Synergistic O3 + OH oxidation pathway to extremely low-volatility dimers revealed in β-pinene secondary organic aerosol

Article abstract of DOI:10.1073/pnas.1804671115

Dimeric compounds contribute significantly to the formation and growth of atmospheric secondary organic aerosol (SOA) derived from monoterpene oxidation. However, the mechanisms of dimer production, in particular the relevance of gas- vs. particle-phase c

Full text of DOI:10.1073/pnas.1804671115

Synergistic O3 + OH oxidation pathway to extremely
low-volatility dimers revealed in β-pinene
secondary organic aerosol
Christopher M. Kensetha, Yuanlong Huangb, Ran Zhaoa, Nathan F. Dalleskab, J. Caleb Hethcoxa, Brian M. Stoltza,
and John H. Seinfelda,c,1
aDivision of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125; bDivision of Geological and Planetary Sciences,
California Institute of Technology, Pasadena, CA 91125; and cDivision of Engineering and Applied Science, California Institute of Technology, Pasadena,
CA 91125
Edited by Joost A. de Gouw, University of Colorado Boulder, Boulder, CO, and accepted by Editorial Board Member A. R. Ravishankara July 2, 2018 (received
for review March 16, 2018)
underlying dimer production and the relative importance of gas-
vs. particle-phase chemistry remain unresolved.
Dimeric compounds contribute significantly to the formation and
growth of atmospheric secondary organic aerosol (SOA) derived
from monoterpene oxidation. However, the mechanisms of dimer
production, in particular the relevance of gas- vs. particle-phase
chemistry, remain unclear. Here, through a combination of mass
spectrometric, chromatographic, and synthetic techniques, we iden-
tify a suite of dimeric compounds (C15–19H24–32O5–11) formed from
concerted O3 and OH oxidation of β-pinene (i.e., accretion of O3- and
OH-derived products/intermediates). These dimers account for an
appreciable fraction (5.9–25.4%) of the β-pinene SOA mass and
are designated as extremely low-volatility organic compounds. Cer-
tain dimers, characterized as covalent dimer esters, are conclusively
shown to form through heterogeneous chemistry, while evidence
of dimer production via gas-phase reactions is also presented. The
formation of dimers through synergistic O3 + OH oxidation repre-
sents a potentially significant, heretofore-unidentified source of
low-volatility monoterpene SOA. This reactivity also suggests that
the current treatment of SOA formation as a sum of products orig-
inating from the isolated oxidation of individual precursors fails to
accurately reflect the complexity of oxidation pathways at play in
the real atmosphere. Accounting for the role of synergistic oxidation
in ambient SOA formation could help to resolve the discrepancy
between the measured atmospheric burden of SOA and that pre-
dicted by regional air quality and global climate models.
In this work, we investigate the formation, identity, and
abundance of molecular products in SOA derived from the O3-
and OH-initiated oxidation of β-pinene, the second-most-abun-
dant monoterpene emitted to the atmosphere (global emissions
estimated at 19 Tg y−1) (28). Through detailed chromatographic
and mass spectrometric analysis, coupled with 13C isotopic la-
beling and OH/SCI scavenging, we identify a reactive pathway to
extremely low-volatility dimeric compounds in SOA formed from
monoterpene ozonolysis involving reaction of O3-derived prod-
ucts/intermediates with those generated from oxidation by OH
produced in situ via vinyl hydroperoxide (VHP) decomposition.
We present evidence for formation of these dimers via both gas-
and particle-phase processes, underscoring the complexity of
atmospheric accretion chemistry. In establishing that O3 and OH
can act in concert to form nontrivial yields of dimeric SOA
constituents, we highlight the potential significance of synergistic
oxidation in ambient aerosol formation.
Results and Discussion
Dimers in β-Pinene SOA. β-Pinene ozonolysis and photooxidation
experiments were carried out in the Caltech dual 24 m3 Teflon
Environmental Chambers (CTEC) (Materials and Methods). A
Significance
secondary organic aerosol synergistic oxidation atmospheric accretion
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chemistry dimer formation monoterpenes
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Secondary organic aerosol (SOA) is ubiquitous in the atmosphere
and plays a pivotal role in climate, air quality, and health.
Monoterpenes, emitted in large quantities from forested re-
gions, are a dominant source of SOA globally, with dimers
having been identified as key contributors to particle formation
and growth. Here, we establish the role of concerted oxidation
by O3 and OH as a significant route to dimer formation in SOA
generated from β-pinene, the second-most-abundant mono-
terpene emitted to the atmosphere. Production of this class of
dimers is found to occur through both gas- and particle-phase
processes. Dimer formation via synergistic O3 + OH oxidation
could represent an appreciable source of “missing” SOA not in-
cluded in current atmospheric models.
he oxidation of monoterpenes (C10H16) represents a sub-
stantial and well-established source of atmospheric second-
T
ary organic aerosol (SOA) (1, 2), which constitutes a dominant
mass fraction (15–80%) of fine particulate matter (PM2.5) (3)
and exerts large but uncertain effects on Earth’s radiative bal-
ance (4) as well as adverse impacts on regional air quality and
human health (5, 6). High-molecular-weight, low-volatility di-
meric compounds have been identified as significant components
of both ambient (7–11) and laboratory-derived (12–22) mono-
terpene SOA, and have been implicated as key players in new
particle formation and growth (9–14, 23–26), particle viscosity
(27), and cloud condensation nuclei (CCN) activity (8, 24, 26).
