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Cite this: DOI: 10.1039/c5cs00375j
Heterogeneous chemistry and reaction dynamics
of the atmospheric oxidants, O
3
,NO
3
, and OH,
on organic surfaces
Robert C. Chapleski Jr., Yafen Zhang, Diego Troya and John R. Morris *
Heterogeneous chemistry of the most important atmospheric oxidants, O
3
,NO
3
, and OH, plays a central
role in regulating atmospheric gas concentrations, processing aerosols, and aging materials. Recent
experimental and computational studies have begun to reveal the detailed reaction mechanisms and
kinetics for gas-phase O
3
,NO
3
, and OH when they impinge on organic surfaces. Through new research
approaches that merge the fields of traditional surface science with atmospheric chemistry, researchers
are developing an understanding for how surface structure and functionality affect interfacial chemistry
with this class of highly oxidizing pollutants. Together with future research initiatives, these studies will
provide a more complete description of atmospheric chemistry and help others more accurately predict
the properties of aerosols, the environmental impact of interfacial oxidation, and the concentrations
of tropospheric gases.
I. Introduction
O
3
,OH,andNO
3
are the most important atmospheric oxidants
due to their high chemical potentials, abundance, and negative
effects on human health. While the importance of these oxidants
has led many scientists to investigate their gas-phase chemis-
try,
1–4
detailed studies into the reactions of O
3
,OH,andNO
3
at
the gas–surface interface have only recently been reported.
Interfacial reactions between organic particles and oxidative
gases result in changes in particulate composition, size, and
physical properties. These changes affect human health and
visibility, climate, and the global carbon cycle.
5–7
Organic parti-
cles in the atmosphere form or grow through the following four
mechanisms (Fig. 1): (i) biogenic emissions in remote regions
and anthropogenic emissions in urban areas, which result
in particles described as soot or primary organic aerosols;
8–10
(ii) adsorption of volatile organic compounds onto liquid or solid
surfaces, which results in aerosols coated by organic films;
11,12
(iii) coagulation of smaller carbonaceous nanoparticles;
13
and
(iv) reactions of atmospheric oxidants on the surfaces of existing
organic particles, which produce so-called secondary organic
aerosols (SOAs).
14,15
The concentrations of these types of organic
aerosols typically range from 1 to 10 mgm
3
and can reach levels
exceeding 15 mgm
3
in heavily industrialized environments.
16,17
Once formed, they nearly immediately begin contributing to the
heterogeneous chemistry of the lower atmosphere.
Surface reactions involving O
3
,NO
3
, and OH are known to
alter the properties and fate of organic particulates, often in
unexpected ways. For example, Fig. 2 shows scanning-electron
Fig. 1 Schematic illustration of the generation and transformation of
organic particles in the atmosphere. Particles can form and grow from
biogenic and anthropogenic emission products, adsorption of volatile
organics onto surfaces, coagulation of carbonaceous nanoparticles, and
reactions of atmospheric oxidants on the surfaces of existing organic
particles. These particles can affect the balance of incoming and outgoing
solar radiation.
Department of Chemistry, Virginia Tech, Blacksburg, Virginia 24061, USA.
E-mail: jrmorris@vt.edu; Tel: +1-540-231-2471
Received 8th May 2015
DOI: 10.1039/c5cs00375j
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microscope images of SOAs generated through the oxidation of
volatile organic emissions from pine seedlings.
18
When expo sed t o
OH in the presence of SO
2
, particles (shown as white spots in the
figure) with an average radius of 100 nm are formed. Conversely,
exposure to O
3
results in the formation of significantly smaller
particles (28 nm). Such modifications in particulate properties due
to atmospheric oxidation most likely affect the scattering and
absorption of light by the particles, thereby altering the balance
between incoming and outgoing solar radiation
5,19
(Fig. 1).
Further, the overall surface area of each individual particle is
altered, thereby affecting the subsequent adsorption and reaction
rates on these surfaces. These findings have motivated scientists
to investigate the role of organic surfaces in atmospheric inter-
facial oxidative reactions—the chemistry contributing to atmo-
spheric organic particle transformation and growth.
20–23
Such
chemistry is particularly important in polluted regions of the
atmosphere where elevated concentrations of both particulates
and oxidative gases often coincide.
Tropospheric O
3
hasbeenrecognizedasaworldwideenviron-
mental problem, hallmarked by the presence of NO and NO
2
in
the lower atmosphere. Anthropoge nic O
3
firstbecameofconcern
as an atmospheric pollutant in the late 1940s,
24
when high
concentrations were reported in the Los Angeles area. Since
then, dangerously elevated tropospheric concentrations have
been measured in Greece, Japan, Sydney, Jerusalem, Mexico
City, and many other locations.
