Inorganic ammonium salts and carbonate salts are efficient catalysts for aldol condensation in atmospheric aerosols

Article abstract of DOI:10.1039/b924443c

In natural environments such as atmospheric aerosols, organic compounds coexist with inorganic salts but, until recently, were not thought to interact chemically. We have recently shown that inorganic ammonium ions, NH ₄⁺, act as catalysts for acetal formation from glyoxal, a common atmospheric gas. In this work, we report that inorganic ammonium ions, NH₄⁺, and carbonate ions, CO₃^2-, are also efficient catalysts for the aldol condensation of carbonyl compounds. In the case of NH₄⁺ this was not previously known, and was patented prior to this article. The kinetic results presented in this work show that, for the concentrations of ammonium and carbonate ions present in tropospheric aerosols, the aldol condensation of acetaldehyde and acetone could be as fast as in concentrated sulfuric acid and might compete with their reactions with OH radicals. These catalytic processes could produce significant amounts of polyconjugated, light-absorbing compounds in aerosols, and thus affect their direct forcing on climate. For organic gases with large Henry's law coefficients, these reactions could also result in a significant uptake and in the formation of secondary organic aerosols (SOA). This work reinforces the recent findings that inorganic salts are not inert towards organic compounds in aerosols and shows, in particular, that common ones, such as ammonium and carbonate salts, might even play important roles in their chemical transformations.

Full text of DOI:10.1039/b924443c

View Article Online / Journal Homepage / Table of Contents for this issue
PAPER
www.rsc.org/pccp | Physical Chemistry Chemical Physics
Inorganic ammonium salts and carbonate salts are efficient catalysts
for aldol condensation in atmospheric aerosols
a
b
b
´
Barbara Noziere,* Pawel Dziedzic and Armando Cordova*
Received 20th November 2009, Accepted 27th January 2010
First published as an Advance Article on the web 24th February 2010
DOI: 10.1039/b924443c
In natural environments such as atmospheric aerosols, organic compounds coexist with inorganic
salts but, until recently, were not thought to interact chemically. We have recently shown that
inorganic ammonium ions, NH4+, act as catalysts for acetal formation from glyoxal, a common
atmospheric gas. In this work, we report that inorganic ammonium ions, NH4+, and carbonate
ions, CO32À, are also efficient catalysts for the aldol condensation of carbonyl compounds. In the
+
case of NH4 this was not previously known, and was patented prior to this article. The kinetic
results presented in this work show that, for the concentrations of ammonium and carbonate ions
present in tropospheric aerosols, the aldol condensation of acetaldehyde and acetone could be as
fast as in concentrated sulfuric acid and might compete with their reactions with OH radicals.
These catalytic processes could produce significant amounts of polyconjugated, light-absorbing
compounds in aerosols, and thus affect their direct forcing on climate. For organic gases with
large Henry’s law coefficients, these reactions could also result in a significant uptake and in the
formation of secondary organic aerosols (SOA). This work reinforces the recent findings that
inorganic salts are not inert towards organic compounds in aerosols and shows, in particular, that
common ones, such as ammonium and carbonate salts, might even play important roles in their
chemical transformations.
residence of aerosols in the atmosphere. In tropospheric
1. Introduction
particles, for instance, of a residence time of 4–5 days,
chemical reactions need to have an overall rate of at
least 10À6 sÀ1 to have significant effects. Until recently, strong
acids such as sulfuric acid were the only catalysts known for
ionic reactions in aerosols but kinetic studies8–10 have shown
that very large acid concentrations are needed for aldol
Over the last decade new analytical techniques for atmospheric
aerosols have shown that many aerosol particles contain
significant amounts of organic compounds mixed with
inorganic salts such as sulfate, ammonium and nitrate.1 These
inorganic salts were not thought to interact chemically with
the organic compounds until recently. The main fate of the
organic compounds in these media was expected to be their
reaction with OH radicals, which, in the oxygen-rich atmosphere,
are equivalent to oxidations and lead ultimately to the break
down of the organic chains. By contrast, ionic reactions
such as aldol condensation and acetal formation form new
carbon–carbon or carbon–oxygen–carbon bonds, respectively,
and thus products with longer carbon chains than their parent
compounds. Although their relevance for atmospheric
aerosols has only been considered for the last 10 years,2 these
ionic reactions have now been suggested as responsible for the
formation of secondary organic aerosols (SOA),3 for the
production of light-absorbing compounds in aerosols, increasing
their warming effect on climate,4–6 and for the depletion of
glyoxal observed in some regions.7 But while these reactions
can qualitatively explain some atmospheric observations, they
need efficient catalysts to be kinetically significant during the
condensation to reach an overall rate of 10À6 À1: for a
s
concentration of 0.1 M of acetaldehyde, corresponding to
the total carbonyl concentrations reported in some aerosols
(0.7 mg mÀ3 11 and an aerosol specific volume of 10À10
)
(m3 mÀ3), the overall rate of reaction reaches 10À6 sÀ1 only
with sulfuric acid concentrations of at least 50% wt8–10
(B8.5 M, where M = mol LÀ1). Such large acid concentrations
are mostly present in stratospheric aerosols,12 but not in
tropospheric ones. Using pH = 0 as an upper limit for the
acid concentration in lower tropospheric aerosols,7,13 the same
concentration of acetaldehyde as above would correspond to
an overall rate of reaction of the order of 10À8 À1, showing
s
that acid catalysis would be negligible in these aerosols.
