Hydroxydicarboxylic acids: Markers for secondary organic aerosol from the photooxidation of α-pinene

Article abstract of DOI:10.1021/es0620181

Detailed organic analysis of fine (PM_2.5) rural aerosol collected during summer at K-puszta, Hungary from a mixed deciduous/coniferous forest site shows the presence of polar oxygenated compounds that are also formed in laboratory irradiated α-pinene/NO_x/air mixtures. In the present work, two major photooxidation products of α-pinene were characterized as the hydroxydicarboxylic acids, 3-hydroxyglutaric acid, and 2-hydroxy-4-isopropyladipic acid, based on chemical, chromatographic, and mass spectral data. Different types of volatile derivatives, including trimethylsilyl ester/ether, methyl ester trimethylsilyl ether, and ethyl ester trimethylsilyl ether derivatives were measured by gas chromatography/mass spectrometry (GC/MS), and their electron ionization (EI) spectra were interpreted in detail. The proposed structures of the hydroxydicarboxylic acids were confirmed or supported with reference compounds. 2-Hydroxy-4-isopropyladipic acid formally corresponds to a further reaction product of pinic acid involving addition of a molecule of water and opening of the dimethylcyclobutane ring; this proposal is supported by a laboratory irradiation experiment with α-pinene/NO_x/air. In addition, we report the presence of a structurally related minor α-pinene photooxidation product, which was tentatively identified as the C₇ homolog of 3-hydroxyglutaric acid, 3-hydroxy-4,4-dimethylglutaric acid. The detection of 2-hydroxy-4-isopropyladipic acid in ambient aerosol provides an explanation for the relatively low atmospheric concentrations of pinic acid found during daytime in forest environments.

Full text of DOI:10.1021/es0620181

Environ. Sci. Technol. 2007, 41, 1628-1634
Introduction
Hydroxydicarboxylic Acids: Markers
for Secondary Organic Aerosol from
the Photooxidation of r-Pinene
It has been firmly established that monoterpenes give rise
to secondary organic aerosol (SOA) through reactions of
ozone, OH, and NO3 radicals (for a review, see Calogirou et
al., ref 1). Monoterpenes are mainly emitted by conifers,
having an annual global emission rate estimated at about
127 Tg with R-pinene as the major terpene emitted (2). Most
laboratory studies of formation of SOA from monoterpenes
have focused on investigations of SOA ozonolysis products
of R-pinene including pinic, norpinic, pinonic, and norpi-
nonic acids with pinic acid as a major reaction product (3-
8), and, conversely, field studies mainly concerned the
measurement of the latter compounds (9-13). The SOA
ozonolysis products of R-pinene were also reported for the
OH radical initiated photooxidation of R-pinene (14); how-
ever, in recent smog chamber experiments chemical analyses
of irradiated R-pinene/NOx mixtures that involve reactions
of R-pinene with OH and NO3 as well as with ozone revealed
the presence of highly oxidized, acyclic, polar compounds
along with the ozonolysis products (15, 16). A number of
these highly oxidized compounds have also been observed
in the fine fraction of ambient PM samples, although in the
absence of standards, these compounds have only been
tentatively identified (15-17). The presence of these com-
pounds in apparently high concentrations in ambient PM
samples along with the relatively low concentrations of pinic
acid during daytime raises the possibility that one or more
of the highly oxidized polar compounds could be formed
from further reaction of pinic acid.
In the present study, we consider the identification of
three of these compounds detected in PM2.5 field samples
collected in K-puszta, Hungary, a rural site with mixed
deciduous, coniferous vegetation. Based on chemical con-
siderations, the synthesis of reference compounds, and the
interpretation of mass spectra, the structures of the com-
pounds, all hydroxydicarboxylic acids, are identified with
greater confidence than could be achieved in the recent study
of Jaoui et al. (16). The compounds, 3-hydroxyglutaric acid,
3-hydroxy-4,4-dimethylglutaric acid, and 2-hydroxy-4-iso-
propyladipic acid (2-HIPAA), are given in Figure 1. In addition,
an R-pinene/NOX smog chamber irradiation was conducted
and the concentrations of pinic acid and 2-HIPAA in the
aerosol were then compared as a function of time to
determine the extent to which pinic acid might be further
reacted in the system.
