CFCs replacements: Reactivity and atmospheric lifetimes of a series of Hydrofluoroolefins towards OH radicals and Cl atoms

Chemical Physics Letters 714 (2019) 190–196

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Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett

Research paper

CFCs replacements: Reactivity and atmospheric lifetimes of a series of Hydrofluoroolefins towards OH radicals and Cl atoms

T

Cynthia B. Rivelaa, Carmen M. Tovarb, Mariano A. Teruela, Ian Barnesb, Peter Wiesenb, ⁎ María B. Blancoa, a

Instituto de Investigaciones en Fisicoquímica de Córdoba (I.N.F.I.Q.C.), CONICET, Dpto. de Fisicoquímica, Facultad de Ciencias Químicas, Universidad Nacional de Córdoba, Ciudad Universitaria, 5000 Córdoba, Argentina b Physikalische Chemie/FBC, Bergische Universitaet Wuppertal, Wuppertal, Germany

HIGHLIGHTS

temperature dependence of k of OH/ Cl with CH CF]CH and (E/Z)-CF CF]CHF. • First temperature dependence of k of Cl with (CF ) C]CH . • First activation energies for all reactions studied. • Negative rates with altitude estimated from 0.12 (0 km) to 8.88 (10 km) (in 10 s ). • OH-loss • Local and regional impact of HFOs with POCPs ranged from 3 to 13. 3

3 2

2

3

2

−5 −1

ARTICLE INFO

ABSTRACT

Keywords: CFCs replacements Hydrofluoroolefins Arrhenius expressions OH radicals Cl atoms

A study of the temperature dependence of the gas-phase reactions of OH radicals and Cl atoms with a series of hydrofluroolefins: 2-fluoropropene (CH3CF]CH2), hexafluoroisobutylene ((CF3)2C]CH2) and (E/Z)-1,2,3,3,3pentafluoropropene ((E/Z)-CF3CF]CHF) has been performed. The relative-rate technique was used to determine rate coefficients over the temperature range 287–313 K. This work constitutes the first temperature dependence study of OH radicals and Cl atoms with 2-fluoropropene and (E/Z)-1,2,3,3,3-pentafluoropropene and for the reaction of Cl with hexafluoroisobutylene. Atmospheric implications are discussed with particular reference to the rate coefficients obtained as a function of the temperature.

1. Introduction The Montreal Protocol (1987) enumerates a series of Ozone depleting substances (ODSs) which are chemical products of anthropogenic origin issued in the atmosphere that depletes the stratospheric ozone layer. The majority of ODSs are also fluorinated substances, including chlorofluorocarbons (CFCs) and bromofluorocarbons (halons), with hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs) in many cases. CFCs have shown adverse effects on the stratospheric ozone layer, but it has been observed that their atmospheric concentration has been substantially reduced. On the other hand, the atmospheric abundances of HCFCs and HFCs, are generally increasing [1]. HCFCs are considered substitutes for transient use and their disposal according to the Montreal Protocol is stipulated [1,2]. HFCs are not ozone-depleting compounds but are important greenhouse gases ⁎

that are included in the Kyoto Protocol (1997) [3]. It is known that, since the recognition of the ozone layer depletion by ODSs, there has been a continued the use of shorter-lived substances. Molecules with lifetimes of the order of days are now being considered; they are commonly referred to as very short-lived substances (VSLSs) in the literature. These substitutes, for example, hydrofluoro-olefins (HFOs) or hydrofluoroalkenes (HFAs) are substances with lifetimes of days to weeks, which is much shorter than the transport to the stratosphere. They are not well mixed in the troposphere; their short-lifetime effectively reduces the fraction of their emission reaching the stratosphere and their accumulation in the atmosphere as compared to the longerlived HCFCs and HFCs. Hydrofluoroolefins (HFOs), have been considered as possible replacement for CFCs since they have high gas-phase reactivity and low global warming potentials (GWPs), and consequently a reduced effect

Corresponding author. E-mail address: [email protected] (M.B. Blanco).

https://doi.org/10.1016/j.cplett.2018.10.078 Received 3 October 2018; Received in revised form 30 October 2018; Accepted 31 October 2018 Available online 02 November 2018 0009-2614/ © 2018 Elsevier B.V. All rights reserved.

