Acid-catalyzed reactions of epoxides for atmospheric nanoparticle growth

Article abstract of DOI:10.1021/ja508989a

Although new particle formation accounts for about 50% of the global aerosol production in the troposphere, the chemical species and mechanism responsible for the growth of freshly nucleated nanoparticles remain largely uncertain. Here we show large size

Full text of DOI:10.1021/ja508989a

Communication
pubs.acs.org/JACS
Acid-Catalyzed Reactions of Epoxides for Atmospheric Nanoparticle
Growth
Wen Xu,§ Mario Gomez-Hernandez, Song Guo, Jeremiah Secrest, Wilmarie Marrero-Ortiz,
Annie L. Zhang, and Renyi Zhang*
Department of Chemistry and Department of Atmospheric Sciences, Texas A&M University, College Station, Texas 77843, United
States
S
* Supporting Information
example, previous experimental studies have demonstrated that
ABSTRACT: Although new particle formation accounts
for about 50% of the global aerosol production in the
troposphere, the chemical species and mechanism
responsible for the growth of freshly nucleated nano-
particles remain largely uncertain. Here we show large size
growth when sulfuric acid nanoparticles of 4−20 nm are
exposed to epoxide vapors, dependent on the particle size
and relative humidity. Composition analysis of the
nanoparticles after epoxide exposure reveals the presence
of high molecular weight organosulfates and polymers,
indicating the occurrence of acid-catalyzed reactions of
epoxides. Our results suggest that epoxides play an
important role in the growth of atmospheric newly
nucleated nanoparticles, considering their large formation
yields from photochemical oxidation of biogenic volatile
organic compounds.
glyoxal and 2,4-hexadienal lead to particle growth for 10 nm
and larger sizes, but negligibly for sizes smaller than 4 nm;
amines are shown to contribute to nanoparticle growth down
to the size of 4 nm.4 The distinct growth patterns of the
different organic species on relative humidity (RH) and particle
size have been explained by the reaction mechanisms and the
Kelvin effect.4,5
Epoxides have been suggested to be produced with a large
yield from the oxidation of biogenic hydrocarbons (e.g.,
isoprene) and to contribute to the formation of secondary
organic aerosols (SOA).6 Isoprene and monoterpenes represent
the most abundant volatile organic compounds (VOCs)
emitted from the biosphere, with a global emission rate of
about 1000 Tg yr−1.7 Epoxides have been shown to be
produced from isoprene oxidation in large yields (up to 75%),
especially for isoprene epoxydiols.6c Also, metharcylic acid
epoxide has been suggested to be produced from methacry-
loylperoxynitrate oxidation (with a yield of 8−32%),6k likely
explaining SOA formation from isoprene under higher NOx
conditions.6b Recent studies have also indicated that the
epoxides derived from 2-methyl-3-buten-2-ol oxidation are
important for SOA and organosulfate formation.6d,m Epoxide
formation from monoterpenes, however, is still debatable.
While recent chamber experiments have shown organosulfate
formation from reactive uptake of gas-phase epoxides (α- and
β-pinene oxides) by acidic aerosols,6a the reactive uptake for α-
pinene oxides has been suggested not to be a major route to
formation of aerosols or organosulfates in the atmosphere,
since a higher particle acidity is required.6o
In the present study, we investigated the growth of sulfuric
acid-water nanoparticles from the heterogeneous reactions of
epoxides (see Supporting Information (SI)). The size-depend-
ent growth factors of 4−20 nm nanoparticles exposed to three
model epoxides (i.e., isoprene oxide, α-pinene oxide, and
butadiene diepoxides) at various RH and reactant concen-
trations were measured using a nanotandem differential
mobility analyzer (n-TDMA). Chemical compositions of the
sulfuric acid-water nanoparticles after epoxide exposure were
analyzed by using a thermo desorption ion drift chemical
ionization mass spectrometer (TD-ID-CIMS).8 In our experi-
ments, the particle acidity is regulated by RH, and the RH range
of 4−68% is relevant to the atmospheric conditions. It should
erosols are ubiquitous in the atmosphere and have many
Aconsequential effects, such as impairing visibility, modify-
ing the microphysical properties of clouds, altering the Earth’s
radiation balance, and affecting human health.1 Currently, the
direct and indirect radiative forcings of aerosols represent the
largest uncertainty in the projections of future climate. As a key
component of atmospheric particulate matter, nanoparticles are
frequently formed through gas-to-particle conversion under
diverse environmental conditions, including urban, coastal, and
forested areas.1b New particle formation in the atmosphere
involves two consecutive steps, i.e., nucleation to form critical
nucleus and growth of freshly nucleated particles to a larger
size. The rate of aerosol nucleation is limited by a free energy
barrier, which needs to be surmounted before the critical
nucleus is formed. In addition, growth of nanoparticles is
restricted by the Kelvin (curvature) barrier, because of
significantly elevated equilibrium vapor pressures over small
particles. Various inorganic and organic species have been
identified for their participation in aerosol nucleation,1b,2,3
including sulfuric acid, basic species, and organic acids. On the
other hand, the growth of newly formed nanoparticles is less
understood, because of the lack of knowledge in the chemical
identities and mechanisms responsible for overcoming the
Kelvin barrier.1b Several organic compounds have been
proposed to contribute to atmospheric nanoparticle growth
via particle-phase reactions, such as acid-catalyzed polymer-
ization/oligomeration, hydration, and acid−base reactions. For
Received: September 1, 2014
Published: October 22, 2014
© 2014 American Chemical Society
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be pointed out that in our present work model epoxides, which
may not exactly correspond to those produced from
atmospheric oxidation biogenic hydrocarbons,6 are employed.
