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Changes of fatty acid aerosol hygroscopicity induced by ozonolysis under humid conditions Atmos. Chem. Phys., 8, 46834690, 2008
www.atmos-chem-phys.net/8/4683/2008/
© Author(s) 2008. This work is distributed under
the Creative Commons Attribution 3.0 License.
Atmospheric
Chemistry
and Physics
Changes of fatty acid aerosol hygroscopicity induced by ozonolysis
under humid conditions
O. Vesna
1,2
, S. Sjogren
3
, E. Weingartner
3
, V. Samburova
4
, M. Kalberer
4
, H. W. G¨aggeler
1,2
, and M. Ammann
1
1
Laboratory of Radio- and Environmental Chemistry, Paul Scherrer Inst., Villigen, Switzerland
2
Department of Chemistry and Biochemistry, University of Bern, Bern, Switzerland
3
Laboratory of Atmospheric Chemistry, Paul Scherrer Institut, Villigen, Switzerland
4
Institute of Organic Chemistry, ETH Z¨urich, Z¨urich, Switzerland
Received: 1 October 2007 Published in Atmos. Chem. Phys. Discuss.: 1 November 2007
Revised: 24 June 2008 Accepted: 9 July 2008 Published: 18 August 2008
Abstract.
Unsaturated fatty acids are important constituents
of the organic fraction of atmospheric aerosols originating
from biogenic or combustion sources. Oxidative process-
ing of these may change their interaction with water and
thus affect their effect on climate. The ozonolysis of oleic
and arachidonic acid aerosol particles was studied under hu-
mid conditions in a ow reactor at ozone exposures close
to atmospheric levels, at concentrations between 0.5 and
2 ppm. While oleic acid is a widely used proxy for such stud-
ies, arachidonic acid represents polyunsaturated fatty acids,
which may decompose into hygroscopic products. The hy-
groscopic (diameter) growth factor at 93% relative humidity
(RH) of the oxidized arachidonic particles increased up to
1.09 with increasing RH during the ozonolysis. In contrast,
the growth factor of oleic acid was very low (1.03 at 93%
RH) and was almost invariant to the ozonolysis conditions,
so that oleic acid is not a good model to observe oxidation
induced changes of hygroscopicity under atmospheric condi-
tions. We show for arachidonic acid particles that the hygro-
scopic changes induced by humidity during ozonolysis are
accompanied by about a doubling of the ratio of carboxylic
acid protons to aliphatic protons. We suggest that, under hu-
mid conditions, the reaction of water with the Criegee inter-
mediates might open a pathway for the formation of smaller
acids that lead to more signicant changes in hygroscopicity.
Thus the effect of water to provide a competing pathway dur-
ing ozonolysis observed in this study should be motivation
to include water, which is ubiquitously present in and around
atmospheric particles, in future studies related to aerosol par-
ticle aging.
Correspondence to: M. Ammann
(markus.ammann@psi.ch)
1
Introduction
Atmospheric aerosols consist of both inorganic and organic
compounds. Field measurements indicate that the organic
fraction of atmospheric aerosols is in the range from 20 to
90%, depending on the location and sources (Kanakidou et
al., 2005; Jacobson et al., 2000; Novakov and Penner, 1993;
Seinfeld and Pandis, 1998). Fatty acids, i.e., n-alkanoic and
n-alkenoic acids, can make up a signicant contribution to
the organic fraction, they have been found in emissions from
biomass burning (Oros and Simoneit, 2001), coal burning
(Oros and Simoneit, 2000), fossil fuel combustion (Rogge et
al., 1993), cooking (Rogge et al., 1991; Schauer et al., 1999),
plants of the terrestrial biosphere (Cheng et al., 2004; Si-
moneit et al., 1988; Simoneit and Mazurek, 1982), and also
in marine aerosols, where they are associated with sea salt
aerosol and stem from degradation of marine biota (Mochida
et al., 2002; Tervahattu et al., 2002). In all these sources,
a signicant fraction of fatty acids are unsaturated alkenoic
acids (Kawamura and Gagosian, 1987; Wang et al., 2006).
Some of these are polyunsaturated fatty acids, i.e., have more
than one double bond, and their concentration may exceed
the concentrations of monounsaturated fatty acids (Polzer
and Bachmann, 1991). Given their amphiphilic nature, fatty
acids can be considered as potentially important aerosol sur-
factants (Ellison et al., 1999; Tervahattu et al., 2002).
