyzer, (LDMA; TSI 3081) and an

ultra-fine condensation particle counter (CPC; TSI 3025A) as

detector.

Proton-transfer-reaction mass spectrometry (PTR-MS)

The PTR-MS, purchased from Ionicon Analytik, Innsbruck,

Austria, has been described in detail elsewhere.34 Briefly, a

stable flow of air and high concentrations of H3O+ ions are

continuously sampled into an ion/molecule reaction flow-drift

tube at a pressure of few mbar. Here, compounds with a

proton affinity greater than that of water, including a large

selection of oxygenated volatile organic compounds (OVOCs),

undergo efficient proton-transfer reactions with the H3O+

ions to produce protonated organic product ions. Species

having a lower proton affinity than water, including the most

abundant species in the air, are non-reactive and are pumped

off undetected. Individual ions can be swiftly differentiated

based on mass to charge ratio using a quadrupole mass filter

and can then be sensitively detected using an electron

multiplier. Compound concentration can be calculated using

known kinetics of the ion/molecule reaction or can be

independently calibrated. Here the PTR-MS drift tube

was operated at 2.2 mbar and 50 1C with a drift field of

600 V cm�1. Sampled air flow into the drift tube was

constant (approximately 15 mL min�1) using a bypass flow

of 300 mL min�1.

HPLC-MS

The aerosol particles formed were collected for 20–25 min on

Teflon (PTFE) filters (45 mm diameter, 0.45 mm pore size),

using a flow rate of 14 L min�1. Afterwards, the filters were

enclosed in a 7 cm3 glass flask with 3 ml pure methanol added

for extraction and sonicated in an ice bath for 30 min in order

to enhance the extraction process. The samples were stored

at �20 1C prior to analysis.

For chromatographic separation, 100 mL of this extract was

directly injected into the HPLC system, which consisted

of a thermostated autosampler (Series 200, Perkin Elmer,

Norwalk, Connecticut, USA), a degasser and a quaternary

pump (Series 1100, Agilent Technologies, Waldbronn,

Germany). The analytical column was a ReproSil-Pur C18-AQ

(250 mm � 2 mm I.D., 5 mm particle size) in a PEEK

(poly(ether–ether–ketone)) cartridge (Dr. Maisch GmbH,

Ammerbuch, Germany). Gradient elution was implemented at

a flow rate of 400 mL min�1. In contrast to the recent study in

this laboratory,28 where different eluents were used for the

separation of the products in order to facilitate the ionisation

by APCI, here we used the method described by Rompp et al.35

For the analysis of carboxylic acids using ESI� ionization,

0.1% (v/v) formic acid in deionised water (eluent A) and

acetonitrile (eluent B) were used as eluents. The gradient of

the mobile phase was as follows: gradient from 100%A to 95%

B in time interval (0–20) min, followed by gradient from 95% B

to 100% A in (20–23) min. The HPLC system was coupled to a

hybrid mass spectrometer (LC-Triple Quad-MS-MS-TOF

QSTAR, Applied BiosystemsMDS-SCIEX, Toronto, Canada).

This instrument combines tandem mass spectrometry (MS-MS)

with the high mass resolution of a time-of-flight detector (TOF).

The mass measurement error was below 10 ppm. Electrospray

ionization was used in negative mode (ESI�) at 400 1C with an

ionization voltage of �4 kV.

Table 4 SOA yields in the ozonolysis of b-caryophyllene (BC)

ExperimentOzone(ppb)

BC(ppb) CI-Scavenger ppm

ReactedBC/mg m�3

Aerosol mass/mg m�3

Aerosolyield (%)

BC2808 100 309 — — 920 50 6BC1608 180 562 — — 2340 190 8BC2308 200 301 — — 1930 170 9BC1708 200 315 — — 2310 230 10BC1618 180 566 — — 2010 210 10BC1408 200 300 — — 2290 280 12BC2509 200 320 — — 1540 270 18BC2908 100 258 — — 1540 370 24BC1910 200 305 HCOOH 0.25 1580 310 19BC1920 200 298 HCOOH 0.25 2190 460 21BC1810 200 298 HCOOH 0.50 1900 470 24BC1820 200 304 HCOOH 0.50 2220 660 30BC1610 200 320 HCOOH 1.00 1900 620 32BC2609 200 301 HCOOH 1.00 2190 820 38BC0610 200 331 HCOOH 1.00 1520 620 41BC0910 200 315 HCHO 7.80 2130 180 9BC2009 100 239 HCHO 6.80 1050 130 12BC1310 200 295 H2O 12 800 1620 440 27BC1210 200 296 H2O 12 800 1510 420 28


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Chemicals

Sources of the chemicals (stated purities in parentheses) were

as follows: cyclohexane (99.9%), cyclohexanone (99%),

formic acid (98%) and paraformaldehyde (95%) from

Sigma-Aldrich, Steinheim, Germany, b-caryophyllene(99.5%) from Fluka, Buchs, Switzerland, CO (99.97%) and

CO2 (99.998%) from Linde, Germany. All chemicals were of

highest purity commercially available and used without

further purification.

Results and discussion

a. Determination of the rate constant for the reaction of O3

with both double bonds of b-caryophyllene

b-Caryophyllene contains two double bonds with different

reactivity with respect to O3. Based on analogy with the

rate constant of simple alkenes, such as 2-methylpropene

(k = 1.13 � 10�17 cm3 molecule�1 s�1) and 2-methyl-2-butene

(k = 4 � 10�16 cm3 molecule�1 s�1),36 it can be estimated that

the rate constant of the endocyclic double bond (1) is about

1 to 2 orders of magnitude larger than for the exocyclic double

bond (2). The aim of these experiments was to determine

the average rate constant of reaction of ozone with all

first-generation products of b-caryophyllene. The knowledge

of these rate constants will allow the estimation of the

atmospheric lifetime of the sesquiterpene and its degradation

products still containing a double bond.

