Chemosphere 46 (2002) 1195–1199
www.elsevier.com/locate/chemosphere
Determination of malic acid and other C4 dicarboxylic acids
in atmospheric aerosol samples
Andreas R€
ohrl
a
a,b
, Gerhard Lammel
a,c,*
Max-Planck-Institut f€ur Meteorologie, Bundesstraße 55, D-20146 Hamburg, Germany
Institut f€ur Anorganische und Angewandte Chemie, Universit€at Hamburg, Germany
c
Meteorologisches Institut, Universit€at Hamburg, Germany
b
Received 2 January 2001; received in revised form 7 September 2001; accepted 17 October 2001
Abstract
An ion chromatographic method was developed which is able to separate five unsubstituted and hydroxy C4 dicarboxylic acids, succinic, malic, tartaric, maleic and fumaric acid, besides the other unsubstituted C2 –C5 dicarboxylic
acids, oxalic, malonic and glutaric acids, as well as inorganic ions in samples extracted from atmospheric particulate
matter. By the application of this method it was found for both rural and urban sites and for various types of air masses
that in the summer-time malic acid is the most prominent C4 diacid (64 ng m3 by average), exceeding succinic acid
concentration (28 ng m3 by average) considerably. In winter-time considerably less, a factor of 4–15, C4 acids occurred
and succinic acid was more concentrated than malic acid. Tartaric, fumaric and maleic acids were less concentrated (5.1,
5.0 and 4:5 ng m3 by average, respectively). Tartaric acid was observed for the first time in ambient air.
The results indicate that in particular anthropogenic sources are important for the precursors of succinic, maleic and
fumaric acids. Biogenic sources seem to influence the occurrence of malic acid significantly. Ó 2002 Elsevier Science
Ltd. All rights reserved.
Keywords: Atmospheric aerosol; Dicarboxylic acids; Malic acid; Particulate organic matter
1. Introduction
Being late products in the photochemistry of aliphatic and aromatic hydrocarbons and due to the low
vapour pressure being almost entirely partitioning to the
particulate phase, the dicarboxylic acids constitute an
important fraction of the water soluble part of particulate organic matter (POM) in atmospheric aerosol particles at remote (Kawamura and Usukura, 1993;
Limbeck and Puxbaum, 1999), rural (R€
ohrl and Lammel, 2001) and urban (Sempere and Kawamura, 1994;
Limbeck and Puxbaum, 1999; R€
ohrl and Lammel, 2001)
sites. Mostly the unsubstituted dicarboxylic acids have
*
Corresponding author. Tel.: +49-40-41173-362; fax: +4940-441787.
E-mail address: lammel@dkrz.de (G. Lammel).
been addressed by determinations based on gas chromatographic and ion chromatographic methods (Dabek-Zlotorzynska and McGrath, 2000), whereas no
adequate method for the determination of the hydroxy
acids is at hand.
Oxalic acid is by far the most abundant dicarboxylic
acid in ambient air. Little is known so far on the photochemical degradation paths of biogenic and anthropogenic volatile organic compounds: based on laboratory
studies (Dumdei et al., 1988; Kleindienst et al., 1999), it
is likely that C4 degradation products are important
intermediates in the atmospheric chemistry of simple
anthropogenic aromatic hydrocarbons. Their incomplete oxidation would lead to substituted and unsubstituted, saturated and unsaturated C4 diacids. Most
biogenic volatile organic compounds are considered not
to form POM. The most significant terrestrial biospheric
precursors of POM are certainly the terpenes with trees
0045-6535/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved.
PII: S 0 0 4 5 - 6 5 3 5 ( 0 1 ) 0 0 2 4 3 - 0
1196
A. R€ohrl, G. Lammel / Chemosphere 46 (2002) 1195–1199
as the predominant emitting species (Griffin et al., 1999).
C4 fragments could be formed in terpene photochemistry, but no low-molecular (<C6 ) products could be
identified so far.