Accumulating studies of α-pinene SOA indicate that a vast
majority of these dimers are formed only through O3- and not
OH-initiated oxidation, despite the apparent monomeric build-
ing blocks being present in both oxidative systems (11–13).
Particle-phase reactions of closed-shell monomers [e.g., aldol
addition/condensation (15–18), (peroxy)hemiacetal/acetal for-
mation (16, 21, 22), esterification (7, 18–21), and gem-diol for-
mation (16, 17)] and gas-phase reactions involving early-stage
oxidation products and/or reactive intermediates [e.g., stabilized
Criegee intermediates (SCIs), carboxylic acids, and organic
peroxy radicals (RO2) (9–14, 23–26)] have been advanced as
possible dimer formation pathways. However, the mechanisms
Author contributions: C.M.K. designed research; C.M.K., Y.H., and R.Z. performed re-
search; J.C.H. and B.M.S. contributed new reagents/analytic tools; C.M.K., Y.H., R.Z., and
N.F.D. analyzed data; and C.M.K. and J.H.S. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission. J.A.d.G. is a guest editor invited by the
Editorial Board.
Published under the PNAS license.
1To whom correspondence should be addressed. Email: seinfeld@caltech.edu.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.
1073/pnas.1804671115/-/DCSupplemental.
www.pnas.org/cgi/doi/10.1073/pnas.1804671115
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357
steady-state β-pinene ozonolysis experiments were conducted in
the Caltech Photooxidation Flow Tube (CPOT) in the presence
and absence of cyclohexane as an OH scavenger (Materials and
Methods). UPLC/(−)ESI-Q-TOF-MS was employed to measure
the molecular composition of SOA samples collected on Teflon
filters and extracted into H2O (Materials and Methods). A group
of 23 dimeric compounds, also present in the CTEC experi-
ments, was identified whose formation was significantly inhibited
(>65%) by introduction of the OH scavenger; four of the dimers
are major peaks in the BPI chromatogram (Fig. 2). These di-
mers, with molecular formulas C15–19H24–32O5–11 and O:C ratios
ranging from 0.26 to 0.61, exhibit saturation mass concentrations
(C*) < 3 × 10−4 μg m−3 and are designated as extremely low-
volatility organic compounds (ELVOC) (Table 1). Compounds
with accurate masses/molecular formulas corresponding to 20 of the
23 identified dimers have been measured in recent monoterpene
SOA formation experiments, and 18 such compounds have been
observed in ambient SOA samples from forested regions dominated
by monoterpene emissions (SI Appendix, Table S2). The clear
conclusion from these experiments is that formation for this par-
ticular collection of dimeric species depends on both O3 and OH
oxidation, in either a concurrent or sequential manner.
(C17H26O8)
185
-pinene + O3
100
80
60
40
20
0
185
389
(C9H14O4)
(C9H14O4)
(C18H30O9)
171
369
(C8H12O4)
199
313
(C16H26O6)
249
(C10H18O5S)
339
(C18H28O6)
(C19H30O7)
329
(C16H26O7)
(C10H16O4)
215
337
(C9H12O6)
377
(C19H30O5)
311
(C16H24O6)
357
(C16H26O10
)
205
(C18H30O7)
(C8H14O6)
3.50
4.00
4.50
5.00
5.50
6.00
6.50
7.00
185
(C9H14O4)
-pinene + OH
100
80
60
40
20
0
249
(C10H18O5S)
171
(C8H12O4) 199
(C10H16O4)
185
217
(C9H14O4)
(C9H14O6)
3.50
4.00
4.50
5.00
5.50
6.00
6.50
7.00
Time (min)
Fig. 1. UPLC/(−)ESI-Q-TOF-MS BPI chromatograms of SOA produced from the
O3- and OH-initiated oxidation of β-pinene after ∼4 h of reaction in the CTEC
(SI Appendix, Table S1, Exps. 1 and 2). Numbers correspond to nominal m/z
values of [M−H]− ions. Molecular formulas (CxHyOz) were assigned with mass
tolerances of <7 ppm and supported by associated 13C isotope distributions.
Chromatograms consist of monomeric (black) and dimeric (green) regions.
