Atmospheric sink of β-ocimene and camphene initiated by Cl atoms: Kinetics and products at NOx free-Air

Article abstract of DOI:10.1039/c8ra04931a

Rate coefficients for the gas-phase reactions of Cl atoms with β-ocimene and camphene were determined to be (in units of 10^-10 cm³ per molecule per s) 5.5 ± 0.7 and 3.3 ± 0.4, respectively. The experiments were performed by the relative technique in an environmental chamber with FTIR detection of the reactants at 298 K and 760 torr. Product identification experiments were carried out by gas chromatography with mass spectrometry detection (GC-MS) using the solid-phase microextraction (SPME) method employing on-fiber carbonyl compound derivatization with o-(2,3,4,5,6-pentafluorobenzyl) hydroxylamine hydrochloride. An analysis of the available rates of addition of Cl atoms and OH radicals to the double bond of alkenes and cyclic and acyclic terpenes with a conjugated double bond at 298 K is presented. The atmospheric persistence of these compounds was calculated taking into account the measured rate coefficients. In addition, tropospheric chemical mechanisms for the title reactions are postulated.

Full text of DOI:10.1039/c8ra04931a

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PAPER
Atmospheric sink of b-ocimene and camphene
initiated by Cl atoms: kinetics and products at NOx
free-air†
Cite this: RSC Adv., 2018, 8, 27054
a
a
Elizabeth Gaona-Colman, Marıa B. Blanco, Ian Barnes,b Peter Wiesenb
and Mariano A. Teruel
*
´
´
a
*
Rate coefficients for the gas-phase reactions of Cl atoms with b-ocimene and camphene were
determined to be (in units of 10ꢀ10 cm3 per molecule per s) 5.5 ꢁ 0.7 and 3.3 ꢁ 0.4, respectively. The
experiments were performed by the relative technique in an environmental chamber with FTIR
detection of the reactants at 298 K and 760 torr. Product identification experiments were carried out
by gas chromatography with mass spectrometry detection (GC-MS) using the solid-phase
microextraction (SPME) method employing on-fiber carbonyl compound derivatization with o-
(2,3,4,5,6-pentafluorobenzyl) hydroxylamine hydrochloride. An analysis of the available rates of
addition of Cl atoms and OH radicals to the double bond of alkenes and cyclic and acyclic terpenes
with a conjugated double bond at 298 K is presented. The atmospheric persistence of these
compounds was calculated taking into account the measured rate coefficients. In addition,
tropospheric chemical mechanisms for the title reactions are postulated.
Received 9th June 2018
Accepted 20th July 2018
DOI: 10.1039/c8ra04931a
rsc.li/rsc-advances
presented for Cl chemistry in continental regions far removed
from coastal and marine regions where observations of nitryl
1. Introduction
chloride (ClNO2), a gaseous photolytic Cl atom precursor, were
made9–12 and in a recent study, different chlorine species have
been detected in particles in urban air.13
The interaction between the biosphere and atmosphere is
essential for all living systems and organisms. Hundreds of
Volatile Organic Compounds (VOCs) are released from biogenic
sources. The rst biogenic volatile organic compound (BVOC)
specically identied to be important for atmospheric compo-
sition was isoprene.1 The biogenic VOCs include isoprenoids
(isoprene and terpenes), alkanes, alkenes, carbonyls, alcohols,
esters, ethers and acids, where the predominant species is
isoprene. VOCs emitted from biogenic sources are more reactive
than VOCs with anthropogenic origin.2
Atmospheric concentrations of BVOCs range from few ppt to
several ppb and their reactivity is high as reected by their
chemical lifetimes which range from minutes to hours.1 Many
species of biogenic VOCs are rapidly oxidized by O3, OH radicals
and/or NO3 radicals and it is also possible through reaction with
chlorine (Cl) atoms. This later reaction has generally only been
considered to be of importance in coastal and marine air
environments.3–8 However, in eld studies evidence has been
Atmospheric degradation of BVOCs leads to the production
of secondary chemical species which can enhance concentra-
tions of tropospheric ozone and other oxidants in areas rich in
nitrogen oxides and monoterpenes.2 On the other hand, the
oxidation of BVOCs under low concentrations of NOx generates
peroxides and acids.14 Acids as well as some other BVOC
oxidation products may undergo gas-to-particle conversion
leading to the growth of secondary organic aerosols (SOA).15
Monoterpene compounds as b-ocimene and camphene are
emitted especially by conifers such as pine, spruce, r, mint or
citrus families16–18 and are known to constitute the main frac-
tion of terpenic or essential oils produced in plant secretory
organs.1 Monoterpene emissions are mostly dependent on
ambient temperature, which typically increases their emissions.