In recent years, our work has thus focused on finding other
catalysts which could play more significant roles in tropospheric
aerosols. Amino acids were first identified as such, as
evidenced by their strong effects on the rate coefficient of aldol
condensation.6,14 More recently, we reported that inorganic
ammonium ions, NH4+, strongly increase the rate coefficient
of acetal formation from glyoxal, and thus also acted as
catalysts.15
a Department of Applied Environmental Science, Svante Arrhenius vag
¨
8c, 106 91 Stockholm, Sweden. E-mail: barbara.noziere@itm.su.se;
Fax: +46 8674 7638; Tel: +46 8674 7281
b Department of Organic Chemistry, Stockholm University,
Svante Arrhenius vag 16c, 106 91 Stockholm, Sweden.
¨
The present work continues this exploration by reporting
+
E-mail: acordova@organ.su.se; Fax: +46 815 4908;
Tel: +46 816 2479
that inorganic ammonium (NH4
) and carbonate ions
ꢀc
This journal is the Owner Societies 2010
3864 | Phys. Chem. Chem. Phys., 2010, 12, 3864–3872

View Article Online
(CO32À), also act as catalysts for the aldol condensation of
carbonyl compounds.
same experiments as described above, but where the products
were identified by direct injection in high resolution mass
spectrometry (HRMS; Bruker MicrOTOF spectrometer) using
electrospray ionization. Yields were determined from
experiments performed at a larger scale and with higher
acetaldehyde concentrations (B30 M), in water at 50 1C or
in dimethylsulfoxide (DMSO) in the presence of 1 M ammo-
nium fluoride. These reactions were quenched by extracting
the reaction mixture with dichloromethane (CH2Cl2) and
drying the organic phase with sodium sulfate, Na2SO4. Then,
filtration and removal of the solvent under reduced pressure
and low temperature gave the products. The yields were
determined from the weights of the products relative to the
initial weight of acetaldehyde. The product distribution
was investigated by 1H NMR (nuclear magnetic resonance)
and HRMS.
2. Experimental
The experimental technique used in this work was identical as
in our previous studies;6,14 the kinetic experiments were
performed in glass vials containing 4 mL of salt solution,
continuously stirred, and protected from light. Carbonyl
compounds were introduced in these solutions with micro-
syringes, to concentrations between 0.1 and 1 M (Table 1). As
explained in the calculations made in the introduction, these
concentrations were consistent with those reported for
carbonyl compounds in tropospheric aerosols.11 The pH of
all the solutions was measured with a pH-meter (VWR pH100)
and with a precision of Æ0.1 units, calibrated with standard
solutions of pH = 4, 7, and 10.
For the kinetic studies, small samples (o0.3 mL) of the
reaction mixtures were placed in a 1 mm quartz cell. Their
absorbance, Ab(l), where l is the wavelength in cm, was
measured by a UV-Vis spectrometer (Agilent 8453) over
Separate series of experiments were dedicated to the
identification and quantification of the reaction products of
acetaldehyde. Product identification was performed in the
Table 1 Summary of the kinetic experiments and results presented in this work
Carbonyl compound
[Carbonyl]/M
Salt
[Salt]/M
pH
Ion activity/M
Rate constant, k/sÀ1
T/K
Acetaldehyde
Acetaldehyde
Acetaldehyde
Acetaldehyde
Acetaldehyde
Acetaldehyde
Acetaldehyde
Acetaldehyde
Acetaldehyde
Acetaldehyde
Acetaldehyde
Acetaldehyde
Acetaldehyde
Acetaldehyde
Acetaldehyde
Acetaldehyde
Acetaldehyde
Acetaldehyde
Acetaldehyde
Acetaldehyde
Acetaldehyde
Acetaldehyde
Acetaldehyde
Acetaldehyde
Acetaldehyde
Acetaldehyde
Acetaldehyde
Acetaldehyde
Acetaldehyde
Acetaldehyde
Acetaldehyde
Acetaldehyde
Acetaldehyde
Acetaldehyde
Acetaldehyde
Acetaldehyde
Acetaldehyde
Acetaldehyde
Acetaldehyde
Acetaldehyde
Acetone
0.1
0.05
0.1
0.5
0.5
0.5
0.5
0.3
0.3
0.3
0.3
0.3
0.3
0.5
0.5
1
0.5
1
0.5
0.5
1
1
0.5
1
0.5
0.1
0.1
0.3
0.3
0.3
0.3
0.1
0.5
1
0.1
0.1
0.1
0.1
0.1
0.1
0.5
0.5
0.5
0.5
(NH4)2SO4
(NH4)2SO4
(NH4)2SO4
(NH4)2SO4
(NH4)2SO4
(NH4)2SO4
(NH4)2SO4
NH4F
NH4F
NH4F
NH4F
NH4F
NH4F
NH4F
NH4Cl
NH4Cl
NH4Br
NH4Br
NH4NO3
NH4NO3
(NH4)HSO4
(NH4)HSO4
(NH4)2CO3
(NH4)2CO3
(NH4)2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
MgCO3
MgCO3
Water
Water
NaCl
Na2SO4
(NH4)2SO4
(NH4)2SO4
NH4F
3.7
3.7
3.7
2.7
2.7
1.7
1.7
2
1
1
1
0.5
0.5
0.5
4
5.5
5.5
5.5
5.5
5.5
5.5
5.5
7.8
6.5
6.5
6.5
6.3
6.3
6.3
4.6
4.6
4.5
4.5
4.5
4.5
