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Atmospheric chemistry of perfluoronitriles: Environmental impact and experimental evidence related to N2O and NO formation
Accepted Manuscript Atmospheric chemistry of perfluoronitriles: Environmental impact and experimental evidence related to N2O and NO formation Qin Guo, Liang Chen, Shuzo Kutsuna, Hengdao Quan, Junji Mizukado PII:
S1352-2310(18)30764-7
DOI:
https://doi.org/10.1016/j.atmosenv.2018.10.066
Reference:
AEA 16363
To appear in:
Atmospheric Environment
Received Date: 11 September 2018 Revised Date:
28 October 2018
Accepted Date: 30 October 2018
Please cite this article as: Guo, Q., Chen, L., Kutsuna, S., Quan, H., Mizukado, J., Atmospheric chemistry of perfluoronitriles: Environmental impact and experimental evidence related to N2O and NO formation, Atmospheric Environment (2018), doi: https://doi.org/10.1016/j.atmosenv.2018.10.066. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Atmospheric
Chemistry
of
Perfluoronitriles:
2
Experimental Evidence Related to N2O and NO Formation
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Qin Guoa, b, Liang Chenb, *, Shuzo Kutsunab, Hengdao Quana, b, Junji Mizukadob
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a
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South Zhonguancun Street, Haidian District, Beijing 100081, PR China.
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b
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Higashi, Tsukuba, Ibaraki 305−8565, Japan.
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*
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l−[email protected] (L. Chen)
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Corresponding author: Tel: +81 29 861 9379. Fax: + 81 29 861 4457. E−mail address:
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and
National Institute of Advanced Industrial Science and Technology (AIST), 1−1−1
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Impact
School of Chemistry & Chemical Engineering, Beijing Institute of Technology, 5
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Environmental
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Abstract The atmospheric chemistry of perfluoronitriles, the proposed replacement gases of
21
SF6, has been investigated using an atmospheric reaction chamber. N2O formation was
22
first observed following the reaction between perfluoronitriles and OH radicals, then
23
NO formation was verified through experimentation. COF2, CF3C(O)F, and CO2
24
generation was observed, and a revised oxidation mechanism for perfluoronitriles in the
25
atmosphere is proposed. Additionally, the rate coefficients related to OH radicals were
26
measured for the perfluoronitriles of CF3CN, CF3CF2CN, CF3CF2CF2CN, and
27
(CF3)2CFCN: their atmospheric lifetimes were 6.6, 10, 12, and 54 years, their radiative
28
efficiencies were evaluated to be 0.188, 0.223, 0.317, and 0.231 W m–2 ppb–1, and their
29
100-year time horizon GWPs were 212, 374, 633, and 1705, respectively. The findings
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contained in this study indicate that perfluoronitriles present an insulating gas
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replacement option with a comparatively low environmental impact.
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Keywords: Perfluoronitriles; atmospheric lifetimes; global warming potentials;
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degradation mechanism.
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1. Introduction Sulfur hexafluoride (SF6) exhibits excellent dielectric strength, heat transfer
39
capacity, and electric arcs interruption and as a result, SF6 is the most common
40
insulation gas in current use. For many years, most of the SF6 that was produced was
41
used in high-voltage gas circuit breakers and gas insulated switchgear (Beroual et al.,
42
2017), with an emission of approximately 10 kt/a (Stocker et al., 2013). Such
43
large-scale usage leads to an abundance of SF6 that is 3 orders of magnitude higher than
44
that in preindustrial times (Rabie et al., 2018). It is noticeable that SF6 is an extremely
45
potent greenhouse gas with an atmospheric lifetime of 3200 years and a 100-year time
46
horizon global warming potential (GWP100) of 23500. SF6 has been a monitored
47
substance since the formation of the Kyoto Protocol and worldwide efforts were taken
48
in order to develop a low-GWP compound to replace SF6. Perfluoronitriles (PFNs), such
49
as CF3CN (PFEN), CF3CF2CN (PFPN), and CF3CF2CF2CN (n-PFBN), were proved to
50
exhibit around 2.7 times the dielectric strength of SF6 in a uniform field (Plump et al.,
51
1962). Considering the fact that perfluoronitriles generally exhibit relatively high
52
toxicity, nitrite esters with a low boiling point were used as additives (Yamauchi, 1985).
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Recently, a branched perfluoronitrile with a high dielectric strength and lower toxicity,
54
(CF3)2CFCN (i-PFBN), has been the center of attention with regards SF6 potential
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replacement. Pilot gas-insulated busbars have been installed in a substation in England,
56
and several 145 kV gas-insulated substations are planned in Europe (Kieffel et al., 2016).
57
Additionally, during its usage as an arc extinguishing gas, the major thermal
58
decomposition products of i-PFBN have been identified as CF3CN and CF3CF2CN by
59
Kieffel et al. (2017). Based on these considerations, it is essential to evaluate the
60
environmental impacts of these perfluoronitriles prior to their large-scale application.
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As for PFEN, PFPN, and n-PFBN, there was no previous study related to their
62
atmospheric chemistry. In contrast, the rate coefficients for the reactions of i-PFBN with
63
respect to OH radicals, Cl atoms and O3 were measured using a relative method at 296 ±
64
2 K by Sulbaek Andersen et al. (2017), while Blázquez et al. (2017) measured the rate
65
coefficients related to OH radicals in the 278–358 K range using the absolute rate
66
method. Although the OH radical reaction rate coefficient measured at 298 K by
67
Blázquez et al. showed a good agreement with that measured by Sulbaek Andersen et al.,
68
its value was significantly higher than that calculated using the Arrhenius expression, as
69
reported by Blázquez et al. Moreover, Blázquez et al. reported i-PFBN radiative
70
efficiency (RE) that was 32% higher than that proposed by Sulbaek Andersen et al. With
71
respect to the study of perfluoronitrile atmospheric chemistry, the key point is that
72
clarification is required with regards the reaction mechanism for OH addition to the –
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C≡N group. Previously, a theoretical study of the reaction pathway between the OH
74
radical and –C≡N group was conducted for the HCN + OH reaction (Cicerone et al.,
75
1983). Recently, the reaction pathways of i-PFBN with OH radicals was studied by
76
Sulbaek Andersen et al. (2017) using experimental and theoretical methods and a
77
reaction scheme of –C≡N with OH was proposed. The proposed reaction pathway was
78
theoretically reasonable, however, corresponding products of NO and NO2 were not
79
observed in the experiment. Therefore, a study of the kinetics and mechanisms related
80
to perfluoronitriles are necessary.
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In this work, we evaluated the OH rate coefficient of four different perfluoronitriles
82
using a relative rate method, in which there persisted a high concentration of OH in the
83
reaction chamber. Infrared absorption cross sections and radiative efficiencies were also
84
obtained for these systems. Moreover, the decay of perfluoronitriles of more than 50%
85
during product experiments allowed for all of the degradation products to be
86
distinguished and observed. N2O and NO were produced following the degradation of
87
perfluoronitriles. Based on the experimental evidence, a revised reasonable reaction
88
mechanism for OH radical with perfluoronitriles is proposed and their environmental
89
impact is evaluated.
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2. Materials and Method
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The experiments were carried out in an 11.5-L cylindrical double-layer quartz
92
chamber, as described in detail in our previous work (Chen et al., 2003). Herein, OH
93
radicals were produced through the photolysis of O3 under 254-nm UV irradiation in the
94
presence of water:
96
O (1D) + H2O → 2OH
(1)
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O3 + hν → O (1D) + O2
(2)
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An O3/O2 (3%, O3) gas mixture was generated from pure O2 using a silent-discharge
98
ozone generator (ECEA-1000, Ebara Jitsugyo Co., Japan). CF3CN (purity: 98%),
99
CF3CF2CN (purity: 98%), and CF3CF2CF2CN (purity: 98%) were purchased from PCR
100
incorporated (USA). (CF3)2CFCN (purity: 99.5%) was purchased from Beijing Yuji
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Sciences & Technology Co., Ltd. (China). The CF3CH2CHF2 reference compound
102
(purity: ≥99.9%) was purchased from Central Glass Co., Ltd. (Japan), CHF2CH3 (purity:
103
>99%) was obtained from Sigma-Aldrich (America), and CF3CH2F (purity: 99.9%) and
104
CH2F2 (purity: 99%) were purchased from SynQuest Labs, Inc. (America). NO (purity:
105
99%) was obtained from Takachiho Chemical Industrial Co., Ltd. (Japan).
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In the relative rate studies performed at 253–343 K, the perfluoronitriles, the
107
reference compound, and H2O were introduced into the reaction chamber. He gas was
108
used to dilute the reactants to an initial pressure of 200 Torr, which led to relatively low
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O (1D) quenching. An O3/O2 gas mixture was introduced into the chamber with a flow
110
rate of 6–10 mL min–1 at STP, and 6–10 UV lamps were used for irradiation. In this
111
manner, a nearly constant OH radical concentration could be maintained at
112
approximately 1010 radical cm–3 during measurement (Chen et al., 2003). The
113
concentrations of perfluoronitriles and reference compounds were monitored by Gas
114
Chromatography with flame ionization detector (GC-FID) at 8–12 min intervals. The
115
capillary column was a TC-Bond Q metal (length: 30 m; i.d.: 0.53 mm; GL Sciences
116
Inc., Japan), and the column temperature was set to 373 or 403 K.