Some species (e.g. oaks) also show light-dependence of emis-
sions suggesting link between monoterpene formation and
photosynthetic process. Therefore, weather extreme conditions
(such as high temperature) have large implications for the
uxes of matter between plants and the atmosphere that will
also impact on the atmospheric chemistry.19,20
aInstituto de Investigaciones en Fisicoquımicas de Cordoba (INFIQC), Dpto. de
´
´
´
´
´
Fisicoquımica, Facultad de Ciencias Quımicas, Universidad Nacional de Cordoba,
´
Ciudad Universitaria, 5000 Cordoba, Argentina. E-mail: mteruel@fcq.unc.edu.ar;
mblanco@fcq.unc.edu.ar; Fax: +54-351-4334180; Tel: +54-351-5353866 int. 53526
bPhysikalische Chemie/FBC, Bergische Universitaet Wuppertal, 42119 Wuppertal,
Germany
In order to assess the impact of these species on air quality,
kinetic and mechanistic information on their tropospheric
degradation is therefore needed.
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c8ra04931a
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In this work, we report room temperature relative kinetic was used to evacuate the reactor to 10ꢀ3 torr. Three magnetically
determinations of the rate coefficients for the reactions of Cl coupled Teon mixing fans are mounted inside the chamber to
atoms with b-ocimene and camphene performed in a long path ensure homogeneous mixing of the reactants. The photolysis
photoreactor coupled to an FTIR spectrometer at room system consists of 32 low-pressure mercury lamps (Philips TUV
temperature and atmospheric pressure of nitrogen:
40 W, lmax ¼ 254 nm), spaced evenly around the reaction vessel.
The lamps are wired in parallel and can be switched individually,
allowing a variation of the light intensity, and thus the photolysis
frequency/radical production rate, within the chamber. The
chamber is equipped with a White type multiple-reection mirror
system with a base length of (5.91 ꢁ 0.01) m for sensitive in situ
long path absorption monitoring of reactants and products in the
IR spectral range 4000–700 cmꢀ1. The White system was operated
at 82 traverses, giving a total optical path length of (484.7 ꢁ 0.8)
m. The IR spectra were recorded with a spectral resolution of
1 cmꢀ1 using a Nicolet Nexus FT-IR spectrometer, equipped with
a liquid nitrogen cooled mercury-cadmium-telluride (MCT)
detector. For the reaction of b-ocimene + Cl, 60 interferograms
were co-added and for the reaction of camphene + Cl, 80 inter-
ferograms were co-added, for both reactions 20 spectra were
recorded per experiment.
(1)
(2)
Chlorine atoms were generated by the photolysis of oxalyl
chloride with the germicidal lamps
To the best of our knowledge, this work provides the rst
kinetic and product study for the reactions of Cl atoms with
both monoterpenes. The aim of this study was to extend our
earlier work on the reactivity of these monoterpenes toward OH
radicals and O3 molecules.21–23 Kinetic data for these Cl reac-
tions are needed to facilitate a better understanding of the
oxidation mechanisms of these monoterpenes so that their role
in tropospheric atmospheric chemistry can be better assessed.
The measurements will also help to develop a more reliable
structure–reactivity relationship for these biogenic compounds.
Moreover, it has been identied reaction products due to the
degradations of the studied monoterpenes with Cl atoms.
In addition to the kinetic experimental investigations,
a correlation between the rate coefficients for the reactions of
OH radicals with the monoterpenes studied in this work
together with other monoterpenes and alkenes and those for
the corresponding reactions with Cl atoms has been examined.
