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Aerosol Formation from OH Oxidation of the Volatile Cyclic Methyl Siloxane (cVMS) Decamethylcyclopentasiloxane Yue Wu, and Murray V Johnston Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b00655 • Publication Date (Web): 23 Mar 2017 Downloaded from http://pubs.acs.org on March 29, 2017
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Aerosol Formation from OH Oxidation of the Volatile Cyclic Methyl Siloxane (cVMS)
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Decamethylcyclopentasiloxane
3
Yue Wu and Murray V. Johnston*
4
Department of Chemistry and Biochemistry
5
University of Delaware, Newark DE, USA 19716
6
*Corresponding author: Phone 302-831-8014, Fax 302-831-6336, Email
[email protected] 7 8
Abstract Aerosol
formation
from
OH oxidation
of decamethylcyclopentasiloxane (D5,
9
C10H30O5Si5), a cyclic volatile methyl siloxane (cVMS) found in consumer products, was studied
10
in a flow-through photo chamber with and without the presence of ammonium sulfate seed
11
aerosol. For the unseeded experiments, chemical characterization with high performance mass
12
spectrometry showed that the molecular composition changed substantially with aerosol mass
13
loading in the 1-12 µg/m3 range.
14
atoms/molecule) dominated the mass spectra of aerosols at higher mass loadings while ring
15
opened species (neither 5 nor 10 Si atoms/molecule) dominated the mass spectra of aerosols at
16
lower mass loadings.
17
area ratio suggest that nonvolatile ring opened species are formed in the gas phase and assist
18
particle formation through condensation, while dimers are formed by accretion reactions within
19
the particle phase as the particles grow.
20
presence of seed aerosol with similar siloxane aerosol mass loading but higher volume to surface
21
area ratio, where ring-opened species are much less prevalent than monomers or dimers and the 1
Monomers (5 Si atoms/molecule) and dimers (10 Si
Molecular signal intensity dependencies on the aerosol volume-to-surface
These conclusions are supported by experiments in the
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aerosol yield is higher.
Because of the importance of accretion chemistry, the aerosol yield
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from D5 oxidation is likely to be strongly dependent on particle size and morphology.
24 25
Introduction Cyclic volatile methylsiloxanes (cVMS) are widely used in personal care products.1,2
26 27
Owing to high vapor pressures, more than 90% of cVMS emissions are into atmosphere.3
28
phase cVMS have been detected in both indoor and outdoor environments,4-8 with generally
29
much
30
decamethylcyclopentasiloxane (D5) and dodecamethylcyclopentasiloxane (D6) at levels up to
31
650 and 79 ng/m3 respectively in ambient air in Zurich, Switzerland.4
32
sources of VOCs in a university classroom and reported a D5 concentration of 60±32µg/m3,
33
which represented >90% of all VOCs detected.8 Silicon has been reported in ambient
34
nanoparticles in a variety of urban locations, and gas phase oxidation of cVMS was suggested as
35
a possible source.9
higher
concentrations
indoors.6-8
For
example,
Buser
et
al.
Gas
reported
Tang et al. examined the
36
The major degradation pathway of cVMS in the atmosphere is oxidation by hydroxyl
37
radical (OH),10 where generation of semivolatile and nonvolatile products can lead to aerosol
38
formation.11,12 An atmospheric transport model with built-in OH and D5 chemistry was used to
39
predict D5 concentrations in air at a rural site in Sweden, with good agreement between measured
40
and predicted concentrations.13 Janechek et al. modified the Community Multiscale Air Quality
41
(CMAQ) model by incorporating cVMS and their gas phase oxidation products, and found that
42
oxidized D5 concentrations could be up to 9 ng/m3 downwind of major urban areas.14 2 ACS Paragon Plus Environment
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In the work reported here, the molecular composition of secondary aerosol obtained from
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D5 oxidation is studied both in the presence and absence of ammonium sulfate seed aerosol using
45
high performance mass spectrometry. Systematic changes in the molecular composition with
46
aerosol mass loading are observed and interpreted on the basis of estimated volatilities of the
47
individual molecular products, which give fundamental insight into the pathways15 involved in
48
secondary aerosol formation.