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In the reaction chamber, the initial concentrations of the reactants were
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approximately 4.7–9.5 × 1014 (perfluoronitriles), 4.7–7.1 × 1014 (CF3CH2F), 4.7–7.1 ×
119
1014 (CHF2CH3), 3.5–5.9 × 1014 (CH2F2), 3.7–4.8 × 1014 (CF3CH2CHF2), and 1.2 × 1018
120
(H2O) molecules cm−3, respectively. After 60–120 min reaction, the loss of any reactant
121
was more than 50%. Additionally, the loss of these reactants due to wall reaction and
122
photolysis was measured to be 0.5–1%, which was less than the analysis uncertainty of
123
GC-FID (2%).
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Considering that reactions with O3 or O (1D) could be negligible compared with
125
those with OH radicals (Sulbaek Andersen et al., 2017; Chen et al., 2003), the loss of
126
perfluoronitriles relative to reference compounds was plotted using the following
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127
128
expression: ln
+
=
!"
#ln
$
$
%
+
%
&
(I)
where [Perfluoronitriles]0(t) and [Reference]0(t) are the concentrations of perfluoronitriles
130
and reference compounds (CF3CH2F, CH2F2, or CF3CH2CHF2) at time 0(t), respectively.
131
Dn = nln0.9983 is a correcting factor accounting for the loss of reactants derived from
132
GC sampling, where n is the number of GC-FID samplings.
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In order to evaluate the integrated absorption cross sections and radiative
134
efficiencies of perfluoronitriles, their IR spectra at room temperature were recorded by
135
averaging 64 interferograms at a resolution of 0.5 cm–1. A FT-IR spectrometer with a
136
DTGS detector and a glass cell (optical path length: 10 cm, KBr window) were used.
137
The concentrations of perfluoronitriles in the glass cell were calculated from the
138
pressure, which was measured through a vacuum line linking two pressure meters
139
(MKS Baratron 626B, measuring range: 0–10 Torr; MKS cold cathode transducer 974B,
140
measuring range: 1 × 10–8–1500 Torr). During the investigation of the products and
141
reaction mechanisms of perfluoronitriles with OH radicals, the reactant mixture was
142
irradiated for 90 minutes. Then, their products were detected using a FT-IR
143
spectrometer (resolution: 0.5 cm–1, detector: MCT) with a nickel-coated aluminum
144
multiple-reflection IR cell (volume: 200 cm3; optical path length: 3 m) at 3-min
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intervals. The degradation products were then identified and quantified according to
146
these recorded spectra.
147
3. Results and discussion
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3.1. Relative rate study of OH + perfluoronitriles
149
The rate coefficients of reaction (3) were measured relative to reaction (7) and (8),
150
the rate coefficients of reaction (4) and (5) were measured relative to reaction (7) and
151
(9), while those of reaction (6) were measured relative to reaction (7) and (10):
152
OH + CF3CN → Products
153
OH + CF3CF2CN → Products
154
OH + CF3CF2CF2CN → Products
(5)
155
OH + (CF3)2CFCN → Products
(6)
156
OH + CF3CH2F → Products
(7)
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OH + CHF2CH3 → Products
(8)
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(4)
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(3)
OH + CH2F2 → Products
(9)
OH + CF3CH2CHF2 → Products
(10)
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Figure 1 shows the loss of perfluoronitriles versus that of the reference compounds.
161
The decay of all the reactants was greater than 30%. Linear least-square fitting for these
162
plots gives reaction rate coefficient ratios of: k3/k7 = 1.76 ± 0.02, k3/k8 = 0.230 ± 0.003,
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k4/k7 = 1.26 ± 0.01, k4/k9 = 0.496 ± 0.007, k5/k7 = 1.02 ± 0.01, k5/k9 = 0.413 ± 0.004,
164
k6/k7 = 0.234 ± 0.005, and k6/k10 = 0.159 ± 0.004. Herein, the errors only represent two
165
standard deviations resulting from using GC-FID. Using the reported data of k7 = 4.5 ×
166
10–15 cm3 molecule–1 s–1 (± 10%), k8 = 3.3 × 10–14 cm3 molecule–1 s–1 (± 7%), k9 = 1.1 ×
167
10–14 cm3 molecule–1 s–1 (± 7%), and k10 = 7.0 × 10–15 cm3 molecule–1 s–1 (± 15%)
168
(Burkholder et al., 2015), the final reaction rate coefficient values of k3, k4, k5, and k6 at
169
298 K were (7.76 ± 1.08) × 10–15, (5.53 ± 0.76) × 10–15, (4.60 ± 0.60) × 10–15, and (1.08
170
± 0.23) × 10–15 cm3 molecule–1 s–1, respectively. Likewise, the rate coefficients of
171
perfluoronitriles at other temperatures were measured, and the results provided in Table
172
S1.
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2.5
PFEN 2.0
PFPN
2.0
CF3CH2F
CF3CH2F 1.5
1.0
CH2F2
0 2.0
CH2F2
0.5
0.5
0
0.5
1.0
1.5
2.0
0 3.0 0 0.4
2.5
0.5
i-PFBN
CF3CH2F 1.5
0.3
1.0
CF3CH2CHF2
0.1
CH2F2
0
2.0
CF3CH2F
0.2
0.5
1.5
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n-PFBN
1.0
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1.5
0
0
0.5
1.0
1.5
2.0
0
0.5
1.0
1.5
2.0
ln[Reference]0/[Reference]t + Dn
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squares); CH2F2 (empty squares); CF3CH2CHF2 (empty circles).
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Figure 1. Loss of perfluoronitriles versus that of reference compounds in the presence of OH radicals under 200 Torr initial pressure in He, at T=298 K. CF3CH2F (solid
174
Figure 2 shows the temperature dependence of k3, k4, and k5 for T=253–328 K, and
178
k6 for T=268–343 K. Non-linear least-square analysis for these plots gives the Arrhenius
179
expressions:
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k3 (253–328 K) = (3.09 ± 0.30) × 10–13 exp [–(1089 ± 28)/T]
181
k4 (253–328 K) = (4.46 ± 0.67) × 10–13 exp [–(1311 ± 44)/T]
182
k5 (253–328 K) = (3.08 ± 0.55) × 10–13 exp [–(1251 ± 53)/T]
183
k6 (268–343 K) = (1.18 ± 0.07) × 10–13 exp [–(1397 ± 18)/T] 11
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k3
-14
1/2 k4
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k5
10
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kOH (molecule cm-3 s-1)
10
-15
k6
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k6 (Blazquez et al.)
k6 (Sulbaek Andersen et al.) 2.6
2.8
3.0
3.2
3.4
3.6
3.8
4.0
4.2
1000/T (K-1)
184
188
The data obtained in this work for k6 was compared with those measured by
189
Blazquez et al. and Sulbaek Andersen et al., and their results are also plotted in Figure 2.
190
The Arrhenius plot of k6 measured in this work showed a good agreement with the
191
majority of the data points provided by Blazquez et al.’s study, although at 298 K, the
192
measured values of Blazquez et al. and Sulbaek Andersen et al. were approximately
193
25% higher than those of both the present study and the data calculated from an
194
Arrhenius expression of k6 by Blazquez et al. Additionally, there have been no previous
195
reports related to the k3, k4, and k5 rate coefficients for these systems. The results shown
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Figure 2. Temperature dependence of rate coefficients for the perfluoronitriles + OH reaction. The solid line is the fitted Arrhenius curve. The error bars include two standard deviations and uncertainties derived using reference compounds.
185
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in Figure 2 indicate that the reactivity of perfluoronitriles towards OH radicals decreases
197
with increasing numbers of fluorine atoms. i-PFBN has a significant lower reactivity
198
than n-PFBN, which may be due to the stronger electron affinity and greater steric effect
199
of –CF(CF3)2 compared with –CF2CF2CF3.
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3.2. Global warming potentials
201
In a real atmosphere, reactions with O3 and Cl atoms will provide a negligible
202
contribution to the degradation of perfluoronitriles, while their wet or dry deposition is
203
not to be expected (Sulbaek Andersen et al., 2017). Moreover, perfluoronitriles will not
204
undergo photolysis because they do not absorb in the actinic region (Sulbaek Andersen
205
et al., 2017). Considering the perfluoronitriles will be well mixed in the atmosphere,
206
their atmospheric lifetime could be calculated relative to that of CH3CCl3 (MCF):
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/0*/,)1
=
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'()*+,-*.
23
(5657)
9:;<=:>?@ :@<9
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207
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200
(5657)
× 'BCD
(II)
208
where '()*+,-.*.
209
CH3CCl3, respectively, and 'CEF CC,F = 6.1 years (WMO, 2014). kMCF(272 K) and
210
kPerfluoronitrles(272 K) are rate coefficients for the reactions with MCF and
211
perfluoronitriles, respectively, and kMCF(272 K) = 6.14 ×10–15 cm3 molecule–1 s–1
212
(Burkholder et al., 2015). And then, the atmospheric lifetimes of PFEN, PFPN, n-PFBN,
213
and i-PFBN were evaluated to be 6.6, 10, 12, and 54 years, respectively. Herein, the
and 'BCD are atmospheric lifetimes for perfluoronitriles and
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lifetime of i-PFBN was close to the value of 47 years reported by Blázquez et al. (2017),
215
while a value of 22 years was reported by Sulbaek Andersen et al. (2017). Generally,
216
uniform mixing of a gas in the atmosphere requires a time scale of years, and so these
217
measured lifetimes could be considered to be the global-mean lifetimes of these
218
perfluoronitriles.