However, the kinetic and mechanistic characteristics of
heterogeneous reactions of the model epoxides are likely
similar to those for atmospherically relevant compounds,
considering the similarity in the molecular functionality (i.e.,
the epoxide group).
Figure 1 displays an example of the size growth factor for
isoprene oxide at various particle mobility sizes and RH. The
Figure 2. (a,b) TD-ID-CIMS spectra of 20 nm sulfuric acid
nanoparticles after exposure to isoprene oxide at 4 and 32% RH,
respectively.
exposure to isoprene oxide, α-pinene oxide, and butadiene
diepoxides, respectively. The peak assignments for the
heterogeneous reactions of epoxides on sulfuric acid nano-
particles are summarized in Table S1. As illustrated in Scheme
1, for the reaction leading to polymerization from isoprene
Figure 1. Size growth factors, defined by the ratio of the particle size
measured after and before the epoxide exposure, of sulfuric acid
nanoparticles of various initial sizes exposed to isoprene oxide vapor of
6 × 1014 molecules cm−3 at 4% (black squares), 43% (red dots), and
68% RH (blue triangles).
Scheme 1. Heterogeneous Reaction Mechanism of Isoprene
Oxide on Sulfuric Acid Nanoparticles
measured growth factor of isoprene oxide is dependent on both
the RH and particle size: the measured growth factor decreases
with increasing RH for a given size but increases with increasing
particle size for a given RH. The growth factor ranges from 1.50
to 1.91 at 4% RH but is close to unity at 68% RH. For 43% RH,
the growth factors are 1.28 and 1.47 for the particle sizes of 4
and 20 nm, respectively. Similar dependence on RH and
particle size is observed for the other two epoxides (Table 1).
Table 1. Size Growth Factors of 4 and 20 nm Sulfuric Acid
Nanoparticles Exposed to Epoxide Vapors at 4% and 30%
a
RH
4 nm
20 nm
species
RH = 4%
RH = 30%
RH = 4%
RH = 30%
isoprene oxide
1.50(0.09)
1.36(0.01)
1.04(0.01)
1.23(0.02)
1.00(0.01)
1.02(0.01)
1.91(0.01)
1.31(0.02)
1.15(0.05)
1.500(0.09)
1.020(0.01)
1.020(0.02)
α-pinene oxide
butadiene
diepoxide
oxide, the α,β-unsaturated ketone formed through 1,2-
methanide shift polymerizes via the Michael addition in the
presence of sulfuric acid,9 and the resulting polymers are
detected by the TD-ID-CIMS at m/z of 169, 253, 337, and 421
for dimer, trimer, tetramer, and pentamer, respectively.
Organosulfates are not identified for isoprene oxide, since
their formation is suppressed by polymerization. In contrast,
the previous study by Lal et al.6h suggested that the
heterogeneous reaction of isoprene oxide in the presence of
acid involves protonation and formation of a carbocation, which
rearranges into an aldehyde. The reaction mechanism in
Schemes S1 and S2 for α-pinene oxide and butadiene
diepoxides is consistent with a previous study.6a Because of
the absence of the double bond in α-pinene oxide and
butadiene diepoxides, the α,β-unsaturated ketone formation
a
Number in the parentheses reflects the 2σ standard deviation. The
concentrations (in molecules cm−3) are 6 × 1014 for isoprene oxide, 5
× 1015 for α-pinene oxide, and 5 × 1016 for butadiene diepoxide.