Ozone was used in a 3-fold excess over the b-caryophylleneconcentration. The mixing ratio of ozone was varied between

810 ppb and 1660 ppb, while that of b-caryophyllene ranged

between 240 and 530 ppb. The ozone consumption is determined

by FTIR spectroscopy and plotted versus the reaction time.

The rate constants were determined with the FACSIMILE

program,37 using the following reaction scheme:

P1: products of the ozonolysis of the first double bond, X,Y

are used to represent all possible products and are not

functional groups

P2: products of the ozonolysis of the second double bond

k1, k2: reaction rate constants

The formation of OH-radicals and their reaction with

b-caryophyllene and the first-generation products were neglected

in these simulations. Fig. 1 shows the results of the simulation

of the ozone consumption for different initial concentrations.

The initial instantaneous ozone consumption at the beginning

of experiments reflects the reaction of ozone with the first

double bond. The lifetime of this double bond is on the order

of a few seconds.

For the simulations, the rate constant for the reaction

of ozone with the first double bond (endocyclic) k1 was

taken from the measured value of Shu and Atkinson (1995)

(k1 = 1.16 � 0.43 � 10�14 cm3 molecule�1 s�1).25 The average

reaction rate constant for the second double (exocyclic) bond

of the first-generation products resulting from the simulation

was k2 = 1.1 � 0.4 � 10�16 cm3 molecule�1 s�1, about

100 times slower than k1.

Using a typical tropospheric ozone mixing ratio of 30 ppb, it

is possible to estimate the atmospheric lifetime t1 of b-caryo-phyllene and of its first-generation products t2 according to

tx = 1/kx [O3]

where kx represents the reaction rate constant of O3 with

each of the double bonds x = 1, 2. Using rate constants

determined here, lifetimes of t1 = 117 s (E2 min) and, for

the first-generation products, t2 = 12318 s (E3.5 h) were

obtained.

b. Determination of the OH–radical yield using cyclohexane

as OH scavenger and PTR-MS

In order to determine the OH yield of the reaction of

ozone with b-caryophyllene, the ozonolysis experiments were

conducted in the presence of excess cyclohexane which served

as an OH scavenger. Cyclohexane reacts with OH radicals

to yield cyclohexanone and cyclohexanol.38 Only cyclo-

hexanone can directly be detected by the PTR-MS in its

protonated form at parent mass m/z 99. The protonated

form of cyclohexanol detected at m/z 101 is not stable and

decomposes to cyclohexene [C6H11]+ at m/z 83 accompanied

by loss of H2O. It was also observed that a very small fraction

(ca. 0.007%) of the excess cyclohexane was oxidized in

the ionization chamber by O2+ to cyclohexanone and cyclo-

hexanol. Since both products were continuously monitored as

Fig. 1 Plot of consumption of ozone versus reaction time for the

experiments BC1008 and BC0619. Symbols represent FTIR data and

solid lines the FACSIMILE results.


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a constant background signal, the observed cyclohexanone

signals at m/z 99 could be corrected.

For the determination of the OH radical yield, the yields of

the products cyclohexanone and cyclohexanol from the reaction

OH+cyclohexane must be known. Berndt et al.38 determined a

yield of (53 � 6)% for cyclohexanone and (35 � 6)% for

cyclohexanol. The OH radicals were produced in the absence of

NOx by photolysis of O3 to yield O(1D) which subsequently

reacts with water vapour to form OH. On the other hand

Atkinson et al.39 measured the sum of both products as

(55� 9)% using the terpene-ozonolysis as an OH-radical source

with cyclohexanone/cyclohexanol ratios from 0.83 to 2.7. In

this work the OH yield was calculated using the yield of

cyclohexanone (53%) of Berndt et al.,28 the yields of Atkinson

et al.39 derived from the reported sum of cyclohexanone and

cyclohexanol (55%), and the respective ratios between cyclo-

hexanone and cyclohexanol (1.1 and 1.59 for a-pinene39 and

b-caryophyllene,24 respectively). These values give a cyclo-

hexanone yield of 29% for a-pinene and 34% for b-caryophyllene.The concentration of cyclohexanone was measured online

by the PTR-MS method. The OH yield measurement was

tested for a-pinene whose OH yield has been determined

previously by several other techniques.38–44 Ozonolysis

experiments with a-pinene and b-caryophyllene were carried

out (Table 2) and the simultaneous formation of cyclo-

hexanone and the ozone consumption were measured during

the course of the reaction as can be seen in Fig. 2 for the

ozonolysis of a-pinene. For a-pinene which contains only one

double bond, it is possible to obtain the OH yield from the

slope of the formation of cyclohexanone versus the ozone

consumption (see Fig. 3).

Under the experimental conditions for the a-pinene–ozonereaction more than 93% of formed OH radicals are scavenged

by cyclohexane. Correcting the observed cyclohexanone for

incomplete scavenging (a yield of 29.5%, see slope in Fig. 3)

and using the 53% yield of cyclohexanone from Berndt et al.38

an OH-radical yield of 60% for the a-pinene–ozone reaction is

obtained. Using the yield of Atkinson et al. (29%)39 an

OH-radical yield of 109% is calculated. The average of both

methods gives an OH-radical yield from the a-pinene–ozonereaction of (84 � 25)%.

This value is in the range of the literature values that vary

between 68 and 91% (see Table 2). Recently Aschmann et al.44

measured the OH-yield of 77% during the a-pinene ozonolysis,independent of relative humidity (5 to 40% RH). Their study

was based on the amount of 2-butanone formed by the

addition of excess 2-butanol as scavenger. The conversion

factor for the yield of 2-butanone 0.69 � 0.06 was used from

direct measurements of the reaction of OH and 2-butanol, in

agreement with an earlier value (0.685) obtained by Chew and

Atkinson.40

For molecules with more than one double bond, such as

b-caryophyllene, it is possible to determine the OH-yield for

the second double bond presuming a sufficiently high time

resolution measurement of cyclohexanone is made.