-0.1
-0.1
8.8
8.8
5.5
5.5
5.5
3.3
3.3
1.9
1.9
1.3
0.6
0.6
0.6
0.3
0.3
0.3
3.2
3.2
1.8
1.8
3.2
3.2
3.0
3.0
1
6.7 Â 10À7
5.7 Â 10À7
5.0 Â 10À7
4.4 Â 10À7
3.1 Â 10À7
2.6 Â 10À7
2.5 Â 10À7
1.7 Â 10À6
1.0 Â 10À6
7.9 Â 10À7
1.3 Â 10À6
4.5 Â 10À7
7.4 Â 10À7
5.9 Â 10À7
1.0 Â 10À6
1.3 Â 10À6
5.4 Â 10À7
7.1 Â 10À7
9.0 Â 10À7
5.2 Â 10À7
3.9 Â 10À7
3.0 Â 10À7
4.9 Â 10À7
4.0 Â 10À7
4.3 Â 10À7
9.8 Â 10À6
1.4 Â 10À5
3.5 Â 10À6
5.5 Â 10À6
1.6 Â 10À6
2.1 Â 10À6
2.5 Â 10À6
1.1 Â 10À6
7.2 Â 10À7
5.3 Â 10À7
5.4 Â 10À7
o 1 Â 10À8
o 1 Â 10À8
o 1 Â 10À8
o 1 Â 1À0À6 8
4.7 Â 10
298
298
298
298
298
298
298
298
298
298
298
298
298
298
308
308
308
308
308
308
308
308
298
298
298
298
298
298
298
298
298
298
298
298
298
298
298
298
298
298
308
308
308
308
4
2.5
2.5
4
4
3
3
1
1
0.5
1
1
0.5
1
11.7
11.7
11.7
11.7
11.4
11.4
11.4
10.9
10.9
10.4
10.4
7.0
7.0
6.7
5.7
5.5
5.5
7.8
11.7
1
1
0.5
0.5
0.1
0.1
0.1
0.005
0.005
0.0005
0.0005
0
0
4
1
3.7
3.7
2
0.5
0.5
0.1
0.1
0.1
0.005
0.005
0.0005
0.0005
/
/
/
/
5.5
5.5
1.3
1
Acetone
Acetone
Acetone
4.4 Â 10À6
3.4 Â 10À7
6.3 Â 10À7
Na2CO3
1
ꢀc
This journal is the Owner Societies 2010
Phys. Chem. Chem. Phys., 2010, 12, 3864–3872 | 3865

View Article Online
190–1100 nm. This absorbance (dimensionless) was converted
into an extinction coefficient, el (cmÀ1), by the Beer–
Lambert law:
Ab(l) = el  l
(1)
where l is the optical pathlength (l = 0.1 cm). Note that el is
the experimental extinction coefficient, containing both the
concentration and absorption cross-section. For a direct
comparison with the optical properties of atmospheric
aerosols, extinction coefficients were converted into absorption
indices, Al (non-dimensional), the imaginary part of the
refractive index:
Al = l el/4p
(2)
The kinetics of the reaction of acetone was studied from its
decay at 270 nm but, as explained in our previous works, this
was not possible for acetaldehyde because its absorption band
(277 nm) strongly overlapped by the intense bands of the
reaction products. The kinetics of this reaction was thus
studied from the formation of a main product at 320 nm
(2,4,6-octatienal) and, whenever possible, from the absorption
of the other main products, crotonaldehyde at 226 nm, and
2,4-hexadienal at 272 nm (Fig. 1). The kinetics observed were
thus specific to the formation of these products and ignored
other potential reaction channels for acetaldehyde, and the
overall rates of reaction obtained were lower limits for the
overall rate of consumption of acetaldehyde.
The variations of the absorption index at these wavelengths,
el(t), were then converted into a first-order expression:
ln (1 À el/eN) = ÀkI Â t,
(3)
Fig. 1 Top: Real-time formation of the products of acetaldehyde
(0.5 M in ammonium sulfate 2.7 M) at 226 (black symbols), 272 (white
symbols), and 320 nm (grey symbols), and fit to first-order expressions
(black lines). Bottom: Conversion of the data into eqn (3) to determine
the rate coefficient, kI (sÀ1).
where eN is the absorption index at long reaction time (or high
degree of conversion), t, time (in s), and kI (sÀ1), the first-order
rate coefficient (Fig. 1, bottom). Obtaining straight lines from
eqn (3) indicated that the reactions were first order in carbonyl
concentration and the slope of eqn (3) gave absolute values for
the rate coefficients, kI, which were independent of the
concentrations of carbonyl used in the experiments. Uncertainties
on kI were estimated to about Æ25%, resulting from Æ10% of
uncertainties on eN, Æ5% of uncertainties on the absorbance,
and to the errors in the linear regressions. The rate coefficients
obtained at the 3 different wavelengths for the reaction of
acetaldehyde were identical, within these uncertainties,
strongly reducing the possibility that other products had
interfered with these analyses (Fig. 1). The experimental
conditions used in these kinetic experiments are summarized
in Table 1.
the model and had to be converted into molarity units
(mol LÀ1) using the following eqn (4).18
gNH+ = gm(NH4+) Â m(NH4+) Â d/[NH4+],
(4)
4
where m(NH4+) is the molality of the ions (mol kgÀ1
)
provided by the model, and d the density of the solution
(kg LÀ1). This expression was re-arranged by replacing the
molar concentration [NH4+] by the number of ions, n(NH4+),
divided by the total volume of solution, V = msol/d, where msol
is the total weight of solution (in kg).