As RH increases from 4% to 30%, the growth factors decrease
from 1.15 for butadiene diepoxides and 1.31 for α-pinene oxide
to close to unity. For 20 nm particles, the measured growth
factors at 4% RH are 1.15 and 1.31 for butadiene diepoxides
and α-pinene oxide, respectively. The increasing growth factor
with increasing particle size is clearly explained by the Kelvin
effect.
Figures 2, S2, and S3 depict the TD-ID-CIMS spectra of
nanoparticles collected on a platinum filament after the
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Communication
from both species is implausible, and only organosulfates and
dimer are detected at the m/z peaks of 269 and 305 for α-
pinene oxide and 203 and 209 for butadiene diepoxides,
respectively. Hence, our results reveal that, for isoprene oxide,
polymers represent the dominant features in the observed mass
spectra at different RH, but the formation of organosulfates is
noticeably absent. However, for α-pinene oxide and butadiene
diepoxides, formation of organosulfates is most prominent at
low RH, but formation of alcohols dominates at high RH. Since
polymers and organosulfates are less or nonvolatile,10 their
formation likely explains the measured large growth factors for
isoprene oxide measured at different RH and for α-pinene oxide
and butadiene diepoxides measured at low RH. In contrast,
alcohols formed from α-pinene oxide and butadiene diepoxides
evaporate into the gas phase, likely explaining the insignificant
growth factors at high RH. Hence, the differences in the
measured growth factors among the three epoxides can be
explained by their respective reaction mechanisms in the
particle phase. Interestingly, isoprene-derived epoxydiols have
been invoked to explain organosulfate formation in both
experimental and field studies,6 in contrast to isoprene oxide.
Our measured growth factors of sulfuric acid nanoparticles
for epoxides can be compared with those previously measured
for other organics, including glyoxal, methylglyoxal, 2,4-
hexadienal, and trimethylamine under comparable reactant
concentrations.4a For particles of 4 nm size, the growth factor is
only significant for epoxide (isoprene oxide and α-pinene
oxide) and trimethylamine, while the growth factor is close to
unity for all other organics. At 4% RH and for 4 nm particles,
the growth factors for isoprene oxide (1.50) and α-pinene oxide
(1.36) are much larger than that for trimethylamine (1.10). The
occurrence of the particle-phase reaction of trimethylamine on
4 nm particles has been explained by its first-order reaction
nature, since reactions of second or higher order are prohibited
by the Kelvin effect in the few nanometer size range. Similarly,
polymerization for isoprene oxide and organosulfate formation
for α-pinene oxide are likely of the first-order with respect to
the acid concentration,6f responsible for their occurrences at 4
nm particles. For 20 nm, the large growth factors for glyoxal are
comparable to those for isoprene oxide, except for the opposite
trend with RH. The particle-phase reaction for glyoxal involves
hydration reaction, followed by oligomerization, in contrast to
the acid-catalyzed polymerization for isoprene oxide.
We derive the size-dependent accommodation coefficient
(αr) on the basis of the measured growth factor (Figure S4).
The accommodation coefficient for the planar surface (α∞) and
characteristic length for Kelvin effect (dσ)11 are obtained by
nonlinear curve fitting (Table S2). The uptake coefficient of
isoprene oxide on the planar sulfuric acid surface at 43% RH is
estimated to be 1.2 × 10−3, which is in the range of the values
of 1 × 10−5 to 3 × 10−3 for 3 to 20 wt % H2SO4 solutions
reported by Wang et al.6i but smaller than the value of 1.7 ×
10−2 reported by Lal et al. on concentrated sulfuric acid (90 wt
% H2SO4) surfaces.6h A most recent experimental study of the
reactive uptake of isoprene-derived epoxydiols on submicron
particles has reported the uptake coefficients on ammonium
bisulfate (0.05) and ammonium sulfate (≤1 × 10−4), showing a
similar dependence on acidity as in our present work (i.e.,
increasing reactivity with increasing acidity).6p Also, that study
has suggested that the reactive uptake of isoprene-derived
epoxydiols on submicron particles may be self-liming, because
of formation of organic coating on the particle surfaces.6p Our
measured large size growths for the three epoxides on sulfuric
acid nanoparticles, however, do not appear to indicate hindered
heterogeneous reactions by organic formation in the particle
phase. Considering the possibility of volatile products, which
may evaporate from nanoparticles, the uptake coefficient
estimated in our present study likely corresponds to a low
limit, compared to those previously measured on bulk
solutions.6h Alcohols have been shown to contribute negligibly
to nanoparticle growth;4 at 30% RH and 298 K organosulfate
b
formation from methanol takes about 96 h, much longer than
the exposure time (ca. 3 s) in our experiment.