49
monomers react in the particle phase to give a nonvolatile dimer, are shown to have a substantial
50
impact on aerosol formation as the aerosol mass loading increases.
Accretion reactions, specifically those where two semivolatile
51 52
Experimental Section
53
Secondary aerosol was generated in a Photo-oxidation chamber (PC) consisting of a 50L
54
(251 × 251 × 800 mm, WxHxL) rectangular bag composed of a perfluoroalkoxy copolymer
55
(Welch Fluorocarbon, Dover, New Hampshire). Before each experiment, the PC was cleaned by
56
exposure to 20 ppmv ozone in the presence of ultraviolet radiation for at least 48 hr and flushed
57
continuously with clean, dry, air for 4-5 days obtained from a Zero-Air generation system
58
(Model 737, Aadco, Cleves, Ohio). A Scanning Mobility Particle Sizer (SMPS; Model
59
3080/3078, TSI, St. Paul, Minnesota) was used to measure particle size distributions.
60
A syringe pump (Model No. 55-1199, Harvard Apparatus, Holliston, Massachusetts) was
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used to feed precursor fluid (D5/MeOH as 1/5 by volume) (CAS No.541-02-6, Gelest,
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Morrisville, Pennsylvania) at a rate of 0.1 µL/min into an airflow through a gently heated Teflon
63
fitting. This airflow was then diluted to adjust the initial concentration of gas phase D5 and a 3 ACS Paragon Plus Environment
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constant flow of the diluted air was sent into the PC. Ozone was generated by passing clean air
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around a mercury lamp (Model No.81-1025-01, BHK Inc., Ontario, California).
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laden air was then bubbled through deionized water and sent into the PC where interaction with
67
ultraviolet radiation produced OH. Using the method described by Hall and Johnston,16 the OH
68
mixing ratio for these experiments was estimated to be about 108 molecules/cm3.
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combined and sent into the PC, the ozone, D5 and a third make-up air flow yielded a nominal
70
residence time of 15 min in the PC.
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ammonium sulfate solution. The particles were then sent through a diffusion drier and entered
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the PC through a portion of the makeup air flow. After mixing, the seed aerosol mass
73
concentration in the PC was 1.9 µg/m3. All experiments were performed with low relative
74
humidity (typically 8-10%) and a temperature of 27 oC.
75
that were performed, five each seeded and unseeded. Figure S1 shows representative particle
76
size distributions.
The ozone
When
Seed particles were generated by atomizing a 5mM
Table S1 summarizes the experiments
77
Particulate matter in the air flow exiting the PC was collected at a flow rate of ~2 L/min
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for 20-24 hrs onto a Teflon coated, glass fiber filter (GF/D, CAT No.1823-025, Whatman, GE
79
Healthcare, Piscataway, NJ) for chemical analysis. Generally, ~0.2 mg of aerosol was collected
80
in each experiment for analysis and four separate filter collections were obtained for each
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experiment in Table S1.
82
to identify and remove artifacts. After particle collection, each filter was sonicated for three
83
hours with 8 ml acetonitrile (ACN)/deionzied H2O 50/50 by volume (CAS No. 75-05-8, Fisher
84
Scientific, Pittsburgh, Pennsylvania) and the resulting solution was filtered through a 4
Background samples were collected prior to the injections of reactants
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polyvinylidenedifluoride (PVDF) filter (CAT No. 6747-2504, GE Healthcare, Piscataway, NJ).
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The filtered solution was then concentrated nearly to dryness ( 60 µg/m3), where accurate mass measurements
104
and MS/MS spectra were used to characterize the molecular products of D5 oxidation.12
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Although the mass spectra are complex, molecular products can be divided into three main types. 5 ACS Paragon Plus Environment
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First are monomer products, defined here as those containing exactly 5 Si atoms, at least 5 O
107
atoms, and a molecular formula (and MS/MS spectra) consistent with one siloxane ring but no
108
other sites of unsaturation.
