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Photooxidation of Nonanoic Acid by Molecular and Complex
Environmental Photosensitizers
Published as part of The Journal of Physical Chemistry A special issue “Vicki H. Grassian Festschrift”.
Grace Freeman-Gallant,
†
Emily J. Davis,
†
Elizabeth Scholer, Onita Alija, and Juan G. Navea*
Cite This: J. Phys. Chem. A 2024, 128, 9792−9803
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sı Supporting Information
ABSTRACT: Photochemical aging and photooxidation of atmospheric particles
play a crucial role in both the chemical and physical processes occurring in the
troposphere. In particular, the presence of organic chromophores within
atmospheric aerosols can trigger photosensitized oxidation that drives the
atmospheric processes in these interfaces. However, the light-induced oxidation
of the surface of atmospheric aerosols, especially those enriched with organic
components, remains poorly understood. Herein, we present a gravimetric and
vibrational spectroscopy study aimed to investigate the photosensitized oxidation
of nonanoic acid (NA), a model system of fatty acids within organic aerosols, in
the presence of complex organic photosensitizers and molecular proxies.
Specifically, this study shows a comparative analysis of the photosensitized
reactions of thin films containing nonanoic acid and four dierent organic
photosensitizers, namely marine dissolved organic matter (m-DOM) and humic
acids (HA) as environmental photosensitizers, and 4-imidazolecarboxaldehyde (4IC) and 4-benzoylbenzoic acid (4BBA) as
molecular proxies. All reactions show predominant photooxidation of nonanoic acid, with important dierences in the rate and yield
of product formation depending on the photosensitizer. Limited changes were observed in the organic photosensitizer itself. Results
show that, among the photosensitizers examined, 4BBA is the most eective in photooxidizing nonanoic acid. Overall, this work
underscores the role of chromophores in the photooxidation of organic thin films and the relevance of such reactions on the surface
of aerosols in the marine environment.
1. INTRODUCTION
Light-absorbing organic chromophores are ubiquitous in the
terrestrial and marine boundary layer (MBL), where they can
act as photosensitizers and are known to initiate daytime
chemistry in the environment.
1−6
These chromophores are
found within the marine boundary layer and are known to
partition into sea spray aerosols (SSA).
7−9
As the largest
source of natural aerosols, the presence of photosensitizer
components within SSA can exert substantial influence on
Earth’s atmosphere and climate. Similarly, atmospheric aerosol
particles have been found to contain chromophores that
resemble terrestrial and aquatic humic and fulvic acids.
10
The
atmospheric impact of these organic chromophores has been
linked to aerosol aging, photooxidation, formation of
secondary organic aerosol (SOA), changes in the chemical
balance of the atmosphere, and aerosol’s ability to act as cloud
condensation nuclei (CCN).
3,11−16
Yet, the extent of photo-
oxidation of these organic complex aerosols in the marine
atmosphere remains poorly understood.
7,17,18
Sea spray aerosols (SSA) are rich in marine organic species,
particularly marine dissolved organic matter (m-DOM), which
represents one of Earth’s largest carbon reservoirs.
19,20
A
fraction of m-DOM, known as marine chromophoric dissolved
organic matter (m-CDOM), absorbs light within the solar
spectral region.
2,19−21
These chromophores have the potential
to act as ecient photosensitizers and are believed to initiate
photochemistry in the marine boundary layer.
20,21
Therefore, it
is imperative to understand the photochemical reactions
induced by m-CDOM at a molecular level. However, m-
CDOM is highly complex, consisting of a mixture of aromatic
and aliphatic hydrocarbon structures with many functional
groups.
21−23
The terrestrial counterpart of m-CDOM, humic
acid (HA), is similarly complex, consisting of a diverse array of
organic substances commonly found in fog, cloudwater, and
coastal environments.
24
Given the high complexity of these
environmental light-absorbing compounds, molecular mimics
are essential for eectively studying photosensitized reactions
and gaining molecular-level insights into their processes.
Received: August 20, 2024
Revised: October 22, 2024
Accepted: October 24, 2024
Published: November 5, 2024
Articlepubs.acs.org/JPCA
© 2024 The Authors. Published by
American Chemical Society 9792
https://doi.org/10.1021/acs.jpca.4c05608
J. Phys. Chem. A 2024, 128, 9792−9803
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Recent studies have explored how these complex environ-
mental photosensitizers, m-CDOM and HA, react with single
components, comparing their eects to those of model systems
such as 4-benzoylbenzoic acid (4BBA), a commonly used
photosensitizer.
6,20,21,25−27
In the case of 4BBA, the high
aromaticity and low solubility closely mimics some of the
physical and chemical properties of m-CDOM and HA.
21
In
addition to these similarities, Alves et al. found that m-CDOM
is enriched in nitrogen and can enhance light-initiated
chemistry. This suggests that these nitrogen-containing
structures within m-CDOM could influence its photosensitiza-
tion properties in ways not captured by 4BBA.
23
To better
understand the role of nitrogen in photosensitization,
imidazole carboxaldehydes have been used to mimic various
light-absorbing, atmospherically complex interfaces.
25−2627
In
this work, we used 4-carboxaldehyde imidazole (4IC) as a
nitrogen-containing photosensitizer. Similar imidazole-derived
molecules, present in secondary organic aerosols (SOA)
formed by the reaction of ammonium salts with α-
dicarbonyls,
28
have recently been used as models for
nitrogen-containing atmospheric chromophores.
29−32
In addition to the chromophoric organic matter found in the
atmospheric boundary layer, fatty acids are also found in
atmospheric aerosols, in particular throughout the marine
environment.
20,33
These organic fractions are known to have
high surface activity and influence the chemistry in the
atmosphere through surface driven reactions.
16,34−38
In this
work, we examined the photosensitized oxidation of nonanoic
acid (NA), a fatty acid commonly found in the surface of
SSA.
39−41
Photosensitization experiments are carried out
through two dierent environmental photosensitizers (m-
CDOM and HA) and two molecular proxies (4BBA and 4IC).
The oxidation of thin films containing a mixture of
photosensitizer and NA was investigated to simulate surface
photochemistry of atmospheric aerosols such as SSA and
coastal systems. Two dierent proportions of fatty acid to
photosensitizer, under atmospherically relevant conditions,
were investigated in the presence of solar radiation. In
addition, this work explores the mechanistic dierences
between the molecular proxies of photosensitizers used.
20,27
Overall, this work reports the kinetics of the light-initiated
oxidation reaction of NA, under dierent photosensitizers and
their proxies.
2. EXPERIMENTAL SECTION
2.1. Materials. Four thin films of a mixture of fatty acid
with dierent photosensitizers were examined. Nonanoic acid
(NA, Sigma-Aldrich) was used as a proxy of fatty acids within
SSA and SSML.
20
Two commonly used molecular photo-
sensitizer models, 4-benzoylbenzoic acid (4BBA, Sigma-
Aldrich) and 4-imidazolecarboxylahyde (4IC, Sigma-Aldrich)
were used to prepare two sets of thin films: one with a mass
ratio of 1:5 photosensitizer to nonanoic acid, and another with
a higher fatty acid content at a mass ratio of 1:10. These two
molecular photosensitizers are both aromatic and have
carbonyl functional groups, making them appropriate mimics
of environmental chromophores. Two complex environmental
samples, humic acid (HA, Sigma-Aldrich) and marine
dissolved organic matter (m-DOM) were also prepared at
the same mass ratios. The m-DOM sample used here was
collected from a large-scale mesocosm campaign, the NSF-
CAICE 2019 SeaSCAPE.
20
2.2. In Situ Flow Reactor. Experiments of the photo-
oxidation of thin films of mixtures containing nonanoic acid
and photosensitizers were performed in a tandem gravimetric
and vibrational spectroscopy flow system modified from a
previously described apparatus.
42
Figure 1 shows the two-
dimensional analysis experimental setup: the first section is a
quartz crystal microbalance (QCM200, SRS) flow system
modified to expose the sample to simulated solar irradiation.