In the experiment in which 10 ppm of cyclohexane is added,

only 80% of the OH radicals are scavenged by cyclohexane. In

the experiment using 100 ppm cyclohexane, more than 97% of

the OH radicals are scavenged. The measured cyclohexanone

mixing ratios have also been corrected to a complete scavenging

ratio of 100%. In Fig. 4 the formation of cyclohexanone

during the ozonolysis of 100 ppb b-caryophyllene (ozone:

240 ppb) is shown. Under these conditions (lifetime 15 s for

the first DB) an ozone consumption of 94 ppb is already

Fig. 2 Plot of the ozone consumption and the formation of

cyclohexanone during the ozonolysis of a-pinene.

Fig. 3 Plot of the formation of cyclohexanone versus ozone

consumption during the ozonolysis of a-pinene.

Fig. 4 Plot of the ozone consumption and the formation of

cyclohexanone during the ozonolysis of b-caryophyllene for

experiment BC0805. The interruptions of the cyclohexanone data

are caused by internal background calibration of the PTR-MS using

a catalytic converter.


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observed in the first FTIR-spectrum (32 scans, duration 29 s).

After reaction of the first DB the ozonolysis of the resulting

first-generation products is much slower. The interruption in

the cyclohexanone data is due to an internal calibration of the

PTR-MS. The introduction of zero air in the intake manifold

of this instrument has the consequence that the wall is

reconditioned, causing the slight decrease of the cyclohexanone

level after each calibration step.

For b-caryophyllene the yield of cyclohexanone due to

OH-radical formation from the first DB can be calculated

from the initial cyclohexanone mixing ratios at reaction times

corresponding to a measured ozone consumption of roughly

100 ppb. The OH-yield for the second double bond is calculated

from the slope of cyclohexanone versus reacted ozone from

100 ppb up to about 150 ppb (Fig. 5). The respective

OH-radical yields are indicated in Table 2 as 1.DB OH-yield

and 2.DB OH-yield. The last column of Table 2 lists OH

yields from other studies. The measured 1.DB OH-yield of

(10.4 � 2.3)% is higher than the literature value of 6%24 with

an uncertainty of a factor 1.5. The average OH-radical yield

from the first-generation products (2.DB OH-yield) was

determined to be (16.4 � 3.6)%.

c. FTIR measurements

FTIR-spectroscopy was used to monitor ozone, b-caryophylleneand small gaseous molecules such as CO, CO2, HCHO and

HCOOH. Interestingly, formic acid, which possesses a sharp

absorption at 1105 cm�1, is not detected in any of the product

spectra. In the experiment with HCOOH as CI-scavenger it was

observed that a large fraction of HCOOH was consumed

corresponding to 60% of the reacted ozone (Table 3). Therefore

it can be concluded that the formation yield of HCOOH is

either very low or that all formed HCOOH reacts efficiently

with the stabilized Criegee intermediates (CI).

CO and CO2 are difficult to measure at this low concentra-

tion, CO on account of its low absorption cross section and CO2

because of its presence in the synthetic air and in the optical path

parts of the FTIR spectrometer outside the reaction chamber.

Although these parts are flushed with nitrogen, the fluctuations

are in the range of several ppb. The measured yields of only a

few ppb reported in Table 3 therefore have a large uncertainty of

a factor of 2. The yields of the gaseous products are given in

Table 3 and have been calculated for the first DB versus reacted

b-caryophyllene and for the second DB (from first-generation

products) versus additionally reacted ozone. The observed yields

for CO range from 1–6% for the first DB with an average

yield � 1s of (2.0 � 1.8)% and from 2–13% for the 2.DB,

average (5.5� 4.8)%. For CO2 the observed yields vary between

1 and 7% for the first DB, average (3.8 � 2.8)%, and between

5 and 11% for the second DB, average (8.2 � 2.8)%.

HCHO was determined using its characteristic absorptions

at 1740 cm�1 and in the range from 2700 to 2800 cm�1. The

observed yield for the first DB ranges from 2–15%, with an

average of (7.7 � 4.0)%. Only one other study has so far

measured this yield, reporting a value of 8% HCHO for the

first DB,20 a value which is in agreement with the obtained

yield in this study. The yield of HCHO from the second DB

ranges from 50 to 63% with an average of (60 � 6)%.

The FTIR spectrum from an experiment with excess ozone

where both DBs are ozonolysed is shown in Fig. 6a. The

top spectrum was recorded after complete b-caryophylleneconsumption. The residual spectrum after subtraction of

b-caryophyllene, ozone and HCHO is shown at the bottom

of Fig. 6a. The strong absorption in the residual spectrum at

1104 cm�1 could originate from secondary ozonides (SOZ).

Typical FTIR-absorptions of SOZ from simple alkenes45,46

and b-pinene31 are in the range from 1050 to 1150 cm�1.

In Fig. 6b the effect of excess b-caryophyllene can be seen.

The top spectrum shows the absorption in the residual spectrum

when only the first DB is oxidised. The peak at 1116 cm�1 is in

the range of C–O stretching vibrations. A similar peak, with

maximum at 1116 cm�1 was reported following liquid phase

ozonolysis of b-caryophyllene using CCl4 as solvent.47 In

liquid phase ozonolysis with non-participating solvents, SOZ

are the main products. From the similarity of the liquid phase

and gas-phase spectra it was concluded that the SOZ of

b-caryophyllene is also formed in the gas-phase.

Further addition of ozone leads to ozonolysis of the second

less reactive DB. The corresponding residual spectrum is

shown at the bottom of Fig. 6b. A change in the region

1100–1150 cm�1 is observed as well as an increased absorption

in the region of the CQO absorption (1650–1750 cm�1), This

change in the spectrum can be attributed to a SOZ, whose

external DB bond has been oxidised by ozone to yield a

carbonyl functional group.

The residual FTIR spectra from experiments with initial

mixing ratios of 300 ppb for b-caryophyllene and 200 ppb for

ozone in the absence and presence of CI-scavenger are

displayed in Fig. 6c. The influence of the CI-scavenger H2O

and HCOOH on the SOZ absorption at 1116 cm�1 is evident

from the FTIR-spectra, indicating the concurrent reaction

between intramolecular SOZ-formation and bimolecular

reactions of the CI with H2O and HCCOH, respectively.