109
various combinations of functionalization where CH3 groups are replaced by OH and/or CH2OH
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groups.
Relative to the D5 precursor molecule, monomer products contain
In Figure 1, monomer products are coded red.
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Second are dimer products, defined here as those containing exactly 10 Si atoms, at least
112
10 O atoms, and a molecular formula (and MS/MS spectra) consistent with two siloxane rings
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but no other sites of unsaturation.
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functionalization where CH3 groups are replaced by OH and/or CH2OH, and the two siloxane
115
rings can be linked by O, CH2 or CH2CH2 groups.
Dimers also contain various combinations of
In Figure 1, dimer products are coded black.
Third are ring-opened products, defined here as those containing neither 5 nor 10 Si
116 117
atoms.
118
siloxane ring structure.
119
consistent with CH3 replacement by OH and/or CH2OH, and sometimes indicate an additional
120
site of unsaturation, for example, Si=O.
121
Formation of these products necessarily requires fragmentation of the original D5 Molecular formulas and MS/MS spectra of these products are
In Figure 1, ring-opened products are coded blue.
Scheme 1 shows candidate structures for three prominent ions of each type, which are
122
starred in the mass spectra of Figure 1.
While not proven, candidate structures are consistent
123
with molecular formulas and MS/MS spectra. Figure S2 shows MS/MS spectra for the three
124
starred ions in Figure 1. As discussed in our previous work,12 accurate mass and MS/MS spectra
125
often permit identification of specific functional groups and dimer linkages, but they generally
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cannot determine functional group locations. 6 ACS Paragon Plus Environment
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In our previous study with a high aerosol mass loading (> 60 µg/m3),12 ring-opened
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products had very low signal intensities that could be explained in part by molecular
129
decomposition within the ESI source.
130
only briefly (referred to as fragmentation products in ref. 12). In contrast, the ring-opened
131
products in Figure 1 have very high signal intensities that cannot be explained simply by ESI
132
decomposition.
133
(dissolved in ACN/H2O 50/50 by volume) for 3 hours under the same experimental condition.
134
Figure S3 shows that the ESI mass spectrum obtained after sonication was generally devoid of
135
ring-opened species, indicating that the high abundance of these products in the filter samples
136
was not from sonochemistry.
137
in plasma polymerization experiments, 17 so it is not unreasonable to expect they can be formed
138
by OH oxidation.
Therefore, those ring-opened products were discussed
Additionally, a control experiment was performed by sonicating 1 mg/ml D5
We note that ring-opened products have been reported previously
139
The mass loading dependence of the three product types are explored further in Figure 2a
140
where the ratio of the summed signal intensities of dimers to ring-opened products is plotted as a
141
function of aerosol volume to surface area ratio. As shown in Figure S4, the volume to surface
142
area ratio of the unseeded aerosols studied in this work scale approximately linearly with mass
143
loading.
144
Figure 2a shows that the ratio of dimers to ring-opened products increases approximately
145
linearly with increasing volume to surface area ratio.
As will be discussed later, all dimers and
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most ring-opened products are expected to be extremely nonvolatile with predicted saturation
147
concentrations (C*) that are orders of magnitude lower than the aerosol mass loadings in these 7 ACS Paragon Plus Environment
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experiments.
149
mechanisms for these two types of products.
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particle formation and/or early growth since they are most prevalent at low volume to surface
151
area ratio.
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when they strike the particle surface.
153
directly at the particle surface.
154
determined by the available surface area.
155
The approximately linear relationship in Figure 2a suggests different formation Ring-opened products appear to be necessary for
These species are likely to be formed directly in the gas phase and then condense It is also possible that ring-opened products are formed
In either case, particle growth via ring-opened species is
Our earlier work, performed with a high aerosol mass loading, speculated that dimers
156
were formed in the gas phase.12
157
study shows that dimers must be formed by accretion reactions in the particle phase, since they
158
are nonvolatile but strongly represented only in higher mass loadings where the aerosol volume
159
is larger.