M A G D A C L A E Y S , * , † R A F A L S Z M I G I E L S K I , †
I V A N K O U R T C H E V , †
P I E T E R V A N D E R V E K E N , †
R E I N H I L D E V E R M E Y L E N , †
W I L L Y M A E N H A U T , ‡ M O H A M M E D J A O U I , §
T A D E U S Z E . K L E I N D I E N S T ,
M I C H A E L L E W A N D O W S K I ,
J O H N H . O F F E N B E R G , A N D
E D W A R D O . E D N E Y
Department of Pharmaceutical Sciences, University of
Antwerp (Campus Drie Eiken), Universiteitsplein 1,
BE-2610 Antwerp, Belgium, Department of Analytical
Chemistry, Institute for Nuclear Sciences, Ghent University,
Proeftuinstraat 86, BE-9000 Gent, Belgium, Alion Science and
Technology, P.O. Box 12313, Research Triangle Park,
North Carolina 27709, and National Exposure Research
Laboratory, Office of Research and Development, United
States Environmental Protection Agency,
Research Triangle Park, North Carolina 27711
Detailed organic analysis of fine (PM2.5) rural aerosol
collected during summer at K-puszta, Hungary from a
mixed deciduous/coniferous forest site shows the presence
of polar oxygenated compounds that are also formed in
laboratory irradiated R-pinene/NOx/air mixtures. In the present
work, two major photooxidation products of R-pinene
were characterized as the hydroxydicarboxylic acids,
3-hydroxyglutaric acid, and 2-hydroxy-4-isopropyladipic
acid, based on chemical, chromatographic, and mass spectral
data. Different types of volatile derivatives, including
trimethylsilyl ester/ether, methyl ester trimethylsilyl ether,
and ethyl ester trimethylsilyl ether derivatives were measured
by gas chromatography/mass spectrometry (GC/MS), and
their electron ionization (EI) spectra were interpreted in detail.
The proposed structures of the hydroxydicarboxylic
acids were confirmed or supported with reference
compounds. 2-Hydroxy-4-isopropyladipic acid formally
corresponds to a further reaction product of pinic acid
involving addition of a molecule of water and opening of
the dimethylcyclobutane ring; this proposal is supported by
a laboratory irradiation experiment with R-pinene/NOx/
air. In addition, we report the presence of a structurally
related minor R-pinene photooxidation product, which was
tentatively identified as the C7 homolog of 3-hydroxyglutaric
acid, 3-hydroxy-4,4-dimethylglutaric acid. The detection
of 2-hydroxy-4-isopropyladipic acid in ambient aerosol
provides an explanation for the relatively low atmospheric
concentrations of pinic acid found during daytime in
forest environments.
Experimental Section
Aerosol Sampling. Field Samples. The ambient aerosol
samples were collected at K-puszta, Hungary, during a field
campaign between 4 June and 10 July 2003. The sampling
station is situated in the clearing of a mixed coniferous/
deciduous forest on the Great Hungarian Plain (46°58′N,
19°33′E, 136 m asl) about 80 km SE of Budapest. The location
is believed to be representative for a rural site and to be free
from local anthropogenic pollution. The campaign was
characterized by stable meteorological conditions and the
weather was especially warm and dry. A dichotomous high-
volume (Hi-Vol) sampler (placed at about 7 m above ground
level) was used to collect samples in two size fractions, a fine
(<2.5 µm aerodynamic diameter (AD)) and a coarse (>2.5
µm AD) fraction (18). Details about the aerosol sampling
and meteorological and ozone data are presented in previous
work dealing with the analysis of the same samples but for
other aerosol constituents (19).
* Corresponding author phone: 32 3 820 27 07; fax: 32 3 820 27
34; e-mail: magda.claeys@ua.ac.be.
