Organic acid formation in the gas-phase ozonolysis of α-pinene

Article abstract of DOI:10.1039/b709880d

The mechanism of formation of pinonic and norpinonic acids from α-pinene ozonolysis has been investigated by studying the products of the ozonolysis of an enone derived from α-pinene using gas chromatography coupled to mass spectrometry. the Owner Societies.

Full text of DOI:10.1039/b709880d

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www.rsc.org/pccp | Physical Chemistry Chemical Physics
Organic acid formation in the gas-phase ozonolysis of a-pinene
Yan Ma, Thomas Luciani, Rachel A. Porter, Andrew T. Russell, David Johnson
and George Marston*
Received 28th June 2007, Accepted 23rd July 2007
First published as an Advance Article on the web 3rd August 2007
DOI: 10.1039/b709880d
The mechanism of formation of pinonic and norpinonic acids
from a-pinene ozonolysis has been investigated by studying the
products of the ozonolysis of an enone derived from a-pinene
using gas chromatography coupled to mass spectrometry.
In recent decades, there has been growing interest in the
formation of secondary organic aerosols (SOA) in the atmo-
sphere for reasons that have both local and global significance.
SOA is the primary cause of photochemical smog, leading to a
reduction in visibility;1 particles can absorb or scatter solar
Scheme 1
radiation, serve as cloud condensation nuclei, hence directly or
indirectly impacting on the Earth’s radiation budget;2–4 and
stituted CI2 when ozonised, while the enone only gives the
they may be involved in heterogeneous chemistry in the atmo-
sphere;5 furthermore, the fine particles are likely to have severe
health implications when inhaled.6,7 The atmospheric oxida-
tion of a-pinene is known to form low-volatility products
which can be incorporated into SOA, and available evidence
indicates that initiation by ozonolysis is the most efficient in
this respect, while initiation by OH or NO3 play relatively
more minor roles.8,9 Thus, a detailed understanding of the
gas-phase ozonolysis of a-pinene, especially with regard to the
generation of condensable species, is crucial in order to better
understand atmospheric SOA formation.
monosubstituted CI1. An investigation of the products of the
ozonolysis of these two compounds compared with products
observed from a-pinene ozonolysis can then help to identify
which of the CIs generates which products. We have
previously reported the results of ozonolysis experiments on
the enal (which was synthesised from a-pinene) with focus on
the formation of cis-pinic acid and cis-pinonic acid, the two
major acidic products in a-pinene ozonolysis.25 Subsequent to
publication of this earlier work, we have now successfully
synthesised the enone, again starting from a-pinene;26
a
product study of its reaction with ozone was carried out which
complements our previous results obtained from the enal
experiments and gives an overall picture of the mechanism
of formation of key acid products in the ozonolysis of
a-pinene.
The procedure for the gas-phase ozonolysis experiments was
described in our earlier paper. Briefly, experiments were
carried out at atmospheric pressure (760 Æ 10 Torr, synthetic
air) at a temperature of 295 Æ 4 K in an 80 L collapsible Teflon
chamber. Concentrations of the unsaturated compounds were
on the order of 15–20 ppmv, with ozone concentrations being
Various multifunctional oxygenated species have been
detected in SOA, and products with acid functionalities fea-
ture highly.10–18 The higher molecular weight organic acids,
such as pinic acid and pinonic acid from a-pinene oxidation,
have been suggested to be key compounds in aerosol crea-
tion,10,12,19 and some have been detected in atmospheric
aerosols.20–24 A number of gas-phase mechanisms have so
far been suggested for the formation of such products in
a-pinene ozonolysis; however, to validate properly these
mechanisms still remains a challenge. A particular difficulty
is that the first step of the reaction gives two Criegee inter-
mediates (CIs), as illustrated in Scheme 1.