160
previously. 18
However, the mass loading dependence observed in the current
The possibility of condensed phase formation of oligomers has been reported
161
A linear dependence on volume to surface area ratio would be expected for a
162
volume-limited process such as accretion chemistry relative to a surface-limited process such as
163
condensation.
164
group for accretion reaction products in biogenic secondary organic aerosol (SOA) as a function
165
of particle size in the 35-110 nm range.19
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particle of size d is 2d/3, making the range of volume to surface area ratios where accretion
167
chemistry becomes important similar for the siloxane secondary aerosol in Figure 2a and the
168
biogenic SOA studied previously.19
The linear relationship in Figure 2a is similar to that recently reported by our
Note that the volume to surface area ratio for a
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Figure 2b shows ratio of monomer to dimer mass-weighted signal intensities.
In
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contrast to ring-opened products, the monomer to dimer ratio is relatively invariant with volume
171
to surface area ratio as would be expected for a quasi-equilibrium process where monomers react
172
to form dimers. Note that monomer partitioning between the gas and particle phases like
173
accretion chemistry is a volume-limited process.
174
the lowest volume to surface area ratio arises from a slight change in the distribution of
175
monomers – compare the mass spectrum in Figure 1a to those in 1b and 1c. As a group, the
176
monomers in Figure 1a are less volatile than those in Figures 1b and 1c based on calculated
177
saturation concentrations of candidate molecular structures (see below).
The slight increase in monomers to dimers at
178 179 180
Aerosol Formation in Seeded Experiments Secondary aerosol formation in the presence of ammonium sulfate seed was found to
181
give different product distributions than those observed in the unseeded experiments.
182
compares the mass spectra of secondary aerosol from seeded and unseeded experiments where
183
the aerosol mass loadings after subtraction of the seed are the same.
184
between the two is the very low contribution of ring-opened products to the seeded spectrum.
185
In fact, the mass spectra obtained from seeded experiments show no significant change with
186
aerosol mass loading.
187
Figure 3
The main difference
These results are consistent with the above conclusions for unseeded aerosol.
In seeded
188
experiments, formation and/or growth of new particles is less important since secondary aerosol
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is able to grow on the preexisting seeds.
The volume to surface area ratio of the seed aerosol 9
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was 18 nm, which then increased to as high as 32 nm depending on the amount of secondary
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aerosol produced.
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represent only a minor contribution to aerosol mass at such high values of volume to surface area
193
ratio.
194
the presence of seed particles), secondary aerosol formation proceeds mainly by a combination
195
of monomer partitioning and particle phase accretion reactions to produce dimers.
Based on the plot in Figure 2a, ring-opened species would be expected to
Once particle formation and early growth has been accomplished (or replaced through
196 197
Product Volatility Estimation
198
The difference in composition between the seeded and unseeded aerosols in Figure 3 is
199
further examined on the basis of molecular volatility as given by the saturation concentration
200
(C*). Measuring or estimating values of C* for individual molecular components is important
201
since in combination with the aerosol mass loading it determines the fraction of that component
202
that exists in the gas vs. particle phase at equilibrium.20, 21 Several volatility estimation methods
203
have been reported in the literature with varying degrees of complexity.22-24 The method
204
introduced by Nannoolal et al. is most relevant to the current study since it considers group
205
contributions involving silicon.24, 25 Beginning with a candidate molecular structure, the method
206
calculates a normal boiling point based on group contributions, a group interaction term and
207
finally the vapor pressure (saturation concentration).
208
We tested this approach by comparing measured and calculated boiling points and
209
saturation concentrations for cyclic siloxanes D3 to D7.
The results, shown in Table S2,
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further test the method, we took the molecular formula for the ring-opened product in Scheme 1
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and calculated C* values for eight candidate structures including the one shown in Scheme 1.
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The results, shown in Table S3, give predicted C* values within a factor of 2-4 of each other.
