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Atmosphere a Chemical ReactorЧFormation Pathways of Secondary Organic Aerosols.

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Atmosphere, a Chemical Reactor—Formation Pathways
of Secondary Organic Aerosols
Wolfgang Schrader*
aerosols · atmospheric chemistry · environmental
chemistry · sulfuric acid
The investigation of atmospheric processes is an extremely difficult task
because of the complexity of the “chemical reactor” atmosphere. The large
number of parameters that can be
involved in one single observation mean
that it is sometimes difficult to design
laboratory experiments for finding an
explanation of such processes, and often
compromises are necessitated, such as
higher concentrations than usually
found in the natural atmosphere. The
results of the experiments are then
frequently criticized with regard to their
significance. Nonetheless, the chemistry
of the atmosphere has been an important area of research, especially since
the pioneering work from Went[1] in the
early 1960s, who correlated the “blue
haze” phenomenon with biogenic emission from plants. In the last few years the
emissions from anthropogenic[2] and
biogenic[3] sources, as well as their
reactions in the atmosphere have been
investigated. The main focus of the
research has centered on the identification[4] and determination of yields[5] of
certain products from these reactions.
Recently, reactions were investigated in
detail that play a role in the formation of
secondary organic aerosols (SOA).[6]
Particles in the atmosphere have
different properties than their gaseous
precursors. They scatter, absorb, or
reflect the solar radiation and additionally, they can play an important role in
cloud droplet formation.[7] Therefore,
[*] Dr. W. Schrader
Max Planck-Institut fr Kohlenforschung
Kaiser Wilhelm Platz 1
45470 Mlheim/Ruhr (Germany)
Fax: (+ 49) 208-306-2982
Dedicated to Professor Dieter Klockow
on the occasion of his 70th birthday
the study of these particles and—more
importantly—their formation pathways
are of great interest. The formation of
the particles and their size distribution
are dependent on the chemical composition of the surrounding air. Accordingly, industrial plumes or other gaseous
components have an effect on the aerosol.[8] Different low-volatile components
of the atmosphere, such as biogenic
compounds and their reaction products,
are being discussed as having an influence on aerosol formation. Scheme 1
shows the gas-phase oxidation of apinene as an example for the different
processes, which can lead to particle
The main problem with aerosol formation is that the basic mechanisms are
not yet completely understood. Different theories are discussed to explain the
formation of atmospheric particles on
the molecular level either directly from
gaseous precursors[12] or on the surface
of already existing particles.[9] One of
these theories describes gas-phase ozonolysis of biogenic precursors, in particular monoterpenes, which lead to highly
oxidized reaction products often containing carboxy groups. Experimental
and theoretical studies of a number of
terpenoids suggested that the formation
of stable heterodimers from organic
acids could be the first step of aerosol
formation.[12] Another theory of SOA
formation uses a model of homogeneous
nucleation in a binary system comprising water and sulfuric acid (BHN, binary
homogeneous nucleation) or a ternary
system comprising water, sulfuric acid,
and ammonia (THN, ternary homogeneous nucleation), which each form a
stable aerosol. Here, a condensation of
low-volatile organic species, such as
acids, can lead to a growth of particles.[13]
The BHN model has been used to
explain aerosols with a particle diameter
of 5–100 nm. In some areas, local effects
led to the observation of much higher
particle formation rates, which was explained by using the THN model.[14] In
the THN model, which includes ammonia, the nucleation rate is much higher
than in the BHN model. Still, there are a
lot of uncertainties in the modeling,
mainly because the classical theory of
homogeneous nucleation involves the
liquid drop model,[11] which is not valid
for small molecular clusters.[10] The first
step in aerosol formation still requires
detailed study to clarify the major
Zhang et al.[15] have recently carried
out studies aimed at explaining aerosol
formation. They have investigated gasphase reactions and particle formation
of anthropogenic compounds by using
aromatic acids, such as benzoic acid, 4and 3-methylbenzoic acids, in the presence of gaseous H2SO4. In laboratory
studies, particles with a diameter of
> 3 nm were produced in an aerosol
chamber. The size and formation rates
were dependent on the concentration of
H2SO4 as well as on the concentration of
the organic acid. It has already been
proposed that sulfate possibly functions
as a catalyst for aerosol formation.[13]
Zhang et al. were able to show that the
influence of sulfuric acid on new particle
formation seems comparable to that
previously reported for ammonia.
For a better understanding of the
complex processes involved, Zhang
et al. performed theoretical calculations,
according to which the critical step of
particle formation is the generation of
an embryo before the growth of the
DOI: 10.1002/anie.200461611
Angew. Chem. Int. Ed. 2005, 44, 1444 –1446
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Scheme 1. The formation of a secondary organic aerosol, using a-pinene ozonolysis as an example.[9]
particle. For the formation of such an
embryo, an energy barrier has to be
overcome, and thus particle growth in a
single-component system only occurs
under supersaturated conditions.[15] In
previous studies carried out in smog
chambers, homogeneous nucleation
from low-volatile compounds was reported; however, these experiments required much higher concentrations than
usually found in natural environments.
On the basis of their quantum-chemical
calculations, Zhang et al. proposed a
particle formation mechanism that involves a stable complex between the
aromatic acid and sulfuric acid. The
structure of this complex appears to be
a planar, eight-membered-ring system
connected by two hydrogen bonds, in
which both the organic acid and the
sulfuric acid act as a hydrogen donor
and acceptor (Figure 1). The stability of
the complex implies that the aromatic
acid bonds irreversibly to the sulfuric
acid, which subsequently reduces the
barrier for continued heteromolecular
Figure 1. Possible structure of the complex
between benzoic acid and sulfuric acid as the
starting point for heteromolecular nucleation.[16]
Angew. Chem. Int. Ed. 2005, 44, 1444 –1446
These results lead to the conclusion
that homomolecular nucleation most
likely does not happen under atmospheric conditions—the formation of the
complex between sulfuric acid and aromatic acid rather supports heteromolecular nucleation. This mechanism explains SOA formation in polluted areas,
because both organic and sulfuric acids
are photochemical degradation products. It also offers a different approach
to the frequently observed SOA formation than the THN model. The aromatic
acid/sulfuric acid model can be related
to some observations from field measurements[17] that can explain particle
growth in the presence of organic and
sulfuric acids. The important first step,
the formation of neutral, stable clusters
could be explained by this theory.
The more aspects that become
known relating to the formation of
secondary organic aerosols, the better
the models can be improved. Further
studies are required to reveal whether
aerosol formation from anthropogenic
and biogenic sources proceed by the
same mechanism. In this context, the
direct measurement of the intermediate
clusters and observation of their growth
under atmospheric conditions would
help to improve the understanding substantially. The formation of new particles is an essential process that has to be
fully understood and incorporated into
climate models.
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