Pinic and pinonic acid formation in the reaction of ozone with α-pinene

Article abstract of DOI:10.1039/b617130c

The mechanism of formation of key compounds in atmospheric secondary aerosol (SOA) has been investigated by studying the products of the ozonolysis of an enal derived from α-pinene using gas chromatography coupled to mass spectrometry. The Royal Society of Chemistry.

Full text of DOI:10.1039/b617130c

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Pinic and pinonic acid formation in the reaction of ozone with a-pinene
Yan Ma, Timothy R. Willcox, Andrew T. Russell and George Marston*
Received (in Cambridge, UK) 23rd November 2006, Accepted 23rd January 2007
First published as an Advance Article on the web 12th February 2007
DOI: 10.1039/b617130c
The mechanism of formation of key compounds in atmo-
spheric secondary aerosol (SOA) has been investigated by
studying the products of the ozonolysis of an enal derived
from a-pinene using gas chromatography coupled to mass
spectrometry.
Scheme 2
Secondary organic aerosol (SOA) is an important component of
Earth’s atmosphere for a variety of reasons. It is the primary
manifestation of photochemical smog, impacting on urban
visibility;1 particles are easily inhaled, inducing various health
problems;2,3 they can impact on the atmospheric radiation budget
directly and indirectly by acting as cloud condensation nuclei;4–6
and they may act as surfaces catalysing heterogeneous reactions in
the atmosphere.7 The atmospheric oxidation of both large and
small volatile organic compounds (VOCs) is an important source
of SOA, and available evidence indicates that initiation by
ozonolysis is often more effective in this respect than initiation
by OH or NO3 reactions.8,9 Of the biogenic C10 VOCs (terpenes),
a-pinene is by far the most commonly emitted.10 Thus, the
ozonolysis of a-pinene is a key process in the formation of SOA
and understanding the reaction mechanism is an essential part of
understanding atmospheric SOA formation.
overcome this problem, we have synthesised an enal (A) derived
from a-pinene20 that can only give rise to CI2 when ozonised; see
Scheme 2. An investigation of the products of ozonolysis of this
compound compared with products obtained from the ozonolysis
of a-pinene can then identify which of the CIs generates which
products. A wide range of acidic products were observed in the
ozonolyses, and will be reported fully elsewhere. For the purposes
of this communication, we focus on cis-pinic and cis-pinonic acids,
which have been identified in SOA, are the major acidic products
and were the main thrust of our original investigation.
Various multifunctional oxygenated species have been detected
in SOA, and products with acid functionalities feature highly.11–19
Christoffersen et al. were the first to suggest that pinic acid could
be the compound leading to aerosol nucleation.11 However, while
there have been a number of suggested mechanisms for the
formation of such products in a-pinene ozonolysis, properly
validating the mechanisms is very difficult. A particular difficulty is
that the first step in the reaction can occur in two ways, to give two
Criegee intermediates (CI), as illustrated in Scheme 1.
Ozonolysis experiments were carried out at atmospheric
pressure (synthetic air) at a temperature of 295 ¡ 4 K in an
80 L collapsible Teflon chamber. Concentrations of the unsatu-
rated compounds were on the order of 15–20 ppmv, with ozone
concentrations being typically 5 ppmv less in order to limit
secondary reactions. Experiments were carried out in the presence
of ca. 3000–4000 ppmv cyclohexane in order to scavenge any OH
radicals (.95%) formed in the reactions. Ozone was generated
using a Fischer (Model 502) ozoniser, and was trapped onto silica
gel at 225 K, before being desorbed into a Pyrex bulb. Its purity
was checked by measuring its ultraviolet absorption at 254 nm.
Reagents were added to the Teflon chamber by flushing with
synthetic air from a vacuum line. The air used could be admitted
dry or wet by passing through a series of bubblers; by varying the
fraction of dry to wet air it was possible to control the relative
humidity of the reaction mixture. Reaction was allowed to occur
long enough that it was estimated to be more than 95% complete,
but not so long that subsequent slower heterogeneous processes
could significantly affect the results. A variety of diagnostic
experiments on the ozone–cyclohexene and ozone–a-pinene
systems were carried out before the main study; these included
looking at the time dependence of product yields, measuring
products on the walls of glass vessels and varying the surface-to-
volume ratio of the reaction vessel. These experiments showed that
we were observing products formed as a result of gas-phase
The two CIs, CI1 and CI2, have never been isolated and so
there is a problem deciding which of these two intermediates gives
rise to the various products that have been observed. In order to
Scheme 1
Department of Chemistry, Reading University, Whiteknights, PO Box
224, Reading, UK RG6 6AD. E-mail: g.marston@rdg.ac.uk;
Fax: +44 118 3788703; Tel: +44 118 3786343
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reactions, and that the yields were not significantly influenced by
wall losses on the timescale that the experiments were performed.