† University of Antwerp.
‡ Ghent University.
§ Alion Science and Technology.
Laboratory Samples. R-Pinene/NOx/air irradiation experi-
ments were performed as described in detail in Jaoui et al.
United States Environmental Protection Agency.
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1628 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 41, NO. 5, 2007
10.1021/es0620181 CCC: $37.00
2007 American Chemical Society
Published on Web 01/23/2007

dryness with nitrogen. The dried residue was converted to
different types of derivatives and analyzed using a Polaris Q
GC/ion trap MS instrument, equipped with an external ion-
ization source (ThermoElectron, San Jose, CA). X-calibur
version 1.2 software was used for data acquisition and pro-
cessing. Details about the GC/MS conditions are provided
in previous work (19). Briefly, the chromatographic system
comprised a fused silica capillary column (30 m × 0.25 mm
i.d., 0.25 µm film thickness, AT-5MS, Alltech, Deerfield, IL).
Helium was used as a carrier gas at a flow rate of 1.2 mL
min-1. The temperature program of the column was as fol-
lows: initial temperature at 50 °C held for 5 min, a gradient
of 3 °C min-1 up to 200 °C, held for 2 min, then 30 °C min-1
to 310 °C, held for 2 min, and the total analysis time was 61
min.
Laboratory Samples. A section of the irradiated R-pinene/
NOx/air filter samples (one-eight for preparation of different
derivatives and half in the case of the time evolution
experiment) was processed in the same way as the field
samples. No internal recovery standards were added to
prepare the different types of derivatives. To examine the
time evolution of theR-pinene oxidation products, an internal
recovery standard (1,4-cyclohexanedicarboxylic acid; Sigma;
2 µg) was added to sections of the samples to correct for
losses of the analytes during sample workup and as such to
determine the concentrations of pinic acid and the novel
compound identified as 2-hydroxy-4-isopropyladipic acid.
Derivatization Procedures. Three different types of de-
rivatives were prepared, i.e., (i) trimethylsilyl (TMS) ether/
ester derivatives, (ii) mixed methyl ester, TMS-ether deriva-
tives and (iii) mixed ethyl ester, TMS-ether derivatives. TMS-
derivatives were obtained by adding to the extract residues
40 µL of a trimethylsilylation mixture containing N-methyl-
N-trimethylsilyltrifluoroacetamide (MSTFA) with 1% tri-
methylchlorosilane (TMCS) (Pierce, Rockford, IL) and an-
hydrous pyridine (2:1, v/v), and heating at 70 °C for 60 min.
For the preparation of deuterated [(CD3)Si] derivatives, the
procedure was the same except that the reagent wasN, O-bis-
(trimethyl-D9-silylacetamide) (Cambridge Isotope Labora-
tories, Andover, MA). Mixed methyl ester, TMS-ether de-
rivatives were prepared by first reacting the residues with an
ethereal solution of diazomethane for 5 min at room
temperature, evaporating the solvent under a nitrogen
stream, and then trimethylsilylating the methylated extract.
Mixed ethyl ester, TMS-ether derivatives were obtained by
first ethylating the residues with 40 µL ethanol and 8 µL
trimethylchlorosilane (Supelco, Bellafonte, PA) and heating
the reaction mixture for 2 h at 60 °C. In addition, mixed
methyl ester, TMS-ether derivatives were prepared (at the
U.S. Environmental Protection Agency (EPA)) by reacting
residues and reference compounds with BF3-methanol
(Aldrich Chemical Co., Milwaukee, WI) followed by bis(tri-
methylsilyl)trifluoroacetamide (BSTFA) containing 1% TMCS
(Aldrich). More details about the derivatization and analysis
procedures can be found in Jaoui et al. (21).
FIGURE 1. GC/MS TICs obtained for the trimethylsilylated extract
of PM2.5 aerosol (a) collected during daytime at K-puszta, Hungary,
on 19 June 2003, and (b) an irradiated r-pinene/NOx/air mixture.