The aim of the present study was to develop a reliable
chromatographic method by which the most important
low-molecular unsubstituted and substituted C2 –C5 dicarboxylic acids could be separated from each other in
aqueous extracts of atmospheric aerosol samples and
also quantitated. It covered the acids succinic (butanedioic), malic (monohydroxybutanedioic), tartaric
(2,3-dihydroxybutanedioic), maleic (cis-butenedioic),
fumaric (trans-butenedioic), oxalic (ethanedioic), malonic (propanedioic) and glutaric acid (pentanedioic
acid). Ion chromatography was preferred, because the
sample preparation procedure does not need any organic solvent, possible analyte losses in extraction steps
would be minimized and the target ions can be determined together with the major inorganic ions present
2
(Cl , NO
3 , SO4 ) in one separation.
with other mono- and di-carboxylic acid anions (and
some inorganic anions). The latter pair of diacid anions
remain incompletely separated but their presence and
relative importance can be judged from the chromatograms. With regard to the application of the method to
atmospheric aerosol samples, this limitation appeared
2. Experimental
The standard mixtures were aqueous solutions (Seralpur) of the carboxylic acids. These reagents were used
as received (p.a.; Merck). The eluents were composed of
aqueous solutions of inorganic salts and methanol
(HPLC gradient grade; Malinckrodt Baker).
2.1. Separation
For analyses we used an ion chromatography system
consisting of guard and separation columns (Dionex,
AG11-HC and AS11-HC, 250 4:6 mm ID, both maintained at constant temperature, 30 °C), anion suppression
system (Dionex, ASRS-ultra) and conductivity detector.
Several eluent compositions containing sodium hydroxide and methanol were tested in eluent gradient programmes. A methanol-free gradient (henceforth called
eluent A), with increasing NaOH (Merck, p.a.; 0.5–16.2
mmol l1 during minutes 5–35 of the separation; modified
but similar as used earlier; Kerminen et al., 1999), proved
adequate for the determination of oxalic, glutaric, pyruvic, and maleic acid. The corresponding peaks did not
show interferences with other anions including the inor2
ganic ions Cl , NO
(Fig. 1(a)). However,
3 and SO4
the malonic and succinic acids coelute with tartaric and
malic acid anions, respectively, such that the peaks represent the sums of two acids each. A gradient programme
sweeping from 0.5 to 18 mmol l1 NaOH and from 0%
to 20% methanol (eluent B; cf. Table 1) proved to meet
the aim best. The addition of methanol to the eluent allows for the satisfactory separation of succinic and malic
acids and the separation of the peaks which correspond
to the malic and tartaric acid anions (Fig. 1(b)) together
Fig. 1. IC separations of mixtures of dicarboxylic acids by ion
chromatography (a) a test sample composed of 5 mg l1 standard mixture concentrations of diacids using a methanol-free
NaOH gradient eluent, eluent A, (b) the same but with methanol gradient, eluent B (see text for details) (c) a sample from a
rural site (eluent B). Flow rate: 2 ml1 min, detection: suppressed conductivity. 1 ¼ Nitrate, 2 ¼ glutarate, 3 ¼ succinate,
4 ¼ malate, 5 ¼ carbonate, 6 ¼ malonate, 7 ¼ tartrate, 8 ¼ maleate, 9 ¼ fumarate, 10 ¼ sulfate, 11 ¼ oxalate. Chloride elutes at
ca. tR ¼ 10 min (not shown). Carbonate (a) originates in the
laboratory air.
A. R€ohrl, G. Lammel / Chemosphere 46 (2002) 1195–1199
1197
Table 1
Eluent gradient programme used for the ion chromatographic determination of C2 –C5 diacids (eluent B)
Time (min)
Vol% H2 O
Vol% NaOH
ð5 mmol l1 Þ
Vol% NaOH
ð60 mmol l1 Þ
Vol% CH3 OH
Remarks
0
8
18
28
38
38.1
45
90
90
65
50
70
90
90
10
10
0
0
0
10
10
0
0
15
30
30
0
0
0
0
20
20
0
0
0
Isocratic phase
Start gradient
Maximum CH3 OH achieved
Maximum NaOH achieved
Stop gradient
Return to starting conditions
End of run
to be acceptable as in all samples we analysed it was
found that the ratio of the peak heights was >5 and the
concentration of tartaric acid was below detection limit
(see below).