The second section is a commercial horizontal attenuated total
reflection Fourier transformed infrared spectrophotometer
(HATR-FTIR, Thermo) designed to allow solar irradiation
of thin films, equivalent to that in the QCM section. The dual
system is connected to an air dryer (Balston 75−60) to purge
the spectrophotometer compartment and the quartz crystal
microbalance (QCM) enclosure.
Both the QCM and IR sections of the experimental setup
use corresponding broadband (λ> 300 nm) xenon arc solar
simulators (Newport 67005) to irradiate the thin films, with an
average output of 130 mW/cm2, approximately equivalent to
one solar constant. Light sources were positioned above the
photochemical cells for the QCM and FTIR flow systems. In
Figure 1. Schematic diagram of the QCM and HATR-FTIR flow system. The experimental apparatus is divided into three segments: (A) Gas
control manifold, (B) HATR flow cell with broadband light source in a purged spectrophotometer compartment, and (C) QCM flow chamber with
broadband light source in a purged enclosure.
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the FTIR setup, samples were uniformly deposited in a 7.3 cm
×0.7 cm germanium horizontal attenuated total reflection
(HATR) crystal designed for 20 internal reflections (PIKE),
enclosed in a custom-made Teflon flow cell with a window on
top to allow photochemistry experiments.
3,43,44
Due to the
limited sample quantity, m-DOM FTIR analysis was
performed using a 1.5 mm diameter iTR ATR ZnSe crystal
(Thermo), also enclosed in a custom-made Teflon flow cell
with a window on top to allow solar radiation to reach the
sample. In a typical experiment, about 50 mg of visually
homogeneous sample was deposited on the ATR crystal. In all
FTIR cases, a beam turning mirror assembly (Newport
66245), equipped with a heat absorber window to eliminate
infrared radiation and ensure isothermal reaction conditions at
298 K, was used to direct the light to the sample. For the
QCM, a focusing assembly (Newport 77776) and a liquid light
line (Newport 77628) were used to direct the light to the
sample and remove infrared radiation, keeping the temperature
constant at 298 K. Films with a photosensitizer to nonanoic
acid ratio of 1:5 or 1:10 were uniformly deposited as visually
homogeneous mixtures on a 1-in. diameter Au/Cr polished
quartz crystal enclosed in a flow cell for QCM analysis. The
photosensitizers used in this study included 4BBA, 4IC, m-
DOM, and HA, In order to compare these molecular
photosensitizers, the absorbance spectra of both 4BBA and
4IC thin films containing NA is shown in SI, Figure S1.
All FTIR measurements were conducted in situ to
qualitatively monitor the oxidation of the sample during
irradiation. The Teflon enclosure of HATR crystal, with the
sample placed directly on the crystal, is designed to allow a
continuous flow of dry air or other gaseous mixtures when
coupled to a flow system, as shown in Figure 1. This flow cell
was positioned in the purged internal compartment of an FTIR
spectrophotometer (Nicolet 6700). Infrared measurements of
the photooxidation of the sample films were collected from
900 to 4000 cm−1at 4 cm−1resolution by averaging 100 scans.
The QCM segment provides quantitative information to
accurately determine the amount of oxygen added to the
sample through a gravimetric measure.
45,46
The QCM
measures changes in the frequency of the polished quartz
crystal based on its piezoelectric properties. When the mass
loading is less than 2% of the unloaded crystal frequency, the
thin sample deposited on the crystal is treated as an extension
of its surface. Thus, the relationship between the frequency
change and the mass change can then be correlated using the
Sauerbrey equation
47
f C m
f
=
(1)
where Δfrepresents the change in frequency of the crystal
(Hz), Δmdenotes the change in mass (μg/cm2), and Cfis the
quartz sensitivity factor, which is 56.6 Hz μg−1cm−2for the 5
MHz crystal and remained constant across the sample mass
ranges tested. Samples on the QCM ranged from 3 to 5 mg per
crystal area. In a typical QCM experiment, the thin film
mixture of nonanoic acid and photosensitizers is exposed to
alternating 20 min intervals of dark and light, under a flow of
2.5 slpm of dry air or argon, for a total duration of 1 h and 40
min. Similarly, a typical experiment for FTIR was conducted
under comparable conditions, with the sample exposed to light
for at least 40 min.
2.3. Ex Situ Analysis of Products. Following exposure to
solar radiation in the QCM and the FTIR, postreaction
samples containing either 4BBA or 4IC mixed with nonanoic
acid were further analyzed using liquid chromatography−mass
spectrometry (LCMS, Thermo Vanquish/ISQ-EC) to deter-
mine the photooxidation products. Samples containing HA
and m-DOM were omitted from this analysis due to the
complexity of the photosensitizer (vide inf ra). Analysis of the
irradiated samples was carried out using an Ultra AQ C18
column with automatic injections and at a flow rate of 0.250
mL min−1, with Chromeleon software package used to assign
molecular signatures.
3. RESULTS AND DISCUSSION
3.1. Photooxidation of Nonanoic Acid in the
Presence of Molecular Photosensitizers. Gravimetric
results of NA mixed with either 4BBA or 4IC under dry air
are shown in Figure 2A,B, respectively. The blue shaded
sections represent the changes in mass of the thin film samples
in the darkness and the yellow shaded sections indicate the
samples exposed to solar simulated light. It is clearly observed
that mass increases during light cycles, which we interpret as
oxygen addition in the samples through photosensitized
oxidation (vide infra).
20
Minimal to no mass change was
observed for both 4BBA and 4IC samples when irradiated
under near-oxygen-free conditions, achieved with a 2.5 slpm
flow of ultrahigh purity argon. Neither 4BBA nor 4IC thin
films showed any mass increase when irradiated without NA.
Finally, irradiation of NA in the absence of either photo-
sensitizers did not result in detectable mass changes.
Figure 2. Percentage of mass increase attributed to photoinduced
oxidation of thin films with varying mass ratios of photosensitizer to
NA (photosensitizer/NA). Two dierent photosensitizers were used:
(A) 4BBA, (B) 4IC. The change in mass labeled “No O2” indicates
the mass analysis of a 1:5 mixture under anaerobic conditions. Shade
represents standard deviation of triplicate experiments. Only 0.1% of
data is plotted for clarity.
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As shown in Figure 2, there is a higher photoinduced mass
increase for mixtures with higher proportion of photosensitizer
(1:5 mass ratio of photosensitizer/NA). The rates of
photooxidation were extracted from the slopes of a linear fit
of the first and second light cycles, with the final rates
summarized in Table 1. These rates do not include the
possibility of minor simultaneous mass loss through decom-
position or fractionation during oxidation reactions, which can
result in the formation of oxidized C7 or C8 products. While
post irradiation analysis indicates that the fraction of these
products is minor, it also suggests that the slopes represent a
lower limit of the rate of oxidation, as there is a concurrent
small mass loss during the oxygen uptake by NA.
As the proportion of photosensitizer decreases from a 1:5 to
a 1:10 mass ratio with nonanoic acid (NA), the rate of mass
increase from oxygen reactive uptake also decreases. This
increase in the reaction rate with more photosensitizer suggests
that the rate is more dependent on the amount of
photosensitizer than on the amount of fatty acid. Comparing
the molecular mimics, 4BBA is a more eective photo-
sensitizer, with samples containing 4BBA exhibiting a mass
increase of 1.5% for the 1:5 mass ratio and 0.5% for the 1:10
ratio. Correspondingly, thin films containing 4IC as photo-
sensitizer reached a mass increase of 0.6% for the 1:5 samples
and 0.3% for the 1:10 samples. This dierence may be partly
due to the varying optical depths of the 4BBA and 4IC thin
films: 4BBA exhibits more intense absorbance bands, while
4IC absorbs lower-energy wavelengths, leading to greater
overlap with the solar simulator’s spectral irradiance (Figure
S1). The 1:5 sample containing 4BBA had a reaction rate
approximately three times faster than that of the 1:10 sample.
Conversely, for 4IC, the 1:5 sample had a reaction rate about
twice that of the 1:10 sample. These changes in mass gain due
to oxygen addition are consistent with similar photosensitizing
studies conducted in the aqueous phase, where isomers of
imidazole carboxaldehyde exhibit a lower quantum yield than
4BBA, resulting in photooxidation when imidazole was present
compared to that of 4BBA.