+
gNH+ = gm(NH4+) Â m(NH4+) Â msol/n(NH4
)
(5)
Because most of the ammonium solutions studied were
concentrated ([NH4+] > 1 M), the kinetic results are
4
Eqn (5) had the advantage of involving only outputs from the
model and avoiding to use the density, which was not known
presented as function of the ion activities, aNH+ (M), rather
4
than their concentrations. The activity coefficients of these
ions, gNH+, were calculated with the AIM Model II16 for the
for many of the solutions studied. The activities, aNH+, were
4
4
then determined from the coefficients calculated in eqn (5):
ammonium sulfate, bisulfate, and nitrate solutions, and Model
III17 for ammonium chloride at 298 K. For ammonium
fluoride and bromide these coefficients were taken equal to
those in ammonium chloride. These activity coefficients,
gm(NH4+), were provided in molality units (mol kgÀ1) by
+
]
aNH+ (M) = gNH+ Â [NH4
(6)
4
4
The activities thus calculated for each of the solutions studied
are given in Table 1. As the carbonate solutions studied in this
ꢀc
This journal is the Owner Societies 2010
3866 | Phys. Chem. Chem. Phys., 2010, 12, 3864–3872

View Article Online
work were diluted ([CO32À] r 1 M), the activities of the
CDCl3 was used as internal standard (d = 77.0 ppm) for
carbonate ions, aCO2À, were assumed equivalent to the molar
13C NMR.
3
concentrations: aCO2À E [CO32À].
3
3. Results
Chemicals and general
3.1 Catalysts identification
The solutions were prepared by mixing milliQ water with
known quantities of the salts of interest. Ammonium
sulfate ((NH4)2SO4), Merck, >99.5%; ammonium bisulfate
((NH4)HSO4), Fluka >99%; ammonium nitrate (NH4NO3),
Merck >99%; ammonium chloride (NH4Cl), Merck 99.8%;
ammonium bromide (NH4Br), Aldrich 99.99+%; ammonium
fluoride (NH4F), Acros >98%; sodium carbonate (Na2CO3),
Sigma >99.0%; ammonium carbonate ((NH4)2CO3), Aldrich
99.999%; magnesium carbonate (MgCO3), Sigma; acetaldehyde
(CH3CHO), Aldrich, >99.5%; acetone (CH3C(O)CH3),
Aldrich 99.5%; propanal (CH3CH2CHO), Aldrich 97%;
butanal (CH3CH2CH2CHO), Aldrich 99.5%. 1H NMR and
13C NMR spectra were recorded on Varian AS 400. Chemical
shifts are given in d relative to tetramethylsilane (TMS), the
coupling constants J are given in Hz. The spectra were
recorded in CDCl3 as solvent at room temperature, TMS
3.1.1 Catalysis by inorganic ammonium ions. The reactivity
of acetaldehyde was investigated in different aqueous salt
solutions. A reaction was observed in all the solutions containing
ammonium ions, NH4+, (ammonium sulfate, nitrate, chloride,
bromide, fluoride and bisulfate), as evidenced by the formation
of products absorbing at 226, 272 and 320 nm. However, in
water and other ammonium-free solutions (sodium sulfate and
sodium chloride), less than 1% of acetaldehyde had reacted
after more than 30 days, indicating that no significant reaction
was taking place. Analyses of the mixtures by HRMS and
1H NMR established that the products formed in the
ammonium solutions were crotonaldehyde, 2,4-hexadienal,
2,4,6-octatrienal, and 2,4,6,8-decatetraenal (Fig. 2 top). These
compounds are the well-known products of the aldol
condensation of acetaldehyde, also previously identified with
amino acid catalysis,6 and proved that the reaction was aldol
1
served as internal standard (d = 0 ppm) for H NMR, and
Fig. 2 HRMS spectra of the aldol and condensation products of acetaldehyde and subsequent oligomers (M + NH4+) in ammonium sulfate
(top), and sodium carbonate (M + Na+) (bottom).
ꢀc
This journal is the Owner Societies 2010
Phys. Chem. Chem. Phys., 2010, 12, 3864–3872 | 3867

View Article Online
condensation. The fact that the reaction was enabled only in
solutions containing ammonium ions but that the main
products did not contain nitrogen atoms was already a strong
indication that ammonium ions acted as catalysts. This will be
confirmed by the kinetic study presented below. The yields for
these unsaturated (olefinic) aldol products were determined in
a limited number of experiments, as described in the Experi-
mental section. Both in water and DMSO the total yield for
these products at complete conversion of acetaldehyde was
over 95%. In all cases, crotonaldehyde was the major product,
representing between 70 and 90% of the products identified.
The highest yields of crotonaldehyde were recovered in water
while the yield of the trimer, 2,4-hexadienal, was found to
reach 30% in DMSO. These yields confirmed that aldol
condensation was the main reaction pathway for acetaldehyde
in these experiments, and that, if compounds other than aldol
products were formed, they represented only a minor fraction.
Similarly, acetone, propanal, and butanal were all found to
undergo aldol condensation in ammonium solutions, in
particular ammonium sulfate. As in acid catalysis, most of
these carbonyl compounds produced linear polyconjugated
compounds displaying absorption bands over 200–800 nm:
acetaldehyde, 226, 272, and 320 nm; propanal, 220, 278, and
318 nm; butanal, 220 and 280 nm (Fig. 3 top). This implied
that ammonium-containing aerosols in the atmosphere
would transform the carbonyl compounds they contain into
light-absorbing products.
Although aldol condensation is a classic reaction in organic
chemistry and a plethora of catalysts are already known for it,
inorganic ammonium ions, NH4+, were not known as
catalysts for this reaction before our work, and were patented
prior to this article.19 To account for the catalytic properties of
these ions, we propose an enamine mechanism proceeding via
Mannich-type and aldol reaction pathways (Scheme 1). This
mechanism is similar to the amino acid-catalyzed aldol
condensation of acetaldehyde, and contains iminium and
enamine intermediates (eqn (1) and (2) in Scheme 1).14 In a
recent study of similar reactions, the aldol reaction between
different carbonyl compounds in water catalyzed by inorganic
ammonium ions20 the presence of iminium and enamine
intermediates was established with HRMS, confirming the
enamine mechanism. Since this mechanism involves the
conversion of an iminium ion into an enamine, it is expected
to be mostly efficient under basic conditions, which will be
confirmed by the kinetic results presented below.