19,25–27
Today, tropospheric O
3
concentrations are monitored to characterize the extent of air
pollution in urban areas, where emissions from transportation and
industrial plants are substantial. Anthropogenic O
3
is primarily
generated from the reaction of atmospheric O
2
with ground-
state O(
3
P) radicals that result from the photolytic dissociation
of ambient NO
2
19,28
(Scheme 1).
Though air pollution is often evaluated using O
3
concentra-
tions, hydroxyl (OH) radicals also play a critical role in daytime
atmospheric chemistry. In fact, concentrations of OH radicals
are difficult to quantify precisely because of their high reactivity
and consequent short atmospheric lifetime (r1 s). Techniques
for quantifying OH have been reviewed by Heard and Pilling.
29
One important reaction of OH involves volatile organic com-
pounds (VOCs) and the conversion of two molecules of NO
to NO
2
(Scheme 2).
30
This conversion is primarily responsible
for the formation of NO
2
, a precursor to O
3
in the atmosphere
(Scheme 1).
30,31
Further, the atmospheric concentration of OH
is dependent on the photodissociation of O
3
by UV radiation,
which produces an electronically excited oxygen atom (O(
1
D)),
able to undergo rapid reactions with water vapor in the air to
form two OH radicals (Scheme 3).
32,33
Because the primary mechanisms of O
3
and OH generation
involve the photodissociation of other atmospheric molecules,
reactions initiated by these two oxidants are of highest importance
during the daytime.
28,32
At night, in the absence of photochemical
reactions, the consumption of O
3
and OH is much faster than
the formation of these two oxidants, and their atmospheric
concentrations diminish.
22,34
However, oxidation of NO
2
by O
3
generates NO
3
, which drives a great deal of tropospheric night-
time chemistry.
26,35
Interestingly, NO
3
chemistry is most relevant
to the atmosphere after sunset because NO
3
photodissociates
rapidly in daylight.
26,36
NO
3
initially reacts with certain VOCs
through addition or hydrogen abstraction reactions, ultimately
yielding peroxyacetyl nitrate (PAN).
37–40
PAN (due to its eventual
photo- and thermal-decomposition into NO
2
) acts as a reservoir
and a transportation medium for NO
2
.
41
In addition, reactions of
NO
3
and NO
2
result in the production of dinitrogen pentoxide
(N
2
O
5
), which is an important source of nitric acid in the
atmosphere.
36
While the rich interplay amongst O
3
, OH, and NO
3
and their
reactions in the gas phase have key implications in tropospheric
chemistry, this review focuses on reactions of each of these gases
with organic surfaces. These heterogeneous reactions alter the
size and composition of atmospheric particulates and modify
the surfaces of other anthropogenic and natural materials as
well, such as metals, metal oxides, and polymers. The impor-
tance of these processes has provided ample motivation for
detailed laboratory investigation. Below, we highlight a subset
of key studies into the chemistry of organic materials with O
3
,
OH, and NO
3
. For each gas, the review begins with a focus on
Fig. 2 SEMimagesofSOAparticlesofdifferen t sizes resulting from the
oxidation of pine-seedling volatile organic compound (VOC) emissions following
(a) OH-initiated oxidation in the presence of SO
2
,and(b)O
3
-initiated oxidation.
The white spots are SOAs supported on a lacey carbon grid (black regions).
Reprinted with permission from Macmillan Publishers Ltd: Nature, 2010,
467, 824. Copyright 2010.
Scheme 1 Generation of O
3
in the atmosphere during the daytime.
Scheme 2 Generation of NO
2
during typical oxidation of volatile organic
compounds (VOC) by OH.
Scheme 3 Formation of OH radicals in the atmosphe re during the daytime.
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reactions at highly relevant, yet relatively uncharacterized, surfaces
and particles (including soot), followedbystudiesperformedwith
model aerosols composed of a single type of molecule or surface-
adsorbate combination. Finally, each section concludes with a
discussion of fundamental research involving highly ordered
synthetic surfaces that helps provide insight into specific
details of the gas–surface reaction dynamics and kinetics, while
serving as a valuable benchmark for theoretical studies.
II. O
3
reactions with organic surfaces
Reactions between ozone and organic surfaces have long been
recognized as critical to the overall chemistry of the atmo-
sphere. It is therefore not surprising that, relative to NO
3
and
OH radicals, there have been many studies into the interfacial
chemistry of ozone. In the gas phase, O
3
initiates reactions with
vinyl-containing organics and polycyclic aromatics via addition
across double bonds to form an unstable primary ozonide,
which triggers a series of subsequent reactions.