27
Overall, for both 4BBA and 4IC, increasing the amount of
photosensitizer results in an increased reaction rate. This
Table 1. Photooxidation Rates of Nonanoic Acid by
Molecular Photosensitizers 4BBA and 4IC in the Presence
of Light and Dry Air
a
photosensitizer/NA rate (×10−5mmol O s−1)
4BBA/NA 1:5 4.7 ±0.8
4BBA/NA 1:10 1.4 ±0.3
4IC/NA 1:5 1.5 ±0.2
4IC/NA 1:10 0.8 ±0.2
a
Estimated rates assume that all mass changes are the net eect of
oxygen reactive uptake.
Figure 3. Selected spectra of the ATR−FTIR, referenced to the initial spectrum of NA, in the presence of (A) 4BBA, (B) 4IC. Spectra presented
with 10 min intervals for at least 40 min of irradiation. Lines become increasingly light with increased time. No significant absorption features are
observed in the region between 2200−2700 cm−1.
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change from 1:10 to 1:5 mass ratio can also be interpreted as a
decrease in the concentration of NA by half. However, the
reaction rate does not decrease proportionally; instead, it
increases, which is consistent with the reaction rate being more
dependent on the amount of photosensitizer than on the
concentration of nonanoic acid. The disproportionate increase
in mass as the proportion of photosensitizer varies indicates a
nonlinear relation between the rate of photooxidation and the
amount of photosensitizer.
Vibrational spectroscopy of NA mixed with either photo-
sensitizer under dry air are shown in Figure 3A,B for 4BBA and
4IC, respectively. Thin film mixtures containing either
photosensitizers show a significant increase in absorption
intensity in the 3000 to 3600 cm−1region relative to the time
of irradiation. The broad positive absorption bands at 3415
and 3320 cm−1in Figure 3A and 3452 and 3250 cm−1in
Figure 3B, are attributed to the O−H stretching from the
formation of oxygenated products.
20
The growth of positive
absorptions bands at 1705 and 1701 cm−1in Figure 3A,B,
respectively, are due to carbonyl CO stretches from the
formation of multiple aliphatic ketone and aldehyde species,
characteristic of oxidation products.
20,48−50
The growth of
negative absorption bands observed at 1730 in Figure 3A and
1720 cm−1in Figure 3B, while low in intensity, arise from
changes in the carboxyl group in the photosensitizer,
suggesting that a small proportion of reactions involve changes
to the photosensitizer itself. Finally, the growth of the
absorption bands at 1403 and 1355 cm−1in Figure 3A and
1352 cm−1in Figure 3B can be attributed to the combination
of C−H bending and O−H in-plane bending from the
formation of aldehydes and other oxygenated species, as well as
a combination of C−O stretching vibrations from the
formation of oxygenated species.
20,51
This growth in positive
absorption bands is consistent with the mass gain observed in
the gravimetric data for 4BBA and 4IC, confirming the
photoinduced oxidation of the samples.
Both samples containing either 4BBA or 4IC show changes
in the absorption intensity in the 2800 to 3000 cm−1region
upon exposure to light. These changes, shown in both Figure
3A,B, are due to variations in the C−H symmetric and
asymmetric stretching vibrations in NA.
20
Here, reactions in
the thin film, including the oxidation of NA, causes the C−H
stretch to shift as the addition of oxygen changes the
vibrational modes of NA, leading to the observed positive
and negative absorptions in this spectral region. The negative
absorptions at 1470 cm−1in Figure 3A and centered at 1474
and 1415 cm−1in Figure 3B can be attributed to the loss of
O−H in-plane bending modes of NA due to the formation of
dimerization products, including 4BBA and 4IC combination
products with NA, as shown in Scheme 1.
20,27
Finally, the
positive absorption band at 1635 cm−1in Figure 3A can be
attributed to the CC stretching mode of unsaturated
aldehydes.
20
Postreaction LC-MS analysis of irradiated NA thin films
containing either 4BBA or 4IC confirms the observations from
gravimetric and vibrational spectroscopy analysis. Figure 4
shows that the oxygen addition reaction takes place primarily
in NA, while some dimerization of NA (2NA-2H) and
combination between the photosensitizer and NA reactions
take place. The disproportionation reaction leads to a minor
product, nonenoic acid, with both photosensitizers.
51
These
reactions are initiated by the photosensitizer (P) absorbing a
photon, leading to a triplet state (P*), as suggested by Tinel et
al. for aqueous phase and summarized for thin-films in Scheme
1.
27
While, these products are not directly detected via
gravimetry, as the change in mass is negligible, these reactions
can lead to changes in the CH-stretch as shown in the
vibrational spectra in Figure 3. Conversely, oxygen addition,
with the concomitant increase in mass, leads to a significant
number of detected products, such as hydroxy-oxo-NA and
hydroxy-NA. Decomposition products of the oxidation
reaction were also detected, such as octanoic acid and
heptanoic acid, with their oxidation products, such as
Scheme 1. Proposed Mechanism for the Photosensitized Oxidation of NA
a
a
Based on mechanism proposed by Tinel et al.,
27
adapted for thin films and the absence of water. The subscript “ox” refers to the reaction products
listed in Figure 4.
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hydroxy-oxo-octanoic acid (hydroxy-oxo-OA). These products,
resulting from oxygen addition, are responsible for mass
increases during irradiation observed via QCM and shown in
Figure 2. Similar oxidation products were observed for both
photosensitizers used, with the major product for 4IC being
hydroxy-oxo-OA. For both photosensitizers, the formation of
an oxidized fatty acid is consistent with the mass gain observed
in the QCM and the increase in O−H and CO bands
observed in the FTIR. For 4BBA, the formation of the dimer
product could provide another explanation for the large
increase in the C−H stretch observed in the FTIR at 2955 and
2833 cm−1.
The light-initiated reaction, shown in Scheme 1 and based
on a previously proposed mechanism by Tinel et al.,
27
shows
that the reaction is initiated by the formation of the triplet state
of the photosensitizer, leading to free-radical chemistry
4,5,52
P P
h j,*
(2)
where jis the photochemical kinetic constant. Here, the rate
constant for the quenching of P*by nonanoic acid is
significantly faster than its relaxation, with the reaction
preferentially proceeding to the H-abstraction of NA, as
shown in Reaction 3.