Interestingly the addition of HCHO did not result in an

apparent decrease of the SOZ-absorption at 1116 cm�1.

d. SOA yield in the ozonolysis of b-caryophyllene

The ozonolysis experiments for the determination of SOA

mass yields were performed in the absence of OH scavenger.

The experiments were performed under 1.5 to 3 fold terpene

Fig. 5 Plot of cyclohexane versus ozone consumption from ozono-

lysis of b-caryophyllene (BC0805) after oxidation of the first DB.


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excess over O3. The typical time evolution of particle size

distributions during the first 60 min of the ozonolysis of

b-caryophyllene is shown in Fig. 7. The aerosol number reaches

its maximum within the first 10 min, with a particle diameter of

about 50 nm. At longer reaction times particles grow in size,

reaching a diameter of 120 nm after 60 min reaction time. Their

number gradually decreases due to coagulation of the aerosol

particles and/or loss by deposition on the reactor wall. The

corresponding aerosol mass is shown in Fig. 8.

From the aerosol mass of the reacted b-caryophyllene, theSOA yields have been calculated and presented in Table 4

using a density of the aerosol particles of 1 g cm�3. The aerosol

yields show a strong dependence on the added Criegee-

intermediate scavenger. In the absence of scavengers under

dry conditions (H2O = 3 ppm), SOA-yields ranging from 5 to

24% were obtained. Following the addition of HCOOH,

yields between 20 and 41% (average 29 � 8%) were obtained,

an increase of about a factor of 2 to 3. It is also observed that

the SOA yield increases at larger added HCOOH concentrations.

By the addition of HCHO the yield varied between 9 and 12%,

and thus shows no effect in comparison to the experiments

done in absence of the Criegee-intermediate scavenger. On

the contrary, the experiments performed in the presence of

12 800 ppm H2O (36% RH) resulted in an increase of the

SOA yield to 28%, representing more than a doubling of the

Fig. 6 (a) FTIR spectrum from experiment BC1008 (top), reference

spectra of ozone, b-caryophyllene and HCHO and residual spectrum

after subtraction of the reference compounds (bottom). The absorption

at 1104 cm�1 is typical for the C–O stretching vibration in organic

peroxides and secondary ozonides. The ‘‘negative’’ absorption of

b-caryophyllene corresponds to the reacted b-caryophyllene, since a

new background spectrum was recorded after addition of b-caryophylleneto the reaction chamber. (b) Residual FTIR spectra from experiment

BC1107. A: obtained under conditions with excess b-caryophyllene(only internal DB oxidized); B: after further ozonolysis of the second

DB. (c) FTIR spectra from experiments (300 ppb b-caryophyllene and200 ppb ozone) in the absence and presence of CI-scavenger. The

SOZ-absorption at 1116 cm�1 is not affected by addition of HCHO,

whereas in the presence of H2O and HCOOH a decrease is observed.

Fig. 7 Particle size distribution during the first 60 min from the

ozonolysis of b-caryophyllene (BC2509).

Fig. 8 The corresponding aerosol mass during the ozonolysis of

b-caryophyllene (BC2509).


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aerosol mass compared to experiments performed under dry

conditions.

The results representing an increase in the SOA yield with

added Criegee intermediate scavenger contrast with findings

obtained from monoterpene ozonolysis where the SOA

yield can be affected upon addition of HCOOH.48 A decrease

of total formed aerosol mass for exocyclic monoterpene

(b-pinene) ozone reactions was reported, but little change for

endocyclic (a-pinene) reactions was observed.48

In order to further analyze the influence of CI scavenger on

aerosol yields, aerosol particles were sampled and analyzed by

HPLC-MS. The extracted ion chromatograms of m/z = 251

from experiments with HCHO, HCOOH and H2O as

CI-scavenger and without CI-scavenger are shown in Fig. 9.

Comparing the observed fragmentation from MS-MS experi-

ments (Fig. 10) with patterns reported by Kanawati et al.28

(Fig. 5 in ref. 28), the peak at 16.2 min was identified as

b-caryophyllonic acid (structure P4 in Table 5). The retention

times in these studies are different because of the different

HPLC conditions applied.

In the presence of HCOOH and H2O a strong increase of

b-caryophyllonic acid was observed compared to the experi-

ment without addition of a CI-scavenger, while the effect

of HCHO is less pronounced. Absolute concentrations of

b-caryophyllonic acid could not be measured due to the lack

of an authentic standard, but the relative increase of its peak

Fig. 9 Extracted ion chromatograms of m/z = 251� from ozonolysis without addition of a CI-scavenger and with the addition of HCHO,

HCOOH and H2O (from top to bottom). The peak at 16.15 to 16.17 min originates from b-caryophyllonic acid (structure P4 in Table 5). The peaks

at 13.6 and 17.2 min could not be identified.

Fig. 10 MS-MS-Spectrum (collision energy: 20 eV) of the anion with m/z = 251� at RT 16.2 min.