Our measurements show that the growth factor increases
with increasing concentrations of the epoxides (Figure S5).
Since epoxides are typically volatile and their partitioning is
highly suppressed because of the curvature effect,4a the rate-
liming step in the heterogeneous reactions of epoxides on
nanoparticles corresponds to mass accommodation.4a Using
our measured growth factors (i.e., for isoprene oxide at different
concentrations) and at 43% RH and assuming an ambient
concentration of epoxides (∼1 part per billion), we estimate the
growth rates of 0.11 and 1.10 nm h−1 for 4 and 20 nm particles,
respectively, close to those typically measured in the
atmosphere.12 Clearly, the contribution of epoxides to
atmospheric nanoparticle growth is dependent on the
abundance of the epoxide species, RH, and particle acidity
under ambient conditions.
In summary, we have measured the size growth factor of
sulfuric acid nanoparticles exposed to three model epoxides, i.e.,
isoprene oxide, α-pinene oxide, and butadiene diepoxides. The
measured growth factors of the epoxides decrease with
increasing RH, reflecting an acid-catalyzed reaction mechanism.
The positive correlation between the particle sizes and
measured size growth factors on sulfuric acid nanoparticles is
explained by the Kelvin effect. Chemical composition analysis
of sulfuric acid nanoparticles after epoxide exposure indicates
that both α-pinene oxide and butadiene diepoxides form
organosulfates, while isoprene oxide only forms polymers.
Organosulfates are formed only in low RH or high acidity
conditions, but isoprene-derived polymers are formed in both
low and high RH conditions by the acid-catalyzed reaction. The
organosulfates and polymers formed from the reactive uptake
of epoxides are sufficiently nonvolatile, helping to overcome the
Kelvin barrier and contributing to growth of nanoparticles. A
lower limit of 1.2 × 10−3 is obtained for the uptake coefficients
of isoprene oxide on planar sulfuric acid surface at 43% RH.
Our measurements show that the growth factors for isoprene
oxide on 4 nm nanoparticles are higher than those previously
4
a
measured for trimethylamine, glyoxal, and 2,4- hexadienal.4
a
Interestingly, several recent studies have suggested that
isoprene suppresses aerosol nucleation events in forested
areas.12 On the other hand, atmospheric measurements have
shown efficient new particle formation in urban locations (i.e.,
southeast US and Beijing, China) with abundant isoprene
presence.13 While the effect of the oxidation products from
isoprene on the nucleation free energy is yet to be elucidated,
our present results reveal that epoxides likely promote the
particle growth by overcoming the curvature barrier. Using our
measured growth factor at atmospherically relevant RH (i.e.,
43%), we estimate a growth rate of nanoparticles from epoxides
that is consistent with those from ambient measurements,14
indicating that epoxides may contribute significantly to growth
of freshly nucleated nanoparticles in the atmosphere. Given the
large yield of epoxides from the photochemical oxidation of
biogenic VOCs (e.g., isoprene) and their role in new particle
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formation, current atmospheric models without inclusion of the
epoxide pathway may considerably underestimate the climate
forcings of biogenic aerosols.
ASSOCIATED CONTENT
* Supporting Information
Experimental details, MS and TDMA data, and analysis. This
material is available free of charge via the Internet at http://
pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
renyi-zhang@tamu.edu
■
Present Address
§Aerodyne Research, Inc., 45 Manning Road, Billerica, MA
01821-3976, United States.
(7) Zhang, R. Y.; Wang, L.; Khalizov, A. F.; Zhao, J.; Zheng, J.;
McGraw, R. L.; Molina, L. T. Proc. Natl. Acad. Sci. U.S.A. 2009, 106,
17650−17654.
Notes
The authors declare no competing financial interest.
(8) Zhao, J.; Zhang, R.; Misawa, K.; Shibuya, K. J. Photochem.
Photobiol., A 2005, 176, 199−207.
(9) Morrison, R.; Boyd, R. Organic Chemistry; 6th ed.; Prentice Hall:
Upper Saddle River, NJ, 1992.
ACKNOWLEDGMENTS
■
This work was supported by Robert A. Welch Foundation
(Grant A-1417) and U.S. National Science Foundation (AGS-
0938352). The authors were grateful to A. Khalizov and L.
Wang for assistance with the experimental setup and helpful
discussions.
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