The atmospheric lifetimes of the BVOCs studied, with respect
to reaction with Cl atoms, have been calculated with the rate
coefficients obtained in this work and compared with the lifetimes
of b-ocimene and camphene due to other homogeneous sinks in
the troposphere. Products of the studied reactions has been
identied using solid-phase microextraction (SPME) technique
employing on-ber carbonyl compound derivatization with
detection system gas chromatograph-mass spectrometer (GC-MS).
ClC(O)C(O)Cl + hn / 2Cl + 2CO
(3)
The initial concentrations of reactants in ppmV (1 ppmV ¼
2.46 ꢂ 1013 molecule per cm3 at 298 K) were: b-ocimene, (2.1–
2.7); camphene, (1.7–2.2); oxalyl chloride, (1.4–2.0); isobutene,
(1.8–2.0) and propene, (3.7–4).
The reactants were monitored at the following infrared
absorption frequencies (in cmꢀ1): b-ocimene at 2700–3150;
camphene at 882; isobutene at 890 and propene at 912.
Room temperature products identication experiments were
carry out using SPME (grey ber, PDMS/CAR/DVB, SUPELCO)/
GC-MS (Varian CP3800) technique. Mixture of b-ocimene/
ClC(O)C(O)Cl/air and camphene/ClC(O)C(O)Cl/air were
ushed into the Teon bag, respectively. The photolysis time
was 10 s for each experiment, it was a experimentally consid-
erable time in which the reactants have not been completely
consumed with appreciable formation of the reaction products.
In order to identify carbonyl products a 2 mL aqueous solution
of o-(2,3,4,5,6-pentauorobenzyl) hydroxylamine hydrochloride
(PFBHA) of 25 mg mLꢀ1 was used as derivatizing agent. The
PFBHA reacts with carbonyl compounds forming a stable
oxime. The PFBHA was loaded on the SPME ber during 1 min
by head-space extraction. The ber-PFBHA was exposed during
2 min for the b-ocimene–Cl reaction and 3 min for camphene–
Cl reaction inside the chamber to produce the oxime on the
ber to be transferred to the GC-MS injector. The desorption
ꢃ
time was 3 min at 200 C. It was employed a capillary column
2. Experimental
HP-5MS (30 m ꢂ 0.250 mm, lm 0.25 mm Agilent Part N 19091S-
433). The temperature program was 80 ꢃC for 3 min, 100 ꢃC for
5 min, 200 ꢃC for 5 min to 250 ꢃC at a rate of 15 ꢃC minꢀ1 for the
all experiments. The exposition times used were appropriate to
detect these compounds, since larger exposition saturate the
detector system used. In the mechanistic study, the time used
between two consecutive withdrawals was 30 minutes.
All the experiments were performed in a 1080 dm3 quartz–glass
reaction chamber at a total pressure of 760 torr (760 torr ¼
101.325 kPa) and 298 ꢁ 3 K in nitrogen. A detailed description of
the reactor can be found elsewhere24 and only a brief description
is provided here. A pumping system consisting of a turbo-
molecular pump backed by a double-stage rotary fore pump
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3. Materials
The following chemicals, with purities as stated by the supplier,
were used without further purication: nitrogen (Air Liquide,
99.999%), b-ocimene (Sigma-Aldrich, $90%), camphene (Aldrich,
95%), isobutene (Messer Griesheim, $99%), propene (Messer
Griesheim, 99.5%) and oxalyl chloride (Sigma-Aldrich, $99%).
4. Results
Rate coefficients for the reactions of Cl atoms with b-ocimene
and camphene were determined by comparing their rate of
decay with that of the corresponding decay of the reference
compounds:
X + BVOC / Products, kBVOC
(4)
(5)
Fig. 1 Relative rate data for the reaction of Cl with b-ocimene using
isobutene (,) and propene (B) as reference compounds at 298 K and
atmospheric pressure of air.