27
The free radicals formed, go on to react
with molecular oxygen, forming the oxidized products
observed via QCM through multiple pathways
P NA NA PH
k3
*+ +
• •
(3)
NA O NA
k
2 ox
4
+
•
(4)
where NAox represents the oxidation products that result in
oxygen addition and mass increase. As mentioned above,
Scheme 1 shows various secondary reactions including
dimerization, disproportionation, and combination, with
products observed via LCMS (Figure 4). These three
secondary reactions have a rate law for the formation of
secondary products (SP) that depends on [NA•]
t
k k k
t
k k k
d SP
dNA P NA NA NA
d SP
dNA ( P NA )
5 6 7
5 6 7
[ ] = [ ][ ] + [ ][ ] + [ ]
[ ] = [ ] [ ] + [ ] +
• • •
•
(5)
in which k5,k6and k7represent the kinetic constants for the
combination, dimerization, and disproportionation reactions,
respectively, as shown in Scheme 1. These secondary products
ultimately do not contribute to mass increases observed
gravimetrically. Thus, the rate of oxygen uptake, summarized in
Table 1, is the result of Reaction 4, with a rate law for the
production of oxidized nonanoic acid (NAox) shown in eq 6
t
k
d NA
d
NA O
ox
4 2
[ ] = [ ][ ]
•
(6)
Here, a steady-state approximation analysis of NA•leads to
an expression of [NA•] that depends on the excited state of
photosensitizer [P*]
t
k k
k k k
k
k k k k
d NA
dP NA NA O
NA ( P NA )
0
NA P NA
O ( P NA )
3 4 2
5 6 7
3
4 2 5 6 7
[ ] = [ *][ ] [ ][ ]
[ ] [ ] + [ ] +
[ ] [*][ ]
[ ] + [ ] + [ ] +
•
•
•
•
(7)
where the triplet state of the photosensitizer is also an
intermediary, with a rate of P*estimated by assuming steady-
state conditions
tj k
j
k
d P
d
P P NA 0
PP
NA
3
3
[*]= [ ] [ *][ ]
[*][ ]
[ ]
(8)
Combining eqs 7 and 8into eq 6 leads to a rate expression
that only depends on the amount of NA based on the
dimerization secondary reaction. The rate expression is
nonlinearly dependent on the amount of photosensitizer
([P]) and the concentration of oxygen ([O2]), consistent
with the nonlinear increase in the rate of oxygen uptake
observed as the amount of photosensitizer increases, as shown
in Table 1
t
jk
k k k k
d NA
d
P O
O ( P NA )
ox 4 2
4 2 5 6 7
[ ] =[ ][ ]
[ ] + [ ] + [ ] +
(9)
Equation 9 suggests that, if secondary reactions are
minimized ((k5[P] + k6[NA] + k7)→0), the oxidation rate
becomes more linear and less dependent on the partial
pressure of oxygen, with a reduced rate of quenching of P*by
nonanoic acid. However, the presence of secondary products
and a (k5[P] + k6[NA] + k7) > 0, ultimately leads to a reaction
rate of oxygen uptake and mass increase due to oxidation that
is not linear with respect to photosensitizer concentration. As
shown in Table 1, as the concentration of 4BBA doubles, the
reaction rate increases approximately 3-fold. This observation
is consistent with ex situ LCMS analysis which shows that a
significant fraction of the products is those produced during
Reaction 5, with the dimerization products being the
predominant product. In this case, the increase in rate suggests
that the rate constants k4and those for secondary reactions are
relatively small, leading to a k4[O2] + (k5[P] + k6[NA] + k7) <
1. Conversely, for 4IC, as the concentration of photosensitizer
doubles, the reaction rate also roughly doubles, suggesting a
nearly linear relationship and thus formation of fewer
secondary products, in agreement with LC-MS findings that
demonstrate showing smaller fractions of dimer product and
higher fractions of oxidation resulting from oxygen addition. As
Figure 4. LC-MS relative intensities of products. NA+P represent the
dimerization between nonanoic acid and the photosensitizer (either
4BBA or 4IC).
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expected from eq 9, an increase in the partial pressure of O2
also leads to a nonlinear increase in the rate of mass gain due
to oxidation products (see Supporting Information, Figure S2).
While the 4BBA/NA thin film, under 60% partial pressure of
O2, nearly doubles the mass of photooxidation products, the
4IC/NA shows just a slight increase in the oxidation of NA.
This low mass increase observed when 4IC is used as the
photosensitizer is consistent with the relatively low fraction of
secondary reactions in 4IC/NA samples, as shown in Figure 4.
Overall, the fewer secondary products observed, with k4[O2]
≫(k5[P] + k6[NA] + k7), the less dependent on [O2] the
reaction becomes.
Overall, thin film experiments discussed above show that
although the photosensitizer is involved in combination
reactions, oxygen uptake in photosensitized oxidation reactions
preferentially oxidizes only the fatty acid. The photosensitized
oxidation reaction is less dependent on the amount of fatty
acid but exhibits a nonlinear dependence on the amount of
photosensitizer. This photoinduced oxygen uptake shows a
possible mechanism for the aging of aerosols and environ-
mental interfaces that can transform highly hydrophobic
components, such as fatty acids, into more complex hydro-
philic and oxidized systems. Yet, real environmental interfaces,
such as HA and m-DOM, are more complex than the
molecular model system, mixing the chromophore with other
components that can aect the chemistry in interfaces.
3.2. Comparison with Complex Environmental
Photosensitizers HA and m-DOM. Gravimetric irradiation
experiments of thin films of NA mixed with either HA or m-
DOM under dry air are shown in Figure 5A,B respectively.
Similar to Section 3.1, the blue shaded sections represent the
periods when the thin film samples were kept in darkness while
the yellow shaded sections indicate the periods when samples
were exposed to solar simulated light. Figure 5 shows
experiments containing either environmental photosensitizers
exposed to light/dark cycles without NA, with NA in a 1:5
ratio, and NA in a 1:5 ratio with no oxygen. As shown in
Figure 5A, all experiments conducted with HA as photo-
sensitizer show little to no mass change, suggesting no
measurable mass gain due to oxidation of NA taking place in
the presence of light and HA. A small but measurable loss in
mass occurs when HA is exposed to light under dry air.
53
In
the presence of NA, this loss in mass can be compensated with
a roughly equivalent mass gain due to light-initiated oxidation,
ultimately leading to changes in the gravimetric data that fall
within the uncertainty of the QCM.
Gravimetric experiments conducted with m-DOM as the
photosensitizer are shown in Figure 5B. When a thin film
containing a 1:5 ratio of m-DOM/NA in the absence of oxygen
was exposed to solar radiation in the QCM, a steep mass loss
was observed, totaling in a 2.5% loss in mass at the end of the
two 20 min light cycles. This mass loss suggests that m-DOM
undergoes fractionation and loss of condensed phase as volatile
organic compounds (VOC), which is consistent with photo-
lytic mass loss observed for secondary organic aerosols of
similar complexity over longer exposures to solar radiation.
27,53
In the absence of oxygen, there is no mass gain due to
oxidation of NA or m-DOM, making the loss in mass for the
thin film m-DOM/NA in the absence of oxygen the largest
mass loss observed. Correspondingly, when m-DOM was
irradiated in the absence of fatty acid but under dry air, the
mass loss decreased substantially, totaling around 0.14% after
two 20 min light cycles. Initially, during the first light cycle, the
m-DOM thin film undergoes a rapid but small mass increase of
about 0.02%. However, after 10 min of irradiation, mass loss
became predominant, resulting in a net decrease in mass. We
interpret this decrease in the overall rate of mass loss, in part,
to the oxidation of m-DOM segments, including non-
chromophoric segments of the complex sample.
2,20
This is
supported by vibrational spectroscopy (vide infra), where clear
absorbance bands suggest the photooxidation of m-DOM.
Here, the mass loss due to its fractionation and VOC evolution
is counterbalanced by a mass gain due to the reactive uptake of
oxygen by m-DOM. Ultimately, the irradiation of thin films
containing a 1:5 ratio of m-DOM/NA in the presence of dry
air results in less mass loss, indicating that the photooxidation
of NA leads to a simultaneous mass gain. As a way to interpret
this smaller gain in mass due to photooxidation of the m-
DOM/NA 1:5 thin film (net Δmm−DOM/NA 1:5), we estimated
the net mass gain in the sample as the dierence between the
oxidations with and without NA
m m mnet
(m DOM/ NA1:5) (m DOM only) m DOM/ NA1:5
=
(10)
where Δm(m−DOM/NA 1:5) represents the mass changes in the m-
DOM/NA 1:5 thin film, and Δm(m−DOM only) represents the
mass changes in m-DOM in the absence of NA. The resulting
net change in mass in the m-DOM/NA 1:5 thin film is shown
in Figure 6, with a final mass increase of (0.11 ±0.02)% after
two 20 min irradiation cycles.
The initial loss in mass shown in Figure 6 reflects the initial
reactive oxygen uptake by the m-DOM thin film during the
first light cycle, which reaches a maximum at around 10 min
(Figure 5). After that point, both thin films, with and without
NA, undergo mass loss, with the sample without NA
experiencing a steeper mass loss. Overall, the result is a net
Figure 5. Percentage of mass change in thin films with varying mass
ratios of photosensitizer to NA (photosensitizer:NA). Two dierent
environmental photosensitizers were used: (A) humic acid (HA), (B)
marine dissolved organic matter (m-DOM). Shade represents
standard deviation of triplicate experiments. Only 0.1% of data is
plotted for clarity.
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9798
gain in mass when NA is present, and suggests that photolytic
mass changes in SOA is a complex processed that can be
influenced by the surface composition of the aerosol.