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214
IR spectra were recorded for the perfluoronitriles in an atmospheric pressure of N2
220
diluent gas at room temperature. Figure 3 shows the IR absorption spectra of
221
perfluoronitriles, which were the average results of more than ten measurements. The
222
integrated cross sections (unit: cm2 molecule–1 cm–1) at 500–2400 cm–1 were (1.65 ±
223
0.07) × 10–16 for PFEN, (1.87 ± 0.06) × 10–16 for PFPN, (2.37 ± 0.04) × 10–16 for
224
n-PFBN, and (2.32 ± 0.04) × 10–16 for i-PFBN. The result for i-PFBN herein was
225
slightly higher than that measured by Sulbaek Andersen et al., which was because –C≡N
226
absorption was also taken into consideration in this work. However, a significantly
227
higher integrated cross section of (2.88 ± 0.01) × 10–16 cm2 molecule–1 cm–1 was
228
reported by Blázquez et al. According to the discussion in Blázquez et al.’s work, the
229
different buffer gas and spectral resolution were not expected to account for this
230
discrepancy. Hence, the most possible source of this discrepancy is the different total
231
pressure (700 Torr in Sulbaek Andersen et al.’s work, 8–100 Torr in Blázquez et al.’s
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work, and 760 Torr in this work). We recorded the spectra under <10 Torr, the results
233
were consistent with that obtained under 760 Torr. Therefore, the source of this
234
significant discrepancy is unclear.
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Using the revised Pinnock curve based on the 1 cm–1 Oslo line-by-line (LBL)
236
model (Hodnebrog et al., 2013), the instantaneous radiative efficiencies (IREs) of PFEN,
237
PFPN, n-PFBN, and i-PFBN were then calculated to be 0.182, 0.213, 0.300, and 0.213
238
W m–2 ppb–1, respectively. Furthermore, a 10% increase in these values was introduced
239
in order to account for the stratospheric temperature adjustment, and an S-shaped
240
lifetime correction factor was also adopted using the following expression: IJK
LMNJO
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G(τ) =
10ST < ' < 10T VWXYZ
(III)
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241
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235
where a, b, c, and d are constants with values of 2.962, 0.9312, 2.994, and 0.9302,
243
respectively, and τ is the atmospheric lifetime. Hence, the final values for the REs were
244
as follow: 0.188 W m–2 ppb–1 (PFEN), 0.223 W m–2 ppb–1 (PFPN), 0.317 W m–2 ppb–1
245
(n-PFBN), and 0.231 W m–2 ppb–1 (i-PFBN).
247
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242
The GWPs of the perfluoronitriles were calculated using the following formula: `ab(c (0) 3de (0)
[\](D^1 (_) = `ab(
=
`JfLS)gh S
i
`ab(3de (0)
j
(IV)
248
where A is RE, τ is the atmospheric lifetime, and t is the given time horizon.
249
k[\]Cle (_) is the absolute GWP of CO2 for time horizon t, with values of 2.495 × 10– 15
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, 9.171 × 10–14 and 32.17 × 10–14 W m–2 yr kg–1 for time horizons of 20, 100, and 500
251
years, respectively (Hodnebrog et al., 2013). Substituting the corresponding values of
252
these parameters, the GWPs of the perfluoronitriles were obtained, and the results listed
253
in Table 1.
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PFEN
PFPN
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15 12 9 6 3 0 4
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14
3 2 1 0 6
n-PFBN
4 2 0 6
TE D
Absorption cross section (10-18 cm2 molecule-1)
250
i-PFBN
4 2
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0 2400
254
2200
2000
1800
1600
1400
1200
1000
800
600
-1
Wavenumber (cm )
257
Table 1. The atmospheric properties of the perfluoronitriles and related compounds.
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Figure 3. IR absorption spectra for perfluoronitriles between 500 and 2400 cm–1 recorded in a N2 diluent gas atmosphere at room temperature.
255
kOH (298 K) × 1015
A × 1013
Ea/R
Lifetime
RE
Compound 3
–1
–1
3
–1
–1
GWP100
Ref.
0.188
212
this work
10
0.223
374
this work
12
0.317
633
this work
(cm molecule s )
(cm molecule s )
(K)
(years)
(W m–2 ppb–1)
PFEN
7.76 ± 1.08
3.09 ± 0.30
1089 ± 28
6.6
PFPN
5.53 ± 0.76
4.46 ± 0.67
1311 ± 44
n-PFBN
4.60 ± 0.60
3.08 ± 0.55
1251 ± 53
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1.08 ± 0.23
1.18 ± 0.07
1397 ± 18
54
0.231
1705
this work
i-PFBN
1.45 ± 0.25
—
—
22
0.217
1490
Sulbaek Andersen et al. (2017)
i-PFBN
1.47 ± 0.19
5.9 ± 3.2
1856 ± 162
47
0.280
3646
Blázquez et al. (2017)
N 2O
—
—
—
116
0.17
298
Stocker et al. (2013)
SF6
—
—
—
850
0.575
22500
Stocker et al. (2013)
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i-PFBN
3.3. Degradation products and mechanism
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Experiments were performed by irradiating a gas mixture consisting of 6–10 × 1014
260
molecule cm–3 perfluoronitriles and 5.57 × 1017 molecule cm–3 H2O, an O3/O2 gas
261
mixture was continuously introduced at a flow rate of 4–8 mL min–1 at 298 ± 1 K. The
262
IR spectra were recorded with a time interval of 3 min, and the perfluoronitrile decays
263
were 40–80% after UV irradiation. Figure 4a and Figure 4b show results recorded
264
before and after 30-min UV irradiation, respectively. Figure 4c shows the absorption
265
spectrum for products observed for the reaction of i-PFBN with OH radicals, which was
266
obtained by subtracting the IR features of i-PFBN from Figure 4b. CO2, COF2,
267
CF3C(O)F, and N2O were identified as the major products through comparison with the
268
reference IR spectra. Figure 4h plots the concentration of COF2, CF3C(O)F, and N2O
269
versus the decreasing i-PFBN concentration, which provided molar yields of 102 ± 2%
270
for COF2, 84 ± 2% for CF3C(O)F, and 9.5 ± 0.1% for N2O. COF2 and CF3C(O)F exhibit
271
significant hydrolysis due to the presence of H2O in the reaction chamber. The results of
272
product identification and quantification for other perfluoronitriles are represented in
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Figures S1–S3. PFPN and n-PFBN gave the same products as i-PFBN, while the PFEN
274
did not produce CF3C(O)F. Additionally, the product molar yields for perfluoronitriles
275
are listed in Table S2.
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It was expected that reactions between perfluoronitriles and OH would initiate in
277
the –C≡N group, and therefore that their reaction pathways would be similar to that of
278
HCN + OH. The mechanisms of the OH addition reaction with HCN and CH3CN have
279
previously been studied using theoretical methods (Cicerone et al., 1983; Galano et al.,
280
2007). It was proposed that OH addition only occur at the C atom of the –C≡N group,
281
giving a –C(OH)=N radical, of which the N atom then reacts with O2 in order to
282
produce a –C(OH)=NOO radical. This radical could easily undergo H migration from
283
OH to OO (Sulbaek Andersen et al., 2017; Galano et al., 2007). Then, the –C(O)NOOH
284
group is converted into a –C(O) radical, NO, and an OH radical. According to their
285
calculations, of Sulbaek Andersen et al. proposed the same reaction lines (Sulbaek
286
Andersen et al., 2017). However, neither the formation of NO/NO2 nor the regeneration
287
of the OH radical was proved by experimental methods. Due to the extremely high OH
288
radical reactivity of NO/NO2 compared to those of perfluoronitriles, NO/NO2 will react
289
rapidly with an OH radical, giving HNO2/HNO3 (Burkholder et al., 2015). In our
290
experimental system, due to the presence of high concentration of water, most of HNO2
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and HNO3 will mainly dissolved in water on the wall of reactor, and their IR features
292
are hardly to be observed. However, NO formation could be manifested due to the
293
generation of CF3C(O)F during the experiments using PFPN and n-PFBN. In the case of
294
perfluoronitriles, the radical of CxF2x+1O is produced during the OH-initiated reaction.
295
In the absence of NO, the fate of the CxF2x+1O radical has been well established (Calvert
296
et al., 2011). The (CF3)2CFO radical is converted into COF2 and CF3C(O)F, while COF2
297
is produced following the degradation of CF3O, CF3CF2O, and CF3CF2CF2O radicals.