Acid products were trapped onto a PTFE membrane filter
(Schleicher and Schuell TE 36, 0.45 mm pore size) by pumping
product mixtures through the filter. The filter was then 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
using quadrupole mass spectrometry (ThermoFinnigan, Trace
MS). cis-Pinic acid and cis-pinonic acid were identified as methyl
esters by comparison of the GC retention times and mass spectra
with those of the methylated authentic standards. Quantification
was performed using the calibration of the relevant standards by
GC-MS. Under dry conditions and in the presence of cyclohexane
as OH scavenger, yields of pinic acid and pinonic acid are
measured to be 2.85 ¡ 0.31% and 1.86 ¡ 0.31% respectively, in
the range of the results from other studies.12,13,15,19
Fig. 2 Acid yields from the ozonolysis of the enal under various
conditions.
Illustrated in Fig. 1 are plots of pinic acid and pinonic acid
yields as a function of relative humidity in the presence and
absence of acetic acid.
acid experiments are also interesting. Three sets of experiments
give very similar results (a-pinene in the presence of acetic acid;
enal in the presence of acetic acid; and enal in the absence of acetic
acid), while one set (a-pinene in the absence of acetic acid) is very
different. What these experiments show is that there are two routes
to pinonic acid. Water can react with CI1 to generate pinonic acid
in a route that is fairly well known in ozone chemistry. However,
in addition there is a further route that occurs via CI2 and does not
involve reaction with water. Thus it can be concluded that the ester
channel illustrated in Scheme 3 is not a major source of pinonic
acid.
In the absence of acetic acid, pinonic acid shows a fairly strong
dependence on RH, while pinic acid shows a much weaker
dependence. This is consistent with the suggestion that water
catalyses the rearrangement of CIs to acid functionalities, probably
via a hydroxyalkylhydroperoxide.21–24 No such mechanism is
expected for the formation of pinic acid. However, pinonic acid
also has a significant source that is not dependent on RH. One
possible explanation is that the acid is formed in the well-known
ester channel,25 and we shall return to this point later.
A number of comments can be made about the mechanism for
this channel. It is generally believed that doubly alkyl-substituted
CIs decompose with high efficiency to give the OH radical and a
co-radical, which is then converted to the peroxy then alkoxy
radical followed by carbon–carbon bond fission, as illustrated for
the dimethyl CI (Scheme 4).
In the presence of acetic acid—a well known SCI scavenger—
the dependence of the pinonic acid yield on RH has been removed,
consistent with acetic acid competing more effectively for CI than
water.26
Fig. 2 illustrates similar results to Fig. 1, except that these are for
the ozonolysis of the enal illustrated in Scheme 2 rather than
a-pinene. The first thing to note is that the yields of pinic acid are
similar to those for a-pinene ozonolysis and in both cases are—at
most—mildly affected by the presence of water vapour or acetic
acid. This is entirely consistent with the generally held view that
this acid species is generated via the decomposition of CI2.13,19,23
However, these experiments are the first to show conclusively that
pinic acid is generated predominantly through CI2. The pinonic
Pinonic acid cannot be generated via this (or part of this)
process; addition of O2 to the initially formed radical introduces
functionality at two carbon centres, while carbon fission reduces
the number of carbon atoms in the product molecule. A possible
route to pinonic acid is that a small fraction of CI2 isomerises
(possibly via an alkyl-bisoxy radical intermediate), abstracts the
aldehydic hydrogen from the other end of the molecule, releases
OH and then leads to the acid as shown in Scheme 5.
The first isomerisation step in this scheme has not previously
been suggested, but is similar to those postulated to explain pinic
Scheme 3
Fig. 1 Acid yields from the ozonolysis of a-pinene under various
conditions.