214
The relatively close agreement for different candidate structures arises in large part from similar
215
Si-O scaffolding and only minor perturbation due to type and location of functional groups.
216
Applying this method to candidate structures for all molecular products leads values spanning
217
many orders of magnitude in C*, whereas the uncertainty associated with any individual
218
prediction is likely to be much less than one order of magnitude.
219
formulas and predicted C* values is given in Table S4.
220
molecular formula vs. log C* is shown in Figure S5. All dimers and most ring-opened products
221
are relatively nonvolatile, having predicted log C* values below -2.
222
semivolatile or volatile, though a few can be considered nonvolatile.
223
A complete list of molecular
A summary plot of O/Si ratio of the
Most monomers are
Figure 4 summarizes the mass spectra in Figure 3 using a volatility basis set
224
representation,21,
26
225
fraction) vs. log C*.
226
mass fraction.
227
the signal intensity of the species multiplied by its molecular mass, then divided by the
228
summation of mass-weighted intensities over all species:
which normally involves a plot of aerosol mass concentration (or mass In Figure 4, the mass-weighted signal intensity is used as a surrogate for
The mass-weighted signal intensity (SImw) of a molecular species is defined as
, =
∑
229
SImw is an approximate measure of the mass fraction of a species, but it is not precise because
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different species can have different ESI detection efficiencies. 11 ACS Paragon Plus Environment
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spectrometer used in this study that ESI detection efficiencies for different cyclic siloxanes can
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vary by up to a factor of 3.
233
ESI has been shown to be able to accurately determine average molecular weights for polymeric
234
samples.27, 28
235
A smaller variation is likely for linear polydimethyl siloxanes since
Figure 4 provides insight into the contributions of different volatility species to seeded vs.
236
unseeded aerosol.
Figure 4a shows that the majority of nonvolatile matter (log C* < -2) in
237
unseeded aerosol consists of ring-opened products with a minor contribution from dimers, while
238
the opposite is true for seeded aerosol in Figure 4b.
239
ring-opened species are needed for particle formation and/or early growth in an unseeded
240
experiment but not in the presence of seeds.
241
is higher in the seeded experiment, which suggests that dimer formation in the particle phase is at
242
least partially reversible.
243
decomposition would have occurred during the sample preparation process or in the reactor itself
244
when the aerosol was formed.
245
then the higher fraction of intermediate volatility monomers in Figure 4b would suggest that they
246
can be trapped within the particle phase owing to hindered diffusion29-31 and/or phase
247
separation.32-35
This observation is not surprising since
The fraction of intermediate volatility monomers
However, it is not clear from the experiments performed whether such
If reversibility occurred on the timescale of aerosol formation,
248 249 250 251
Aerosol Yield and Atmospheric Implications Adding seed particles had the effect of significantly increasing the aerosol yield.
For
example, the unseeded experiment in Figure 1a had identical reaction conditions to the seeded 12 ACS Paragon Plus Environment
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experiment in Figure 3b, yet the secondary aerosol mass concentration was a factor of 2 greater
253
for the seeded experiment.
254
reflection on the kinetics of aerosol formation.
255
was slow since it relied on formation of ring-opened species that represented a small fraction of
256
the D5 oxidation product yield.
257
significant amount of time needed to reach a volume to surface area ratio that became favorable
258
for dimer formation.
259
level much faster in the presence of seed particles, enhancing the formation of dimers through
260
accretion chemistry and hence the amount of secondary aerosol produced within the residence
261
time of the reactor.
262
smaller impact due to vapor wall loss than in the unseeded experiments.
The higher aerosol yield in the seeded experiment is likely a In the unseeded experiment, particle formation
The consequence of slow particle formation would be a
In contrast, the aerosol volume to surface area ratio reached a favorable
The higher aerosol yields of the seeded experiments may also reflect a
263
Overall, we found the aerosol yield to decrease with decreasing reacted D5 mass
264
concentration, reaching values of 0.15 and 0.08 respectively for seeded and unseeded
265
experiments at the lowest secondary aerosol mass concentrations studied.