Inset: mass chromatograms showing the coelution between
2-hydroxyglutaric acid (m/z 321; RT, 37.17 min) and U1 that was
identified as 3-hydroxyglutaric acid (m/z 349; RT, 37.23 min).
Identifications: (1) succinic acid; (2) 2-methylglyceric acid; (3)
glyceric acid; (4) and (6) cis- and trans-2-methyl-1,3,4-trihydroxy-
1-butene; (5) 3-methyl-2,3,4-trihydroxy-1-butene; (7) malic acid; (8)
norpinic acid; (9) 2-methylthreitol; (10) 2-methylerythritol; (11)
2-hydroxyglutaric acid; (12) pinic acid; (13) octanoic acid; (14)
levoglucosan; (15) arabitol; (16) tetradecanoic acid; (17) glucose1;
(18) mannitol; (19) sorbitol; (20) palmitic acid; (21) glucose2; (22)
stearic acid; and (*) unknown. The superscripts 1 and 2 refer to
isomeric forms of glucose. Chemical structures of the novel
compounds identified in the present study: U1, 3-hydroxyglutaric
acid; U2, 3-hydroxy-4,4-dimethylglutaric acid; and U3, 2-hydroxy-
4-isopropyladipic acid (2-HIPAA).
TABLE 1. Initial Conditions for the r-Pinene Irradiation
Experiments
chamber
NO
NOx r-pinene
T
(C)
initial RH
(%)
Exp ID
operation (ppb) (ppb)
(ppb)
ER-177 dynamica 317 317
ER-275 static 491 501
313
478
27.2
22.8
30.9
30.8
a Average chamber residence time (τ): 5.9 h.
(16). Briefly, the experiments were carried out in a rectangular
14.5 m3 smog chamber similar to the one described by
Kleindienst et al. (20). The chamber is fabricated from stain-
less steel with interior walls bonded with a 40-µm TFE Teflon
coating and equipped with four banks of fluorescent bulbs
(40 per bank) that provide irradiation distributed over the
UV spectrum from 300 to 400 nm. The smog chamber was
operated in the dynamic mode for the collection of the sample
used for structural elucidation purposes and in the static
mode for the time evolution experiment. The samples were
collected by a 47-mm Zefluor filter (Pall Gelman Laboratory,
Ann Arbor, MI) and the filters were weighed before and after
sampling for gravimetric determination of the SOA mass.
After obtaining the SOA mass, the filters were kept in a freezer
at -25 °C until GC/MS analysis. The initial conditions for the
chamber experiments are given in Table 1
Analyses for Polar Organic Marker Compounds. Field
Samples. Only selected fine size fraction (<2.5 µm AD) filters
of the Hi-Vol samples were analyzed. For each filter, one-
sixteenth part was placed in a 25 mL Pyrex glass bottle and
acidified with a 0.1% aqueous formic acid solution; then the
sample was extracted three times with 20 mL of methanol
for 15 min under ultrasonic agitation. The combined filtrates
were concentrated to 1 mL at 35 °C using a rotary evaporator,
then transferred to a 1 mL reaction glass vial and blown to
For ion trap MS2 experiments, selected ions were colli-
sionally activated by applying a percentage of a 10 V
supplementary a.c. voltage to the end caps of the ion trap
at the resonance frequency of the selected ion. This percent-
age, the collision energy level, was 16%, while the excitation
time was 15 ms. The fragmentation pathways that were
supported by ion trap MS2 experiments are indicated by an
asterisk in the Schemes, while shifted m/z values obtained
by deuterium labeling of the TMS methyl groups are given
in parentheses.
Synthesis of Reference Compounds. The detailed pro-
cedures for the preparation of the reference compounds,
i.e., 3-hydroxyglutaric acid and the ethyldi-ester of 3-hydroxy-
5-isopropyladipic acid, are provided in the Supporting
Information.