Dimers from Ozonolysis of 13C-β-Pinene. To determine the point in
the O3-initiated dimer formation pathway at which OH radical
chemistry occurs (e.g., oxidation of the precursor hydrocarbon,
gas-phase reaction with first-generation products, or heteroge-
neous aging of particle-bound dimers), the exocyclic double bond
of β-pinene was exploited. 13C-β-Pinene, labeled at the terminal
vinylic carbon, was synthesized from 13C-iodomethane and
nopinone via Wittig olefination (Scheme 1). Ozonolysis of 13C-
β-pinene was carried out in the CPOT in the absence of an OH
scavenger; SOA samples were collected on Teflon filters and
extracted into H2O. The m/z of 18 of the identified dimers (Table
1, types 1–3), including the four major BPI peaks, shifted by one
custom-modified particle-into-liquid sampler (PILS) integrated
with ultra-performance liquid chromatography/electrospray ion-
ization quadrupole time-of-flight mass spectrometry operated in
negative ion mode [UPLC/(−)ESI-Q-TOF-MS] was used to
characterize the time-resolved SOA molecular composition
(Materials and Methods) (29). Base peak ion (BPI) chromato-
grams of the O3- and OH-derived β-pinene SOA are shown in
Fig. 1. The chromatographic fingerprint of the O3 system displays
distinct monomeric and dimeric regions. Conversely, while the
identities of the monomers in both systems are similar, dimers
measureable by PILS + UPLC/(−)ESI-Q-TOF-MS were not
formed above the detection limit in the OH system, consistent
with previous LC/(−)ESI-MS studies on dimer formation in
α-pinene SOA (11–13). The monomers, on average, exhibit higher
O:C ratios than the dimers, suggesting that deoxygenation (e.g.,
condensation) is operative in dimer formation (18, 21). That the
dimers in the O3-derived SOA elute at retention times (RT) dis-
tinct from those of the monomers and are undetected in SOA
produced from OH oxidation demonstrates that they are authentic
β-pinene SOA products rather than ion-source artifacts (e.g.,
noncovalent adducts) formed during the (−)ESI process.
The absence of detectable dimers in pinene photooxidation sys-
tems has prompted SCIs, specifically reaction of SCIs with first-
generation carboxylic acids forming hydroperoxide esters (11, 12,
15), to be implicated as potential drivers of monoterpene accretion
chemistry. However, we recently demonstrated that dimers identi-
fied in α-pinene SOA using LC/ESI-MS methods do not contain
(hydro)peroxide moieties (30). Further, no clear reduction in dimer
abundance was observed for SOA generated from β-pinene ozo-
nolysis when either water vapor or formic acid was introduced as an
SCI scavenger (SI Appendix, Fig. S1). These findings, together with
modest SCI yields (15%) measured for α-pinene ozonolysis (31, 32),
experimental and theoretical studies showing thermal unimolecular
decay to be a dominant SCI loss process (32–34), and reported
increases in dimer concentrations in α-pinene SOA with increasing
relative humidity (RH) (11–13) despite the probable role of water
vapor as an SCI scavenger, call into question the importance of SCI
chemistry in monoterpene dimer production. The lack of peroxide
dimers in α-pinene SOA also implies that gas-phase dimers formed
via RO2 self/cross-reactions (RO2 + RO2 → ROOR + O2) (23–26)
do not retain their peroxide character (i.e., undergo chemical
transformation/decomposition) following condensation to the par-
ticle phase, as previously suggested (13, 20, 23).
mass unit on formation from 13C-β-pinene, indicative of 13C in-
corporation, while their RT remained unchanged. Recalling that
reaction of 13C-β-pinene with O3 will cleave the 13C label
whereas on reaction with OH the label will be retained, the one-
unit mass shift suggests that formation of the 18 dimers occurs
via reaction, in either the gas or particle phase, of O3-derived
products/intermediates with products/intermediates generated
from oxidation of β-pinene by OH produced via VHP de-
composition. For those dimers that did not undergo a mass shift
(Table 1, type 4), the observed dependence of their formation on
OH can be rationalized, in addition to the scenarios above, in
terms of a photooxidative pathway in which the labeled carbon is
100
0
Control
-25
-50
OH Scavenger
-75
80
60
40
20
0
-100
369
(C19H30O7)
337
389
(C19H30O5)
(C18H30O9)
357
(C18H30O7)
3
4
5
6
7
Time (min)
Fig. 2. UPLC/(−)ESI-Q-TOF-MS BPI chromatograms of SOA produced from
the O3-initiated oxidation of β-pinene in the CPOT in the presence and ab-
sence of cyclohexane as an OH scavenger (SI Appendix, Table S1, Exps. 7 and
8). Numbers correspond to nominal m/z values of [M−H]− ions; molecular
formulas are given in parentheses. (Inset ) Dimers that exhibited a significant
decrease in abundance due to cyclohexane addition (Table 1), reported as a
percent change relative to the control experiment and precise to <10%.
Data for Control and OH Scavenger experiments are normalized to the total
organic carbon content of the corresponding SOA samples (SI Appendix,
S3.4) and are reported as averages of replicates (n = 3).