15,16,54
The rate of net mass increases due to oxygen uptake, estimated
from the second irradiation cycle in the m-DOM:NA sample in
Figure 6, was 3.54 ±0.01 ×10−6mmol O s−1. This rate is
lower compared to the photooxidation rates observed when
4IC and 4BBA were used as photosensitizers. Several factors
may contribute to this dierence. First, not all components in
m-DOM are chromophores, leading to a lower eective
proportion of photosensitizer to NA in the m-DOM:NA
sample.
23
Second, fractionation and degradation of oxidized m-
DOM species can result in a more significant and simultaneous
loss in mass. While m-DOM:NA sample shows an initial direct
mass increase during the first few minutes of irradiation
(Figure 5), similar to that observed for m-DOM alone during
the first light cycle, the average mass increase was significantly
lower (∼0.01%), suggesting that the reactive uptake of oxygen
by NA is slower compared to reactive components within m-
DOM.
The vibrational spectroscopy analysis results for HA, shown
in Figure 7A, is consistent with the gravimetric results shown
in Figure 5A. Upon exposure to light in the presence of dry air,
the 1:5 HA/NA thin film shows no significant changes in
absorption in the 3000 to 3600 cm−1region which is consistent
with the lack of mass change observed in the QCM
experiment. Negative absorption bands at 2912 and 2850
cm−1are likely due to a slight loss of C−H stretch due to
fractionation and loss of mass as VOC.
53,55
This minor mass
loss, observed gravimetrically, occurred when HA was exposed
to light under dry air in the absence of NA. These possible
changes in the HA/NA thin film are also observed as positive
absorption band at 1718 cm−1, attributed to a growth in the
CO stretching mode for aldehyde and ketone products,
Figure 6. Net mass gain in m-DOM:NA thin film, calculated using eq
10. Shade represents standard deviation of triplicate experiments.
Only 0.1% of data is plotted for clarity.
Figure 7. Selected spectra of the ATR−FTIR, referenced to the initial spectrum, of NA in the presence of (A) HA, (B) m-CDOM. Spectra
presented with 10 min intervals. Lines become increasingly light with increased time.
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likely due to oxidation of NA.
20
Yet, a simultaneous growth of
a negative absorption band at 1704 cm−1, also attributed to the
stretching mode of CO functional groups, indicates a loss of
mass due to fractionation or decarboxylation of the complex
sample, leading to VOC and CO2formation.
33,55,56
Slight
positive absorptions at 1404 and 1370 cm−1, corresponding to
the bending modes of aldehydic C−H and O−H product
functional groups, further support these observations.
20
These
simultaneous processes of oxidation and decarboxylation oset
one another, resulting in no measurable mass change in the
QCM.
Figure 7B shows the vibrational spectroscopy results of 1:5
m-DOM/NA thin film, where distinct features of oxygenated
product formation are identified. The broad positive
absorption band at 3345 cm−1is attributed to the O−H
stretching from the formation of oxygenated species. This
observation is consistent with the gravimetric measurements
shown in Figure 5B, in which a lower fraction of mass loss is
observed in the sample containing both m-DOM and
NA.
20,33,56
The large negative absorption bands at 2962 and
2827 cm−1are likely due to the loss of C−H stretches due to
fractionation and mass loss as VOC. This interpretation is
supported by mass loss observed during gravimetric experi-
ments. Figure 7B also shows a sharp negative absorption at
1707 cm−1as a result of the loss of CO species and
decarboxylation within m-CDOM.
20,56
The slight positive
absorption at 1718 cm−1can be attributed the reactive uptake
of oxygen leading to the formation of carbonyl functional
groups, with the concomitant growth of the CO stretching
mode band for aldehyde and ketone products.
20
Small but
observable absorptions bands between 1550 and 1400 cm−1
can be attributed to the bending modes of aldehydic C−H and
O−H product functional groups.
Overall, components of m-DOM and HA, including
chromophoric and nonchromophoric, undergo photolytic
mass loss, which slows down in the presence of oxygen, with
the possible formation of reactive oxygen species.
53,57
This
mass loss is only observed in the more complex environmental
samples, HA and m-DOM. Notably, when m-DOM is used as
a photosensitizer, the mass loss is oset by a mass gain in the
presence of NA.
20
No mass loss is observed using the
molecular models 4IC and 4BBA. The complexity of the
environmental samples also results in a lower eective ratio
between the photosensitizer and the fatty acid, which may
explain the dierences in reaction rates between m-DOM and
the molecular proxies. Overall, 4BBA and 4IC are more
eective photosensitizers than m-DOM, producing oxygenated
species and unsaturated ketones/aldehydes. This finding is in
good agreement with aqueous phase experiments conducted
using the m-DOM same sample by Trueblood et al.
20
4. CONCLUSIONS
In this work, we estimated the rates of photooxidation of
nonanoic acid (NA), a model fatty acid, using molecular
photosensitizers as model systems for environmental chromo-
phores. Recent work suggests that marine derived DOM
contains more nitrogen organic compounds than their
terrestrial counterparts. We compared the potential for
initiating photosensitized oxidation of NA using two molecular
modelsa nitrogen-containing photosensitizer (4IC) and a
non-nitrogen photosensitizer (4BBA)to two complex
environmental photosensitizers: a terrestrial humic acid
(HA) and a marine dissolved organic matter (m-DOM)
system. Gravimetric and vibrational spectroscopy results
demonstrate that the oxidation takes place primarily in NA,
with 4BBA being the most ecient photosensitizer among
those examined, with an increase in mass due to oxygen uptake
of 1.5% when the mixture had a 1:5 photosensitizer to NA
ratio. Assuming that all the mass is the result of oxygen uptake,
the rate of oxygen uptake for mixtures containing 4BBA as
photosensitizer was (4.7 ±0.8 ×10−5) mmol O s−1.
Conversely, similar thin film composition using 4IC as
photosensitizer shows a mass increase of up to 0.8% of the
initial mass, with a rate of oxygen uptake of (1.5 ±0.2 ×10−5)
mmol O s−1. The relative eectiveness of 4BBA as a
photosensitizer in NA oxidation is due to the higher presence
of aromatic species, which has been shown to increase
photoactivity, as seen in its more intense absorption bands
compared to 4IC.
20,21
Overall, the rate of photooxidation is
dependent on the amount of photosensitizer, and independent
of the amount of NA, with dierences potentially linked to the
optical density of the samples, as 4BBA and 4IC absorb light in
dierent spectral regions. Irradiation of NA in the presence of
m-DOM led to a decrease in mass, indicating that the
fractionation of m-DOM results in the formation of volatile
organic compounds (VOCs). However, the consistently larger
mass loss observed when m-DOM was irradiated without NA,
along with FTIR spectra, suggests a net mass gain when NA
present, although at a slower rate than in the model systems.
This overall decrease in mass loss observed when the m-
DOM:NA sample is irradiated may involve multiple eects
that require further study. HA was found to be a less ecient
photosensitizer than 4BBA, 4IC, and m-DOM. While m-DOM
and HA show lower photosensitivity activity, the relative
abundance of these environmental photosensitizers in both
terrestrial and marine boundary layers can lead to higher
functionalization and oxidation of aerosol organic frac-
tions.
58−61
The photosensitized oxidation of NA using 4-benzoylben-
zoic acid (4BBA) as a photosensitizer produces leads to the
formation of oxo and hydroxy C9, C8 and C7 products, as well
as the combination product (4BBA + NA) and the
dimerization of NA (2NA-2H). Although the presence of
4IC photooxidation of NA leads to a lower fraction of
dimerization, combination, or disproportionation, the products
formed upon irradiation of the thin film are similar to those
observed with 4BBA. All products formed in the thin films
containing either 4BBA or 4IC with NA yield unsaturated and
oxidized products analogous to those found in previous
experiments using complex environmental photosensi-
tizers.
20,27
Kinetic analysis suggests that the photooxidation
rate of NA is nonlinearly dependent on the amount of
photosensitizer. A decrease in photosensitized nonoxidation
secondary reactions, such as the combination or dimerization
of NA, leads to a rate of reaction becoming more linear with
respect to the amount of photosensitizer present in the
mixture.