298
However, as shown in Figures S2 and S3, the formation of CF3C(O)F was observed for
299
both PFPN and n-PFBN, with molar yields of 11 ± 1% and 28 ± 1%, respectively, and
300
F-abstraction by NO was considered as the only pathway capable of producing
301
CF3C(O)F (Taketani et al., 2005; Wallington et al., 1995):
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(11)
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Consequently, the formation of NO during OH-initiated reactions of perfluoronitriles
304
was proved, although the IR absorption of NO was not observed. Herein, the IR features
305
of FNO were not observed due to its very short lifetime (Wallington et al., 1995). In
306
addition, reactions between NO and CF3O were also possible, producing COF2 and
307
FNO, while it is assumed that reactions between NO and (CF3)2CFO or CF3CF2CF2O
308
do not occur due to the fact that no CF3C(O)CF3 and CF3CF2C(O)F generation was
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detected, which mainly depends on the lifetimes of these radicals. Similar observations
310
related to the CF3O and CF3CF2O radicals have been reported (Bark et al., 1995). In the
311
case of the CF3O radical, it could react with the CF3OO radical to produce the
312
CF3OOOCF3 trioxide compound. However, the CF3CF2O radical will not react with the
313
CF3OO radical and instead undergoes β-scission giving CF3 radical and COF2 due to its
314
shorter lifetime.
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0.6 (a) i-PFBN
(h) Products yield
0.2 0.0 0.6 (b) i-PFBN + products 0.4
0.02
N2O
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CF3C(O)F
COF2
N2O
0
1x1014
[i-PFBN]decay/moecule cm
(g) N2O
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2200 2000 1800 1600 1400 1200 1000 800
316 317 318 319 320
-3
(f)
(e) CF3C(O)F
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0 3x1014
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1x1014
COF2
O3
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CF3C(O)F
-3
Absorbance
0.2
2x1014
COF2
[Products]/moecule cm
0.4
2200
2100
2000
Wavenumber (cm-1)
Figure 4. IR spectra recorded before (a) and after (b) 30-min UV irradiation of a gas mixture containing 8.49 × 1014 molecule cm–3 i-PFBN and 5.57 × 1017 molecule cm–3 H2O with continuous O3/O2 gas introduction at 298 ± 1 K; (c) IR spectra of products obtained by subtracting (a) from (b); (f) IR spectra of (a) at 2200–2350 cm–1; (d, e, g) reference IR spectra of COF2, CF3C(O)F, and N2O; (h) yields of products.
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It is of note that N2O was also identified as a product of the reaction between
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perfluoronitriles and the OH radical. As shown in Table S2, all the perfluoronitriles lead
323
to N2O formation with a molar yield of approximately 10%, which indicates that there
324
was another reaction pathway competing with that described by Sulbaek Andersen et al.
325
Observing the fact that N2O has a structure of N=N=O, it was expected that its
326
formation resulted from an attack on a N atom with a lone pair of electrons by another
327
N atom in a –N–O structure. According to the reaction mechanism proposed in previous
328
studies, the former could be a RF–C(OH)=N radical (RF: CF3, CF3CF2, CF3CF2CF2,
329
(CF3)2CF), while the latter may be NO or RF–C(O)NOOH. In order to investigate the
330
formation pathway of N2O, a different O3/O2 gas flow rate (4 mL min–1 and 8 mL min–1)
331
was adopted during the product experiments for n-PFBN. It was found that the molar
332
yield of N2O was independent of the concentration of O2 in the reaction chamber
333
(Figure S4), which indicated that the RF–C(OH)=N radical is not related to the
334
generation of N2O, and that O2 addition to N is its sole pathway. Therefore, reaction
335
pathways for perfluoronitriles with an OH radical are proposed by combining the results
336
of previous studies (Cicerone et al., 1983; Galano et al., 2007; Sulbaek et al., 2017) with
337
the results in this work (Figure 5). The reaction lines are as follow:
338
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RF–C≡N + OH → RF–C(OH)=N•
(12)
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RF–C(OH)=N• + O2 → RF–C(OH)=NOO•
(13)
The radical is then considered to be transformed into RF–C(O)NOOH with a very low
341
energy barrier, which leads to the formation of NO:
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RF–C(OH)=NOO• → RF–C(O)NOOH
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RF–C(O)NOOH → RF–C(O)NO• + •OH→ RF–C(O)• + •OH + NO
(14)
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Additionally, the peroxy radical could also react with NO or another peroxy radical in
345
order to generate a corresponding alkoxy radical:
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RF–C(OH)=NOO• + NO/ RF–C(OH)=NOO• → RF–C(OH)=NO•
(16)
The RF–C(OH)=NO radical then undergoes H-migration through a 5-membered
348
transition state in order to produce RF–C(O)NOH (Atkinson et al. 2007):
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RF–C(OH)=NO• → RF–C(O)NOH
(17)
Noting that the N atom in RF–C(O)NOH also has a lone electron, the compound is
351
expected to react with another N atom with a lone electron, leading to N2O formation.
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In the reaction chamber, the following reactions may result in the generation of N2O:
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RF–C(O)NOH + RF–C(O)NOOH → 2 RF–C(O)• + 2 •OH + N2O
(18)
RF–C(O)NOH + NO → RF–C(O)• + •OH + N2O
(19)
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The atmospheric fate of the RF–C(O) radical has been well established, where CO, CO2,
356
COF2, and CF3C(O)F are the degradation products (Guo et al., 2018; Sulbaek et al.,
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2017). In this work, CO was not observed due to its rapid conversion into CO2.
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In order to verify the proposed mechanism, a product experiment was performed
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Figure 5. The reaction mechanism of perfluoronitriles with an OH radical. The black pathway was proposed by Sulbaek Andersen et al. (2017), and the blue pathway is proposed in this work.
359
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with the introduction of NO. PFEN was chosen because its IR absorption features have
364
no overlap with the products. As shown in Figure 6, in the absence of NO, the molar
365
yield of N2O was approximately 13%, which is comparable with the above results.
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During the proceeding of the reaction, pure NO gas was then introduced into the
367
reaction chamber at a flow rate of 0.05 mL min–1 at STP, which resulted in a magnitude
368
of NO of the order 1013, and the molar yield of N2O increased to 98%. These
369
observations indicate that reaction (16) was practically the sole reaction pathway for the
370
CF3C(OH)=NOO radical, due to the high concentration of NO in the reaction chamber.
371
These inferences indicate that the proposed reaction mechanism in this work is
372
reasonable. Additionally, reaction 14 and 16 lead to the formation of NO and N2O,
373
respectively. According to the N2O molar yield of approximately 10% for all the
374
perfluoronitriles, k16 × [NO]/k14 = 1/9 could be obtained. During the OH reaction of the
375
perfluoronitriles, the upper limit for the average NO concentrations was determined to
376
be approximately 0.05 ppb through a Facsimile simulation (Supporting Information).
377
Therefore, in the urban area, the N2O formation will dominate as atmospheric NO
378
concentration (several ppb) will be much higher than that of the reaction chamber
379
(Corradi et al., 1998; Han et al., 2011). In the remote area such as Antarctic, the
380
concentration of NO was measured to be 0.01–0.09 ppb (Masclin et al., 2013; Frey et al.,
381
2015), where the formation of N2O due to the degradation of perfluoronitriles will have
382
a limited importance.
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6x1013
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4x1013
slope = 0.98
NO was added 13
2x1013 1x1013
slope = 0.13
0 5x1013
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[N2O]/molecule cm-3
5x1013
1x1014
2x1014
2x1014
3x1014
[PFEN]decay/molecule cm-3 383
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4. Conclusion
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Figure 6. N2O concentration versus the loss of PFEN obtained without (solid square) and with (empty square) introduction of NO at 298 K.
384
In this work, the atmospheric lifetimes of PFEN, PFPN, n-PFBN, and i-PFBN
388
were measured to be 6.6, 10, 12, and 54 years, respectively. And the contribution to
389
climate change of these perfluoronitriles is significantly lower than that of SF6. The
390
main degradation products of perfluoronitriles were CO/CO2, NO/N2O, COF2, and
391
CF3C(O)F. In the atmosphere, NO will be rapidly converted into HONO through the
392
reaction with the OH radical, which is then removed by precipitation (Bark et al., 1995).
393
N2O was noted as an important monitored greenhouse gas in the IPCC report (Stocker
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et al., 2013), however, its global warming potential is significantly lower than that of
395
SF6. Additionally, the concentration of N2O derived from the degradation of
396
perfluoronitriles will remain within several ppq, which is a negligible source of N2O
397
considering its global mean concentration was 324 ppb in 2011 (Stocker et al., 2013).
398
COF2 and CF3C(O)F could be removed from the atmosphere by clouds within one to
399
several weeks, producing HF, CF3C(O)OH and CO2. Even assuming that the global
400
emissions of perfluoronitriles are 10 kt per year, the level of HF in precipitation due to
401
the degradation of perfluonitriles would be order of 10-10-10-9 molar, and the additional
402
acidity could be negligible (Wallington et al., 2015). CF3C(O)OH is ubiquitous in
403
precipitation and ocean water even in remote area. CF3C(O)OH containing in ocean
404
water is estimated to be 268 million tonnes (Frank et al., 2002), hence, the natural
405
environmental loading of CF3C(O)OH will extremely exceeds that from the degradation
406
of perfluoronitirls. Therefore, the secondary degradation products of perfluoronitriles
407
are considered to have no significant environmental impact.
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Acknowledgments This work was financially supported by the internal funds of AIST.
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kOH of four kinds of perfluoronitriles were measured by the relative method.