Although dimers in pinene SOA seem to form only through
O3- and not OH-initiated oxidation, scavenging of OH radicals
produced as a byproduct of the hot Criegee VHP channel during
α-pinene ozonolysis has been found to suppress the formation of
certain dimeric species (11). To further explore this effect,
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Table 1. Dimers identified in SOA produced from the O3-initiated oxidation of β-pinene that exhibited a significant decrease in
abundance (>65%) due to OH scavenging
Dimer
type
Observed
m/z (−)
RT,
min
Molecular
formula
Error,
ppm
Exchangeable
hydrogens
SOA mass
†
O:C
OSC
logC*,‡ μg m−3
fraction,§,{
%
1
323.1860
355.1754
419.1525
373.1851
405.1764
351.1828
337.2027
369.1916
341.1960
357.1919
373.1851
389.1814
353.1961
315.1448
347.1346
375.1653
417.1769
403.1963
345.1549
359.1714
339.1822
355.1754
371.1707
6.86
6.24
4.70
5.99
4.49
5.73
7.16
6.48
6.87
6.91
5.70
6.18
6.15
5.83
5.19
6.24
6.76
6.28
5.36
5.89
6.34
5.27
5.46
C18H28O5
0.6
−0.8
−6.7
−2.9
0.7
5.7
3.6
0.8
−1.2
1.7
−2.9
0.8
−0.8
1.3
1.2
−0.5
1.9
−1.2
0.1
2.2
4.1
−0.8
0.3
0.28
0.39
0.61
0.44
0.56
0.32
0.26
0.37
0.33
0.39
0.44
0.50
0.32
0.47
0.60
0.53
0.53
0.48
0.50
0.47
0.33
0.39
0.44
−1.00
−0.78
−0.33
−0.78
−0.56
−0.84
−1.05
−0.84
−1.00
−0.89
−0.78
−0.67
−0.95
−0.67
−0.40
−0.59
−0.53
−0.74
−0.63
−0.71
−0.89
−0.78
−0.67
−3.9
−7.6
−15.2
−9.4
−13.2
−6.1
−4.3
−7.9
−5.7
−7.6
−9.4
−11.3
−6.1
−6.6
−10.5
−11.0
−13.5
−11.6
−8.8
−9.1
−5.7
−7.6
−9.4
2
4
4
4
5
2
2
4
2
3
3
3
3
3
3
3
2
3
2
3
2
5
3
0.04−0.17
0.11−0.36
0.02−0.16
0.07−0.23
0.06−0.62
0.10−0.41
0.46−2.01
2.23−9.24
0.06−0.24
0.39−1.60
0.14−0.47
1.88−7.81
0.14−0.60
0.02−0.10
0.03−0.25
0.02−0.24
0.06−0.62
0.07−0.24
0.05−0.50
0.32−1.07
0.46−1.93
0.51−1.72
0.30−1.22
C
18H28O7
C
C
18H28O11
C
18H30O8
18H30O10
C
C
19H28O6
19H30O5
19H30O7
18H30O6
18H30O7
18H30O8
18H30O9
19H30O6
15H24O7
15H24O9
17H28O9
C
C
C
C
C
C
C
C
C
2
3
4
C19H30O10
C
C
C
C
C
C
19H32O9
16H26O8
17H28O8
18H28O6
18H28O7
18H28O8
Dimers are grouped into four types based on structure and formation mechanism (see main text).
†Average carbon oxidation state (OSC = 2 O:C − H:C).
‡Saturation mass concentration (C*). Estimated using empirical model developed by Donahue et al. (35).
§Calculated for β-pinene SOA produced from O3-initiated oxidation in the CTEC (SI Appendix, Table S1, Exp. 1) after ∼4 h of reaction.
{Upper and lower bounds represent mass fraction estimates derived from experimental and computational approaches, respectively. Uncertainties in
experimental and computational approaches are estimated to be 23% (relative) and a factor of 3, respectively. Details are provided in SI Appendix, S7.
eliminated following OH addition to β-pinene (36, 37) but prior
in dimer formation: cis-pinic acid (C9H14O4; Fig. 3A), cis-norpinic
acid (C8H12O4; Fig. 3B), and diaterpenylic acid (C8H14O5; Fig.
to reaction with the O3-derived species.
The steady-state CPOT experiments enabled collection of suf- 3C) (7, 39) (SI Appendix, S6.1). Conversely, the OH-derived mo-
nomeric units are characterized by the ionic [M−H]− formulas
ficient quantities of SOA mass for detailed structural analysis via
[C10H15,17O2–7]−, indicative of OH addition to β-pinene (C10H16)
collision-induced dissociation (CID) (SI Appendix, S3.5). MS/MS
spectra of the 13C-labeled dimers and their 12C isotopologues
followed by varying degrees of O2 incorporation, isomerization,
revealed distinct OH-derived (13C-mass-shifted) and O3-derived
and bimolecular radical chemistry (36, 37). The daughter ions of
−
(unshifted) fragmentation patterns (SI Appendix, Table S3). A
group of eight dimeric compounds (Table 1, type 1) was identified
with fragmentation patterns and relative peak intensities charac-
teristic of covalent dimer esters, which have been reported to be
significant components of monoterpene SOA (7, 10–13, 18–21).
Specifically, (i) the elemental composition of the dimers is given
by condensation of the O3- and OH-derived monomeric product
the dicarboxylic acid and [C10H15,17O2–7
]
monomeric units are
rationalized by successive neutral losses of H2O (18 Da), CO2
(44 Da), CH2O (30 Da), and C3H6O (58 Da), all of which are
established CID pathways (40–42). Notably, the loss of 13CH2O
(31 Da) from the [C10H15,17O2–7]− fragment is observed in the MS/MS
spectrum of almost every 13C-labeled dimer ester (Fig. 3 and SI
Appendix, Table S3), consistent with the expected major addition of
OH to β-pinene (83% of total OH reactivity) at the terminal vinylic
carbon (36). Moreover, while the three dicarboxylic acids implicated
as O3-derived precursors were identified as major products in
β-pinene SOA formed from ozonolysis (SI Appendix, S6.1), SOA
products corresponding to the OH-derived monomeric building
blocks (i.e., C10H16O3,7 and C10H18O2,4,6) were not detected.