The photooxidation mechanism discussed in this work
provides insights on the proportion of hydroxy and hydroxy-
oxo fatty acids components within the surface of marine and
coastal aerosols.
9,62
This oxidation process contributes to our
understanding of how hydrophobic components, such as fatty
acids, influence various aerosol processes, such as hygro-
scopicity, interface reactivity, and cloud condensation nuclei
(CCN) activity of SSA.
37,38,63,64
Although this study shows
lower activity of m-DOM and HA in fatty acid photooxidation
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J. Phys. Chem. A 2024, 128, 9792−9803
9800
compared to molecular models, it also highlights the relevance
of such reactions at ocean surfaces and within SSA due to the
abundance of organic photosensitizer sources. Given the
complexity of environmental chromophores like m-DOM
and HA, molecular proxies such as 4BBA and 4IC are essential
for molecular-level studies. The results shown in this work
provides further insight on the formation of reactive organic
species within SSA and how naturally occurring chromophores
can influence the pathways and rates of formation for these
atmospheric components.
65
■ASSOCIATED CONTENT
Data Availability Statement
Data for this study can be accessed in the Center for Aerosol
Impacts on Chemistry of the Environment (CAICE)
University of California San Diego Library Digital Collections
(10.6075/J0KD1Z7Z).
*
sı Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.jpca.4c05608.
UV−vis absorbance spectra of 4BBA and 4IC containing
NA plotted with the spectral irradiance of the solar
simulator. Percentage of mass increase due to photo-
induced oxidation of NA in the presence of 4BBA and
4IC at both 20 and 60% O2(PDF)
■AUTHOR INFORMATION
Corresponding Author
Juan G. Navea −Chemistry Department, Skidmore College,
Saratoga Springs, New York 12866-1632, United States;
orcid.org/0000-0002-7723-6033; Email: jnavea@
skidmore.edu
Authors
Grace Freeman-Gallant −Chemistry Department, Skidmore
College, Saratoga Springs, New York 12866-1632, United
States
Emily J. Davis −Chemistry Department, Skidmore College,
Saratoga Springs, New York 12866-1632, United States
Elizabeth Scholer −Chemistry Department, Skidmore College,
Saratoga Springs, New York 12866-1632, United States
Onita Alija −Chemistry Department, Skidmore College,
Saratoga Springs, New York 12866-1632, United States
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.jpca.4c05608
Author Contributions
†
G.F-G. and E.J.D. contributed equally to this work.
Conceptualization of the study: J.G.N. Method development:
G.F-G., O.A., and J.G.N. Measurements: G.F.-G., E.J.D., E.S.,
and O.A. Data analysis: E.J.D., E.S., and J.G.N. Discussion: all.
Interpretation of results: G.F-G., E.J.D., E.S., and J.G.N.
Writing−original draft: E.J.D., E.S., and J.G.N. Final editing:
J.G.N.
Notes
The authors declare no competing financial interest.
■ACKNOWLEDGMENTS
This work is supported by the National Science Foundation
through grants CHE-2003814 and the NSF Center for Aerosol
Impacts on the Chemistry of the Environment, a Center for
Chemical Innovation (CHE-1801971). The authors would like
to thank Dr. Vicki H. Grassian for helpful discussions. Authors
also like to acknowledge Anthony Peraza and Sofia Chihade for
help with preliminary experiments and analysis, as well as Dr.
Grassian and Dr. Michael R. Alves for their eorts in collecting
and sharing m-DOM from the NSF-CAICE 2019 SeaSCAPE
campaign. J.G.N acknowledges support from the Henry
Dreyfus Teacher-Scholar Awards Program.
■REFERENCES
(1) Rapf, R. J.; Dooley, M. R.; Kappes, K.; Perkins, R. J.; Vaida, V.
pH Dependence of the Aqueous Photochemistry of α-Keto Acids. J.
Phys. Chem. A 2017,121, 8368−8379.
(2) Trueblood, J. V.; Wang, X.; Or, V. W.; Alves, M. R.; Santander,
M. V.; Prather, K. A.; Grassian, V. H. The Old and the New: Aging of
Sea Spray Aerosol and Formation of Secondary Marine Aerosol
through OH Oxidation Reactions. ACS Earth Space Chem. 2019,3,
2307−2314.
(3) Ricker, H. M.; Leonardi, A.; Navea, J. G. Reduction and
Photoreduction of NO2 in Humic Acid Films as a Source of HONO,
ClNO, N2O, NOx, and Organic Nitrogen. ACS Earth Space Chem.
2022,6, 3066−3077.
(4) Navea, J. G.; Grassian, V. H. Photochemistry of Atmospheric
Particles. In Encyclopedia of Interfacial Chemistry: Surface Science and
Electrochemistry; Wandelt, K.; Kolasinski, K., Eds.; Elsevier Publishing:
Amsterdam, 2018; pp 553−562.
(5) Leonardi, A.; Ricker, H. M.; Gale, A. G.; Ball, B. T.; Odbadrakh,
T. T.; Shields, G. C.; Navea, J. G. Particle Formation and Surface
Processes on Atmospheric Aerosols: A Review of Applied Quantum
Chemical Calculations. Int. J. Quantum Chem. 2020,120, No. e26350.
(6) Garcia, S. L. M.; Pandit, S.; Navea, J. G.; Grassian, V. H. Nitrous
Acid (HONO) Formation from the Irradiation of Aqueous Nitrate
Solutions in the Presence of Marine Chromophoric Dissolved
Organic Matter: Comparison to Other Organic Photosensitizers.
ACS Earth Space Chem. 2021,5, 3056−3064.
(7) Schiffer, J. M.; Mael, L. E.; Prather, K. A.; Amaro, R. E.; Grassian,
V. H. Sea Spray Aerosol: Where Marine Biology Meets Atmospheric
Chemistry. ACS Cent. Sci. 2018,4, 1617−1623.
(8) Rosati, B.; Christiansen, S.; Dinesen, A.; Roldin, P.; Massling, A.;
Nilsson, E. D.; Bilde, M. The Impact of Atmospheric Oxidation on
Hygroscopicity and Cloud Droplet Activation of Inorganic Sea Spray
Aerosol. Sci. Rep. 2021,11, No. 10008.
(9) Cochran, R. E.; Ryder, O. S.; Grassian, V. H.; Prather, K. A. Sea
Spray Aerosol: The Chemical Link between the Oceans, Atmosphere,
and Climate. Acc. Chem. Res. 2017,50, 599−604.
(10) Graber, E. R.; Rudich, Y. Atmospheric HULIS: How humic-like
are they? A comprehensive and critical review. Atmos. Chem. Phys.
2006,6, 729−753.
(11) Rudich, Y.; Donahue, N. M.; Mentel, T. F. Aging of Organic
Aerosol: Bridging the Gap between Laboratory and Field Studies.
Annu. Rev. Phys. Chem. 2007,58, 321−352.
(12) Borgatta, J. In Fate of Aqueous Iron Leached from Tropospheric
Aerosols during Atmospheric Acidic Processing: Study of the Eect of
Humic-Like Substances; WIT Transactions on Ecology and the
Environment; WIT Press, 2015; pp 155−166.
(13) García, S. L. M.; Gutierrez, I.; Nguyen, J. V.; Navea, J. G.;
Grassian, V. H. Enhanced HONO Formation from Aqueous Nitrate
Photochemistry in the Presence of Marine Relevant Organics: Impact
of Marine-Dissolved Organic Matter (m-DOM) Concentration on
HONO Yields and Potential Synergistic Effects of Compounds within
m-DOM. ACS ES&T Air 2024,1(6), 525−535.
(14) Kruse, S. M.; Slade, J. H. Heterogeneous and Photosensitized
Oxidative Degradation Kinetics of the Plastic Additive Bisphenol-A in
Sea Spray Aerosol Mimics. J. Phys. Chem. A 2023,127, 4724−4733.
(15) Malecha, K. T.; Cai, Z.; Nizkorodov, S. A. Photodegradation of
Secondary Organic Aerosol Material Quantified with a Quartz Crystal
Microbalance. Environ. Sci. Technol. Lett. 2018,5, 366−371.