Both CIs can go on to decompose and/or react with other
molecules, producing a variety of products. CI1 and CI2 have
never been isolated and so it is difficult to determine which of
these two intermediates generates which of the products that
have been observed. One solution to this problem is to react
ozone with compounds which are ‘‘cleaner’’ sources of either
one or other of the two CIs: enal (A) and enone (B). As
illustrated in Scheme 2, the enal only gives rise to the disub-
Department of Chemistry, Reading University, Whiteknights, PO Box
224, Reading, UK RG6 6AD. E-mail: g.marston@rdg.ac.uk;
Fax: +44 (0)118 378 8703; Tel: +44 (0)118 3786 343
Scheme 2
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typically 5 ppmv less, in order to minimise secondary ozone
reactions. 3000–4000 ppmv of cyclohexane was added to
scavenge the 495% of any OH radicals formed in the reac-
tions. Ozone was generated by passing oxygen through a
Fischer ozone generator, its concentration being determined
spectrophotometrically using its well-characterised absorption
at l = 254 nm. All reagents were added to the Teflon chamber
by flushing with synthetic air from a vacuum line. Water vapor
was introduced by passing the synthetic air through a series of
bubblers containing deionized water. The reaction was started
by adding ozone to the chamber, and allowed to occur long
enough that it was simulated to be more than 95% complete,
but not so long that subsequent slower heterogeneous pro-
cesses could significantly affect the results. A variety of
diagnostic experiments carried out on the ozone–cyclohexene
and ozone–a-pinene systems have shown that we were obser-
ving products formed as a result of gas-phase reactions, and
that the yields were not significantly influenced by wall losses
on the timescale that the experiments were performed. Reac-
tion products were trapped onto a PTFE membrane filter
(Schleicher and Schuell TE 36, 0.45 mm pore size) by pumping
reactant–product mixtures through the filter. The filter sample
was immediately methylated using 14% BF3–methanol to
generate the methyl esters of the acids, which were extracted
with hexane and then separated using gas chromatography
(ThermoFinnigan, Trace GC) and detected by quadrupole
mass spectrometry (ThermoFinnigan, Trace MS).
pinonic acid can be generated from CI1.25 Its formation yield
was quantified using the calibration of the methylated stan-
dard by GC-MS. Under dry conditions, the measured yield of
pinonic acid from the enone was 0.18%, which rose to 1.36%
at 80% RH. Fig. 1 illustrates the yields of pinonic acid from
enone ozonolysis at different relative humidities (RH), to-
gether with results from our previous studies of the ozonolysis
of a-pinene and the enal. In addition, the sum of the yields
from the enal and enone experiments are also shown.
The pinonic acid yield from the enone–O3 reaction shows
a clear dependence on RH, which is very similar to that
observed previously for a-pinene ozonolysis. This strongly
supports the suggestion that water can react with CI1 to
generate pinonic acid, probably via a hydroxyalkylhydroper-
oxide intermediate as illustrated in Scheme 3.19,27–29 It is also
noted that a small amount of pinonic acid was observed from
the ozonolysis of the enone in the absence of water vapour,
indicating that there is an additional route via CI1 that does
not involve reaction with water. One possible explanation is
that the acid is formed from the rearrangement of CI1 through
the well-known ester channel,30 as also shown in Scheme 3.
Combining the observations on pinonic acid formation from
a-pinene, the enal and the enone, it can be concluded that the
RH-dependent formation of pinonic acid in the a-pinene
ozonolysis is due to CI1, while the RH-independent formation
is mainly due to CI2, and in a small fraction to CI1.
In addition to pinonic acid, norpinonic acid was tentatively
identified as its methyl ester in the ozonolysis of the enone. The
assignment was made on the basis of an observed electron
impact (70 eV) mass spectrum identical to that reported by
Koch et al.12 for this compound. Due to the lack of authentic
standard, norpinonic acid was semi-quantified using pinonic
acid as the surrogate. Norpinonic acid was also observed from
the ozonolysis of a-pinene, but not from the ozonolysis of the
enal. These findings show clearly that its route of formation is
via CI1. Yields of norpinonic acid from the enone compared
with those from a-pinene are depicted in Fig. 2 as a function of
relative humidity.