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Table

5Ozonolysisproductsofb-caryophyllene

Product

IDused

intext

Structure

Mw

Product

ID(K

anawati

etal.,2008)28

Product

ID(Jaouiet

al.,

2003)13

Product

ID(C

alogirou

etal.,1997)11

Terpene

nomenclature

(Larsen

etal.,2000)48

IUPAC

name

(ACD/C

hem

Sketch

Freew

are

11.01)50

Comment

204

——

Caryophyllane

2,6,10,10-Tetramethyl-

bicyclo-[7.2.0]undecane

Parent

hydrocarbon

skeleton

204

—I

b-Caryophyllene

(4E)-4,11,11-Trimethyl-8-

methylidenebicyclo[7.2.0]-

undec-4-ene

206

—II

b-Caryophylla

ketone

(5E)-6,10,10--

Trimethylbicyclo[7.2.0]-

undec-5-en-2-one

Oxidationof

exocyclic

DB,

intact

endocyclic

DB

P1

236

—IV

�1b-Caryophyllon

aldehyde

4-[3,3-D

imethyl-2--

(3-oxobutyl)cyclobutyl]-

pent-4-enal

P2a

252

252-E-C

1—

Notconsidered

Methyl3-[2,2-dim

ethyl-

4-(5-oxopent-1-en-2-yl)-

cyclobutyl]propanoate

P2b

252

——

Notconsidered

2-[2,2-D

imethyl-4-

(5-oxopent-1-en-2-

yl)cyclobutyl]ethyl

acetate

Notidentified

sofar,predicted

bytheoretical

study(N

guyen

etal.2008)29

P3

208

208-E-C

1—

Notconsidered

4-(3,3-D

imethyl-2-propyl-

cyclobutyl)-pent-4-enal

P4

252

252-E-C

2VIII

�7b-Caryophyllonic

acid

4-[3,3-D

imethyl-2-(3-

oxobutyl)cyclobutyl]-

pent-4-enoic

acid


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Table

5(continued

)

Product

IDused

intext

Structure

Mw

Product

ID(K

anawati

etal.,2008)28

Product

ID(Jaouiet

al.,

2003)13

Product

ID(C

alogirou

etal.,1997)11

Terpene

nomenclature

(Larsen

etal.,2000)48

IUPAC

name

(ACD/C

hem

Sketch

Freew

are

11.01)50

Comment

P5

252

252-N

P-H

P2-

C1

XIII

b-14-H

ydroxy

caryophyllon

aldehyde

4-[2-(4-H

ydroxy-3-oxo-

butyl)-3,3-dim

ethyl-

cyclobutyl]-pent-4-enal

P6

250

250-H

P2-C

1—

b-14-O

xo

caryophyllon

aldehyde

4-[2-(3,4-D

ioxo-butyl)-

3,3-dim

ethyl-cyclobutyl]-

pent-4-enal

P7

250

250-H

P1-C

1—

b-8-O

xocaryophyllon

aldehyde

4-[2-(2,3-D

ioxo-butyl)-

3,3-dim

ethyl-cyclobutyl]-

pent-4-enal

P8

252

252-N

P-H

P1-

C1

—b-8-H

ydroxy

caryophyllon

aldehyde

4-[2-(2-H

ydroxy-3-oxo-

butyl)-3,3-dim

ethyl-

cyclobutyl]-

pent-4-enal

P9

252

252-N

P-H

P-

C2

—b-5-H

ydroxy

caryophyllon

aldehyde

4-[3,3-D

imethyl-2-

(3-oxo-butyl)-cyclobutyl]-

2-hydroxy-pent-4-enal

P10

254

254-D

i-HP2-

C1

VI

b-Caryophyllinic

acid

4-[2-(2-C

arboxyethyl)-3,3-

dim

ethylcyclobutyl]pent-

4-enoic

acid

P11

222

—V

�4b-Norcaryophyllon

aldehyde

3-[3,3-D

imethyl-2-

(3-oxo

butyl)cyclobutyl]-

but-3-enal


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Table

5(continued)

Product

IDused

intext

Structure

Mw

Product

ID(K

anawati

etal.,2008)28

Product

ID(Jaouiet

al.,

2003)13

Product

ID(C

alogirou

etal.,1997)11

Terpene

nomenclature

(Larsen

etal.,2000)48

IUPAC

name

(ACD/C

hem

Sketch

Freew

are

11.01)50

Comment

P11b

224

—XI

�5b-Nornocaryophyllon

aldehyde

3-[3,3-D

imethyl-2-(3-

oxobutyl)cyclobutyl]-3-

oxopropanal

Second-

generation

product

ofP11

238

—X

�2b-Nocaryophyllon

aldehyde

4-[3,3-D

imethyl-2-(3-


oxobutanal

Second-

generation

product

P12b

210

——

�6Notconsidered

Second-

generation

product

220

—III

�3b-Caryophyllene

oxide

4,12,12-Trimethyl-9-

methylidene-5-

oxatricyclo[8.2.0.0

4,6]

dodecane

238

238-H

E—

—4,11,11-Trimethyl-8-

methylidenebicyclo

[7.2.0]undecane-4,5-diol

b-Caryophyllene

oxide+

H2O

254

—VII

b-Nocaryophyllonic

acid

4-[3,3-D

imethyl-2-(3-


oxobutanoic

acid

Second-

generation

product

ofP4


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Table

5(continued)

Product

IDused

intext

Structure

Mw

Product

ID(K

anawati

etal.,2008)28

Product

ID(Jaouiet

al.,

2003)13

Product

ID(C

alogirou

etal.,1997)11

Terpene

nomenclature

(Larsen

etal.,2000)48

IUPAC

name

(ACD/C

hem

Sketch

Freew

are

11.01)50

Comment

238

—XII

b-Norcaryophyllonic

acid

3-[3,3-D

imethyl-2-(3-

oxobutyl)cyclobutyl]but-

3-enoic

acid

268

—XIV

b-14-H

ydroxy

caryophyllonic

acid

4-[2-(4-H

ydroxy-3-

oxobutyl)-3,3-

dim

ethylcyclobutyl]pent-

4-enoic

acid

270

—XV

b-14-H

ydroxy

nocaryophyllonic

acid

4-[2-(4-H

ydroxy-3-

oxobutyl)-3,3-

dim

ethylcyclobutyl]-4-

oxobutanoic

acid

Second-

generation

product

192

—XVI

—3,3-D

imethyl-2-(3-

oxobutyl)

cyclobutanecarbaldehyde

154

—XVII

—3,3-D

imethyl-2-(2-

oxoethyl)

cyclobutanecarbaldehyde

266

—IX

b-9-

Oxocaryophyllonic

acid

4-[3,3-D

imethyl-2-(3-

oxobutanoyl)

cyclobutyl]pent-4-enoic

acid


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area was found to be a factor of 5 upon addition of HCHO, 25

for H2O and 76 for HCOOH. Since b-caryophyllonic acid is a

non-volatile organic acid and might largely condense onto

existing aerosol particles, the increase of b-caryophyllonic acidupon addition of CI-scavenger could partly account for the

increased aerosol yield.