The Journal of Physical Chemistry A pubs.acs.org/JPCA Article
https://doi.org/10.1021/acs.jpca.4c05608
J. Phys. Chem. A 2024, 128, 9792−9803
9801
(16) O’Brien, R. E.; Kroll, J. H. Photolytic Aging of Secondary
Organic Aerosol: Evidence for a Substantial Photo-Recalcitrant
Fraction. J. Phys. Chem. Lett. 2019,10, 4003−4009.
(17) Gaston, C.; Cahill, J.; Collins, D.; Suski, K.; Ge, J.; Barkley, A.;
Prather, K. A. The Cloud Nucleating Properties and Mixing State of
Marine Aerosols Sampled along the Southern California Coast.
Atmosphere 2018,9, No. 52.
(18) Collins, D. B.; Ault, A. P.; Moffet, R. C.; Ruppel, M. J.; Cuadra-
Rodriguez, L. A.; Guasco, T. L.; Corrigan, C. E.; Pedler, B. E.; Azam,
F.; Aluwihare, L. I.; et al. Impact of Marine Biogeochemistry on the
Chemical Mixing State and Cloud Forming Ability of Nascent Sea
Spray Aerosol. J. Geophys. Res.: Atmos. 2013,118, 8553−8565.
(19) Castillo, C. R.; Sarmento, H.; Alvarez-Salgado, X. A.; Gasol, J.
M.; Marrasé, C. Production of Chromophoric Dissolved Organic
Matter by Marine Phytoplankton. Limnol. Oceanogr. 2010,55, 446−
454.
(20) Trueblood, J. V.; Alves, M. R.; Power, D.; Santander, M. V.;
Cochran, R. E.; Prather, K. A.; Grassian, V. H. Shedding Light on
Photosensitized Reactions within Marine-Relevant Organic Thin
Films. ACS Earth Space Chem. 2019,3, 1614−1623.
(21) Karimova, N.; Alija, O.; García, S. L. M.; Grassian, V. H.;
Gerber, R. B.; Navea, J. G. pH Dependence of the Speciation and
Optical Properties of 4-Benzoylbenzoic Acid. Phys. Chem. Chem. Phys.
2023,25, 17306−17319.
(22) Leenheer, J. A.; Croué, J. Peer Reviewed: Characterizing
Aquatic Dissolved Organic Matter. Environ. Sci. Technol. 2003,37,
18A−26A.
(23) Alves, M. R.; Coward, E. K.; Gonzales, D.; Sauer, J. S.; Mayer,
K. J.; Prather, K. A.; Grassian, V. H. Changes in Light Absorption and
Composition of Chromophoric Marine-Dissolved Organic Matter
across a Microbial Bloom. Environ. Sci.: Processes Impacts 2022,24,
1923−1933.
(24) Enev, V.; Sedlácek, P.; Kubíková, L.; Sovová, S.; Doskocil, L.;
Klucáková, M.; Pekar, M. Polarity-Based Sequential Extraction as a
Simple Tool to Reveal the Structural Complexity of Humic Acids.
Agronomy 2021,11, No. 587.
(25) Fu, H.; Ciuraru, R.; Dupart, Y.; Passananti, M.; Tinel, L.;
Rossignol, S.; Perrier, S.; Donaldson, D. J.; Chen, J.; George, C.
Photosensitized Production of Atmospherically Reactive Organic
Compounds at the Air/Aqueous Interface. J. Am. Chem. Soc. 2015,
137, 8348−8351.
(26) Arroyo, P. C.; Bartels-Rausch, T.; Alpert, P. A.; Dumas, S.;
Perrier, S.; George, C.; Ammann, M. Particle-Phase Photosensitized
Radical Production and Aerosol Aging. Environ. Sci. Technol. 2018,52,
7680−7688.
(27) Tinel, L.; Rossignol, S.; Bianco, A.; Passananti, M.; Perrier, S.;
Wang, X.; Brigante, M.; Donaldson, D. J.; George, C. Mechanistic
Insights on the Photosensitized Chemistry of a Fatty Acid at the Air/
Water Interface. Environ. Sci. Technol. 2016,50, 11041−11048.
(28) Maxut, A.; Noziere, B.; Fenet, B.; Mechakra, H. Formation
Mechanisms and Yields of Small Imidazoles from Reactions of
Glyoxal with NH4+in Water at Neutral pH. Phys. Chem. Chem. Phys.
2015,17, 20416−20424.
(29) Chen, P.; Lin, Q.; An, T. A New Method for the Determination
of Imidazole-Like Brown Carbons Using Gas Chromatography-Mass
Spectrometry. Atmos. Pollut. Res. 2023,14, No. 101701.
(30) Teich, M.; van Pinxteren, D.; Kecorius, S.; Wang, Z.;
Herrmann, H. First Quantification of Imidazoles in Ambient Aerosol
Particles: Potential Photosensitizers, Brown Carbon Constituents, and
Hazardous Components. Environ. Sci. Technol. 2016,50, 1166−1173.
(31) Ackendorf, J. M.; Ippolito, M. G.; Galloway, M. M. pH
Dependence of the Imidazole-2-carboxaldehyde Hydration Equili-
brium: Implications for Atmospheric Light Absorbance. Environ. Sci.
Technol. Lett. 2017,4, 551−555.
(32) Sharp, J. R.; Grace, D. N.; Ma, S.; Woo, J. L.; Galloway, M. M.
Competing Photochemical Effects in Aqueous Carbonyl/Ammonium
Brown Carbon Systems. ACS Earth Space Chem. 2021,5, 1902−1915.
(33) Ciuraru, R.; Fine, L.; van Pinxteren, M.; D’Anna, B.; Herrmann,
H.; George, C. Photosensitized Production of Functionalized and
Unsaturated Organic Compounds at the Air-Sea Interface. Sci. Rep.
2015,5, No. 12741.
(34) Woden, B.; Skoda, M. W. A.; Milsom, A.; Gubb, C.; Maestro,
A.; Tellam, J.; Pfrang, C. Ozonolysis of Fatty Acid Monolayers at the
Air-Water Interface: Organic Films May Persist at the Surface of
Atmospheric Aerosols. Atmos. Chem. Phys. 2021,21, 1325−1340.
(35) Karre, A. V.; Valsaraj, K. T.; Vasagar, V. Review of Air-Water
Interface Adsorption and Reactions between Trace Gaseous Organic
and Oxidant Compounds. Sci. Total Environ. 2023,873, No. 162367.
(36) Dibley, M.; Jaffe, A.; O’Brien, R. E. Molecular Insights into
Dissolved Organic Matter in Natural Dew Water: Biogrime Films on
Leaf Surfaces. ACS Earth Space Chem. 2022,6, 775−787.
(37) Facchini, M. C.; Mircea, M.; Fuzzi, S.; Charlson, R. J. Cloud
Albedo Enhancement by Surface-Active Organic Solutes in Growing
Droplets. Nature 1999,401, 257−259.
(38) Ovadnevaite, J.; Ceburnis, D.; Martucci, G.; Bialek, J.;
Monahan, C.; Rinaldi, M.; Facchini, M. C.; Berresheim, H.;
Worsnop, D. R.; O’Dowd, C. Primary Marine Organic Aerosol: A
Dichotomy of Low Hygroscopicity and High CCN Activity. Geophys.
Res. Lett. 2011,38, No. L21806.
(39) Milsom, A.; Squires, A. M.; Skoda, M. W. A.; Gutfreund, P.;
Mason, E.; Terrill, N. J.; Pfrang, C. The Evolution of Surface Structure
during Simulated Atmospheric Ageing of Nano-Scale Coatings of an
Organic Surfactant Aerosol Proxy. Environ. Sci.: Atmos. 2022,2, 964−
977.
(40) Vesna, O.; Sjogren, S.; Weingartner, E.; Samburova, V.;
Kalberer, M.; Gäggeler, H. W.; Ammann, M. Changes of Fatty Acid
Aerosol Hygroscopicity Induced by Ozonolysis under Humid
Conditions. Atmos. Chem. Phys. 2008,8, 4683−4690.