X + Reference / Products, kreference
Provided that the reference compound and the reactant are
lost only by reactions (4) and (5), then it can be shown that:
In order to place on an absolute basis the rate coefficients for
the reactions of Cl with the monoterpenes, the following values
for the reactions of Cl with the reference compounds at 298 K
were used: (3.40 ꢁ 0.28) ꢂ 10ꢀ10 cm3 per molecule per s for Cl +
isobutene25 and (2.64 ꢁ 0.21) ꢂ 10ꢀ10 cm3 per molecule per s for
Cl + propene.25 The errors for the ratios kBVOC/kreference are only
the 2s statistical errors, and the errors in the kBVOC was calcu-
lated using the propagation of relative error calculation
method.
For both compounds, there is a good agreement between the
values of kBVOC determined using two different reference
compounds.
Averaging the values of the rate coefficients and taking errors
which encompass the extremes of both determinations for each
reaction result in the following nal values for the reaction rate
coefficients at 298 K:
ꢀ
ꢁ
ꢀ
ꢁ
½BVOCꢄ
k4
k5
½Referenceꢄ
½BVOCꢄ0
½Referenceꢄ0
ln
¼
ln
(6)
t
t
where [BVOC]0, [Reference]0, [BVOC]t and [Reference]t are the
concentrations of the compounds studied and reference
compound at times t ¼ 0 and t, respectively, and k4 and k5 are
the rate coefficients of reactions (4) and (5), respectively.
The relative rate technique relies on the assumption that both
the monoterpenes and reference organics are removed solely by
reaction with Cl atoms. To verify this assumption, various tests
were performed to assess the loss of the reactants via reaction
with oxalyl chloride, photolysis and wall deposition. These
processes that could interfere with the kinetic determinations
were found to be negligible for both the monoterpenes and the
reference compounds. Mixtures of the monoterpenes and refer-
ence compounds with oxalyl chloride were stable in the dark
when le in the chamber for the typical time span of the kinetic
experiments (10–20 minutes). Moreover, in the absence of oxalyl
chloride, photolysis of the mixtures (monoterpenes and reference
compounds in air) did not show any decrease in the reactant
concentrations over the time span of the experiments.
Fig. 1 and 2 show the kinetic data obtained from the exper-
iments plotted according to eqn (6) for the reactions of Cl with
the individual compounds studied measured relative to
different reference compounds. Each plot represents
a minimum of 2–3 experiments for each reference compound.
Good linear relationships were obtained in all cases. The line-
arity of the plots with near-zero intercepts, combined with the
fact that similar results were obtained for different initial
concentrations of the unsaturated compound and reference
organics, supports that complications due to secondary reac-
tions in the experimental systems were negligible.
The kBVOC/kreference ratios determined from the slopes of the
straight-line plots in Fig. 1 and 2 are listed in Table 1 together
with the absolute values of the rate coefficients, kBVOC, calcu-
lated from the kBVOC/kreference ratios.
Fig. 2 Relative rate data for the reaction of Cl with camphene using
isobutene (,) and propene (B) as reference compounds at 298 K and
atmospheric pressure of air.
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Table 1 Reference compound, measured rate coefficient ratios, kBVOC/kreference, and the obtained rate coefficients for the reactions of Cl atoms
with b-ocimene and camphene at 298 K in 760 torr of air
Reaction
Reference
kBVOC/kreference
kBVOC ꢂ 1010 (cm3 per molecule per s)
b-Ocimene + Cl
Isobutene
Isobutene
Isobutene
Propene
Propene
Propene
1.63 ꢁ 0.05
1.69 ꢁ 0.05
1.63 ꢁ 0.04
2.13 ꢁ 0.09
1.96 ꢁ 0.07
2.15 ꢁ 0.07
Average
0.93 ꢁ 0.02
0.93 ꢁ 0.01
0.94 ꢁ 0.02
1.26 ꢁ 0.04
1.27 ꢁ 0.01
1.27 ꢁ 0.02
Average
5.5 ꢁ 0.6
5.7 ꢁ 0.6
5.5 ꢁ 0.6
5.6 ꢁ 0.7
5.2 ꢁ 0.6
5.7 ꢁ 0.6
5.5 ꢁ 0.7
3.2 ꢁ 0.3
3.2 ꢁ 0.3
3.2 ꢁ 0.3
3.3 ꢁ 0.4
3.4 ꢁ 0.3
3.4 ꢁ 0.3
3.3 ꢁ 0.4
Camphene + Cl
Isobutene
Isobutene
Isobutene
Propene
Propene
Propene
cm3 per molecule per s).27 However, the structure itself of these
monoterpenes has a minor inuence when they react with Cl
atoms, which is expected due to the reactivity of VOCs with Cl
atoms is collision-controlled.