Species recovery based on multiple standard mixture
application on filter membranes were 103:7 5:3%,
100:4 4:0%, 100:2 7:0%, 100:9 6:0% and 105:1 3:9% for the C4 DCAs succinic, malic, tartaric, maleic
and fumaric acids, respectively, at concentrations
corresponding to atmospheric concentrations of ca.
10 ng m3 , and better at higher concentrations. Recovery upon addition to the matrix were determined for
succinic acid using standard reference material (‘urban
aerosol’ SRM1649a, NIST) and found to be 104%.
Precision of analyses: Based on multiple standard
mixture determination the uncertainty related to the
recommended chromatographic determination is estimated to be <4% for malic, <6% for succinic, maleic and
glutaric, <10% for malonic and tartaric, <15% for oxalic
acid (in the 1–10 mg l1 range, based on 2r). Peak
identification was based on a match of retention times
between standard samples and was confirmed by addition
of authentic standard substances to selected samples.
Detection limits achieved corresponded to 10–50 ng
for the acids investigated. They were determined as the
mean +3 S.D. of the field blank value or, if higher, the
level corresponding to S/N ¼ 10 in chromatograms.
2.2. Application to samples of atmospheric origin: sampling and sample treatment
Samples were taken from five inland sites in Germany: three rural sites during summer-time (Falkenberg,
52°130 N/14°080 E, July–August 1998) and winter-time
(Merseburg, 51°210 N/11°580 E, and Eningen, 48°240 N/
9°150 E, November–December 1999 and March 2000,
respectively) and two urban sites in the summer-time
(Eichst€adt at the rim of the Berlin conurbation, 52°420 N/
13°080 E, and Leipzig, 51°190 N/12°250 E, July–August
1998 and July–August 1999, respectively). During the
summer campaigns the weather was almost dry with
very few light precipitation events. During the wintertime campaigns rain and snow fall occurred frequently.
Total suspended particulate matter was continuously
collected on quartz fiber filters (Munktell MK360, 15 cm
diameter) using high-volume samplers without an upper
cut-off (flow rate 420 l min1 ). Filters were exposed for
periods of 3–48 h. Negative sampling artefacts, e.g.
evaporative losses, and positive artefacts, e.g. adsorption
of gaseous precursors on the filters, cannot be excluded.
The filter samples were kept frozen until analysis. The
membranes were cut into pieces (for various analyses).
For DCA analysis these were then extracted with pure
water (7.5 ml for a quarter of the entire filter, ultrasonic
agitation, Seralpur). The extracts were spiked with trifluoroacetic acid as internal standard and filtered.
2.3. Application to samples of atmospheric origin: data
analysis
We classified the samples according to the air mass
origin and trajectory based on 72 h near-ground backtrajectories analysed with state-of-the-art weather forecast models of German Weather Service (DWD;
Europa-Modell) and National Oceanic and Atmospheric
Administration (NOAA; HYSPLIT). The samples were
grouped into two categories: Marine air masses which
underwent only little influence from the continent and,
in particular, had not passed industrialized regions
(northern sector, henceforth called ‘‘marine’’) and marine air masses which experienced continental influence
during rapid transport from the Atlantic over parts of
western Europe or when residing over western, southern
or central Europe for considerable time (‘‘marine-continental’’). Samples associated with trajectories covering
more than one sector are considered ambiguous and not
attributed to one of the classes.