(41) Milsom, A.; Squires, A. M.; Boswell, J. A.; Terrill, N. J.; Ward,
A. D.; Pfrang, C. An Organic Crystalline State in Ageing Atmospheric
Aerosol Proxies: Spatially Resolved Structural Changes in Levitated
Fatty Acid Particles. Atmos. Chem. Phys. 2021,21, 15003−15021.
(42) Navea, J. G.; Richmond, E.; Stortini, T.; Greenspan, J. Water
Adsorption Isotherms on Fly Ash from Several Sources. Langmuir
2017,33, 10161−10171.
(43) Ostaszewski, C. J.; Stuart, N. M.; Lesko, D. M. B.; Kim, D.;
Lueckheide, M. J.; Navea, J. G. Effects of Coadsorbed Water on the
Heterogeneous Photochemistry of Nitrates Adsorbed on TiO2. J.
Phys. Chem. A 2018,122, 6360−6371.
(44) Lesko, D. M. B.; Coddens, E. M.; Swomley, H. D.; Welch, R.
M.; Borgatta, J.; Navea, J. G. Photochemistry of Nitrate Chemisorbed
on Various Metal Oxide Surfaces. Phys. Chem. Chem. Phys. 2015,17,
20775−20785.
(45) Schuttlefield, J.; Al-Hosney, H.; Zachariah, A.; Grassian, V. H.
Attenuated Total Reflection Fourier Transform Infrared Spectroscopy
to Investigate Water Uptake and Phase Transitions in Atmospheri-
cally Relevant Particles. Appl. Spectrosc. 2007,61, 283−292.
(46) Schuttlefield, J. D.; Cox, D.; Grassian, V. H. An Investigation of
Water Uptake on Clays Minerals Using ATR-FTIR Spectroscopy
Coupled with Quartz Crystal Microbalance Measurements. J. Geophys.
Res. 2007,112, No. D21303.
(47) Lu, C.-S.; Lewis, O. Investigation of Film-Thickness
Determination by Oscillating Quartz Resonators with Large Mass
Load. J. Appl. Phys. 1972,43, 4385−4390.
(48) Patino, P.; Méndez, M.; Pastrán, J.; Gambus, G.; Navea, J.;
Escobar, O.; Castro, A. Oxidation of Cycloalkanes and Diesel Fuels by
Means of Oxygen Low Pressure Plasmas. Energy Fuels 2002,16,
1470−1475.
(49) Gambus, G.; Patino, P.; Navea, J. Spectroscopic Study of Low-
Pressure Water Plasmas and Their Reactions with Liquid Hydro-
carbons. Energy Fuels 2002,16, 172−176.
(50) Gambus, G.; Patino, P.; Méndez, B.; Sifontes, A.; Navea, J.;
Martín, P.; Taylor, P. Oxidation of Long Chain Hydrocarbons by
Means of Low-Pressure Plasmas. Energy Fuels 2001,15 (4), 881−886.
(51) Xiao, P.; Wang, Q.; Fang, W.; Cui, G. Quantum Chemical
Investigation on Photochemical Reactions of Nonanoic Acids at Air−
Water Interface. J. Phys. Chem. A 2017,121 (22), 4253−4262.
The Journal of Physical Chemistry A pubs.acs.org/JPCA Article
https://doi.org/10.1021/acs.jpca.4c05608
J. Phys. Chem. A 2024, 128, 9792−9803
9802
(52) Martins-Costa, M. T. C.; Anglada, J. M.; Francisco, J. S.; Ruiz-
López, M. F. Photosensitization Mechanisms at the Air−Water
Interface of Aqueous Aerosols. Chem. Sci. 2022,13, 2624−2631.
(53) Sun, M.; Smith, G. D. Photolytic Mass Loss of Humic
Substances Measured with a Quartz Crystal Microbalance. ACS Earth
Space Chem. 2024,8, 1623−1633.
(54) Anglada, J. M.; Martins-Costa, M.; Francisco, J. S.; Ruiz-López,
M. F. Photoinduced Oxidation Reactions at the Air−Water Interface.
J. Am. Chem. Soc. 2020,142, 16140−16155.
(55) Walhout, E. Q.; Yu, H.; Thrasher, C.; Shusterman, J. M.;
O’Brien, R. E. Effects of Photolysis on the Chemical and Optical
Properties of Secondary Organic Material Over Extended Time
Scales. ACS Earth Space Chem. 2019,3, 1226−1236.
(56) Perez, E. H.; Schleif, T.; Messinger, J. P.; Buxó, A. R. G.; Moss,
O. C.; Greis, K.; Johnson, M. A. Structures and Chemical
Rearrangements of Benzoate Derivatives Following Gas Phase
Decarboxylation. J. Am. Soc. Mass Spectrom. 2022,33 (11), 1914−
1920.
(57) Gerritz, L.; Wei, J.; Fang, T.; Wong, C.; Klodt, A. L.;
Nizkorodov, S. A.; Shiraiwa, M. Reactive Oxygen Species Formation
and Peroxide and Carbonyl Decomposition in Aqueous Photolysis of
Secondary Organic Aerosols. Environ. Sci. Technol. 2024,58, 4716−
4726.
(58) Stubbins, A.; Spencer, R. G. M.; Chen, H.; Hatcher, P. G.;
Mopper, K.; Hernes, P. J.; Mwamba, V. L.; Mangangu, A. M.;
Wabakanghanzi, J. N.; Six, J. Illuminated Darkness: Molecular
Signatures of Congo River Dissolved Organic Matter and Its
Photochemical Alteration as Revealed by Ultrahigh Precision Mass
Spectrometry. Limnol. Oceanogr. 2010,55, 1467−1477.
(59) Dittmar, T. The Molecular Level Determination of Black
Carbon in Marine Dissolved Organic Matter. Org. Geochem. 2008,39,
396−407.
(60) Hoffer, A.; Gelencsér, A.; Guyon, P.; Kiss, G.; Schmid, O.;
Frank, G. P.; Artaxo, P.; Andreae, M. O. Optical Properties of Humic-
Like Substances (HULIS) in Biomass-Burning Aerosols. Atmos. Chem.
Phys. 2006,6, 3563−3570.
(61) Monge, M. E.; Rosenørn, T.; Favez, O.; Muller, M.; Adler, G.;
Riziq, A. A.; Rudich, Y.; Herrmann, H.; George, C.; D’Anna, B.
Alternative Pathway for Atmospheric Particles Growth. Proc. Natl.
Acad. Sci. U.S.A. 2012,109 (17), 6840−6844.
(62) Bertram, T. H.; Cochran, R. E.; Grassian, V. H.; Stone, E. A.
Sea Spray Aerosol Chemical Composition: Elemental and Molecular
Mimics for Laboratory Studies of Heterogeneous and Multiphase
Reactions. Chem. Soc. Rev. 2018,47, 2374−2400.
(63) Bikkina, P.; Kawamura, K.; Bikkina, S.; Kunwar, B.; Tanaka, K.;
Suzuki, K. Hydroxy Fatty Acids in Remote Marine Aerosols over the
Pacific Ocean: Impact of Biological Activity and Wind Speed. ACS
Earth Space Chem. 2019,3, 366−379.
(64) Nguyen, Q. T.; Kjær, K. H.; Kling, K. I.; Boesen, T.; Bilde, M.
Impact of Fatty Acid Coating on the CCN Activity of Sea Salt
Particles. Tellus B: Chem. Phys. Meteorol. 2022,69, No. 1304066.
(65) Dou, J.; Lin, P.; Kuang, B.; Yu, J. Z. Reactive Oxygen Species
Production Mediated by Humic-like Substances in Atmospheric
Aerosols: Enhancement Effects by Pyridine, Imidazole, and Their
Derivatives. Environ. Sci. Technol. 2015,49, 6457−6465.
The Journal of Physical Chemistry A pubs.acs.org/JPCA Article
https://doi.org/10.1021/acs.jpca.4c05608
J. Phys. Chem. A 2024, 128, 9792−9803
9803