3.1.2 Carbonate catalysis. Carbonate salts were known to
catalyze aldol condensation in organic solvents21,22 and
aqueous solvents at high temperature,23 but to our knowledge,
this catalytic role has not been explored for environmental
conditions. The reactions of acetaldehyde, propanal, butanal,
acetone and glyoxal were studied in 5 mM–2 M aqueous
solutions of sodium carbonate, Na2CO3, magnesium carbonate,
MgCO3, and ammonium carbonate (NH4)2CO3 at 298 and
310 K (for acetone). All these carbonyl compounds were found
to react. HRMS analyses of the products of acetaldehyde
identified the usual aldol condensation products and some
polyhydroxylated products such as deoxy sugars (Fig. 2
bottom), also confirming that aldol condensation was the
reaction occurring. Acetaldehyde, propanal and butanal all
produced compounds absorbing light between 280 and 400 nm,
typical of aldol condensation (Fig. 3 bottom). Carbonate ions
act as strong bases and these reactions thus follow the classical
base mechanisms.23,24 In atmospheric aerosols, carbonate ions
are usually associated with crustal material25,26 and carbonate
catalysis could thus take place in primary mineral aerosols.
Carbonate ions are also present in some highly alkaline
environments at the Earth’s surface, such as spring and surface
waters,27 where they could contribute to form high molecular
weight organic compounds by condensation reactions
(Noziere and Chabert, submitted manuscript, 2010).
3.2 Reactions within phases and at interfaces
The present work focuses on aqueous salt solutions only to
emphasize the importance of these reactions in inorganic
aerosols but these reactions would also take place in organic
solutions. This was already shown in previous studies for
carbonate salt catalysis21,22 and was confirmed in this work
for the ammonium ion catalysis by observing the reactions in
DMSO. Furthermore, experiments with butanal in ammonium
sulfate 40 wt% (B3.7 M) showed that when the carbonyl
compound or its products are not soluble in the inorganic
Fig. 3 Absorption spectra of the products of acetaldehyde (black
line), propanal (long dashes), butanal (short dashes), and glyoxal
(dots) in ammonium sulfate 40 wt% (3.7 M) (top), and sodium
carbonate 0.1 M (bottom).
ꢀc
This journal is the Owner Societies 2010
3868 | Phys. Chem. Chem. Phys., 2010, 12, 3864–3872

View Article Online
Scheme 1 Proposed mechanism for the aldol condensation of acetaldehyde catalyzed by inorganic ammonium ions.
phase, the reaction takes place or the products accumulate at
the inorganic–organic interface. The reactions presented in
this work could thus take place inside or at the surface of
inorganic aerosols, inside organic aerosols, or at the inorganic–
organic interface in mixed-phase particles. They might also
take place at the surface of solid particles, as previously
observed for aldol condensation on mineral clays.28
catalysis by ammonium ions in tropospheric aerosols. For
acetone in ammonium sulfate 40 wt% (3.7 M), kI
(4.5 Æ 1.5) Â 10À6 sÀ1 at 308 K.
=
Neither acid nor base catalysis contributed significantly to
the reactions in the ammonium solutions studied (pH = 0–8).
For acid catalysis, this was obvious since the rate coefficients
increased with pH (i.e. decreased with acidity). Moreover, the
rate coefficient for the acid-catalyzed aldol condensation of
3.3 Kinetics
Studying the kinetics of these reactions was essential to
determine if ammonium and carbonate ions increase the
reaction rate coefficients, and thus verified the definition of a
catalyst, and to estimate the importance of these catalytic
processes in aerosols. Rate coefficients were determined for the
reactions of acetaldehyde and acetone using the method
described in the Experimental section. The results are
summarized in Table 1 and Fig. 4. Both for acetaldehyde
and acetone, the kinetics were found to be first order in
carbonyl compounds, and the rate coefficients to increase with
ion activity, confirming that ammonium and carbonate ions
acted as catalysts. Note that the log scale used in Fig. 4 is for
clarity only, and does not imply that the rate coefficients vary
exponentially with ion activity. The exact variation of these
rate coefficients with ion activity will be studied elsewhere. For
acetaldehyde at 298 K the rate constant increased from
2.5 Â 10À7 to 6 Â 10À7 sÀ1 between ammonium sulfate
20 and 40 wt% (B1.7 to 3.7 M). The second value is identical
to the rate coefficient of the same reaction in sulfuric acid
75 wt% (13 M), illustrating the potential importance of
Fig. 4 Rate constant, kI (sÀ1), for the aldol condensation of
acetaldehyde (diamonds) and acetone (triangles) as function of ion
activity (M), in solutions of ammonium sulfate (red), fluoride (orange),
nitrate (black), chloride (clear blue), bromide (dark blue), and
bisulfate (pink), and in sodium carbonate (dark green), ammonium
carbonate (clear green) and magnesium carbonate (brown-green).
ꢀc
This journal is the Owner Societies 2010
Phys. Chem. Chem. Phys., 2010, 12, 3864–3872 | 3869

View Article Online
acetaldehyde8–10 confirmed that, with the concentrations used
in our experiments, the overall rate would be 10À7 sÀ1 at
pH = 0 and much less at higher pH, thus much smaller than
the overall rates measured in these experiments. Similarly, at
pH = 8, the most basic conditions studied, the rate coefficient
for the base-catalyzed aldol condensation of acetaldehyde is
apparent first-order rates, assuming the typical concentrations
of carbonyl compounds in aerosols indicated in Table 2. This
comparison is also illustrated by the percentage consumption
of the parent compounds over 4 days (the residence time of
tropospheric particles) corresponding to these first-order rates.