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NO formation following the degradation of perfluoronitriles was proved. N2O formation was observed and a revised mechanism was proposed. Environmental impact of perlfuoronitriles was evaluated.
S1352-2310(18)30764-7
DOI:
https://doi.org/10.1016/j.atmosenv.2018.10.066
Reference:
AEA 16363
To appear in:
Atmospheric Environment
Received Date: 11 September 2018 Revised Date:
28 October 2018
Accepted Date: 30 October 2018
Please cite this article as: Guo, Q., Chen, L., Kutsuna, S., Quan, H., Mizukado, J., Atmospheric chemistry of perfluoronitriles: Environmental impact and experimental evidence related to N2O and NO formation, Atmospheric Environment (2018), doi: https://doi.org/10.1016/j.atmosenv.2018.10.066. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Atmospheric
Chemistry
of
Perfluoronitriles:
2
Experimental Evidence Related to N2O and NO Formation
3
Qin Guoa, b, Liang Chenb, *, Shuzo Kutsunab, Hengdao Quana, b, Junji Mizukadob
4
a
5
South Zhonguancun Street, Haidian District, Beijing 100081, PR China.
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b
7
Higashi, Tsukuba, Ibaraki 305−8565, Japan.
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*
9
l−[email protected] (L. Chen)
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Corresponding author: Tel: +81 29 861 9379. Fax: + 81 29 861 4457. E−mail address:
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and
National Institute of Advanced Industrial Science and Technology (AIST), 1−1−1
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School of Chemistry & Chemical Engineering, Beijing Institute of Technology, 5
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Environmental
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Abstract The atmospheric chemistry of perfluoronitriles, the proposed replacement gases of
21
SF6, has been investigated using an atmospheric reaction chamber. N2O formation was
22
first observed following the reaction between perfluoronitriles and OH radicals, then
23
NO formation was verified through experimentation. COF2, CF3C(O)F, and CO2
24
generation was observed, and a revised oxidation mechanism for perfluoronitriles in the
25
atmosphere is proposed. Additionally, the rate coefficients related to OH radicals were
26
measured for the perfluoronitriles of CF3CN, CF3CF2CN, CF3CF2CF2CN, and
27
(CF3)2CFCN: their atmospheric lifetimes were 6.6, 10, 12, and 54 years, their radiative
28
efficiencies were evaluated to be 0.188, 0.223, 0.317, and 0.231 W m–2 ppb–1, and their
29
100-year time horizon GWPs were 212, 374, 633, and 1705, respectively. The findings
30
contained in this study indicate that perfluoronitriles present an insulating gas
31
replacement option with a comparatively low environmental impact.
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Keywords: Perfluoronitriles; atmospheric lifetimes; global warming potentials;
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degradation mechanism.
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1. Introduction Sulfur hexafluoride (SF6) exhibits excellent dielectric strength, heat transfer
39
capacity, and electric arcs interruption and as a result, SF6 is the most common
40
insulation gas in current use. For many years, most of the SF6 that was produced was
41
used in high-voltage gas circuit breakers and gas insulated switchgear (Beroual et al.,
42
2017), with an emission of approximately 10 kt/a (Stocker et al., 2013). Such
43
large-scale usage leads to an abundance of SF6 that is 3 orders of magnitude higher than
44
that in preindustrial times (Rabie et al., 2018). It is noticeable that SF6 is an extremely
45
potent greenhouse gas with an atmospheric lifetime of 3200 years and a 100-year time
46
horizon global warming potential (GWP100) of 23500. SF6 has been a monitored
47
substance since the formation of the Kyoto Protocol and worldwide efforts were taken
48
in order to develop a low-GWP compound to replace SF6. Perfluoronitriles (PFNs), such
49
as CF3CN (PFEN), CF3CF2CN (PFPN), and CF3CF2CF2CN (n-PFBN), were proved to
50
exhibit around 2.7 times the dielectric strength of SF6 in a uniform field (Plump et al.,
51
1962). Considering the fact that perfluoronitriles generally exhibit relatively high
52
toxicity, nitrite esters with a low boiling point were used as additives (Yamauchi, 1985).
53
Recently, a branched perfluoronitrile with a high dielectric strength and lower toxicity,
54
(CF3)2CFCN (i-PFBN), has been the center of attention with regards SF6 potential
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replacement. Pilot gas-insulated busbars have been installed in a substation in England,
56
and several 145 kV gas-insulated substations are planned in Europe (Kieffel et al., 2016).
57
Additionally, during its usage as an arc extinguishing gas, the major thermal
58
decomposition products of i-PFBN have been identified as CF3CN and CF3CF2CN by
59
Kieffel et al. (2017). Based on these considerations, it is essential to evaluate the
60
environmental impacts of these perfluoronitriles prior to their large-scale application.
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As for PFEN, PFPN, and n-PFBN, there was no previous study related to their
62
atmospheric chemistry. In contrast, the rate coefficients for the reactions of i-PFBN with
63
respect to OH radicals, Cl atoms and O3 were measured using a relative method at 296 ±
64
2 K by Sulbaek Andersen et al. (2017), while Blázquez et al. (2017) measured the rate
65
coefficients related to OH radicals in the 278–358 K range using the absolute rate
66
method. Although the OH radical reaction rate coefficient measured at 298 K by
67
Blázquez et al. showed a good agreement with that measured by Sulbaek Andersen et al.,
68
its value was significantly higher than that calculated using the Arrhenius expression, as
69
reported by Blázquez et al. Moreover, Blázquez et al. reported i-PFBN radiative
70
efficiency (RE) that was 32% higher than that proposed by Sulbaek Andersen et al. With
71
respect to the study of perfluoronitrile atmospheric chemistry, the key point is that
72
clarification is required with regards the reaction mechanism for OH addition to the –
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C≡N group. Previously, a theoretical study of the reaction pathway between the OH
74
radical and –C≡N group was conducted for the HCN + OH reaction (Cicerone et al.,
75
1983). Recently, the reaction pathways of i-PFBN with OH radicals was studied by
76
Sulbaek Andersen et al. (2017) using experimental and theoretical methods and a
77
reaction scheme of –C≡N with OH was proposed. The proposed reaction pathway was
78
theoretically reasonable, however, corresponding products of NO and NO2 were not
79
observed in the experiment. Therefore, a study of the kinetics and mechanisms related
80
to perfluoronitriles are necessary.
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In this work, we evaluated the OH rate coefficient of four different perfluoronitriles
82
using a relative rate method, in which there persisted a high concentration of OH in the
83
reaction chamber. Infrared absorption cross sections and radiative efficiencies were also
84
obtained for these systems. Moreover, the decay of perfluoronitriles of more than 50%
85
during product experiments allowed for all of the degradation products to be
86
distinguished and observed. N2O and NO were produced following the degradation of
87
perfluoronitriles. Based on the experimental evidence, a revised reasonable reaction
88
mechanism for OH radical with perfluoronitriles is proposed and their environmental
89
impact is evaluated.
90
2. Materials and Method
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The experiments were carried out in an 11.5-L cylindrical double-layer quartz
92
chamber, as described in detail in our previous work (Chen et al., 2003). Herein, OH
93
radicals were produced through the photolysis of O3 under 254-nm UV irradiation in the
94
presence of water:
96
O (1D) + H2O → 2OH
(1)
SC
O3 + hν → O (1D) + O2
(2)
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An O3/O2 (3%, O3) gas mixture was generated from pure O2 using a silent-discharge
98
ozone generator (ECEA-1000, Ebara Jitsugyo Co., Japan). CF3CN (purity: 98%),
99
CF3CF2CN (purity: 98%), and CF3CF2CF2CN (purity: 98%) were purchased from PCR
100
incorporated (USA). (CF3)2CFCN (purity: 99.5%) was purchased from Beijing Yuji
101
Sciences & Technology Co., Ltd. (China). The CF3CH2CHF2 reference compound
102
(purity: ≥99.9%) was purchased from Central Glass Co., Ltd. (Japan), CHF2CH3 (purity:
103
>99%) was obtained from Sigma-Aldrich (America), and CF3CH2F (purity: 99.9%) and
104
CH2F2 (purity: 99%) were purchased from SynQuest Labs, Inc. (America). NO (purity:
105
99%) was obtained from Takachiho Chemical Industrial Co., Ltd. (Japan).
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In the relative rate studies performed at 253–343 K, the perfluoronitriles, the
107
reference compound, and H2O were introduced into the reaction chamber. He gas was
108
used to dilute the reactants to an initial pressure of 200 Torr, which led to relatively low
6
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O (1D) quenching. An O3/O2 gas mixture was introduced into the chamber with a flow
110
rate of 6–10 mL min–1 at STP, and 6–10 UV lamps were used for irradiation. In this
111
manner, a nearly constant OH radical concentration could be maintained at
112
approximately 1010 radical cm–3 during measurement (Chen et al., 2003). The
113
concentrations of perfluoronitriles and reference compounds were monitored by Gas
114
Chromatography with flame ionization detector (GC-FID) at 8–12 min intervals. The
115
capillary column was a TC-Bond Q metal (length: 30 m; i.d.: 0.53 mm; GL Sciences
116
Inc., Japan), and the column temperature was set to 373 or 403 K.