k1 ¼ (5.5 ꢁ 0.7) ꢂ 10ꢀ10 cm3 per molecule per s
k2 ¼ (3.3 ꢁ 0.4) ꢂ 10ꢀ10 cm3 per molecule per s
The errors quoted are twice the standard deviation arising
from the least-squares t of the straight lines, to which
a contribution has been added to cover uncertainties in the
reference rate coefficients.
Regarding to products identication, for the reaction of b-
ocimene with Cl atoms the only identied product was acetone
observed as oxime due to the derivatization with PFBHA.
Chromatogram and mass spectra are presented on the ESI S1a
and S1b,† respectively. For the reaction of camphene with Cl
atoms, the positively identied products were formaldehyde
and acetone, both were observed as formaldoxime and acetox-
ime, the chromatogram and mass spectra are presented in the
ESI S2a, b, S3a and b,† respectively.
The rate coefficients of VOCs due to the reactions with Cl atoms
and with OH radicals tend to present a linear correlation, in which
this linearity indicates that the reaction mechanisms have similar
pathway. In Fig. 3, it is shown the correlation between the rate
coefficients due to reactions of OH radicals and Cl atoms with
5. Discussion
Kinetics
To the best of our knowledge, no kinetic data on the reaction of
Cl atoms with b-ocimene and camphene have been reported.
The present study, thus, is the rst measurement of the rate
coefficients of the reactions (1) and (2) and therefore no direct
comparison with the literature can be made for the same
reactions. However, the rate coefficients for the reaction of b-
ocimene with Cl atoms ((5.5 ꢁ 0.7) ꢂ 10ꢀ10 cm3 per molecule
per s) can be compared with the rate coefficients of the reactions
of myrcene + Cl atoms ((6.6 ꢁ 1.5) ꢂ 10ꢀ10 cm3 per molecule per
s),26 taking into consideration the experimental errors, these
rate coefficients present a reasonable agreement between them.
On the other hand, comparing the reactivity toward Cl atoms of
camphene and other bicyclic and exocyclic monoterpene, b-
pinene, it can be observed that the value of the rate coefficient
for the reactions of camphene + Cl atoms ((3.3 ꢁ 0.4) ꢂ 10ꢀ10
cm3 per molecule per s) is 1.6 times lower than the rate coeffi-
Fig. 3 Free energy correlation log kOH vs. log Cl for acyclic unsatu-
cient for the reaction of b-pinene + Cl atoms ((5.3 ꢁ 1.5) ꢂ 10ꢀ10 rated terpenes and alkenes, bold square correspond to b-ocimene.
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Table 2 Rate coefficients for the reactions of OH radicals and Cl atoms with acyclic alkenes and terpenes at 298 K in 760 torr
No.
Compounds
kOH ꢂ 1011 (cm3 per molecule per s)
kCl ꢂ 1010 (cm3 per molecule per s)
1
2
3
4
5
6
7
8
1-Butene
1-Pentene
3.15a
3.12a
6.46a
8.72a
3.17a
5.60a
6.37a
6.66a
10a
3.80d
3.97d
3.58d
3.95d
3.29d
3.76d
3.31d
4.20a
4.30e
6.60f
5.5c
2-Methyl-1-butene
2-Methyl-2-butene
3-Methyl-1-butene
(Z)-2-Butene
(E)-2-Butene
1,3-Butadiene
Isoprene
9
10
11
Myrcene
b-Ocimene
33.5b
24c
a
c
f
Ref. 36. b Ref. 26. This work. d Ref. 25. e Ref. 28. Ref. 27.