There are only a few papers that reported SOA-yields from

the dark ozonolysis of b-caryophyllene. Grosjean et al.20

measured the particulate carbon in trans-caryophyllene–ozone

experiments and obtained 12% aerosol products. Jaoui et al.13

reported a SOA yield of 39.3% from ozonolysis studies

performed at high RH (80–85%) and slightly lower than

room temperature (287–290 K) in the absence of OH-radical

scavenger. Lee et al.14 measured the total SOAmass yield to be

45 � 2% at much lower RH (6.2%) and 293 K, but in the

presence of cyclohexane as OH scavenger and ammonium

sulfate as a seed aerosol. The aerosol yield of 28% obtained

in this work under moderate RH conditions (36%) is thus

slightly lower than the yield obtained by Jaoui et al.13 and Lee

et al.14 It has to be noted that in the photo-oxidation of

b-caryophyllene, a SOA yield of 103–125% was observed at

temperatures 316–322 K by Hoffmann et al.,12 whereas Griffin

et al.21 reported SOA yields from photo-oxidation in the range

37–79% at 308 K.

e. Products in the aerosol phase

A summary of all identified oxidation products observed in

aerosol samples of b-caryophyllene ozonolysis is presented in

Table 5. The different product identification numbers from

Jaoui et al.13 and from Kanawati et al.28 are listed together

with the names according to a simple terpene nomenclature

proposed by Larsen et al.49 and the systematic IUPAC names

obtained with the software ACD/ChemSketch.50 In the Larsen

nomenclature the original numbering system of the parent

hydrocarbon skeleton is kept also in the resulting oxidation

products. Since there are several numbering systems used for

b-caryophyllene in the literature and the methyl groups are

not explicitly labelled for b-caryophyllene in the paper of

Larsen et al.,49 the numbering system of the Dictionary of

Terpenoids51 was used for the names according to the Larsen

nomenclature. It should be noted that this system is also

different from the IUPAC numbering system.

In the study of Kanawati et al.28 all experiments were

carried out with excess b-caryophyllene (300 ppb) over ozone

(200 ppb) and the identified products still contain the intact

exocyclic DB. Jaoui et al.13 used a slight excess of ozone

(640 ppb) over b-caryophyllene (601 ppb) and some of the

identified products contain a keto group instead of the

methylene group due to further oxidation of the first-

generation products characterized by Kanawati et al.28 The

exchange of the CH2 through an O-atom results in an increase

of the Mw by 2 amu. Interestingly Jaoui et al.13 also identified

a product (b-caryophylla ketone) where only the exocyclic DB

is oxidised and the endocyclic DB is still intact.

Many of the observed products can be assigned to reaction

pathways generally accepted for the reaction of ozone

with simple alkenes, such as the hydroperoxide and ester

channels,52 which will be explicitly discussed in the following

section. A simple scheme is presented in Fig. 11 in order to

summarize the observed products (P1 to P11), which are

thought to be formed by these channels.

f. Reaction mechanism

b-Caryophyllene contains two double bonds, of which the

reactivity toward O3 is different. The rate constant of the

internal (endocyclic) double bond (1) is nearly 100 times larger

than of the external (exocyclic) double bond (2). The internal

double bond (1) will therefore react first with O3, producing a

primary ozonide (POZ-1 and POZ-2, R1a). Ozone attack at

the exocyclic DB (R1b) is much slower and contributes to less

than 5% of the initial ozone consumption according to

theoretical calculations.29

The POZ exists in two conformations (POZ-1 and POZ-2,

see accompanying theoretical paper29), which can rapidly

interconvert. The theoretical calculations of Nguyen et al.29

predict a stabilisation for the POZ of 64.5%, even though

earlier extrapolations by Chuong et al.53 predicted stabilisation

to occur at the later CI stage. The crucial difference between

these studies is that the theoretically evaluated POZ lifetime

for b-caryophyllene29 is much longer than the extrapolation

estimate of Chuong et al.,53 such that substantial collisional

energy loss already occurs at the POZ stage. The stabilized

POZ should yield mainly thermalized CI, which may then

react further with hydroxylic or carbonyl compounds (such as

H2O, HCOOH or HCHO) in accordance with the general

mechanism of ozonolysis of alkenes. The vicinity of a carbonyl

group in the CI from b-caryophyllene also allows facile

intramolecular formation of the secondary ozonide (R3),


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which is not observed in the case of the monoterpene a-pinenedue to the large ring strain in the resulting SOZ.54 For

atmospheric conditions, the major stabilized CI is expected39

to yield SOZ faster than reacting with H2O, and faster

than thermal OH formation through the hydroperoxide

channel.55

The remainder of the POZ decomposes to the four

chemically activated Criegee-intermediates. The theoretical

calculations39 predict a yield of 6.6% for CI-1a (syn), 17.6%

for CI-2a (anti), 0.3% for CI-2b (syn) and 11% for CI-1b

(anti). The excited Criegee intermediates CI-1 and CI-2 can

rearrange via the ester and hydroperoxide channel, or become

collisionally stabilized and react as already indicated. In the

following, the discussion focuses on the reaction pathways of

the two CI formed from the internal double bond, leading to

the variety of first-generation products.

Fig. 11 Reaction channels leading to identified first-generation products from b-caryophyllene ozonolysis (see Table 5).


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1. Reaction channels of the exited CI-1

Hydroperoxide channel

The excited CI-1b can rearrange via a 1,4-hydrogen shift to an

unsaturated hydroperoxide (R4), which then dissociates

into an OH-radical and a 2-oxo-alkyl radical (a vinoxy-type

radical) (R5). This channel is generally used to explain

OH radical formation during alkene ozonolysis and is

commonly referred to as the hydroperoxide channel. Further

reactions of the 2-oxo-alkyl radicals lead to the formation

of a variety of multifunctional products such as acids,

hydroxyl and carbonyl compounds, which have been charac-

terised in the aerosol phase by mass spectrometric methods

(see Table 5).