Our previous results showed that pinic acid and pinonic acid
are generated from the ozonolysis of a-pinene and the enal,
indicating that both these acids can be formed from CI2.25
Results from the enal experiments also indicated that CI1
should contribute to pinonic acid formation. By comparison
of the GC retention time and mass spectrum with those of the
methylated authentic standard, it was confirmed in the present
study that no pinic acid is formed from enone ozonolysis in the
presence or absence of water vapour. This provides conclusive
evidence that pinic acid is generated exclusively from CI2 and
not from CI1 in the ozonolysis of a-pinene, consistent with our
recent results25 and inferences from previous studes.12,18,19
cis-Pinonic acid was positively identified in the collected
enone ozonolysis products as its methyl ester, confirming that
The yields observed for norpinonic acid from the ozonolysis
of a-pinene and the enone are indistinguishable. This observa-
tion is interesting in terms of what it tells us about the yields of
CI1 in the two sets of experiments. Generally, ozonolysis leads
to higher yields of the more substituted CI (Jenkin et al.19
propose a 40 : 60 ratio for the yield of CI1 to CI2). For
a-pinene, CI1 is the less substituted CI, while for the enone, it
is the more substituted (the other CI being CH2OO); i.e. a
higher yield of norpinonic acid is expected for the enone
Fig. 1 Pinonic acid yields from the ozonolysis of the enone precursor,
the enal precursor and a-pinene at different relative humidities.
Scheme 3
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In summary, the results presented here show conclusively,
for the first time, that pinonic acid and norpinonic acid are
formed in the ozonolysis of a-pinene via CI1. For pinonic acid,
there is a channel with a strong dependence on RH, and a
minor source that is independent of RH; from our previous
experiments, we know that there is also a more important
RH-independent channel via CI2. For norpinonic acid, the
only channel is via CI1, and the close agreement between yields
from a-pinene and the enone strongly suggests that the
branching ratio for CI1 from both compounds is 0.5.
The results of the experiments described here and in our
previous paper have significantly improved our understanding
of the mechanism of the early stages of the ozonolysis of
a-pinene and can be used in detailed atmospheric chemistry
models such as the Master Chemical Mechanism.33 In addi-
tion, the experimental approach of generating and studying
the Criegee intermediates individually in separate experiments
is clearly very useful for understanding alkene ozonolysis
mechanisms. It is worth noting that the concentration of
reactants used here is very much greater than those encoun-
tered in the atmosphere—as is the case in the majority of
laboratory studies. Nevertheless, the results are applicable to
conditions where the fate of peroxy radicals is to react with
RO2 or HO2, rather than NOx; i.e. the results apply to the
chemistry of the rural, unpolluted atmosphere.
Fig. 2 Norpinonic acid yields from the ozonolysis of the enone
precursor and a-pinene at different relative humidities.
experiments. The most likely explanation is that the branching
ratio for CI1 in both systems is close to 0.5. Indeed, there is
evidence that for double bonds attached to long chains, there
is little differentiation in yield between the two possible CIs.31
A recent theoretical study in our laboratory suggests that
ozonolysis of a-pinene results in equal yields of CI1 and
CI2.32 Assuming that CI1 and CI2 are formed with equal
yields from a-pinene, the enal and the enone, allows the
pinonic acid yields from a-pinene to be compared with the
sum of those from the enal and the enone. Fig. 1 shows that
there is reasonable agreement (within 20%) between the two.
It is also worth noting that in both systems, the norpinonic
acid yield shows no clear dependence on the water vapour
concentration. This would suggest that the pathway leading to
its formation does not involve the interaction of CI with water;
thus a possible formation mechanism is through the radical
mediated decomposition of CI1. However, to date, no plau-
sible mechanism has been proposed to explain the direct
formation of norpinonic acid in the gas-phase ozonolysis of
a-pinene.
Based on the mass spectra and GC retention time informa-
tion, a range of other organic acids, including pinalic
acid, norpinic acid, norpinalic acid, OH-pinonic acid and
OH-pinalic acid, were tentatively identified from the ozono-
lysis of a-pinene and the enal, but none of these were observed
from the ozonolysis of the enone. This gives evidence that all
of these acids are generated exclusively from CI2 during
a-pinene ozonolysis. A systematic study of the detailed
mechanisms of formation of pinic acid, norpinonic acid and
other acids is underway and will be reported fully elsewhere.
Acknowledgements
The authors wish to thank NERC for support through the
COSMAS Programme (NER/T/S/2000/01083). DJ wishes to
thank NERC for an Advanced Fellowship (NE/C518230/1).
YM wishes to thank the University of Reading for an
Overseas Postgraduate Research Studentship.
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