To further constrain the structures of the dimer esters, β-pinene
SOA filters were extracted into D2O and analyzed via UPLC/
(−)ESI-Q-TOF-MS using D2O as the polar eluent (SI Appendix,
ions (M1 + M2 − MH O = MD) (7, 16), and (ii) assuming that the
2
principal fragmentation occurs at the ester linkage, producing
carboxyl and alkoxy fragment ions through neutral loss of the al-
cohol and dehydrated acid moieties, respectively, the carboxyl
fragments and associated daughter ions are more intense than the
alkoxy fragments and their daughter ions (Fig. 3) (20, 21, 38), in
line with conventional charge accommodation behavior in (−)ESI-MS.
For the remaining 10 dimers (Table 1, types 2 and 3), the O3- and
OH-derived fragmentation patterns were not indicative of known
accretion chemistry (e.g., formal addition or condensation), and in
most instances a reasonable OH-derived (13C-mass-shifted)
monomeric building block either could not be identified or
failed to account for the observed OH-derived daughter ions (SI
Appendix, Table S3). The structures of these dimers were not
investigated further.
O
13C
Ph3P
n-BuLi
I
PPh3
PPh3
13CH3I
13CH3
13CH3
Based on comparison with MS/MS data for commercial stan-
dards and/or previously published MS/MS spectra, the O3-
derived monomeric building blocks of the dimer esters are
attributed to one of three dicarboxylic acids, each a well-
characterized pinene oxidation product that has been implicated
benzene,
RT, 72 h
THF, reflux, 18 h
30 min
Scheme 1. Synthesis of 13C-β-pinene via Wittig olefination (SI Ap-
pendix, S5).
Kenseth et al.
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100
50
0
100
50
0
particle-phase abundances of the dimer esters and their pre-
sumed dicarboxylic acid building blocks points toward a mech-
anism of heterogeneous formation, wherein the semivolatile
dicarboxylic acids undergo traditional equilibrium gas-particle
partitioning (43) with subsequent reactive uptake of the gas-
phase, OH-derived monomers/intermediates on collision with
particle surfaces to form ELVOC dimers. A heterogeneous
rather than purely particle-phase mechanism is supported by the
absence of monomers corresponding to the OH-derived dimer
ester precursors in SOA generated from β-pinene ozonolysis.
Such heterogeneous/multiphase accretion processes, leading to
the production of high-molecular-weight oligomeric products,
have recently been shown to contribute significantly to SOA
formation in both biogenic and anthropogenic systems (44, 45).
The growth dynamics of the dimer esters can be contrasted
against those shown in Fig. 4D for another group of dimers
(Table 1, type 2) also produced from coupled O3 and OH oxi-
dation. These dimers achieve essentially their maximum, steady-
state SOA concentrations after ∼2 h of reaction, whereas within
the same time frame the dimer esters reach only ∼60% of their
highest measured particle-phase abundances. Prompt formation
and rapid particle-phase growth of the dimers in Fig. 4D, at rates
faster than those observed for the monomers (Fig. 4 A–C), in-
dicate production via gas-phase reactions of first-generation
oxidation products/intermediates followed by irreversible con-
densation of the resulting ELVOC dimers onto existing aerosol
surfaces. As the formation of these dimers was not significantly
inhibited by introduction of SCI scavengers (SI Appendix, Fig.
S1), they likely originate from closed-shell/radical species unique
to the hot Criegee VHP channel. Although the identified dimers
were found not to contain (hydro)peroxide functionalities (SI
Appendix, S3.7), production of the dimers in Fig. 4D may be
explained by gas-phase RO2 self/cross-reactions (23–26, 46) with
subsequent particle-phase decomposition leading to nonperoxide
species (13, 20). Fast formation of particle-bound dimers during
monoterpene ozonolysis has been reported in several recent
studies (10–13, 20) and, together with their estimated low vola-
tility, has been advanced to explain nucleation and initial growth
of monoterpene SOA, both in laboratory studies without seed
aerosol and in regions, such as the boreal forest, dominated by
biogenic emissions (47).