266
values for aerosol yields are not based on direct measurement of the change in D5 concentration,
267
but calculated from the OH mixing ratio which itself was estimated from a different set of
268
measurements whose error was difficult to assess.
We caution that these
269
This work shows that condensation, partitioning and particle phase chemistry (in this case
270
accretion reactions) all contribute to secondary aerosol formation by gas phase D5 oxidation.
271
Condensation appears to be driven mainly by the formation of ring-opened products in the gas
272
phase, though a few monomer oxidation products are also nonvolatile. 13 ACS Paragon Plus Environment
Partitioning is
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dominated by monomer oxidation products, most of which are semivolatile.
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nonvolatile but formed mainly by accretion reactions within the particle phase.
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Dimers are
275
Future experiments should focus on accurate and precise measurements of aerosol yield.
276
Since aerosol formation from D5 oxidation is strongly influenced by accretion reactions, work is
277
also needed to study this chemistry under a wider range of experimental conditions and particle
278
phase morphologies that might be encountered in ambient aerosol.
279 280
Supporting Information
281
Four tables (S1-S4) and five figures (S1-S5).
282 283 284
Acknowledgement This research was supported by the National Science Foundation under grant number
285
CHE-1408455.
The Orbitrap mass spectrometer used in this study was purchased under grant
286
number S10 OD016267-01 and supported by grant number 1 P30 GM110758-01, both from the
287
National Institutes of Health.
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17. Fouquet, T.; Petersen, J.; Bomfim, J. A. S.; Bour, J.; Ziarelli, F.; Ruch, D.; Charles, L.,
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Electrospray tandem mass spectrometry combined with authentic compound synthesis for
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structural characterization of an octamethylcyclotetrasiloxane plasma polymer. Int J Mass
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Spectrom. 2012, 313, 58-67.
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18. Cypryk, M.; Apeloig, Y., Mechanism of the acid-catalyzed Si-O bond cleavage in siloxanes and siloxanols. A theoretical study. Organometallics. 2002, 21, (11), 2165-2175.
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19. Tu, P.; Johnston, M. V., Particle Size Dependence of Biogenic Secondary Organic Aerosol
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Molecular Composition, Atmos. Chem. Phys. Discuss. 2017, doi:10.5194/acp-2017-53, in
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review.
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20. Donahue, N. M.; Robinson, A. L.; Trump, E. R.; Riipinen, I.; Kroll, J. H., Volatility and Aging of Atmospheric Organic Aerosol. Top. Curr. Chem. 2014, 339, 97-143.
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21. Donahue, N. M.; Robinson, A. L.; Stanier, C. O.; Pandis, S. N., Coupled partitioning,
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dilution, and chemical aging of semivolatile organics. Environ. Sci. Technol. 2006, 40, (8),
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2635-2643.
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22. Donahue, N. M.; Epstein, S. A.; Pandis, S. N.; Robinson, A. L., A two-dimensional volatility
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basis set: 1. organic-aerosol mixing thermodynamics. Atmos. Chem. Phys. 2011, 11, (7),
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3303-3318.
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23. Donahue, N. M.; Kroll, J. H.; Pandis, S. N.; Robinson, A. L., A two-dimensional volatility
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basis set - Part 2: Diagnostics of organic-aerosol evolution. Atmos. Chem. Phys. 2012, 12, (2),
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615-634.
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24. Nannoolal, Y.; Rarey, J.; Ramjugernath, D., Estimation of pure component properties. Part 2.
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Estimation of critical property data by group contribution. Fluid Phase Equilibr. 2007, 252,
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(1-2), 1-27.
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25. Nannoolal, Y.; Rarey, J.; Ramjugernath, D.; Cordes, W., Estimation of pure component
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properties Part 1. Estimation of the normal boiling point of non-electrolyte organic
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compounds via group contributions and group interactions. Fluid Phase Equilibr. 2004, 226,
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45-63.