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TABLE 2. Concentration Data (ng m-3) for 3-Hydroxyglutaric
Acid, 2-Hydroxy-4-isopropyladipic Acid and Pinic Acid in PM2.5
Aerosol Collected at K-puszta during a 2003 Summer Period
daytime (N ) 28)
median (range)
nighttime (N ) 28)
median (range)
species
3-hydroxyglutaric acid
2-hydroxy-4-isopropyl- 14.9 (1.9 - 39.2) 12.7 (6.0 - 74.1)
adipic acid
16.8 (5.8 - 40.5) 20.2 (5.4 - 113)
pinic acid
2.3 (0.6 - 3.1)
10.4 (3.9 - 134)
Compound U1 was tentatively identified by Jaoui et al.
(16) as 3-isopropyl-1,2-dihydroxybutanol for which no
standard was available. As will be discussed below, its
structure is revised in the present study and elucidated as
the monohydroxy dicarboxylic acid, 3-hydroxyglutaric acid.
This compound was found to coelute under our GC condi-
tions with the known monohydroxy dicarboxylic acid,
2-hydroxyglutaric acid, which is also present in the PM2.5
K-puszta aerosol and has been previously reported in
Amazonian wet season aerosol (33). The very similar GC
retention time of the TMS-derivative of U1 compared to that
of 2-hydroxyglutaric acid (difference of only 2.4 s) hinted at
a positional isomer. However, the EI mass spectral features
were quite different (Figure 2a, c). The EI mass spectral data
for the TMS-derivative of compound U1 allow us to infer
that the underivatized compound has a MW of 148 (MW of
364 for the TMS-derivative; [M - CH3]+ ion at m/z 349) and
thus indicate that it is isobaric with 2-hydroxyglutaric acid.
Detailed interpretation of the EI mass spectral data led to
the structural proposal of 3-hydroxyglutaric acid. EI frag-
mentation mechanisms resulting in the major ions formed
from the TMS-derivative are summarized in Scheme S4
(Supporting Information). The proposed structure was
subsequently confirmed by synthesis of a reference com-
pound and comparison of its GC and MS behaviors with that
of the unknown compound U1. To our knowledge, 3-hy-
droxyglutaric acid has not been previously reported as an
oxidation product of R-pinene in ambient aerosol.
Furthermore, both the field and the chamber aerosol
extracts were subjected to additional derivatization proce-
dures in order to obtain supporting information on the
functional groups present in the unknown molecule; these
experiments are reported in detail in the Supporting Infor-
mation.
Structural Characterization of the Unknown Compound
U3 as 2-Hydroxy-4-Isopropyladipic Acid. Compound U3
has first been reported in a previous study on Amazonian
wet season aerosol and urban aerosol from Ghent, Belgium
(17), and was tentatively identified as a C8 tricarboxylic acid,
3-carboxyheptanedioic acid (MW 204).
The EI mass spectral data for the TMS-derivative of U3
allow us to infer that the MW of the underivatized compound
is 204 (MW of 420 for the TMS-derivative; [M - CH3]+ ion
at m/z 405) and thus indicates that compound U3 has three
hydroxyl groups (Figure 3a, b). In order to distinguish between
neutral and carboxylic hydroxyl groups, the extracts of both
the field and the chamber aerosol extracts were subjected to
a double derivatization procedure, i.e., methylation followed
by trimethylsilylation, by which carboxylic acid groups are
converted into methyl ester groups while hydroxyl groups
are converted into trimethylsilyl ether groups. These me-
thylation/trimethylsilylation experiments (results not shown)
suggested three carboxylic acid groups as has been reported
in detail in previous studies (16, 17). However, a double
derivatization procedure consisting of ethylation followed
by trimethylsilylation indicated the presence of two carboxylic
acid groups and one neutral hydroxyl group. The unexpected
reactivity of the hydroxyl group of 2-hydroxy-4-isopropyla-
dipic acid in the methylation reaction with diazomethane
FIGURE 2. EI mass spectra obtained for trimethylsilylated (a)
unknown U1 (K-puszta aerosol), identified as 3-hydroxyglutaric acid;
(b) 3-hydroxyglutaric acid, synthesized reference compound; (c)
peak 11 (K-puszta aerosol), identified as 2-hydroxyglutaric acid;
and (d) unknown U2 (r-pinene/NOx/air mixture), tentatively identified
as 3-hydroxy-4,4-dimethylglutaric acid.