m/z 337.2027
[M H]
A1
A3
A2
C19H30O5
RT = 7.16 min
3
4
5
6
7
8
9
100 200 300 400 500
Time (min)
m/z
100
185
141
(C8H13O2)
50
40
30
20
10
0
(C9H13O4)
167
(C9H11O3)
123
169*
(C8H11O)
139
71
(C3H3O2)
113*
99
151*
(C10H17O2)
(C9H15O)
(C6H9O2)
(C5H7O2)
(C10H15O)
60
80
100
120
140
160
180
200
m/z
100
50
0
100
m/z 355.1754
RT = 6.24 min
[M H]
B1
B2
C18H28O7
50
0
3
4
5
6
7
8
9
100 200 300 400 500
Time (min)
m/z
100
171
B3
127
50
40
30
20
10
0
(C8H11O4)
171
(C7H11O2)
(C9H15O3)
95
183*
(C6H7O)
201*
71
(C3H3O2)
153
(C9H13O2)
99
113
135
(C10H15O3)
(C10H17O4)
(C5H7O2) (C6H9O2)
(C9H11O)
60
80
100
120
140
m/z
160
180
200
100
50
0
100
m/z 405.1764
RT = 4.49 min
[M H]
C1
C2
C18H30O10
50
0
3
4
5
6
7
8
9
100 200 300 400 500
Time (min)
m/z
100
60
171
203
C3
189
(C8H11O4)
(C9H15O5)
127
(C8H13O5)
95*
(C7H11O2)
141
40
20
0
(C6H7O)
157*
233*
(C8H13O2)
109
185
145 (C7H9O4)
215*
(C10H17O6)
113*
85
(C9H13O4)
(C7H9O)
(C7H13O3)
(C10H15O5)
(C6H9O2)
(C5H9O)
80
100
120
140
160
m/z
180
200
220
240
Fig. 3. Representative (1) extracted ion chromatograms, (2) MS spectra, and
(3) MS/MS spectra, measured by UPLC/(−)ESI-Q-TOF-MS, of dimer esters in
SOA produced from the O3-initiated oxidation of β-pinene (Table 1, type 1)
presumed to form from (A) cis-pinic acid (C9H14O4), (B) cis-norpinic acid
(C8H12O4), and (C) diaterpenylic acid (C8H14O5). MS/MS peaks are colored to
denote O3-derived (red) and OH-derived (blue) fragment ions. Numbers in
MS/MS spectra correspond to nominal m/z values of [M−H]− fragment ions;
ionic formulas [CxHyOz]− are given in parentheses. *Indicates peaks that
underwent a one-unit mass shift on formation from 13C-β-pinene.
1.0
0.8
0.6
0.4
0.2
0
1.0
0.8
0.6
0.4
0.2
0
S3.6). This approach facilitated deuterium substitution (H → D)
of labile hydrogens (e.g., −OH, −OOH, and −COOH) in the SOA
constituents while preserving chromatographic separation, enabling
quantification of the number of exchangeable hydrogens in the
identified dimer molecules based on systematic shifts in m/z due to
H/D exchange (Table 1 and SI Appendix, Fig. S4). Additionally,
using our recently developed iodometry-assisted LC/ESI-MS assay
for the molecular-level identification of organic peroxides (30), it
was established that the 23 identified dimers do not contain hy-
droperoxide (ROOH) or organic peroxide (ROOR) functionalities
(SI Appendix, S3.7). On the basis of these supporting experiments,
the accurate mass (MS) and fragmentation (MS/MS) data, the
inferred dicarboxylic acid structures of the O3-derived monomers,
and the prevailing mechanism of β-pinene photooxidation (36,
37), tentative molecular structures and fragmentation path-
ways for the dimer esters are proposed in SI Appendix, Table
S4 and Fig. S10, respectively (SI Appendix, S6.1).
A
C
185 (C9H14O4)
351 (C19H28O6)
337 (C19H30O5)
369 (C19H30O7)
189 (C8H14O5)
373 (C18H30O8)
405 (C18H30O10
)
419 (C18H28O11
)
0
1
2
3
4
0
1
2
3 4
1.0
0.8
0.6
0.4
0.2
0
1.0
0.8
0.6
0.4
0.2
0
B
D
341 (C18H30O6)
357 (C18H30O7)
373 (C18H30O8)
389 (C18H30O9)
353 (C19H30O6)
171 (C8H12O4)
323 (C18H28O5)
355 (C18H28O7)
0
1
2
3
4
0
1
2
3
4
Reaction Time (h)
Fig. 4. Temporal profiles of molecular products in SOA produced from the
O3-initiated oxidation of β-pinene in the CTEC (SI Appendix, Table S1, Exp. 1),
measured by PILS
+ UPLC/(−)ESI-Q-TOF-MS. Profiles are plotted as the
particle-phase abundance of each species, normalized to the highest abun-
dance observed over 4 h of reaction, as a function of reaction time. Discrete
data points (5-min resolution) are presented as lines to aid the eye. Numbers
correspond to nominal m/z values of [M−H]− ions; molecular formulas are
given in parentheses. (A–C) Profiles of dimer esters (Table 1, type 1) and
dicarboxylic acids implicated as precursors based on MS/MS analysis: (A) cis-
pinic acid (m/z 185; C9H14O4; RT 4.33), (B) cis-norpinic acid (m/z 171; C8H12O4;
RT 4.06), and (C) diaterpenylic acid (m/z 189; C8H14O5; RT 3.32). (D) Profiles of
dimers (Table 1, type 2) characterized by almost immediate formation and
rapid particle-phase growth. Dashed lines (2 h, 0.6 abundance) are drawn to
aid in comparison between growth profiles in A–C and D.
Mechanisms of Dimer Formation. To assess the relative contribu-
tions of gas- vs. particle-phase chemistry to the formation of
dimers derived from concerted O3 and OH oxidation, the tem-
poral evolution of individual products in β-pinene SOA formed
from ozonolysis in the CTEC was examined. Shown in Fig. 4 A–C
are the particle-phase growth profiles of the eight dimer esters
(Table 1, type 1) overlaid upon those corresponding to the di-
carboxylic acid monomers (cis-pinic acid, cis-norpinic acid, and
diaterpenylic acid) that were implicated as precursors based on
detailed MS/MS analysis. The strong correlation between the
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As a means of evaluating the hypothesized mechanism of
heterogeneous dimer ester formation, β-pinene photooxidation
experiments were conducted in the CTEC in the presence of
seed aerosol composed of varying mass ratios of ammonium
sulfate [(NH4)2SO4] and cis-pinic acid, the only commercially
available pinene-derived dicarboxylic acid (Materials and Meth-
ods). Consistent with the idea that introducing the O3-derived
dimer ester precursors into the particle phase of the OH system
should promote otherwise negligible heterogeneous dimeriza-
tion, the presence of cis-pinic acid in the (NH4)2SO4 seed
resulted in significant formation, proportional to the mass ratio
of cis-pinic acid to (NH4)2SO4, of the major dimer ester at m/z
337 (C19H30O5) (Fig. 5), corroborating the indirect MS/MS (Fig.