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26. Robinson, A. L.; Donahue, N. M.; Shrivastava, M. K.; Weitkamp, E. A.; Sage, A. M.;
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Grieshop, A. P.; Lane, T. E.; Pierce, J. R.; Pandis, S. N., Rethinking organic aerosols:
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Semivolatile emissions and photochemical aging. Science 2007, 315, (5816), 1259-1262.
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27. Yan, W. Y.; Gardella, J. A.; Wood, T. D., Quantitative analysis of technical polymer
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mixtures by matrix assisted laser desorption/ionization time of flight mass spectrometry. J.
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28. Liu, X. M.; Maziarz, E. P.; Heiler, D. J.; Grobe, G. L., Comparative studies of poly(dimethyl
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siloxanes) using automated GPC-MALDI-TOF MS and on-line GPC-ESI-TOF MS. J. Amer.
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of organic aerosol volatility and composition. Atmos. Meas. Technol. 2009, 2, (1), 15-31.
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Environ. Sci. Technol. 2016, 50, (14), 7434-7442.
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3
Relative Intenisty (%)
a) 1.2 µg/m
3
b) 5.6 µg/m
3
c) 12 µg/m
200
400
398
600 m/z (+)
800
1000
399
Figure 1. ESI mass spectra in positive ion mode of secondary aerosol from D5 oxidation under
400
various aerosol mass loadings.
401
measurements from four different samples. Peaks considered as ring opened products are coded
402
blue, dimers are coded black, and monomers are coded red. Candidate structures for starred ions
403
are given in Scheme 1.
Ion signal intensities are averaged over four separate
404
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.
Monomer ( )
Ring opened product ( )
Dimer ( )
405 406
Scheme 1. Candidate structures for the three types of reaction products found in siloxane
407
secondary aerosol. These structures are consistent with molecular formulas and MS/MS spectra
408
of the starred ions in Figure 1.
409 410 411
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SI Ratio of Dimers/Ring-opened
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0.6
a)
0.4
0.2
0.0 10
20
30
40
SI Ratio of Monomers/Dimers
Volume/Surface Area (nm) 5
b)
4 3 2 1 0 10
20
30
40
Volume/Surface Area (nm)
412 413
Figure 2. Signal intensity ratio of a) dimers to ring-opened products and b) monomers to dimers
414
vs. volume/surface area ratio of the secondary aerosol.
415
deviation.
Error bars represent one standard
Error bars smaller than the symbols are not shown.
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Relative Intenisty (%)
a) Unseeded
b) Seeded
200
400
600 m/z (+)
800
1000
416 417
Figure 3. ESI mass spectra in positive ion mode for a) unseeded and b) seeded aerosol produced
418
by OH oxidation of D5 under conditions that give similar mass loadings (after subtraction of seed
419
aerosol) of ~3 µg/m3.
420
four different samples.
421
coded black, and monomers are coded red.
Ion signal intensities are averaged over four separate measurements from Peaks considered as ring opened products are coded blue, dimers are
422
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Mass-weighted Signal Intensities (%)
Mass-weighted Signal Intensities (%)
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423
0.8
NVOC
LVOC
SVOC
IVOC
(a) Unseeded
0.6 0.4 0.2 0.0 2
3
logC (saturation conc., µg/m ) 0.8
NVOC
LVOC
SVOC
IVOC
(b) Seeded
0.6 0.4 0.2 0.0 2
3
logC (saturation conc., µg/m )
424
Figure 4. Mass-weighted signal intensities summed over the three types of ions in a) unseeded
425
and b) seeded aerosols vs. log C* for candidate structures that correspond to individual ions.
426
These plots are for assigned ions in the mass spectra of Figures 3a and 3b, respectively.
427
Ring-opened products are coded blue, dimers are coded black, and monomers are coded red. 25 ACS Paragon Plus Environment
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TOC Graphic
428
200
400
600
800
1000
m/z (+)
NVOC
LVOC
SVOC
IVOC
Ring-opened Dimers Monomers
2
logC (µ µg/m )
430
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