Addition of oxygen to the formed 2-oxo-alkyl radical yields

a peroxy radical (R6), which either disproportionates into an

a-hydroxycarbonyl (P5) and a-dicarbonyl compound (P6) by

self reaction (R7a) or is converted to the corresponding alkoxy

radical by reaction with any peroxy radical present (R7b). This

alkoxy radical should fragment to HCHO and an acetyl type

radical (R8),56 much faster than it can react with oxygen to

yield the a-dicarbonyl compound (P6).

Addition of oxygen to the acetyl-type radical (R9) yields a

peroxyacetyl-type radical whose possible reactions with HO2

lead to a peroxycarboxylic acid (R10a) and a carboxylic acid

(R10b), both with an aldehyde functional group.

The peroxycarboxylic acid might produce the dicarboxylic

acid b-caryophyllinic acid (P10)13,28 through a Baeyer–Villiger-

type reaction between the aldehyde and the peroxycarboxylic

acid functional group (R11a). In other words, the aldehyde is

oxidized to the carboxylic acid by the peroxycarboxylic acid

functional group, which is itself reduced to the carboxylic acid

(intramolecular comproportionation). Most likely, this type of

reaction will occur only in the condensed phase.

An alternative formation route for b-caryophyllinic acid

(P10) would be one analogous to the mechanism proposed

originally by Jenkin et al. for pinic acid formation from

a-pinene.57 This route involves an H-shift reaction from an

acyloxy radical (R11b) formed from the acetylperoxy radical

generated in reaction R9. However, as argued before58 and

again investigated in recent theoretical work [L. Vereecken,

and J. Peeters, manuscript in preparation], the hydrogen shift

in the acyloxy radical is not expected to be competitive with

CO2 loss (R11c). Recent experimental work by Ma et al.59


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suggests that the Jenkin mechanism is consistent with the

experimental data, but does not confirm the reaction steps

being proposed.

The syn conformer CI-1a can also undergo a 1,4-H-shift,

leading to an unsaturated hydroperoxide (R12) and further

decomposing to OH and another vinoxy-type radical (R13).

Consecutive reactions, in analogy to the reactions described

above for the anti conformer CI-1b, lead to the isomeric

a-hydroxycarbonyl (P7) (R15a) and a-dicarbonyl compounds

(P8) (R15a) or to the alkoxy radical. This alkoxy radical

should quickly fragment into a dialdehyde and the acetyl

radical (R15b and R16).

Ester channel

Both conformers of the excited CI-1 can also form a dioxirane

intermediate (R17a,b), which can either rearrange to one of

the esters (P2a, experimentally observed; P2b, possibly

formed, see theoretical calculations29) (R18a,b) or form

a bis-oxy radical (R18c), whose further reactions include

elimination of CO2 and formation of a methyl and a alkyl

radical (R19a) or direct production of the aldehyde (P3) and

CO2 (R19b). The experimentally observed CO2 yield (Table 3)

of the internal DB (3.8 � 2.8)% can be assigned to the

ester channel of CI-1 and/or to the ester channel of CI-2 (see

below).

2. Reaction channels of the exited CI-2

Hydroperoxide channel

A 1,4-H-shift (R20) is possible only for the syn conformer

CI-2b. For the anti conformer CI-2a, the hydroperoxide channel

does not exist. Analogous to the reactions discussed for the

CI-1, the possible products from this channel are as follows:


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The a-hydroxy carbonyl compound (P9) derived from CI-2b

has been identified by Kanawati et al., (2008)28 and the

dicarbonyl compound (P11) has been identified by Jaoui

et al., (2003).13 The resulting formyl radical in R24b eventually

produces CO and HO2 by reaction with O2.

Ester channel

Both conformers of the excited CI-2 can form the dioxirane

intermediate (R25a,b), which either rearranges to a formyl

ester (R26a) or, via reaction (R26b), to the organic acid b-caryophyllonic acid (P4).11,13,28 Formation of the bis-oxy

radical (R26c) and elimination of CO2 could yield a hydrogen

atom and an alkyl radical (R27a) or the carbonyl compound

(P12a) and CO2 (R27b). The unsaturated carbonyl compound

(P12a) formed in the latter reaction has not been observed so

far. However, in the study of Caligirou et al. (1997)11 a

dicarbonyl compound withMw 210 (P12b) has been identified.

This dicarbonyl compound might be formed by ozonolysis of

the remaining DB of the first-generation product (R28).

3. Stabilisation of the CI and bimolecular reactions

The extent of collisional stabilisation of the POZ and/or the

CI-1 and CI-2 can be estimated by the consumption of

HCOOH (60% of the ozone consumption) in experiments

with added HCOOH (Table 3). This consumption is due to

reaction (R29c) and (R31c) for CI-1 and CI-2, respectively.

Theoretical calculations predict 74% stabilized intermediates

(POZ and CI) for the first-generation products.29 A large

fraction of the POZ and CI could therefore form a SOZ by

intramolecular rearrangement.

b-Caryophyllon aldehyde (P1) is a main product with a total

yield of 17% (gas and aerosol phase) under humid conditions

(dew point of 285–287 K at T = 288–290 K).13 Also in the

study of Caligirou et al. (1997),11 b-caryophyllon aldehyde and

the corresponding b-nocaryophyllon aldehyde were the main

products identified based on experiments in humidified air.

The formation of b-caryophyllon aldehyde is only partly

understood. It is thought to originate from the reaction of

both CI with water (R29a and R31a) yielding an intermediate

a-hydroxy-hydroperoxide which decomposes to H2O2 and

b-caryophyllon aldehyde (R30 and R32). It was not observed

by Kanawati et al. (2008)28 in the absence of water (dew

point 193 K).


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The reaction with HCHO (R29b) and (R31b) could yield

other types of secondary ozonides which have so far not been

detected. The fact that the FTIR-absorption of the cyclic SOZ

did not decrease upon HCHO addition (Fig. 6c) indicates that

the reaction is too slow to compete with the intramolecular

SOZ formation.