Scheme 4
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3 R. W. Atkinson, H. R. Anderson, D. P. Strachan, J. M. Bland and
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Scheme 5
acid formation from a-pinene23 and the proposed reaction step for
the CI formed in cyclohexene ozonolysis.27,28 Furthermore, some
kind of rearrangement of this type is required in order that the acid
moiety can be generated at the opposite end of CI2 from the
original CI centre. The other steps in this mechanism are consistent
with known chemistry.
13 S. Koch, R. Winterhalter, E. Uherek, A. Kolloff, P. Neeb and
G. K. Moortgat, Atmos. Environ., 2000, 34, 4031.
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In summary, we have provided, for the first time, conclusive
evidence that pinic acid is formed via CI2 in the ozonolysis of
a-pinene; this observation is consistent with inferences from
previous studies. On the other hand, we have shown that pinonic
acid is formed via both CI1 and CI2. Formation via CI1 is RH-
dependent and our observations are consistent with previous
studies that suggest monosubstituted CIs can react with water to
give organic acids. Formation via CI2 is unexpected and is not
dependent on RH; a plausible mechanism for formation of pinonic
acid via this route is proposed. The results are important for
atmospheric chemistry because they provide mechanistic informa-
tion that can be used in detailed models such as the Master
Chemical Mechanism. It is worth noting that the concentration of
reactants used here is very much greater than those observed in the
atmosphere—as is the case in the majority of laboratory studies.
Nevertheless, the results are applicable under conditions where the
fate of peroxy radicals is reaction with RO2 or HO2 rather than
NO; i.e. the results apply to the chemistry of the rural, unpolluted
atmosphere.
19 R. Winterhalter, R. Van Dingenen, B. R. Larsen, N. R. Jensen and
J. Hjorth, Atmos. Chem. Phys. Discuss., 2003, 3, 1.
20 Enal (A) was prepared using the method of Odinokov et al. from (2)-
a-pinene: V. N. Odinokov, O. S. Kukovinets, R. A. Zainullin,
V. G. Kasradze, A. V. Dolidze and G. A. Tolstikov, Zh. Org. Chim.,
1992, 28, 1619. (1R,3S)-3-Isopropenyl-2,2-dimethylcyclobutaneacetalde-
hyde (A) was found to have the following spectroscopic properties: [a]D20
23 (c 1, CHCl3); nmax(KBr plates, thin film)/cm21 2958s, 2871s, 1724s,
1682s, 1646s, 1460s, 1385s, 886s; dH (250 MHz, CDCl3) 0.77 (3H, s,
CH3), 1.19 (3H, s, CH3), 1.58–1.75 (4H, m, CH3CLCH2 and CH), 2.00–
2.04 (1H, m, CH), 2.15–2.58 (4H, m, four of CHCH2CHCH2CHO),
4.56–4.59 (1H, m, one of CLCH2), 4.79–4.82 (1H, m, one of CLCH2),
9.74 (1H, t, J 1.9 Hz, CHO); dC (63 MHz, CDCl3) 17.08, 23.31, 26.60,
30.82, 36.18, 42.26, 45.59, 49.94, 109.84, 145.38, 202.58; m/z (CI, NH3)
found 166.1358; C11H18O requires 166.1358 (M+). Note that in a natural
aerosol either or both enantiomers of a-pinene may be present. We have
used and illustrated a single enantiomer but this will not affect the
chemistry reported herein.
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In addition to the acids described here, information was also
obtained about a range of other products, including norpinic acid,
norpinonic acid, pinalic acid, norpinalic acid, OH-pinonic acid,
OH-pinalic acid, pinonaldehyde and norpinonaldehyde. Further
information was obtained on the mechanisms of formation of
pinic acid and the other compounds through examining the
dependence of product yields on a variety of conditions. Results
from these studies will be published elsewhere.
25 S. Hatakeyama, K. Izumi, T. Fukuyama and H. Akimoto, J. Geophys.
Res., [Atmos.], 1989, 94, 13013.
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2 I. G. Kavouras, N. Mihalopoulos and E. G. Stefanou, Nature, 1998,
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27 P. J. Ziemann, J. Phys. Chem. A, 2002, 106, 4390.
28 S. M. Aschmann, E. C. Tuazon, J. Arey and R. Atkinson, J. Phys.
Chem. A, 2003, 107, 2247.
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