Processing of organic particulate matter by oxidants in the
atmosphere is believed to signicantly contribute to aerosol
hygroscopicity and CCN activity that in turn inuence the
direct and indirect aerosol effects on the Earths radiation
balance (Kanakidou et al., 2005). A number of studies sug-
gested the importance of the transformation of fatty acid sur-
factants by oxidative processing (Ellison 1999; Finlayson-
Pitts, 1999). Thereby, the organic layer can be transformed
from an inert, hydrophobic lm to a reactive, optically active
Published by Copernicus Publications on behalf of the European Geosciences Union. 4684
O. Vesna et al.: Fatty acid aerosol hygroscopicity induced by ozonolysis
(2)
(3)
(4)
(1)
(5)
O
R
H
O
R
H
O
R
OH
OH
R
H
OOH
O
O
O
R
R
O
R
OH
O
O
O
R
R
O
H
O+
H
O
R
O+
H
O
R
O
R
OH
+
+
H
2
O
2
H
2
O
H
2
O
Criegee Int.
acid
Criegee Int.
aldehyde
hydroxyhydroperoxide
-hydroxyalkyl
peroxide
diperoxide
acid
acid
aldehyde
secondary
ozonide ozonide
R
R
R
O O O
R
primary ozonide
O
+ O
3
R
H
aldehyde
O
O
R
R
O
O
+
+
+
+
+
Fig. 1.
Reaction mechanism for the ozonolysis of an unsaturated
double bond. Formation pathways of acids, aldehydes, secondary
ozonides, -hydroxyalkyl peroxides and diperoxides in presence
and absence of water.
hydrophilic layer. Unsaturated fatty acids are particularly
susceptible to oxidation due to the reactivity of the double
bond with ozone.
A number of laboratory studies has used ozonolysis of
oleic acid as a model system of fatty acids to investigate the
chemistry of organic particle aging and its relation to aerosol
hygroscopicity (Zahardis and Petrucci, 2007). However, sig-
nicant absorption of water by oxidized oleic acid drops and
lms was only evident at very high ozone concentration (up
to 500 ppm) and relative humidity above 90% (Asad et al.,
2004; Hung et al., 2005; Hung and Ariya, 2007), and it has
been argued that these conditions may not have been relevant
for the atmosphere (Zahardis and Petrucci, 2007). Only very
recently, Lee and Chan (2007a) have reported a very low hy-
groscopic growth factor after exposure of oleic acid to low
concentrations of ozone over extended periods of time.
In this work, we exposed oleic acid (OA) and arachidonic
acid (AR) aerosol particles to ozone and humidity under con-
ditions as close as possible to ambient to establish a link
between condensed phase alkene oxidation and hygroscopic
growth. The goal was to determine the hygroscopic growth
factor (GF) as function of ozone concentration and humid-
ity and to relate the observed hygroscopic changes with the
functional group composition of product particles. OA is a
C18 monounsaturated fatty acid with a double bond at the
C9 position, and AR is a polyunsaturated C20 fatty acid
with four double bonds at the C5, C8, C11 and C14 po-
sitions. Ozone attacks at the carbon-carbon double bonds,
which leads to primary ozonides that then decompose into a
range of products. A simplied scheme is given in Fig. 1.
For OA the products are non-hygroscopic C9 acids, aldehy-
des and oligomers thereof (Zahardis and Petrucci, 2007). The
AR-ozone reaction may lead to shorter chain species, includ-
ing hygroscopic C3 and C5 dicarboxylic acids. The effect of
water on condensed phase alkene oxidation has not been en-
tirely claried so far, even though the reaction of the Criegee
intermediates with water had been suggested to be signi-
cant in the gas phase (Hasson et al., 2001a). Very recently,
Chen et al. (2008) have suggested from an ozonolysis study
of methyl vinyl ketone and methacrolein that pathways lead-
ing to hydrogen peroxide via hydroxy-hydroperoxides (see
Fig. 1) might be important. The decomposition of the hy-
droperoxides may also lead to acids. Therefore, humidity
might have a signicant effect on the yield of hygroscopic
products, the target of the present study.
OA and AR have been chosen as laboratory model com-
pounds as the product particles from both OA and AR oxi-
dation are expected to generally represent the organic frac-
tion of aged particles from emissions rich in fatty acids from
sources listed above. For the purpose of these laboratory
studies, the precursor particles are of sufciently low volatil-
ity, and the product particles (for AR) are expected to contain
a sufcient density of hygroscopic products.
2
Experimental
The aerosol particles were generated by passing a ow of
nitrogen (1 l min 1
)
over a reservoir containing OA or AR
in an oven kept at 130 C±2 C or 160 C±2 C, respectively,
downstream of which OA or AR particles form by homoge-
neous nucleation by cooling to laboratory temperature. The
particle number concentration and size distribution were con-
tinuously measured with a Scanning Mobility Particle Sizer
system (SMPS). The resulting aerosol particles had a geo-
metric mean diameter of 77 nm and 55 nm for OA and AR
(±2% for both types), respectively, with a number concen-
tration of 5×10
6
particles cm 3
(±30% (1 )).
Ozone w