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109
In the reaction chamber, the initial concentrations of the reactants were
118
approximately 4.7–9.5 × 1014 (perfluoronitriles), 4.7–7.1 × 1014 (CF3CH2F), 4.7–7.1 ×
119
1014 (CHF2CH3), 3.5–5.9 × 1014 (CH2F2), 3.7–4.8 × 1014 (CF3CH2CHF2), and 1.2 × 1018
120
(H2O) molecules cm−3, respectively. After 60–120 min reaction, the loss of any reactant
121
was more than 50%. Additionally, the loss of these reactants due to wall reaction and
122
photolysis was measured to be 0.5–1%, which was less than the analysis uncertainty of
123
GC-FID (2%).
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124
Considering that reactions with O3 or O (1D) could be negligible compared with
125
those with OH radicals (Sulbaek Andersen et al., 2017; Chen et al., 2003), the loss of
126
perfluoronitriles relative to reference compounds was plotted using the following
7
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127
128
expression: ln
+
=
!"
#ln
$
$
%
+
%
&
(I)
where [Perfluoronitriles]0(t) and [Reference]0(t) are the concentrations of perfluoronitriles
130
and reference compounds (CF3CH2F, CH2F2, or CF3CH2CHF2) at time 0(t), respectively.
131
Dn = nln0.9983 is a correcting factor accounting for the loss of reactants derived from
132
GC sampling, where n is the number of GC-FID samplings.
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In order to evaluate the integrated absorption cross sections and radiative
134
efficiencies of perfluoronitriles, their IR spectra at room temperature were recorded by
135
averaging 64 interferograms at a resolution of 0.5 cm–1. A FT-IR spectrometer with a
136
DTGS detector and a glass cell (optical path length: 10 cm, KBr window) were used.
137
The concentrations of perfluoronitriles in the glass cell were calculated from the
138
pressure, which was measured through a vacuum line linking two pressure meters
139
(MKS Baratron 626B, measuring range: 0–10 Torr; MKS cold cathode transducer 974B,
140
measuring range: 1 × 10–8–1500 Torr). During the investigation of the products and
141
reaction mechanisms of perfluoronitriles with OH radicals, the reactant mixture was
142
irradiated for 90 minutes. Then, their products were detected using a FT-IR
143
spectrometer (resolution: 0.5 cm–1, detector: MCT) with a nickel-coated aluminum
144
multiple-reflection IR cell (volume: 200 cm3; optical path length: 3 m) at 3-min
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intervals. The degradation products were then identified and quantified according to
146
these recorded spectra.
147
3. Results and discussion
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3.1. Relative rate study of OH + perfluoronitriles
149
The rate coefficients of reaction (3) were measured relative to reaction (7) and (8),
150
the rate coefficients of reaction (4) and (5) were measured relative to reaction (7) and
151
(9), while those of reaction (6) were measured relative to reaction (7) and (10):
152
OH + CF3CN → Products
153
OH + CF3CF2CN → Products
154
OH + CF3CF2CF2CN → Products
(5)
155
OH + (CF3)2CFCN → Products
(6)
156
OH + CF3CH2F → Products
(7)
157
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OH + CHF2CH3 → Products
(8)
159
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(4)
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158
(3)
OH + CH2F2 → Products
(9)
OH + CF3CH2CHF2 → Products
(10)
160
Figure 1 shows the loss of perfluoronitriles versus that of the reference compounds.
161
The decay of all the reactants was greater than 30%. Linear least-square fitting for these
162
plots gives reaction rate coefficient ratios of: k3/k7 = 1.76 ± 0.02, k3/k8 = 0.230 ± 0.003,
9
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k4/k7 = 1.26 ± 0.01, k4/k9 = 0.496 ± 0.007, k5/k7 = 1.02 ± 0.01, k5/k9 = 0.413 ± 0.004,
164
k6/k7 = 0.234 ± 0.005, and k6/k10 = 0.159 ± 0.004. Herein, the errors only represent two
165
standard deviations resulting from using GC-FID. Using the reported data of k7 = 4.5 ×
166
10–15 cm3 molecule–1 s–1 (± 10%), k8 = 3.3 × 10–14 cm3 molecule–1 s–1 (± 7%), k9 = 1.1 ×
167
10–14 cm3 molecule–1 s–1 (± 7%), and k10 = 7.0 × 10–15 cm3 molecule–1 s–1 (± 15%)
168
(Burkholder et al., 2015), the final reaction rate coefficient values of k3, k4, k5, and k6 at
169
298 K were (7.76 ± 1.08) × 10–15, (5.53 ± 0.76) × 10–15, (4.60 ± 0.60) × 10–15, and (1.08
170
± 0.23) × 10–15 cm3 molecule–1 s–1, respectively. Likewise, the rate coefficients of
171
perfluoronitriles at other temperatures were measured, and the results provided in Table
172
S1.
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2.5
PFEN 2.0
PFPN
2.0
CF3CH2F
CF3CH2F 1.5
1.0
CH2F2
0 2.0
CH2F2
0.5
0.5
0
0.5
1.0
1.5
2.0
0 3.0 0 0.4
2.5
0.5
i-PFBN
CF3CH2F 1.5
0.3
1.0
CF3CH2CHF2
0.1
CH2F2
0
2.0
CF3CH2F
0.2
0.5
1.5
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n-PFBN
1.0
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1.0
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1.5
0
0
0.5
1.0
1.5
2.0
0
0.5
1.0
1.5
2.0
ln[Reference]0/[Reference]t + Dn
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173
176
squares); CH2F2 (empty squares); CF3CH2CHF2 (empty circles).
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175
Figure 1. Loss of perfluoronitriles versus that of reference compounds in the presence of OH radicals under 200 Torr initial pressure in He, at T=298 K. CF3CH2F (solid
174
Figure 2 shows the temperature dependence of k3, k4, and k5 for T=253–328 K, and
178
k6 for T=268–343 K. Non-linear least-square analysis for these plots gives the Arrhenius
179
expressions:
180
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k3 (253–328 K) = (3.09 ± 0.30) × 10–13 exp [–(1089 ± 28)/T]
181
k4 (253–328 K) = (4.46 ± 0.67) × 10–13 exp [–(1311 ± 44)/T]
182
k5 (253–328 K) = (3.08 ± 0.55) × 10–13 exp [–(1251 ± 53)/T]
183
k6 (268–343 K) = (1.18 ± 0.07) × 10–13 exp [–(1397 ± 18)/T] 11
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k3
-14
1/2 k4
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k5
10
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kOH (molecule cm-3 s-1)
10
-15
k6
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k6 (Sulbaek Andersen et al.) 2.6
2.8
3.0
3.2
3.4
3.6
3.8
4.0
4.2
1000/T (K-1)
184
188
The data obtained in this work for k6 was compared with those measured by
189
Blazquez et al. and Sulbaek Andersen et al., and their results are also plotted in Figure 2.
190
The Arrhenius plot of k6 measured in this work showed a good agreement with the
191
majority of the data points provided by Blazquez et al.’s study, although at 298 K, the
192
measured values of Blazquez et al. and Sulbaek Andersen et al. were approximately
193
25% higher than those of both the present study and the data calculated from an
194
Arrhenius expression of k6 by Blazquez et al. Additionally, there have been no previous
195
reports related to the k3, k4, and k5 rate coefficients for these systems. The results shown
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Figure 2. Temperature dependence of rate coefficients for the perfluoronitriles + OH reaction. The solid line is the fitted Arrhenius curve. The error bars include two standard deviations and uncertainties derived using reference compounds.
185
12
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in Figure 2 indicate that the reactivity of perfluoronitriles towards OH radicals decreases
197
with increasing numbers of fluorine atoms. i-PFBN has a significant lower reactivity
198
than n-PFBN, which may be due to the stronger electron affinity and greater steric effect
199
of –CF(CF3)2 compared with –CF2CF2CF3.
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3.2. Global warming potentials
201
In a real atmosphere, reactions with O3 and Cl atoms will provide a negligible
202
contribution to the degradation of perfluoronitriles, while their wet or dry deposition is
203
not to be expected (Sulbaek Andersen et al., 2017). Moreover, perfluoronitriles will not
204
undergo photolysis because they do not absorb in the actinic region (Sulbaek Andersen
205
et al., 2017). Considering the perfluoronitriles will be well mixed in the atmosphere,
206
their atmospheric lifetime could be calculated relative to that of CH3CCl3 (MCF):
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/0*/,)1
=
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'()*+,-*.
23
(5657)
9:;<=:>?@ :@<9
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207
SC
200
(5657)
× 'BCD
(II)
208
where '()*+,-.*.
209
CH3CCl3, respectively, and 'CEF CC,F = 6.1 years (WMO, 2014). kMCF(272 K) and
210
kPerfluoronitrles(272 K) are rate coefficients for the reactions with MCF and
211
perfluoronitriles, respectively, and kMCF(272 K) = 6.14 ×10–15 cm3 molecule–1 s–1
212
(Burkholder et al., 2015). And then, the atmospheric lifetimes of PFEN, PFPN, n-PFBN,
213
and i-PFBN were evaluated to be 6.6, 10, 12, and 54 years, respectively. Herein, the
and 'BCD are atmospheric lifetimes for perfluoronitriles and
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lifetime of i-PFBN was close to the value of 47 years reported by Blázquez et al. (2017),
215
while a value of 22 years was reported by Sulbaek Andersen et al. (2017). Generally,
216
uniform mixing of a gas in the atmosphere requires a time scale of years, and so these
217
measured lifetimes could be considered to be the global-mean lifetimes of these
218
perfluoronitriles.