Note, however, that for the reactions of acetaldehyde, the rates
coefficients have been determined from the formation of the
products (except in the case of acid catalysis), and therefore
underestimate the consumption of acetaldehyde. The rate
coefficient for acetone was extrapolated from the one at
308 K and the rate coefficient for glyoxal in ammonium sulfate
was taken from ref. 15. The concentrations of carbonyl
compounds and ions chosen for this comparison are based
on measurements made on real tropospheric aerosols and
therefore very realistic: As justified above, pH = 0 has been
chosen as an upper limit for the acid concentration in lower
tropospheric aerosols,7,13 corresponding to a rate coefficient of
10À7 MÀ1 sÀ1 for the acid-catalyzed reactions of acetaldehyde
and acetone. Concentrations of carbonates ions of 270 ng mÀ3
reported in aerosols,26 and a specific aerosol volume of
10À11 m3 mÀ3 corresponded to a molar concentration of
0.45 M, and justified the use of the rate coefficients obtained
with 0.5 M of Na2CO3, (for acetaldehyde) or extrapolated
from 1 M Na2CO3 (for acetone). The concentration of liquid
ammonium sulfate particles in the troposphere is estimated
between 3 and 7 M, based on the water-ammonium sulfate
phase diagram.29 This justified the use of the rate coefficients
measured in 3.7 M ammonium sulfate in this comparison,
which might even underestimate the actual rate coefficients in
aerosols. For amino acids, in particular glycine, concentrations
up to about 20 mM have been reported in aerosols.30,31 This
concentration and the corresponding rate coefficient measured
in our previous work14 were thus used in Table 2. These ionic
reactions are also compared with the reactions of these
carbonyl compounds with OH radicals, for which the concen-
tration of OH radicals had to be estimated. Such estimates
were based on complete model calculations for cloud droplets,
reporting a wide range of values: 10À18 32 to 5 Â 10À13 33 M.
An intermediate value of 10À15 M was chosen for the comparison,
consistent with the fact that OH radical concentrations should
be much lower in aerosol particles than in cloud droplets.
Using all these data, the estimates presented in Table 2 show
that catalysis by inorganic ammonium ions or carbonate ions
would make aldol condensation and acetal formation fast in
tropospheric aerosols, and possibly in competition with the
reactions of these compounds with OH radicals. As discussed
in the introduction, acid catalysis should be negligible.
Because ammonium ions are ubiquitous in tropospheric
1.2 Â 10À7 MÀ1 sÀ1 24
,
leading to an overall rate in our
experiments of 6 Â 10À8 sÀ1 or less, also much smaller than
those measured. This implies that the rate coefficients
measured in this study resulted from a different type of catalysis,
involving NH4+, such as the mechanism proposed in Scheme 1.
The rate coefficients appeared to vary significantly between
different ammonium salts, even at identical ammonium
concentrations. They were generally larger in the solutions
of higher pH, such as in NH4F, for instance. We recently
reported similar effects for the reaction of glyoxal.15 The first
order kinetics in acetaldehyde observed for this reaction
implied that the rate-limiting step of the system is the
formation of the first enamine (‘‘step 2’’ in Scheme 1). Larger
rate coefficients in solutions of higher pH could thus be due
either to larger concentrations of ammonia, and to a faster
addition of the ammonia onto acetaldehyde, or to a faster
deprotonation of the iminium into the enamine. In any case,
an effect of ionic strength can be excluded as the latter should
be identical for ammonium fluoride, chloride, and bromide
solutions of similar concentrations, while the results and Fig. 4
show that this is not the case. The details of the mechanism,
and the reason for the different rate coefficients in different
solutions, are being further investigated.
The rate coefficients were generally faster with carbonate
salts than with ammonium salts. For sodium carbonate 5 mM
and ammonium carbonate 1 M (pH = 8.9), kI(acetaldehyde)
B 10À6 sÀ1, as in sulfuric acid 78 wt% (14 M). Below 0.1 M of
sodium carbonate this rate coefficient seemed to decrease
more markedly. For acetone in sodium carbonate 1 M,
kI = (6.3 Æ 2.0) Â 10À7
s
À1. A more complete study of the
kinetics of the reaction of acetaldehyde in carbonate solutions
is presented elsewhere (Noziere and Chabert, Manuscript
submitted, 2010) and confirms that the catalytic effects
observed in the present work (pH Z 11.0) are indeed mostly
due to CO32À, and not to OHÀ or HCO3
.
À
4. Discussion and conclusion
The efficiencies of the different possible catalysts for the ionic
reactions of carbonyl compounds in aerosols are compared in
Table 2. As some of these reactions have first-order kinetics
and others second-order ones, this comparison is made on
Table 2 Apparent first-order rate of reaction (sÀ1) and percentage consumption after 4 days for various carbonyls compounds in the presence of
different catalysts in aerosols, and comparison with their reaction with OH radicals
Catalyst
Acetaldehyde, 0.1 M
Acetone, 0.1 M
Glyoxal, 5 mM
OH, 10À15 Ma
H2SO4 1 M (pH = 0)
Na2CO3 0.5 M
(NH4)2SO4 3.6 M
Glycine 20 mM
3 Â 10À6 38
1 Â 10À8 9
5 Â 10À6
2 Â 10À7 14
40%
0.3%
80%
20%
7%
3 Â 10À6 38
1 Â 10À8 9
3 Â 10À7
40%
0.3%
10%
30%
Not measured
1 Â 10À6 39
16%
6 Â 10À7
1 Â 10À6
1 Â 10À4 15
100%
Not measured
a
Assumes 12 h of daylight, i.e. only 48 h of reaction over 4 days.
ꢀc
This journal is the Owner Societies 2010
3870 | Phys. Chem. Chem. Phys., 2010, 12, 3864–3872

View Article Online
aerosols they could be important catalysts at the global scale,
while carbonate ions or amino acids might play some roles at
regional scales.