3. Results and discussion
The most abundant C4 diacids were malic and succinic acid with 42 and 20 ng m3 , by average, respectively. The concentrations were significantly above
detection limit in all except one sample. Tartaric, fumaric and maleic acids, were less concentrated (5.1, 5.0
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A. R€ohrl, G. Lammel / Chemosphere 46 (2002) 1195–1199
and 4:5 ng m3 , by average, respectively) and found
below detection limits in 33%, 53% and 51%, respectively, of all analysed samples. To our knowledge this is
the first observation of tartaric acid in the atmosphere.
The 5 C4 diacids determined accounted for 0.37% of the
total suspended particulate matter and for 28% of the
low-molecular weight diacids: In the same samples
the concentrations of oxalic, malonic and glutaric acids
were 188, 40 and 12 ng m3 . Obviously higher concentrations of the C4 diacids were observed at the urban
sites with highest ratios for succinic and maleic acids,
curban =crural ¼ 2.
The analysis of the samples grouped according to air
mass class and season (Table 2) shows that malic and
succinic acids are enriched in air when transported
over the European continent: cmarine–continental =cmarine ¼
38=12 3 for malic acid and and cmarine–continental =
cmarine ¼ 18=8:8 2 for succinic acid. Summer-time
concentrations are much higher than winter-time
(csummer =cwinter ¼ 15 and 4, respectively). The latter ratios
reflect the seasonalities of emission patterns and photochemical activity. Of course, the concentration ranges
at different sites vary. This variability is small, however,
when data sets from the same season are considered
(Table 2).
What are the sources of the acids, primary emissions
or emissions of photochemical precursors? Primary
sources are unknown. In principle, any emission of
volatile organic compounds (VOC) must be considered
as a potential precursor emission. Road traffic is the
most prominent emittor of non-methane volatile organic compounds in Europe while nature ranks second
(EEA – European Environment Agency, 1998). Biogenic
emissions are much stronger in summer than in winter in
the cool and cold temperate climate zones in Europe and
in the neighbouring ocean areas (Simpson et al., 1999;
Lindfors et al., 2000). Anthropogenic emissions on the
other hand show a less pronounced annual variation and
an inverse seasonality: for Germany and other European
countries emissions originating from fuel use in households, power plants and road traffic are higher in winter
than in summer (up to a factor of two in some countries;
Lenhart and Friedrich, 1995). In conclusion the budget
of VOC over Europe is considered to be dominated by
biogenic emissions during summer-time (Simpson et al.,
1999) while in the winter-time anthropogenic emissions
will prevail, at least in central Europe.
The various influences, i.e. season, type of site and air
mass, point to secondary, photochemical sources of biogenic or to anthropogenic origin or to primary, biogenic sources (summer prevalence in both cases) of malic
and succinic acid. This finding is in accordance with the
behaviour of other low-molecular diacids (Kawamura
et al., 1996; R€
ohrl and Lammel, 2001). Oceanic sources –
potential precursors, too – obviously did not contribute
significantly – at least not within the time scale addressed by this data analysis, i.e. several hours to several
days. The small marine air mass data set does not support any further conclusions. Malic acid exceeded succinic acid in 45 out of 48 summer-time samples and by
130% by average, whereas succinic acid exceeded malic
acid in 21 out of 27 winter-time samples (64% by
average). This indicates that anthropogenic sources
(winter-time maximum) are more important for the hydrocarbon precursors of succinic acid and biogenic
sources seem to influence the occurrence of malic acid
even in the polluted near-ground atmosphere. Note that
the same emission strength throughout the year would