The reaction with HCOOH (R29c) and (R31c) could lead to

hydroperoxy formates, in analogy with the formation of

hydroperoxymethyl formate from C1-CI and HCOOH.60

The hydroperoxy formates formed in R29c and R31c have

not been identified so far, but could further react to yield

b-caryophyllonic acid (P4) and HCOOH, possibly in the

aerosol phase. This reaction may constitute the origin of

the increased b-caryophyllonic acid in the presence of

HCOOH. Another possible reaction that could account for

the observed increase of b-caryophyllonic acid is the acid

catalysed decomposition of the cyclic SOZ in the aerosol

phase (or during the analytical procedure) yielding also

b-caryophyllonic acid.

4. Ozonolysis of the exocyclic DB of

b-caryophyllene

Theoretical calculations predict a rate coefficient for O3 attack

on the exocyclic DB having less than 5% of the value of that

for O3 attack on the endocyclic DB.29 The initial POZ-3 and

the possible CIs originating from POZ-3 are given below. In

the presence of excess ozone it can be presumed that the

remaining internal DB of any of the first generation products

of this reaction will rapidly be oxidised according to the

reactions already discussed. For clarity the reactions are given

for the intact internal DB considering only the exocyclic DB.

It should kept in mind, that oxidation of the DB of all

first-generation products (roughly 95% are oxidized at the

internal DB29) leads to production of HCHO and a large CI or

to C1-CI and a large carbonyl compound (e.g. products P11b

and P12b in Table 5).

The average HCHO yield from ozonolysis of the exocyclic

DB is 60 � 6%, which is slightly higher than the predicted

value of 51% from theoretical calculations (see the electronic

supporting information of ref. 29). Decomposition reactions

of the C1-CI yield mainly CO and CO230 and account for the

higher yields of CO and CO2 for the second DB compared to

the first DB (Table 3).

Conclusion

The gas-phase reaction between b-caryophyllene and ozone

was investigated under various experimental conditions, including

the addition of cyclohexane as an OH-radical scavenger and

HCHO, HCOOH and H2O as Criegee-intermediate (CI)

scavengers. The average rate coefficient at 296 K for the

reaction of ozone with the first-generation ozonolysis

products, i.e. for O3 addition to the exocyclic double bond

(second DB), was determined as 1.1� 10�16 cm3 molecule�1 s�1.

This is approximately 100 times slower than the initial

reaction of O3 with b-caryophyllene, i.e. the addition to the

endocyclic double bond (first DB). The atmospheric lifetime of

b-caryophyllene and its first-generation products is 2 min and

3.5 h, respectively, at typical tropospheric ozone mixing ratios

of 30 ppb. Although the ozonolysis of the first-generation is

two orders of magnitude slower than b-caryophyllene, it is stillfaster than the ozonolysis of the monoterpenes a-pineneand sabinene (4.3 h), 3-carene (10 h), b-pinene (24.7 h) and

b-phellandrene (7.9 h).

The OH-radical yield was derived from the yield of

cyclohexanone measured by PTR-MS. The OH-radical yield

for the first DB was 10.4% and 16.4% for the second

less reactive DB of the first-generation products. Previous

measurements by Shu and Atkinson21 yielded 6% for the first


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DB. The low yields imply that the contribution of the hydro-

peroxide channel is a minor process in the mechanism of

ozonolysis of b-caryophyllene. In contrast to the ozonolysis

of simple alkenes and several monoterpenes,23 the reaction of

b-caryophyllene with ozone can be regarded as a minor source

of OH-radicals in the troposphere.

The yields of the gas phase products (CO, CO2, HCHO and

HCOOH) arising from ozonolysis of both double bonds were

measured by FTIR-spectroscopy. The most important finding

is the identification of the secondary ozonide (SOZ) from

analysis of the residual FTIR-spectra. Furthermore, the

consumption of HCOOH added as Criegee-intermediate

scavenger indicated a collisional stabilization of more than

60%. This value is in agreement with the theoretical calculations

predicting a stabilization of 74%.29 The addition of H2O and

HCOOH caused a decrease of the SOZ absorption in the

FTIR-spectra, whereas HCHO did not alter the specific

SOZ absorption features. It could not be resolved in this

study whether the reaction of the CI with HCOOH and

H2O competes with intramolecular SOZ formation or if the

observed change in the product distribution is caused by

secondary reactions of the initially formed SOZ, e.g. by

hydrolysis of the SOZ in the condensed phase.

Aerosol yields have also been determined in the absence of

CI-scavengers and range between 5 and 24%. In the presence

of CI-scavengers (HCOOH and H2O), the aerosol yield was

found to increase. A possible explanation for this observation

could be the reaction of HCOOH and H2O with the CI to form

the low-volatility compound b-caryophyllonic acid (P4), as

identified by HPLC-MS in the aerosol samples. The presence

of HCHO caused a moderate decrease of the aerosol yield.

A detailed reaction mechanism based on the identified

products has been proposed and compared with the results

from the accompanying theoretical paper.29 A main reaction

product is the secondary ozonide (SOZ), which is formed by

intramolecular ring closure of the stabilized CI. The high

estimated yield of the SOZ can be explained by the fact that

the primary ozonide (POZ) is partially stabilized leading

to four different stabilized CI. The fraction of the excited

CI that decomposes via the hydroperoxide channel is estimated

as 10% based on the OH-radical yield. Many of the multi-

functional oxygenated products (P5 to P11) identified in this

study originate from consecutive reactions of the intermediates

formed in the hydroperoxide channel. Evidence of the

contribution of the ester channel was found by the identifica-

tion of CO2 and specific products like b-caryophyllonic acid

(P4) and an ester (P2).

Acknowledgements

Financial support through the BELSPO SSD project

‘‘IBOOT’’ (contract SD/AT/03A and B) is gratefully

acknowledged.

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The gas-phase ozonolysis of β-caryophyllene (C15H24). Part II: A theoretical study

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