Notes and references
1 D. M. Murphy, D. J. Cziczo, K. D. Froyd, P. K. Hudson,
B. M. Matthew, A. M. Middlebrook, R. E. Peltier, A. Sullivan,
D. S. Thomson and R. J. Weber, Atmos. Chem. Phys, 2009, 9,
4363; Q. Zhang, J. L. Jimenez, M. R. Canagaratna, J. D. Allan,
H. Coe, I. Ulbrich, M. R. Alfarra, A. Takami, A. M. Middlebrook,
Y. L. Sun, K. Dzepina, E. Dunlea, K. Docherty, P. F. DeCarlo,
D. Salcedo, T. Onasch, J. T. Jayne, T. Miyoshi, A. Shimono,
S. Hatakeyama, N. Takegawa, Y. Kondo, J. Schneider,
F. Drewnick, S. Weimer, K. Demerjian, P. Williams, K. Bower,
R. Bahreini, L. Cotrell, R. J. Griffin, J. Rautiainen, J. Y. Sun,
Y. M. Zhang and D. R. Worsnop, Geophys. Res. Lett., 2007, 34,
L13801, DOI: 10.1029/2007GL029979; K. D. Froyd,
D. M. Murphy, T. J. Sanford, D. S. Thomson, J. C. Wilson,
L. Pfister and L. Lait, Atmos. Chem. Phys., 2009, 9, 4363.
2 J. L. Duncan, L. R. Schindler and J. T. Roberts, Geophys. Res.
Lett., 1998, 25, 631.
3 M. Jang, N. M. Czoschke, S. Lee and R. M. Kamens, Science,
2002, 298, 814.
4 B. Noziere and W. Esteve, Geophys. Res. Lett., 2005, 32, L03812,
DOI: 10.1029/2004GL021942.
5 B. Noziere and W. Esteve, Atmos. Environ., 2007, 41, 1150.
6 B. Noziere, P. Dziedzic and A. Cordova, Geophys. Res. Lett., 2007,
´
34, L21812, DOI: 10.1029/2007GL031300.
7 R. Volkamer, F. San Martini, L. T. Molina, D. Salcedo,
J. L. Jimenez and M. J. Molina, Geophys. Res. Lett., 2007, 34,
L19807, DOI: 10.1029/2007GL030752.
An important effect of aldol condensation in aerosols would
be to transform carbonyl compounds into light-absorbing
products,4–6 in turn, increasing the absorption index of these
aerosols and their radiative forcing on climate.6 As recently
pointed out,6 condensation reactions might be at the origin of
the ‘‘fulvic’’ compounds found in aerosols and believed to be of
secondary origin.34 These ‘‘fulvic’’ compounds accounted for up
to 25% of the total organic mass in some aerosols,34 and the
+
estimates presented in Table 2 show that catalysis by NH4
could account for such quantities, while acid catalysis can not.
The ammonium-catalyzed reaction of glyoxal has recently
been shown15 to potentially account for the depletion of this
compound in the atmosphere of Mexico City and the
corresponding formation of SOA.7 If this is confirmed, this
reaction would be in competition with the gas-phase chemistry
of glyoxal, but for most other carbonyl gases, having a much
smaller Henry’s law coefficients (H o 105 MatmÀ1) than
glyoxal, an uptake from the gas and formation of SOA should
be much more limited. This is consistent with smog chamber
investigations reporting that, among various carbonyl gases,
only glyoxal produces SOA.35
8 L. M. Baigrie, R. A. Cox, H. Slebocka-Tilk, M. Tencer and
T. Tidwell, J. Am. Chem. Soc., 1985, 107, 3640.
9 W. Esteve and B. Noziere, J. Phys. Chem. A, 2005, 109, 10920.
10 M. T. Casale, A. R. Richman, M. J. Elrod, R. M. Garland,
M. R. Beaver and M. A. Tolbert, Atmos. Environ., 2007, 41, 6212.
11 S. F. Maria, L. M. Russel, B. J. Turpin and R. J. Porcja, Atmos.
Environ., 2002, 36, 5185.
12 Y. A. Zasetsky, J. J. Sloan, R. Escribano and D. Fernandez,
Geophys. Res. Lett., 2002, 29, 15816.
13 Q. Zhang, J. L. Jimenez, D. R. Worsnop and M. Canagaratna,
Environ. Sci. Technol., 2007, 41, 3213.
This work thus represents a new step in the understanding
of the reactivity of organic compounds in aerosols, which has
evolved over the last ten years from considering exclusively
radical reactions to finding increasing evidence for the
importance of ionic reactions. Our group has already
contributed to this evolution by identifying new classes of
catalysts relevant for aerosols, such amino acids6,14 and
ammonium ions,15,19,20 but the present work provides more
quantitative evidences that very common inorganic material in
aerosols, such as ammonium or carbonate salts, can play
important roles in the transformations of organic compounds.
As pointed out above, these ionic reactions might even
compete with radical reactions, and would dominate them at
night. The important aspects of these reactions, in particular
those involving NH4+, such as the production of light-
absorbing compounds and the uptake and formation of
SOA from highly soluble compounds such as glyoxal, have
also been examined by other recent works,36,37 and since
ammonium and carbonate ions are also common in other
environments at the Earth’s surface, they might participate to
the transformation of organic compounds in these environments
as well, and in particular, to the abiotic formation of C–C and
C–O–C bonds. For instance, the potential role of carbonate
catalysis in highly alkaline waters, is discussed elsewhere
(Noziere and Chabert, submitted manuscript, 2010). However,
because of their much higher molar concentrations (both of
ions and organic compounds) atmospheric aerosol particles
are expected to be especially reactive media.