42–44
Analogous
chemistry may occur on surfaces;
45–47
however, scientists are
only beginning to decipher how surface structure and function-
ality affect ozone accommodation, diffusion, and reaction
pathways. A quintessential result for these processes is the
reactive uptake coefficient: the probability that a gas-phase
molecule that collides with the surface will react with the
surface. For a thorough explanation of this coefficient in terms
of reactions of oxidative gases with organic surfaces, the reader
is referred to work by Houle et al.
48
In an investigation of the reaction kinetics of ozone with
several different polycyclic aromatic hydrocarbon (PAH) com-
pounds on laboratory-generated soot, Bedjanian and Nguyen
49
found that fast initial consumption of ozone was followed by
rapid irreversible changes in the molecules at the interface,
rendering the surface unreactive to further ozone exposure. In
complementary work, Disselkamp and coworkers
22
investi-
gated the reaction between soot and ozone in a static aerosol
chamber. By interpreting ozone and CO
2
infrared signals over
the course of ozone exposure, they obtained a stoichiometry
of two O
3
molecules for every molecule of CO
2
product formed
and reported a very small pseudo first-order reaction probability
of 10
8
for ozone with ‘‘aged’’ soot. The interfacial reactivity of
ozone on soot has also been reported in a recent study by Browne
et al., which yielded an uptake coefficient of 2 10
7
.
50
The He
group further augmented these findings by performing in situ
Raman and infrared studies
51,52
of ozone reactions with soot.
Their work suggested that amorphous carbon and disordered
graphitic sites were responsible for initiating the reactions. They
also revealed that some of the reaction products included surface-
bound ketone, lactone, and anhydride groups. Other research
teams exte nded this work to specific surface-bound compounds
(including polycyclic aromatics,
43,53–58
biogenic volatile organics,
59
biomass burning products,
60
and fungicides
61
) and functionalities
at well-characterized aerosol surfaces.
62–66
Within the body of literature on O
3
reactions with aerosols,
investigations of the ozonolysis of oleic acid aerosols are of
particular interest because these particulates appear in relative
high abundance in certain regions of the atmosphere.
67–70
From
these studies, reactive uptake coefficients have been determined
to be generally on the order of 10
3
.
67–69,71
Variations in this
coefficient have been attributed to the effects of particle size on
diffusion of O
3
into the bulk
68
and the likelihood of secondary
reaction pathways beyond ozonolysis.
69
Further, in an experi-
ment in which the substrate was not uniformly covered by oleic
acid, a reactive uptake coefficient on the order of 10
5
was
found.
71
Importantly, the use of fresh surfaces in these studies
contributed to a higher reactive uptake coefficient than in
experiments employing passivated surfaces.
22
Motivated by the challenge of building a highly fundamental
understanding of interfacial ozone chemistry, several groups
have explored ozone reactions with organic particles adsorbed
onto extended (planar) solid surfaces. In their work, Ham and
Wells
72
identified products formed during exposure of alpha-
terpineol on glass and vinyl flooring tile to ozone. Kahan et al.
implemented fluorescence spectroscopy in a Teflon reaction
chamber to investigate the degradation kinetics of PAHs
adsorbed on a microscope slide,
73
and Kwamena et al.
74
inves-
tigated the role of relative humidity and ozone concentration in
the kinetics of ozonolysis of anthracene deposited on a pyrex
flow tube. In research highlighted in these two works, the
Langmuir–Hinshelwood mechanism, whereby the impinging
ozone molecule thermally accommodates on the surface before
reacting, adequately describes the time-resolved data.
To add insight to the role of the interaction between organic
molecules and the underlying surfaces on which they are supported
on oxidation reactions, Chu et al.
21
used density functional
theory methods to model the ozonolysis of surface-bound planar
PAHs. They found an energetic expense to reaction resulting
from the ‘‘lift-off’’ of the molecules from the surface as the
planar PAH reactants form nonplanar intermediates or products.
Fig. 3 shows structures and energies for the ozone reaction with
one of the available double bonds in pyrene. The first step in the
reaction is an exoergic addition to form a primary ozonide,
which retains planarity. Decomposition of the primary ozonide
in step 2 leads to a non-planar Criegee intermediate. The reaction
Gibbs energy (DG
rxn
) for this step is +14.8 kcal mol
1
,andthe
energetic penalty resulting from loss of interactions with the under-
lying surface increases the reaction Gibbs energy by an additional
B14 kcal mol
1
, with variations depending on the nature of the
surface (e.g., fly ash vs. NaCl). The last step of the reaction is the
thermodynamically favorable formation of a secondary ozonide
(step 3), which results in a planar species.