3A) and kinetic (Fig. 4A) evidence for production from particle-
bound cis-pinic acid. That the dimer ester at m/z 337 was not
observed in OH-derived β-pinene SOA, with pure (NH4)2SO4
seed, collected on a Teflon filter coated with cis-pinic acid ex-
cludes the possibility of dimer formation via accretion of con-
densed monomers, either on the filter or during the extraction
process (SI Appendix, Fig. S11). Although the details of the
mechanism forming the ester linkage remain unclear, the observed
increase in dimer ester abundance at elevated RH (SI Appendix,
Fig. S1) argues against production via conventional esterification
(i.e., carboxylic acid + alcohol). Overall, these findings conclu-
sively demonstrate the role of heterogeneous accretion chemistry
in monoterpene SOA formation (SI Appendix, S6.2).
Surprisingly, these experiments also imply that a simple mass
limitation of monomeric precursors in the particle phase is re-
sponsible for the absence of dimer esters in β-pinene SOA
formed from OH-initiated oxidation. However, increased mass
fractions of cis-pinic acid in SOA from α-pinene ozonolysis,
relative to photooxidation, have been observed in a number of
previous studies and have been invoked as a possible explanation
for the lack of dimers containing cis-pinic acid in α-pinene
photooxidation experiments (7, 12, 48). Indeed, in this study
cis-pinic acid was found to comprise a much higher fraction of
β-pinene SOA mass when formed from O3- (13.4 3.1%) rather
than OH-initiated (1.7 0.4%) oxidation (see m/z 185; RT 4.33
in Fig. 1), while significant but less pronounced disparities in
SOA mass fraction between ozonolysis and photooxidation ex-
oxidative environment in which the OH-derived RO2 isomeri-
zation and bimolecular reaction channels are vastly different
from those operative in the comparatively low-HO2 (∼107
molecules cm−3) ozonolysis system (SI Appendix, S4) that form
the [C10H17O4]− and [C10H15O3]− building blocks of the dimer
esters at m/z 369 and 351, respectively (SI Appendix, Table S3).
That RO2 + HO2 rate coefficients are typically order(s) of
magnitude larger than those for RO2 + RO2 reactions (49)
further compounds the disparity between these two RO2 re-
gimes. Conversely, for the dimer ester at m/z 337, although the
structure of the [C10H17O2]− building block is tentative (SI Ap-
pendix, Fig. S10 and Table S3), theoretical studies indicate that
this monomer is able to form under both ozonolysis and pho-
tooxidation conditions from the C10H17O3 hydroxy peroxy radi-
cal, produced from OH and subsequent O2 addition to β-pinene,
either via traditional RO2 + RO2 chemistry or through an OH-
recycling pathway in the reaction with HO2 (36, 50).
Atmospheric Implications. The production of dimeric compounds
(Table 1, types 1–3) through concerted O3 and OH oxidation
accounts for an appreciable, heretofore-unidentified fraction (5.9–
25.4%) of the total mass of β-pinene SOA derived from ozonolysis
under the conditions employed in this work (Table 1). Although
specifically revealed in SOA formation from β-pinene, this re-
active pathway represents a potentially significant source of high-
molecular-weight ELVOC to the atmosphere that is expected to
be broadly applicable to other monoterpenes (SI Appendix, S8).
Through detailed molecular composition and kinetic analysis,
certain dimers are definitively shown to form through heteroge-
neous processes, while indirect evidence for dimer production via
gas-phase routes is also presented. The importance of both gas-
and particle-phase reactions to the formation of dimeric SOA
constituents demonstrated in the current work underscores the
complexity of atmospheric accretion chemistry, as well as the
significant shortcomings in scientific understanding that preclude
adequate characterization of its impact.
The O3 + OH reactivity elucidated in β-pinene SOA also
highlights the importance of understanding and accounting for the
likely role of oxidative synergism in ambient aerosol formation,
where SOA precursors are susceptible to concurrent oxidation by
O3 and OH. At present, SOA formation in atmospheric models is
treated as an additive combination of products originating from the
isolated oxidation of individual precursors (e.g., α-pinene + O3 or
isoprene + OH). In establishing that oxidants can act in concert to
produce extremely low-volatility dimers in nontrivial yields, how-
ever, this study suggests that the tendency of current atmospheric
models to systematically underpredict ambient SOA mass (1) may
be due in part to the fact that discrete SOA formation mechanisms,
parameterized by laboratory experiments that typically feature only
one oxidant and a single SOA precursor, do not accurately reflect
the intricate oxidation pathways at play in the real atmosphere.