Results and Discussion
Structural Characterization of Unknown Compound U1
as 3-Hydroxyglutaric Acid. Figure 1a shows the total ion
current (TIC) chromatogram of a trimethylsilylated extract
of a fine (PM2.5) daytime summer aerosol sample collected
at K-puszta, Hungary. Major peaks in the TIC correspond to
malic acid, the 2-methyltetrols, 2-methylthreitol and 2-me-
thylerythritol, levoglucosan, arabitol, mannitol, palmitic, and
stearic acid. Aerosol sources and source processes for the
latter compounds have been discussed in detail by Ion et al.
(19) and include photooxidation of unsaturated fatty acids
(i.e., malic acid (22-24)), photooxidation of isoprene [i.e.,
the 2-methyltetrols, 2-methylthreitol, and 2-methylerythritol
(25-28), 2-methylglyceric acid (26, 28, 29), and C5 alkene
triols (28, 30)], biomass burning (i.e., levoglucosan (31)) and
fungal spores (i.e., arabitol and mannitol (32)). Among the
other relatively intense peaks in the TIC, two peaks marked
U1 and U3 are worth noting. They are attributed to the
photooxidation of R-pinene since they could be generated
from a laboratory irradiated R-pinene/NOx/air mixture as
demonstrated in Figure 2b and reported by Edney et al. (15)
and Jaoui et al. (16). The latter mixture contains the known
photooxidation products pinic and norpinic acid, the un-
known products labeled U1, U2, and U3, which will be
discussed in detail below, and a major unknown product
(marked with an asterisk in Figure 1b) that remains to be
identified. Since the latter product was not detected in PM2.5
K-puszta aerosol, no further attention was given to it in the
present study. Pinic acid could only be detected in the daytime
aerosol sample using the more sensitive selected ion
monitoring mode (results not shown); however, it was
observed as a minor compound in the corresponding
nighttime sample and other day- and nighttime samples from
the same 2003 summer field campaign (vide infra; Table 2).
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FIGURE 3. EI mass spectra of the (a) TMS-derivative of unknown U3 (K-puszta aerosol); (b) TMS-derivative of unknown U3 (r-pinene/NOx/air
mixture); (c) ethyl ester TMS-ether derivative of unknown U3 (r-pinene/NOx/air mixture) - peak with RT 41.2 min (the same spectrum
was obtained for the diastereoisomer eluting at 40.8 min); and (d) ethyl ester TMS-ether derivative of 3-hydroxy-5-isopropyladipic acid
(synthesized)speak with RT 43.9 min (the same spectrum was obtained for the diastereoisomer eluting at 43.6 min).
can be explained by steric interaction with an electronegative
carboxylic acid group rendering acidic character to the
hydroxyl group. Indeed, a simple molecular model reveals
that the 2-hydroxyl group and the 6-carboxyl group can
spatially interact. It is also possible that the isopropyl group
through its positive inductive effect renders acidic character
to the hydroxyl group. Two chromatographic peaks were
observed for the mixed ethyl ester TMS-derivative (TIC not
shown; retention time data are given in the legend of Figure
3), which indicates diastereoisomeric compounds and is
consistent with two chiral centers in the molecule. The EI
mass spectral data (Figure 3c) are in agreement with a MW
of 204 for the underivatized compound (MW of 332 for the
mixed ethyl ester, TMS-ether derivative; [M-CH3]+ and
[M-OCH2CH3]+ ions at m/z 317 and 267, respectively) and
support that the unknown compound is a C9 monohydroxy
dicarboxylic acid that is structurally related to 3-hydroxy-
glutaric acid. Comparison of mixed ethyl ester TMS-deriva-
tives of the unknown compound with that of a synthesized
reference compound, namely, 3-hydroxy-5-isopropyladipic
acid, pointed to 2-hydroxy-4-isopropyladipic acid. The EI
mass spectral data of the mixed ethyl ester, TMS-ether
derivatives of the unknown compound U3 and 3-hydroxy-
5-isopropyladipic acid (Figure 3c, d) suggest that both
compounds have common structural features since the
majority of ions have the same m/z value though at different
relative abundances as could be expected for isomeric
compounds.