Additionally, attenuated total reflectance infrared (ATR-IR)
experiments
75–78
have been used to investigate the effects of
temperature,
75–77
relative humidity,
75–77
and film thickness
77
on reaction mechanisms,
76
product yields,
76,78
product hygro-
scopicity,
75,77,78
and redox activity.
75
As ATR-IR allows for in situ
collection of infrared spectra, the change in the IR intensity of
specific vibrational modes during exposure can be used to
obtain kinetic information such as ozone’s reactive uptake
coefficient, which has been determined to be in the range of
1.0 10
5
,
76
to 5.1 10
4
.
75
The measured variations in ozone
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uptake coefficients are likely related to differences in the reaction
conditions, surface properties, and the particular infrared band
analyzed to extract kinetic information.
Beyond solid surfaces, the ozonolysis of organic compounds
at the air–water interface has also been studied in thin films.
Raja and Valsaraj
79
investigated the impact of ozone exposure
on the rate of naphthalene vapor uptake onto single droplets
of water. They reported that reactions between ozone and
naphthalene at the air–water interface increased the rate of
naphthalene uptake onto the droplet surface. Oxidation products
with higher water solubility than naphthalene more readily
diffused into the bulk, thus allowing for greater mass transfer
of naphthalene from the gas phase to the surface of the droplet.
Moreover, the rate of the heterogeneous ozone-naphthalene
reaction—15 times greater than that of the homogeneous gas-
phase reaction—was found to be dependent upon the size of the
droplet. Wadia and coworkers
80
used an experimental flow
chamber and molecular dynamics simulations to investigate
the ozonolysis of saturated and terminally unsaturated phospho-
lipid molecules at the air–water interface. No reaction with
the saturated compound was observed, yet reaction with the
unsaturated compound yielded an aldehydic product. This
reaction was facilitated by the availability of the CQCdouble
bond at the interface, which was observed to be invariant to
surface film compression. Recently, Mmereki et al.
81
provided
additional insight into the ozone-anthracene reaction in organic
films on water via laser-induced fluorescence. Interestingly, the
presence of organic acids in the aqueous phase reduced the
reaction rate with the organic film, and the presence of alcohols
enhanced the overall rate of reaction.
The dependence of the ozone reaction rate with organic
surfaces on the physical properties of the surface has received
additional scrutiny. Of particular interest, Moise and Rudich
82
noted an order-of-magnitude increase in the reactive uptake
coefficient for ozone at liquid organic surfaces relative to solid
samples (Fig. 4). This result was attributed to the participation
of subsurface layers in liquid uptake, which is not as prevalent
when the organic surface is frozen. In the figure, dashed lines
mark differences in coefficients of the same compound resulting
from a liquid/solid phase change. The coefficient for linoleic
acid decreased from (1.2 0.2) 10
3
to (1.4 0.1) 10
4
upon
freezing. For oleic acid, a decrease from (8.3 0.2) 10
4
to
(5.2 0.1) 10
5
was observed. This change of phase also
caused a decrease in the coefficient of 1-hexadecene from
(3.8 0.6) 10
4
to (2.5 0.4) 10
5
. These differences are
similar in magnitude to those observed for liquid-phase uptake
compared to a well-ordered thin film.
83
In particular, 1-octene
exhibited a decrease in the uptake coefficient from 1 10
3
for
liquid to 1 10
4
for a monolayer in the same temperature range.
As with reaction rate, the mechanism of ozonolysis has been
shown to depend on the characteristics of the surface. Enami
et al. performed a series of studies that employed electrospray
mass spectrometry to investigate ozonolysis of aqueous micro-
droplets of several organic compounds including uric acid,
84
ascorbic acid,
85
sulfonic acid,
86
phenol and a-tocopherol,
87
cysteine,
88
and b-caryophyllene.
89
After identifying unique
products from these reactions that were distinct from those
in the bulk solution, they concluded that an air–water shell
a few nanometers thick presents a reaction environment
that is fundamentally different from that found in the bulk.
Fig. 3 (a) Structures of minima in the oxidation of pyrene by ozone show a diversion from planarity in an intermediate. (b) Energetics of reaction steps in
(a) show an energetic penalty for surface liftoff as a result of loss of planarity. DG
rxn
is determined with gas-phase molecules (surface-absent), B is defined
as the fraction of carbon atoms that leave the plane of the molecule, and DG
des
is the energy of complete desorption from the surface. Reprinted
(adapted) with permission from J. Am. Chem. Soc., 2010, 132, 15968. Copyright 2010 American Chemical Society.