acyclic alkenes and monoterpenes (these rate coefficients are
If the addition of Cl atoms is produced in the C6 of the
presented in Table 2), where the expression obtained is the double bond (Fig. 5, channel A), the alcoxy radical formed could
following (rate coefficient in cm3 per molecule per s, r2 ¼ 0.72):
decompose forming acetone and CH2]CHC(CH3)]CHCH2-
CH$Cl radical (channel A1) or decompose forming
CH3C(CH3)]CHCH2CH(Cl)C(O)CH3 and $CH3 radical that
could lead to the formaldehyde formation.
log kOH¼ (3.2 ꢁ 0.6) log kCl + (20 ꢁ 6)
When the addition of Cl atoms is produced in C7 the alkoxy
radical formed could generate through a decomposition CH2]
CHC(CH3)]CHCH2C(O)H and $C(CH3)2Cl radical (channel B1)
that aer the subsequent reactions with O2 in the absence of
NOx could produce $OC(CH3)2Cl radical that through an elim-
ination of Cl atom could lead to the formation of acetone. If the
alcoxy radical formed decompose by channel B2 of Fig. 5, the
products formed will be ClC(CH3)2C(O)H and CH2]
CHC(CH3)]CHCH2c radical.
According to the products identied in the experimental
conditions performed in this work, we have demonstrated that
the addition of Cl atoms is produced on C6–C7 of the double
bond of b-ocimene leading to the formation of acetone through
channels A1 and B1 of Fig. 5.
A reasonable correlation has been obtained, therefore, is an
indication of that the presented acyclic alkenes and mono-
terpenes has a similar mechanism when they react with OH
radicals and Cl atoms. Additionally, in this correlation it can be
observed that those compounds with a single double bond are
group together, and that the reactivity increases when the
compound presents two conjugated double bonds and it is even
higher for the tri-unsaturated monoterpenes.
In Fig. 4, it is presented the correlation of the rate coeffi-
cients for the reactions of cyclic alkenes and terpenes with OH
radicals and Cl atoms (these rate coefficients are shown in Table
3), which to our knowledge is the rst correlation of this type
involving cyclic compounds. The following expression has been
obtained (rate coefficient in cm3 per molecule per s, r2 ¼ 0.82):
On the other hand, for the reaction of camphene with Cl
atoms, the proposed reaction mechanisms is presented in
log kOH ¼ (1.4 ꢁ 0.2) log kCl + (3 ꢁ 2)
This correlation presents good linearity and also indicates
similarity on the reaction mechanisms between the reactions of
cyclic compounds with Cl atoms and OH radicals, respectively.
Both correlations can be used to estimate the value of a rate
coefficient when only one of them is available as experimental
data.
Product identication
According to the mechanism presented in Fig. 5, the primary
formation of acetone can be generated through the addition of
Cl atom to the C6 and C7 in the double bond of the group
(CH3)2C]CH– in the structure of b-ocimene. However,
secondary formation of acetone is also possible since b-ocimene
degradation leads to the formation of several products which
can also react with Cl atoms (only acetone primary formation is
presented in Fig. 5).
Fig. 4 Free energy correlation log kOH vs. log Cl for cyclic unsaturated
terpenes and alkenes, bold square correspond to camphene.
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Table 3 Rate coefficients for the reactions of OH radicals and Cl atoms with cyclic terpenes and alkenes at 298 K in 760 torr
No.
Compounds
kOH ꢂ 1011 (cm3 per molecule per s)
kCl ꢂ 1010 (cm3 per molecule per s)
1
2
3
4
5
6
7
8
a-Pinene
b-Pinene
3-Carene
2-Carene
Camphene
Cyclohexene
Cyclopentene
Cycloheptene
Limonene
g-Terpinene
p-Cymene
5.00a
6.92a
8.8b
4.80g
5.30g
5.60g
5.80g
3.3d
7.95c
5.1d
6.74e
5.71e
7.09e
15.9a
17.7f
1.51c
3.97b
3.39b
5.12b
6.40g
10.4h
2.10g
9
10
11
a
c
f
Ref. 37. b Ref. 38. Ref. 39. d This work. e Ref. 36. Ref. 40. g Ref. 27. h Ref. 41.