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214
IR spectra were recorded for the perfluoronitriles in an atmospheric pressure of N2
220
diluent gas at room temperature. Figure 3 shows the IR absorption spectra of
221
perfluoronitriles, which were the average results of more than ten measurements. The
222
integrated cross sections (unit: cm2 molecule–1 cm–1) at 500–2400 cm–1 were (1.65 ±
223
0.07) × 10–16 for PFEN, (1.87 ± 0.06) × 10–16 for PFPN, (2.37 ± 0.04) × 10–16 for
224
n-PFBN, and (2.32 ± 0.04) × 10–16 for i-PFBN. The result for i-PFBN herein was
225
slightly higher than that measured by Sulbaek Andersen et al., which was because –C≡N
226
absorption was also taken into consideration in this work. However, a significantly
227
higher integrated cross section of (2.88 ± 0.01) × 10–16 cm2 molecule–1 cm–1 was
228
reported by Blázquez et al. According to the discussion in Blázquez et al.’s work, the
229
different buffer gas and spectral resolution were not expected to account for this
230
discrepancy. Hence, the most possible source of this discrepancy is the different total
231
pressure (700 Torr in Sulbaek Andersen et al.’s work, 8–100 Torr in Blázquez et al.’s
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work, and 760 Torr in this work). We recorded the spectra under <10 Torr, the results
233
were consistent with that obtained under 760 Torr. Therefore, the source of this
234
significant discrepancy is unclear.
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Using the revised Pinnock curve based on the 1 cm–1 Oslo line-by-line (LBL)
236
model (Hodnebrog et al., 2013), the instantaneous radiative efficiencies (IREs) of PFEN,
237
PFPN, n-PFBN, and i-PFBN were then calculated to be 0.182, 0.213, 0.300, and 0.213
238
W m–2 ppb–1, respectively. Furthermore, a 10% increase in these values was introduced
239
in order to account for the stratospheric temperature adjustment, and an S-shaped
240
lifetime correction factor was also adopted using the following expression: IJK
LMNJO
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G(τ) =
10ST < ' < 10T VWXYZ
(III)
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241
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235
where a, b, c, and d are constants with values of 2.962, 0.9312, 2.994, and 0.9302,
243
respectively, and τ is the atmospheric lifetime. Hence, the final values for the REs were
244
as follow: 0.188 W m–2 ppb–1 (PFEN), 0.223 W m–2 ppb–1 (PFPN), 0.317 W m–2 ppb–1
245
(n-PFBN), and 0.231 W m–2 ppb–1 (i-PFBN).
247
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242
The GWPs of the perfluoronitriles were calculated using the following formula: `ab(c (0) 3de (0)
[\](D^1 (_) = `ab(
=
`JfLS)gh S
i
`ab(3de (0)
j
(IV)
248
where A is RE, τ is the atmospheric lifetime, and t is the given time horizon.
249
k[\]Cle (_) is the absolute GWP of CO2 for time horizon t, with values of 2.495 × 10– 15
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, 9.171 × 10–14 and 32.17 × 10–14 W m–2 yr kg–1 for time horizons of 20, 100, and 500
251
years, respectively (Hodnebrog et al., 2013). Substituting the corresponding values of
252
these parameters, the GWPs of the perfluoronitriles were obtained, and the results listed
253
in Table 1.
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PFEN
PFPN
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3 2 1 0 6
n-PFBN
4 2 0 6
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Absorption cross section (10-18 cm2 molecule-1)
250
i-PFBN
4 2
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0 2400
254
2200
2000
1800
1600
1400
1200
1000
800
600
-1
Wavenumber (cm )
257
Table 1. The atmospheric properties of the perfluoronitriles and related compounds.
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Figure 3. IR absorption spectra for perfluoronitriles between 500 and 2400 cm–1 recorded in a N2 diluent gas atmosphere at room temperature.
255
kOH (298 K) × 1015
A × 1013
Ea/R
Lifetime
RE
Compound 3
–1
–1
3
–1
–1
GWP100
Ref.
0.188
212
this work
10
0.223
374
this work
12
0.317
633
this work
(cm molecule s )
(cm molecule s )
(K)
(years)
(W m–2 ppb–1)
PFEN
7.76 ± 1.08
3.09 ± 0.30
1089 ± 28
6.6
PFPN
5.53 ± 0.76
4.46 ± 0.67
1311 ± 44
n-PFBN
4.60 ± 0.60
3.08 ± 0.55
1251 ± 53
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1.08 ± 0.23
1.18 ± 0.07
1397 ± 18
54
0.231
1705
this work
i-PFBN
1.45 ± 0.25
—
—
22
0.217
1490
Sulbaek Andersen et al. (2017)
i-PFBN
1.47 ± 0.19
5.9 ± 3.2
1856 ± 162
47
0.280
3646
Blázquez et al. (2017)
N 2O
—
—
—
116
0.17
298
Stocker et al. (2013)
SF6
—
—
—
850
0.575
22500
Stocker et al. (2013)
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i-PFBN
3.3. Degradation products and mechanism
259
Experiments were performed by irradiating a gas mixture consisting of 6–10 × 1014
260
molecule cm–3 perfluoronitriles and 5.57 × 1017 molecule cm–3 H2O, an O3/O2 gas
261
mixture was continuously introduced at a flow rate of 4–8 mL min–1 at 298 ± 1 K. The
262
IR spectra were recorded with a time interval of 3 min, and the perfluoronitrile decays
263
were 40–80% after UV irradiation. Figure 4a and Figure 4b show results recorded
264
before and after 30-min UV irradiation, respectively. Figure 4c shows the absorption
265
spectrum for products observed for the reaction of i-PFBN with OH radicals, which was
266
obtained by subtracting the IR features of i-PFBN from Figure 4b. CO2, COF2,
267
CF3C(O)F, and N2O were identified as the major products through comparison with the
268
reference IR spectra. Figure 4h plots the concentration of COF2, CF3C(O)F, and N2O
269
versus the decreasing i-PFBN concentration, which provided molar yields of 102 ± 2%
270
for COF2, 84 ± 2% for CF3C(O)F, and 9.5 ± 0.1% for N2O. COF2 and CF3C(O)F exhibit
271
significant hydrolysis due to the presence of H2O in the reaction chamber. The results of
272
product identification and quantification for other perfluoronitriles are represented in
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Figures S1–S3. PFPN and n-PFBN gave the same products as i-PFBN, while the PFEN
274
did not produce CF3C(O)F. Additionally, the product molar yields for perfluoronitriles
275
are listed in Table S2.
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273
It was expected that reactions between perfluoronitriles and OH would initiate in
277
the –C≡N group, and therefore that their reaction pathways would be similar to that of
278
HCN + OH. The mechanisms of the OH addition reaction with HCN and CH3CN have
279
previously been studied using theoretical methods (Cicerone et al., 1983; Galano et al.,
280
2007). It was proposed that OH addition only occur at the C atom of the –C≡N group,
281
giving a –C(OH)=N radical, of which the N atom then reacts with O2 in order to
282
produce a –C(OH)=NOO radical. This radical could easily undergo H migration from
283
OH to OO (Sulbaek Andersen et al., 2017; Galano et al., 2007). Then, the –C(O)NOOH
284
group is converted into a –C(O) radical, NO, and an OH radical. According to their
285
calculations, of Sulbaek Andersen et al. proposed the same reaction lines (Sulbaek
286
Andersen et al., 2017). However, neither the formation of NO/NO2 nor the regeneration
287
of the OH radical was proved by experimental methods. Due to the extremely high OH
288
radical reactivity of NO/NO2 compared to those of perfluoronitriles, NO/NO2 will react
289
rapidly with an OH radical, giving HNO2/HNO3 (Burkholder et al., 2015). In our
290
experimental system, due to the presence of high concentration of water, most of HNO2
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and HNO3 will mainly dissolved in water on the wall of reactor, and their IR features
292
are hardly to be observed. However, NO formation could be manifested due to the
293
generation of CF3C(O)F during the experiments using PFPN and n-PFBN. In the case of
294
perfluoronitriles, the radical of CxF2x+1O is produced during the OH-initiated reaction.
295
In the absence of NO, the fate of the CxF2x+1O radical has been well established (Calvert
296
et al., 2011). The (CF3)2CFO radical is converted into COF2 and CF3C(O)F, while COF2
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is produced following the degradation of CF3O, CF3CF2O, and CF3CF2CF2O radicals.