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They showed, specifically in b-caryophyllene ozonolysis, that
different products were formed at different depths within this
shell, in correlation with varying water densities. As the water
density changes, the rate of vibrational relaxation changes as
well, allowing for the activation of different processes.
89
To
further elucidate the role of water molecules at the interface,
Beauchamp and coworkers
90
investigated the ozonolysis of
phospholipid surfactant mixtures using field-induced droplet
ionization mass spectrometry. They found known metastable
species in the bulk as major products of ozonolysis at the air–
water interface. Lower water density at the interface relative to
the bulk may play a key role, as proton transfer from water leads
to rapid decomposition of these products. Also, as found in the
study of ozone exposure to naphthalene on water droplets by
Raja and Valsaraj,
79
ozonolysis of unsaturated reactants resulted
in increased molecular hydrophilicity, leading to dissolution of
products into the aqueous phase. Most recently, Beauchamp and
coworkers have developed a method for generating controlled
nanoliter-sized droplets for mass-spectrometric sampling using
bursting bubble ionization and interfacial sampling with an
acoustic transducer.
91
This method has been used to investigate
the time-dependent ozonolysis of oleic acid.
Investigations of the heterogeneous ozonolysis of oleic acid
have more recently been expanded to include macroscopic sur-
face samples.
82,92
For example, flow-tube methods have been
used to examine interfacial reactions of oleic acid on a variety of
substrates.
71,82,92
Further, ATR-IR has been utilized to monitor
oxidation at the interface of large and small oleic acid droplets.
93
The reactive uptake coefficients measured for extended surfaces
were found to range from 10
5
to 10
3
, depending on experi-
mental technique, sample geometry, the influence of secondary
reactions,
93
and whether the coefficient was calculated using
changes in O
3
92
or in oleic acid concentrations or properties.
82,93
Additionally, Reid and coworkers investigated oxidative aging
of aerosol particles containing environmentally relevant organic
acids using optical tweezers and cavity enhanced ringdown spectro-
scopy.
94,95
For mixed NaCl/oleic acid particles, they report an uptake
coefficient of 2.3 10
4
, which is consistent with measurements
cited above that employed ATR-IR experiments.
82,92,93
In another
droplet-based study, O
3
reactions with oleic acid surfaces were
investigated through the use of a pendant drop of water coated
with a monolayer of oleic acid.
96
The authors attributed a
relatively low uptake coefficient ((2.6 0.1) 10
6
) to the
decrease in accessibility of ozone to the CQC bond, which was
submerged within the oleic acid monolayer (Fig. 5).
42
From a much more fundamental perspective, studies employing
self-assembled monolayers (SAMs) as model surfaces have provided
detailed insight into the dynamics of gas–surface collisions
involving ozone. Specifically, dynamic properties including
energy transfer and thermal accommodation coefficients, as
well as reaction probability of a single surface-bound functional
group, have been revealed using SAMs. These surfaces present
many advantages for fundamental work: they can be synthe-
sized in a highly reproducible manner, they can be character-
ized in situ with infrared spectroscopic probes, and they enable
one to locate a specific functional group precisely at the gas–
surface interface. With this strategy, Dubkowski et al.
97
and
Vieceli et al.
98
employed time-resolved changes in atmospheric-
pressure ATR-IR band intensities as well as molecular dynamics
simulations of alkene-terminated SAMs to describe the inter-
action of O
3
with a SAM. Their experimental results suggested a
surprisingly long surface residence time for ozone: B7 s, which
was many orders of magnitude longer that that shown by their
simulations: B17 ps. As nonreactive molecular dynamics were
used in the simulations, the vast difference in residence time
was attributed to the formation of covalent bonds initially
leading to the primary ozonide in the experiment, which was
not modeled in the simulations. The simulations also showed
Fig. 4 Ozone reactive uptake coefficients, g, vs. temperature (K) for several
organic liquids, frozen liquids, and a monolayer.
82,83
The dashed lines show
an order of magnitude difference in uptake coefficient between liquid and
frozen samples. Reprinted (adapted) with permission from J. Phys. Chem. A,
2002, 106,6469–6476. Copyright 2002 American Chemical Society.
Fig. 5 An oleic acid monolayer on a pendant drop of water . The non-
terminal positioning of the CQC double bond hinders reaction with O
3
,
resulting in a decreased reactive uptake relative to a monolayer with a
terminal CQC bond. Reproduced from ref. 42 with permission from the
PCCP Owner Societies.
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