Detailed interpretation of the EI mass spectral data
obtained for the TMS-derivative of the unknown compound
U3 led us to the structural proposal of 2-hydroxy-4-isopro-
pyladipic acid (2-HIPAA). The fragmentation mechanisms
resulting in the major ions formed from the TMS derivative
are summarized in Scheme S5 (Supporting Information).
Structurally informative ions include m/z 287, explained by
the combined loss of a (CdO)OTMS radical and a hydrogen
radical from the [M-CH3]+ ion, m/z 243 and 245, due to
sequential loss of C3H8 and C3H6, respectively, and m/z 204
due to a rearrangement of a TMS group in the M+• ion. The
m/z83 and 69 ions can be explained by fragments that contain
the isopropyl group and inner carbon atoms [i.e., C(3)-C(5)
and C(3)-C(4)], respectively.
To our knowledge, 2-hydroxy-4-isopropyladipic acid has
not been previously reported in atmospheric aerosol. How-
ever, it is worth mentioning that an R-pinene oxidation
product with MW 204 has been detected by high performance
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VOL. 41, NO. 5, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 1631

FIGURE 4. Concentrations of 2-hydroxy-4-isopropyladipic acid and pinic acid, and of the SOA mass in an irradiated r-pinene/NOx/air
mixture as a function of irradiation time, and mechanism proposed for the formation of 2-hydroxy-4-isopropyladipic acid through further
reaction of pinic acid.
liquid chromatography/electrospray ionization mass spec-
trometry in PM2.5 forest aerosol from European forest sites
(i.e., Hyytia¨la¨, Finland, and Ju¨lich, Germany (34)) as well as
from several sites in the U.S. (35).
could be more clearly revealed at trace levels (results not
shown).
Formation Mechanism of 2-Hydroxy-4-Isopropyladipic
Acid. The formation of 2-HIPAA can be formally explained
by further reaction of pinic acid, which is known to be a
major photooxidation product of R-pinene in smog chamber
experiments (3-8, 14), and involves addition of a molecule
of water and opening of the dimethylcyclobutane ring. To
gain insights into the formation pathway a static irradiation
experiment with R-pinene/air/NOx was conducted to ex-
amine the time evolution of the organic products including
2-HIPAA and pinic acid. Figure 4 presents the concentrations
of 2-HIPAA and pinic acid as a function of the irradiation
time. It can be seen that the concentration of 2-HIPAA
increases with time, reaching 12.3 µg m-3 at the end of the
experiment, while that of pinic acid starts to decrease after
a reaction time of 7 h, reaching 7.5 µg m-3 at the end of the
experiment. These results show that 2-HIPAA is an oxidation
product of R-pinene and is consistent with pinic acid serving
as the precursor for 2-HIPAA, although it cannot be ruled
out that there are other formation pathways for 2-HIPAA.
Figure 4 presents a possible mechanism for the formation
of 2-HIPAA from further reaction of pinic acid, involving
addition of a molecule of water and opening of the di-
methylcyclobutane ring. The addition of water to the
dimethylcyclobutane ring likely is an acid-catalyzed process
occurring within the aerosol; however, further research is
needed to obtain insights into the formation mechanism of
5-HIPAA. Several gas-phase mechanisms involving opening
of the dimethylcyclobutane ring of R-pinene following OH
radical-initiated cleavage of the double bond have been
proposed in theoretical studies (36, 37); however, it is noted
that in the latter mechanisms opening of the dimethylcy-
clobutane ring is induced by a radical site. It is also worth
mentioning that several photooxidation products ofR-pinene
with an opened dimethylcyclobutane ring have been previ-
ously reported in forest and urban aerosol (12, 15-17).