lead to higher concentrations in the winter-time due to a
then by average less pronounced mixing of the nearground atmosphere. Succinic acid could not be found at
a rain forest site (Kub
atov
a et al., 2000). There is one
observation of malic acid (Sempere and Kawamura,
1994) in the literature to compare with: 40 ng m3 of
Table 2
C4 acid concentrations (ng m3 ) and concentration ratios malic/succinic (mol/mol) for all samples, site types (rural and urban),
individual sites, seasons and classes of air mass origin
All
Succinic acid
ðng m3 Þ
Malic acid
ðng m3 Þ
Tartaric acid
ðng m3 Þ
Maleic acid
ðng m3 Þ
Fumaric acid
ðng m3 Þ
20 (1.5–70)
42 (0.5–194)
5.1 (<0.2–40)
4.5 (<1.2–13)
5.0 (<0.4–14)
Site type
Rural
Urban
14 (< 2.5–70)
30 (<9.1–68)
34 (<2.5–194)
54 (<7.3–129)
4.0 (<0.2–23)
6.7 (<1.0–40)
3.2 (<0.2–13)
6.2 (<1.2–14)
4.8 (<0.4–26)
5.3 (<1.2–31)
Season
Winter
Summer
7.2 (<2.5–25)
28 (<7.2–70)
4.4 (<2.5–19)
64 (<7.3–194)
1.1 (<0.2–6.54)
5.8 (<1.0–40)
1.1 (<0.2–4.5)
4.9 (<1.2–14)
1.2 (<0.2–3.3)
6.2 (<1.2–31)
Air mass class
Marine
Marine-continental
8.8 (3.4–14)
18 (<5.2–61)
12 (1.4–20)
38 (<5.2–129)
0.6 (<1.6–0.8)
4.6 (<0.2–23)
1.8 (<1.8–2.3)
3.9 (<0.2–14)
1.8 (<1.2–3.5)
5.2 (<0.2–31)
Concentration data listed as: time-weighted means (minimum–maximum). Concentration ratios listed as: Time-weighted
means standard deviation. n is the number of samples, dt is the total sampling time (h). For derivation of means of data subsets
which include values below the detection limit, these were set equal to half the detection limit.
A. R€ohrl, G. Lammel / Chemosphere 46 (2002) 1195–1199
malic acid and 170 ng m3 of succinic acid have been
determined in Tokyo air (by average, 4 samples) using a
gas-chromatographic method after derivatization of the
dried samples to the butyl esters and organic extraction
(n-hexane). By this method the hydroxyl groups are not
derivatized and the recovery of polar substances should
be limited due to significant partitioning to the polar
phase (which was not quantified by Sempere and Kawamura, 1994). Therefore, a systematic underestimation
of hydroxy acids could result.
We observed maleic and fumaric acids at 3.2 and
4:8 ng m3 , respectively, at the rural sites and 6.4 and
5:5 ng m3 , respectively, at the urban sites (mean values,
cf. Table 2). Higher concentrations were reported from
Tokyo, 28 and 17 ng m3 (Sempere and Kawamura,
1994). Anthropogenic sources, namely toluene emissions
(from vehicle exhaust, besides other), can be considered
as a significant source of maleic and fumaric acids:
Laboratory experiments (Dumdei et al., 1988; Bierbach
et al., 1994; Kleindienst et al., 1999) showed that toluene
photochemistry leads to cis-butenedial as a major
product which is then oxidized to maleic acid. (In these
experiments, the anhydride of maleic acid was observed
only, which supposedly resulted from the relatively dry
(rh < 30%) conditions applied in these experiments.)
Through isomerization fumaric acid might be formed in
a subsequent step and, as it is the thermodynamically
favoured isomer and if no other paths of formation
of fumaric acid are significant, its concentration levels
might tend to exceed those of maleic acid. This pathway
is speculative but seems to be perfectly supported by the
results: these suggest that close to the precursor sources
the concentrations are higher and the ratio cfumaric acid =
cmaleic acid is small (0.9), while at rural sites, more distant
from the sources the concentrations have decreased and
the ratio of fumaric acid over maleic acid has increased
(cfumaric acid =cmaleic acid ¼ 1:5) in the aged air mass. Much
more laboratory and field research is needed to understand the atmospheric chemistry which leads to the C4
diacids.
Acknowledgements
We thank Christine Stahlschmidt for help during the
analyses. This research received financial support from
BMBF in the frame of the German Aerosol Research
Focus, AFS.
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