298
However, as shown in Figures S2 and S3, the formation of CF3C(O)F was observed for
299
both PFPN and n-PFBN, with molar yields of 11 ± 1% and 28 ± 1%, respectively, and
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F-abstraction by NO was considered as the only pathway capable of producing
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CF3C(O)F (Taketani et al., 2005; Wallington et al., 1995):
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CF3CF2O• + NO → CF3C(O)F + FNO
(11)
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Consequently, the formation of NO during OH-initiated reactions of perfluoronitriles
304
was proved, although the IR absorption of NO was not observed. Herein, the IR features
305
of FNO were not observed due to its very short lifetime (Wallington et al., 1995). In
306
addition, reactions between NO and CF3O were also possible, producing COF2 and
307
FNO, while it is assumed that reactions between NO and (CF3)2CFO or CF3CF2CF2O
308
do not occur due to the fact that no CF3C(O)CF3 and CF3CF2C(O)F generation was
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detected, which mainly depends on the lifetimes of these radicals. Similar observations
310
related to the CF3O and CF3CF2O radicals have been reported (Bark et al., 1995). In the
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case of the CF3O radical, it could react with the CF3OO radical to produce the
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CF3OOOCF3 trioxide compound. However, the CF3CF2O radical will not react with the
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CF3OO radical and instead undergoes β-scission giving CF3 radical and COF2 due to its
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shorter lifetime.
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0.6 (a) i-PFBN
(h) Products yield
0.2 0.0 0.6 (b) i-PFBN + products 0.4
0.02
N2O
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0.0 0.04 (c) Products
CF3C(O)F
COF2
N2O
0
1x1014
[i-PFBN]decay/moecule cm
(g) N2O
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2200 2000 1800 1600 1400 1200 1000 800
316 317 318 319 320
-3
(f)
(e) CF3C(O)F
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0 3x1014
2x1014
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(d) COF2
1x1014
COF2
O3
0.00
CF3C(O)F
-3
Absorbance
0.2
2x1014
COF2
[Products]/moecule cm
0.4
2200
2100
2000
Wavenumber (cm-1)
Figure 4. IR spectra recorded before (a) and after (b) 30-min UV irradiation of a gas mixture containing 8.49 × 1014 molecule cm–3 i-PFBN and 5.57 × 1017 molecule cm–3 H2O with continuous O3/O2 gas introduction at 298 ± 1 K; (c) IR spectra of products obtained by subtracting (a) from (b); (f) IR spectra of (a) at 2200–2350 cm–1; (d, e, g) reference IR spectra of COF2, CF3C(O)F, and N2O; (h) yields of products.
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It is of note that N2O was also identified as a product of the reaction between
322
perfluoronitriles and the OH radical. As shown in Table S2, all the perfluoronitriles lead
323
to N2O formation with a molar yield of approximately 10%, which indicates that there
324
was another reaction pathway competing with that described by Sulbaek Andersen et al.
325
Observing the fact that N2O has a structure of N=N=O, it was expected that its
326
formation resulted from an attack on a N atom with a lone pair of electrons by another
327
N atom in a –N–O structure. According to the reaction mechanism proposed in previous
328
studies, the former could be a RF–C(OH)=N radical (RF: CF3, CF3CF2, CF3CF2CF2,
329
(CF3)2CF), while the latter may be NO or RF–C(O)NOOH. In order to investigate the
330
formation pathway of N2O, a different O3/O2 gas flow rate (4 mL min–1 and 8 mL min–1)
331
was adopted during the product experiments for n-PFBN. It was found that the molar
332
yield of N2O was independent of the concentration of O2 in the reaction chamber
333
(Figure S4), which indicated that the RF–C(OH)=N radical is not related to the
334
generation of N2O, and that O2 addition to N is its sole pathway. Therefore, reaction
335
pathways for perfluoronitriles with an OH radical are proposed by combining the results
336
of previous studies (Cicerone et al., 1983; Galano et al., 2007; Sulbaek et al., 2017) with
337
the results in this work (Figure 5). The reaction lines are as follow:
338
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RF–C≡N + OH → RF–C(OH)=N•
(12)
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RF–C(OH)=N• + O2 → RF–C(OH)=NOO•
(13)
The radical is then considered to be transformed into RF–C(O)NOOH with a very low
341
energy barrier, which leads to the formation of NO:
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RF–C(OH)=NOO• → RF–C(O)NOOH
343
RF–C(O)NOOH → RF–C(O)NO• + •OH→ RF–C(O)• + •OH + NO
(14)
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Additionally, the peroxy radical could also react with NO or another peroxy radical in
345
order to generate a corresponding alkoxy radical:
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RF–C(OH)=NOO• + NO/ RF–C(OH)=NOO• → RF–C(OH)=NO•
(16)
The RF–C(OH)=NO radical then undergoes H-migration through a 5-membered
348
transition state in order to produce RF–C(O)NOH (Atkinson et al. 2007):
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RF–C(OH)=NO• → RF–C(O)NOH
(17)
Noting that the N atom in RF–C(O)NOH also has a lone electron, the compound is
351
expected to react with another N atom with a lone electron, leading to N2O formation.
352
In the reaction chamber, the following reactions may result in the generation of N2O:
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RF–C(O)NOH + RF–C(O)NOOH → 2 RF–C(O)• + 2 •OH + N2O
(18)
RF–C(O)NOH + NO → RF–C(O)• + •OH + N2O
(19)
355
The atmospheric fate of the RF–C(O) radical has been well established, where CO, CO2,
356
COF2, and CF3C(O)F are the degradation products (Guo et al., 2018; Sulbaek et al.,
22
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2017). In this work, CO was not observed due to its rapid conversion into CO2.
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In order to verify the proposed mechanism, a product experiment was performed
360
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Figure 5. The reaction mechanism of perfluoronitriles with an OH radical. The black pathway was proposed by Sulbaek Andersen et al. (2017), and the blue pathway is proposed in this work.
359
363
with the introduction of NO. PFEN was chosen because its IR absorption features have
364
no overlap with the products. As shown in Figure 6, in the absence of NO, the molar
365
yield of N2O was approximately 13%, which is comparable with the above results.
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During the proceeding of the reaction, pure NO gas was then introduced into the
367
reaction chamber at a flow rate of 0.05 mL min–1 at STP, which resulted in a magnitude
368
of NO of the order 1013, and the molar yield of N2O increased to 98%. These
369
observations indicate that reaction (16) was practically the sole reaction pathway for the
370
CF3C(OH)=NOO radical, due to the high concentration of NO in the reaction chamber.
371
These inferences indicate that the proposed reaction mechanism in this work is
372
reasonable. Additionally, reaction 14 and 16 lead to the formation of NO and N2O,
373
respectively. According to the N2O molar yield of approximately 10% for all the
374
perfluoronitriles, k16 × [NO]/k14 = 1/9 could be obtained. During the OH reaction of the
375
perfluoronitriles, the upper limit for the average NO concentrations was determined to
376
be approximately 0.05 ppb through a Facsimile simulation (Supporting Information).
377
Therefore, in the urban area, the N2O formation will dominate as atmospheric NO
378
concentration (several ppb) will be much higher than that of the reaction chamber
379
(Corradi et al., 1998; Han et al., 2011). In the remote area such as Antarctic, the
380
concentration of NO was measured to be 0.01–0.09 ppb (Masclin et al., 2013; Frey et al.,
381
2015), where the formation of N2O due to the degradation of perfluoronitriles will have
382
a limited importance.
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6x1013
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4x1013
slope = 0.98
NO was added 13
2x1013 1x1013
slope = 0.13
0 5x1013
0
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3x10
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[N2O]/molecule cm-3
5x1013
1x1014
2x1014
2x1014
3x1014
[PFEN]decay/molecule cm-3 383
386
4. Conclusion
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Figure 6. N2O concentration versus the loss of PFEN obtained without (solid square) and with (empty square) introduction of NO at 298 K.
384
In this work, the atmospheric lifetimes of PFEN, PFPN, n-PFBN, and i-PFBN
388
were measured to be 6.6, 10, 12, and 54 years, respectively. And the contribution to
389
climate change of these perfluoronitriles is significantly lower than that of SF6. The
390
main degradation products of perfluoronitriles were CO/CO2, NO/N2O, COF2, and
391
CF3C(O)F. In the atmosphere, NO will be rapidly converted into HONO through the
392
reaction with the OH radical, which is then removed by precipitation (Bark et al., 1995).
393
N2O was noted as an important monitored greenhouse gas in the IPCC report (Stocker
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et al., 2013), however, its global warming potential is significantly lower than that of
395
SF6. Additionally, the concentration of N2O derived from the degradation of
396
perfluoronitriles will remain within several ppq, which is a negligible source of N2O
397
considering its global mean concentration was 324 ppb in 2011 (Stocker et al., 2013).
398
COF2 and CF3C(O)F could be removed from the atmosphere by clouds within one to
399
several weeks, producing HF, CF3C(O)OH and CO2. Even assuming that the global
400
emissions of perfluoronitriles are 10 kt per year, the level of HF in precipitation due to
401
the degradation of perfluonitriles would be order of 10-10-10-9 molar, and the additional
402
acidity could be negligible (Wallington et al., 2015). CF3C(O)OH is ubiquitous in
403
precipitation and ocean water even in remote area. CF3C(O)OH containing in ocean
404
water is estimated to be 268 million tonnes (Frank et al., 2002), hence, the natural
405
environmental loading of CF3C(O)OH will extremely exceeds that from the degradation
406
of perfluoronitirls. Therefore, the secondary degradation products of perfluoronitriles
407
are considered to have no significant environmental impact.
409
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Acknowledgments This work was financially supported by the internal funds of AIST.
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kOH of four kinds of perfluoronitriles were measured by the relative method.
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NO formation following the degradation of perfluoronitriles was proved. N2O formation was observed and a revised mechanism was proposed. Environmental impact of perlfuoronitriles was evaluated.