14 B. Noziere and A. Co
15 B. Noziere, P. Dziedzic and A. Co
113, 231.
´
rdova, J. Phys. Chem. A, 2008, 112, 2827.
´
rdova, J. Phys. Chem. A, 2009,
16 S. L. Clegg, P. Brimblecombe and A. S. Wexler, J. Phys. Chem. A,
1998, 102, 2137.
17 S. L. Clegg, P. Brimblecombe and A. S. Wexler, J. Phys. Chem. A,
1998, 102, 2155.
18 R. A. Robinson and R. H. Stokes, in Electrolyte Solutions,
Butterworth & Co., London, UK, pp. 30–32, 1965.
19 B. Noziere and A. Co
PCT/SE2008/051091, WO 2009/045156, 01/10/2007.
20 P. Dziedzic, A. Bartoszewicz and A. Cordova, Tetrahedron Lett.,
2009, 50, 7242.
´
rdova, International Patent Application
´
21 C. Bertini, Gazz. Chim. Ital., 1901, 31(i), 265.
22 S. Ma, S. Yin, L. Li and F. Tao, Org. Lett., 2002, 4, 505.
23 Z. Zhang, Y.-W. Dong and G.-W. Wang, Chem. Lett., 2003, 32,
966.
24 L. C. Gruen and P. T. McTigue, Aust. J. Chem., 1964, 17, 953.
25 Z. Krivacsy and A. Molnar, Atmos. Res., 1998, 46, 279.
26 V. M. Kerminen, R. Hillamo, K. Teinila, T. Pakkanen, I. Allegrini
and R. Sparapani, Atmos. Environ., 2001, 35, 5255.
27 I. Barnes, J. R. O’Neil and J. J. Trescases, Geochim. Cosmochim.
Acta, 1978, 42, 144; J. F. Branthaver and R. E. Barden, Org.
Geochem., 1983, 4, 117; R. E. Barden, E. R. Logan,
J. F. Branthaver and K. E Neet, Org. Geochem., 1984, 5, 217;
C. Neal and P. Shand, Hydrolog. Earth System Sci., 2002, 6, 797;
H. N. Khoury, E. Salameh, I. D. Clark, P. Fritz, W. Baljalli,
A. E. Milodowska, M. R. Cave and W. R. Alexander, J. Geochem.
Explor., 1992, 46, 117; K. Pederson, E. Nilsson, J. Arlinger,
L. Hallbeck and A. O’Neill, Extremophiles, 2004, 8, 151.
28 P. Li, K. A. Perreau, E. Covington, C. H. Song, G. R. Carmichael
and V. H. Grassian, J. Geophys. Res., 2001, 106, 5517.
29 J. Xu, D. Imre, R. McGraw and I. Tang, J. Phys. Chem. B, 1998,
102, 7462.
Acknowledgements
B. N. acknowledges the European Commission for a Marie
Curie Chair (EXC-025026) and the Swedish Research Council
for research funding (NT-2006-5066). A. C. acknowledges
support from the Swedish National Research Council.
30 Q. Zhang and C. Anastasio, Atmos. Environ., 2003, 37, 2247.
31 K. Matsumoto and M. Uematsu, Atmos. Environ., 2005, 39, 2163.
ꢀc
This journal is the Owner Societies 2010
Phys. Chem. Chem. Phys., 2010, 12, 3864–3872 | 3871

View Article Online
32 H.-J. Lim, A. G. Carlton and B. J. Turpin, Environ. Sci. Technol.,
2005, 39, 4441.
33 H. Herrmann, B. Ervens, H.-W. Jacobi, R. Wolke, P. Nowacki and
R. J. Zellner, J. Atmos. Chem., 2000, 36, 231.
35 J. H. Kroll, N. L. Ng, S. M. Murphy, V. Varutbangkul,
R. C. Flagan and J. H. Seinfeld, J. Geophys. Res., 2005, 110,
D23207, DOI: 10.1029/2005JD006004.
36 M. M. Galloway, P. S. Chhabra, A. W. H. Chan, J. D. Surratt,
R. C. Flagan, J. H. Seinfeld and F. N. Keutsch, Atmos. Chem.
Phys., 2009, 9, 3331.
37 E. L Shapiro, J. Szprengiel, N. Sareen, C. N. Jen, M. R. Giordano
and V. F. McNeill, Atmos. Chem. Phys., 2009, 9, 2289.
38 G. V. Buxton, C. L. Greenstock, W. P. Helman and A. B. Ross,
J. Phys. Chem. Ref. Data, 1988, 17, 513.
39 G. V. Buxton, T. N. Malone and G. A. Salmon, J. Chem. Soc.,
Faraday Trans., 1997, 93, 2889.
34 S. Zappoli, A. Andracchio, S. Fuzzi, M. C. Facchini, A. Gelencse
´
ros, H.-C. Hansson,
r,
´
csy, A. Molna
G. Kiss, Z. Kriva
´
K. Rosman and Y. Zebuhr, Atmos. Environ., 1999, 33, 2733;
´
r, E. Me
´
sza
´
¨
´
Z. Kriva
´
A. Hoffer, T. Me
csy, A. Gelencse
´
´
r, G. Kiss, E. Meszaros, A. Molnar,
rvari, D. Temesi, B. Varga,
´
´
´
szaros, Z. Sa
´ ´
´
U. Baltensperger, S. Nyeki and E. Weingartner, J. Atmos. Chem.,
2001, 39, 235; G. Kiss, B. Varga, I. Galambos and I. Gansky,
J. Geophys. Res., 2002, 107, 8339, DOI: 10.1029/2001JD000603.
ꢀc
This journal is the Owner Societies 2010
3872 | Phys. Chem. Chem. Phys., 2010, 12, 3864–3872