Atmospheric Implications. In the present study, unknown
photooxidation products of R-pinene were structurally
It appears that in prior work, both the Belgian (17) and
U.S. laboratories (15, 16) have wrongly assigned the non-
carboxylic hydroxyl group in the unknown U3 compound to
a carboxylic acid group, owing to the hydroxyl group being
reactive with either ethereal diazomethane or BF3-methanol
reagent. Caution is thus appropriate when using these
derivatizing reagents for distinguishing between carboxylic
and non-carboxylic hydroxyl groups in an unknown mol-
ecule. We are not aware if this phenomenon has been
previously reported.
Characterization of a C7 Homolog of 3-Hydroxyglutaric
acid (U2) as a Minor Aerosol Component. Based on
interpretation of the EI mass spectral data of both the TMS-
derivative (Figure 2d) and the mixed ethyl ester TMS-ether
derivative (Figure S1d), compound U2, tentatively identified
in previous work as 4-isopropyl-2,4-dihydroxyhexanol (16),
could be elucidated as a C7 homolog of 3-hydroxyglutaric
acid with a revised tentative structure of 3-hydroxy-4,4-
dimethylglutaric acid. Taking into account the structure of
R-pinene which serves as precursor, the C7 homolog likely
incorporates the (CH3)2C part of the dimethylcyclobutane
ring. Detailed interpretation of the EI mass spectral data
allowed us to support the structural proposal of 3-hydroxy-
4,4-dimethylglutaric acid. The fragmentation mechanisms
resulting in the major ions formed from the TMS derivative
are summarized in Scheme S6 (Supporting Information).
Structurally informative ions include m/z 287, due to loss of
TMSOH from the [M-CH3]+ ion, m/z 259, explained by the
combined loss of a (CdO)OTMS radical and a hydrogen
radical from the [M-CH3]+ ion, and m/z 232, due to a
rearrangement of a TMS group in the M+• ion. The C7 homolog
3-hydroxy-4,4-dimethylglutaric acid only corresponds to a
minor peak in the TIC of the trimethylsilylated extract of the
K-puszta aerosol sample (Figure 1a); however, when using
mass chromatographic detection at m/z 377 ([M-CH3]+), it
9
1632 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 41, NO. 5, 2007

characterized as hydroxydicarboxylic acids, namely, 3-hy-
droxyglutaric acid, 3-hydroxy-4,4-dimethylglutaric acid, and
2-hydroxy-4-isopropyladipic acid, by detailed interpretation
of chemical, chromatographic and mass spectral data, and
synthesis of reference compounds. Attempts to synthesize
2-hydroxy-4-isopropyladipic acid, a majorR-pinene oxidation
product detected in ambient fine aerosol at K-puszta during
summer, are currently underway in the Antwerp laboratory.
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Research at the Universities of Antwerp and Ghent was
supported by the Belgian Federal Science Policy Office
(contract SD/AT/02A), the Research FoundationsFlanders
(FWO) and the Flemish Government. The U.S. Environmental
Protection Agency through its Office of Research and
Development funded and collaborated in the research
described here under contract EP-D-05-065 to Alion Science
and Technology. The manuscript has been subjected to
external peer review and has been cleared for publication.
Mention of trade names or commercial products does not
constitute an endorsement or recommendation for use. We
thank Luc Vereecken (University of Leuven, Belgium) for
useful discussions on the formation mechanism of 2-HIPAA.
Supporting Information Available
Detailed procedures for the preparation of the reference
compounds, 3-hydroxyglutaric acid and the ethyl di-ester of
3-hydroxy-5-isopropyladipic acid, three schemes containing
the EI fragmentation pathways of the TMS-ated derivatives
of compounds U1, U2 and U3, and results and discussion
of additional derivatization experiments carried out on
compound U1. This material is available free of charge via
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