Fig. 6(a and b) by the addition of Cl atom to the C8 and C3 of also be formed due to the alkoxide decomposition C3–C8
this double bond, respectively. Acetone can be formed through formed from the addition of Cl atom to C3 (Fig. 6b). The
decomposition of C2–C3 bond from the formed alkoxide when degradation of these two studied monoterpenes were expected
Cl atom is added to C8 (Fig. 6a), and through decomposition of to generate acetone since they contain on their structures the
C3–C8 bond from the alkoxide, which was formed due to the pC]C(CH3)2 (b-ocimene) and pC(CH3)2 (camphene) entities,
addition of Cl atom to C3 of the double bond (Fig. 6b), while, product that was also formed due to the reaction of these
´
formaldehyde can be generated through the decomposition of terpenes with OH radicals (Gaona-Colman et al., 2016a and
C3–C8 and C3–C4 bonds from the alkoxides (Fig. 6a), and it can 2017)21,23. Ketones and aldehydes are functional groups also
Fig. 5 Proposed reaction mechanism for b-ocimene initiated by addition of Cl atom to C6 and C7 on the double bond.
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Fig. 6 (a) Proposed reaction mechanism for camphene initiated by addition of Cl atom to C8 on the double bond. (b) Proposed reaction
mechanism for camphene initiated by addition of Cl atom to C3 on the double bond.
reported as products during the reaction of acyclic and cyclic molecules per cm3,30 [O3] ¼ 7 ꢂ 1011 molecules per cm3,31 [NO3]
alkenes with Cl atoms.28,29
¼ 5 ꢂ 108 molecules per cm3 (ref. 32) and [Cl] ¼ 1 ꢂ 104
molecules per cm3.33 Additionally, it is presented the global
lifetimes taking into account the tropospheric losses of the
studied compounds according to the following expression:
Atmospheric implications
In Table 4 is presented the estimated tropospheric lifetimes of
b-ocimene and camphene respect to their degradations toward
the main atmospheric oxidants. These lifetimes have been
estimated using the rate coefficients and the typical tropo-
spheric concentrations of the oxidants: [OH] ¼ 2 ꢂ 106
ꢂ
ꢃꢀ1
1
1
1
1
1
1
sglobal
¼
þ
þ
þ þ þ
sOH
s
O3
s
sCl sphotolysis sdeposition
NO3
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Table 4 Atmospheric lifetimes of b-ocimene and camphene with respect to its reactions with OH radicals, O3 molecules, NO3 radicals and Cl
atoms
Compound kOH ꢂ 1011a sOH (hours) kO ꢂ 1018a sO (hours) kNO ꢂ 1013a sNO (hours) kCl ꢂ 1010a sCl (hours) sglobal (hours)
3
3
3
3
b-Ocimene
25b
0.6
0.6
385d
556e
374f
1
0.7
1
778
441
882
240g
0.017
5.5h
51
0.02
23.6c
Camphene
5.1i
3
3
0.51i
0.90j
0.45k
6.54j
0.85
3.3h
84
0.7
5.33j
a
b
j
k
k in (cm3 per molecule per s). Ref. 42. c Ref. 21. d Ref. 26. e Ref. 43. f Ref. 22. g Ref. 44. h This work. i Ref. 23. Ref. 45. Ref. 46.
Since photolytic loss of b-ocimene and camphene is not
relevant in the troposphere, because they do not absorb in the
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both compounds. The calculated tropospheric lifetimes due to
reaction with each oxidants listed in Table 4 indicate that the
reactions with OH radicals, NO3 radicals and O3 molecules are
most important atmospheric degradation pathways for b-oci-
mene, while the reaction with Cl atoms could be important in
regions where they are more available, i.e. coastal region or
highly industrialized areas. On the other hand, the main
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with OH and NO3 radicals, and the degradation due to reactions
with Cl atoms could also be important in the previously
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risk for human health due to the exposure of formaldehyde35
and this requires the identication all the primary and
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