Revising the chemistry of monoterpene SOA formation in regional
air quality and global climate models to account for the role of
synergistic oxidation could help to resolve the discrepancy between
model predictions and ambient measurements. However, param-
eterization of this reactivity requires additional work.
periments were observed for cis-norpinic acid (1.4
0.3% vs.
0.24 0.06%) and diaterpenylic acid (0.71 0.16% vs. 0.28
0.06%) (SI Appendix, S7).
Although also suggested to arise from heterogeneous reaction
of particle-phase cis-pinic acid (Fig. 4A), neither the prominent
dimer ester at m/z 369 (C19H30O7) nor the minor dimer ester at
m/z 351 (C19H28O6) was detected in the photooxidation experi-
ments carried out with cis-pinic acid and (NH4)2SO4 seed. One
plausible explanation for the absence of these dimers is that the
high HO2 concentrations (∼1010 molecules cm−3) inherent in the
use of H2O2 as an OH precursor (SI Appendix, S1.1) produce an
185
(C9H14O4)
100
80
60
40
20
0
(NH4)2SO4 + Cis-Pinic Acid (1:1)
(NH4)2SO4 + Cis-Pinic Acid (2:1)
(NH4)2SO4
Materials and Methods
337
(C19H30O5)
β-Pinene ozonolysis and photooxidation experiments were carried out in the
CTEC and CPOT at ambient temperature (∼295 K) and atmospheric pressure
(∼1 atm), under dry conditions (<10% RH), in the presence of (NH4)2SO4 seed
aerosol, and at mixing ratios of NOx typical of the pristine atmosphere
(<0.5 ppb). CTEC experiments were designed to mimic oxidation in ambient
air, while those in the CPOT were used to elucidate dimer structures and
formation mechanisms. For CTEC experiments with ∼4-h duration, β-pinene
(∼120 ppb) was oxidized by O3 (∼200 ppb) in the absence of an OH scavenger
or OH (∼2 × 106 molecules cm−3). Select photooxidation experiments were
performed with mixed (NH4)2SO4 and cis-pinic acid seed. For steady-state CPOT
experiments, oxidation of β-pinene (∼150 ppb) by O3 (∼1 ppm) proceeded in
the presence and absence of scavengers for both OH (cyclohexane) and SCIs
(water vapor and formic acid). In certain CPOT experiments, 13C-β-pinene,
synthesized from 13C-iodomethane and nopinone via Wittig olefination
3
4
5
6
7
Time (min)
Fig. 5. UPLC/(−)ESI-Q-TOF-MS BPI chromatograms of SOA produced from
the OH-initiated oxidation of β-pinene after ∼4 h of reaction in the CTEC (SI
Appendix, Table S1, Exps. 2–4). Experiments were conducted in the presence
of seed aerosol at similar mass loadings but different mass ratios (see leg-
end) of (NH4)2SO4 and cis-pinic acid (C9H14O4). Numbers correspond to
nominal m/z values of [M−H]− ions; molecular formulas are given in pa-
rentheses. Shaded peaks correspond to cis-pinic acid and the dimer ester at
m/z 337 (C19H30O5). Chromatograms for each experiment are normalized to
the area of the peak at RT 3.93 (m/z 171; C8H12O4).
Kenseth et al.
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(Scheme 1), was used. Experimental conditions are reported in SI Appendix,
Table S1 and described in detail in SI Appendix, S1.
β-pinene SOA (SI Appendix, S3.7). The Master Chemical Mechanism version 3.2
(MCMv3.2) (51) was used to simulate the concentration profiles of OH, HO2,
and RO2 during β-pinene ozonolysis in the CTEC, as well as the fractions of
β-pinene that react with O3 vs. OH (SI Appendix, S4). Mass fractions of indi-
vidual organic compounds in β-pinene SOA, along with associated uncer-
tainties, were calculated as described in SI Appendix, S7.
β-Pinene mixing ratios were quantified with
a gas chromatograph
equipped with flame ionization detector (GC/FID) (SI Appendix, S2).
a
Aerosol size distributions and number concentrations were measured with a
custom-built scanning mobility particle sizer (SMPS) (SI Appendix, S3.1).
“Bulk” aerosol chemical composition was quantified with an Aerodyne high-
resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) (SI Ap-
pendix, S3.2). The molecular composition of β-pinene SOA collected by PILS
during CTEC experiments (SI Appendix, S3.3) and on Teflon filters during
steady-state CPOT and select CTEC experiments (SI Appendix, S3.4) was
characterized off-line by UPLC/(−)ESI-Q-TOF-MS (SI Appendix, S3.5). Certain
SOA filter samples were analyzed using D2O in place of H2O as the extraction
solvent/polar eluent (SI Appendix, S3.6). Iodometry was coupled to UPLC/
(−)ESI-Q-TOF-MS (30) to identify organic peroxides at the molecular level in
ACKNOWLEDGMENTS. We thank Xuan Zhang, John Crounse, and Paul
Wennberg for useful discussions. UPLC/(−)ESI-Q-TOF-MS was performed
in the Caltech Environmental Analysis Center. This work was supported
by National Science Foundation Grants AGS-1523500 and CHE-1508526.
R.Z. acknowledges support from a Natural Science and Engineering Re-
search Council of Canada Postdoctoral Fellowship. J.C.H. acknowledges
support from the Camille and Henry Dreyfus Postdoctoral Program in
Environmental Chemistry.
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