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C.B. Rivela et al.

on climate forcing [4]. The HFOs under study have many industrial applications including their use in pharmaceuticals, recently HFOs are considered as potential candidates as medical propellants in metered dosed inhalers (MDIs) [5], for example as potential MDI replacement propellants include CF3CF]CHF (Z-1,2,3,3,3-pentafluoropropene, HFO-1225yeZ) [6]. On the other hand, HFOs are considered as replacements for CFCs in air conditioning units [7] and alternative refrigerants like (Z)-CF3CF]CHF (1,2,3,3,3-pentafluoropropene, HFO1225ye) [8]. The kinetic studies on the reactions of OH radicals and Cl atoms with 2-fluoropropene (2FP), hexafluoroisobutylene (HFIB), and E/Zisomeric mixture of 1,2,3,3,3-pentafluoropropene (PFP) presented here have been performed in an environmental chamber using the relative kinetic technique over the temperature range 287–313 K. CH3CF]CH2 + OH → Products

(1)

(CF3)2C]CH2 + OH → Products

(2)

(E/Z)-CF3CF]CHF + OH → Products

(3)

CH3CF]CH2 + Cl → Products

(4)

(CF3)2C]CH2 + Cl → Products

(5)

(E/Z)-CF3CF]CHF + Cl → Products

(6)

Cl2 + h (

X+ HFO X+ Ref

ln

All the experiments were performed in a 1080 L quartz-glass reaction chamber over the temperature range 287–313 K with a precision of ± 2 K and a total pressure of (760 ± 10) Torr of synthetic air 760 Torr. A detailed description of the reactor can be found elsewhere [10] and only a brief description is given here. A pumping system consisting of a turbo-molecular pump backed by a double stage rotary fore pump was used to evacuate the reactor to 10−3 Torr. Three magnetically coupled Teflon mixing fans are inside the chamber to ensure homogeneous mixing of the reactants. The photolysis system consists of 32 super actinic fluorescent lamps (Philips TL05 40 W: 320–480 nm, λmax = 360 nm) and 32 low-pressure mercury vapor lamps (Philips TUV 40 W; λmax = 254 nm), which are spaced evenly around the reaction vessel. The chamber is equipped with a White type multiplereflection mirror system with a base length of (5.91 ± 0.01) m for sensitive in situ long path absorption monitoring of reactants in the IR spectral range 4000–700 cm−1. The White system was operated at 82 traverses, giving a total optical path length of (484.7 ± 0.8) m. The IR spectra were registered with a spectral resolution of 1 cm−1 using a Nicolet Nexus FTIR spectrometer, equipped with a liquid nitrogen cooled mercury-cadmium-telluride (MCT) detector. Typically, 64 interferograms were co-added per spectrum over a period of approximately 1 min and 15–20 such spectra were registered per experiment. OH radicals were generated by the photolysis of hydrogen peroxide with the UV lamps:

2 OH

(8)

Products Products

(9) (10)

[HFO]0 [HFO]t

=

kHFO [Ref]0 ln kref [Ref]t

(11)

where, [HFO]0, [Ref]0, [HFO]t and [Ref]t are the concentrations of the hydrofluoroolefin and reference compound at times t = 0 and t, respectively and kHFO and kref are the rate coefficients of reactions (9) and (10), respectively. The relative rate technique relies on the assumption that both the HFOs and reference organics are removed solely by reaction with OH radicals and Cl atoms. To verify this assumption, various tests were performed to assess possible loss of the reactants via deposition to the chamber walls, reaction with H2O2 or Cl2, and photolysis. Wall deposition, reaction with the precursor of OH or Cl and photolysis were found to be negligible for both the HFOs and the reference compounds. The initial concentrations used in the experiments for the HFOs and reference compounds in ppm (1 ppm = 2.46 × 1013 molecule cm−3 at 298 K and 760 Torr of total pressure) were: ∼1 for hexafluoroisobutylene, ∼1 for 1,2,3,3,3-pentafluoropropene, (E/Z) mixture, 3 for 1-butene, ∼2 for isobutene, ∼2 for propane and ∼2 for propene. The initial concentrations for the precursor of the oxidants were typically ∼6 ppm for Cl2 and ∼5 ppm for H2O2. The following chemicals were used: synthetic air (Air Liquide), 2fluoropropene (Apollo Scientific), hexafluoroisobutylene (Apollo Scientific), 1,2,3,3,3-pentafluoropropene (Apollo Scientific), 1-butene (Messer Griesheim); isobutene (Messer Griesheim), propane (Messer Griesheim), propene (Sigma Aldrich), Cl2 (Messer Griesheim) and H2O2 (Peroxide Chemie, 85%).

2. Material and methods

254 nm)

2 Cl

Provided that the reference compound and the HFO are lost only by reactions (9) and (10), then it can be shown that:

To the best of our knowledge, there are no temperature dependence studies on the reactions of OH radicals and Cl atoms with 2FP and PFP, and for the reaction of HFIB + Cl. In this sense, this study reports the first temperature dependence of the OH/Cl rate coefficients for these reactions. The only previous temperature dependent rate study on the above reactions available in the literature is that from Tokuhashi et al., 2018 [9] on the reaction of OH radicals with HFIB which was performed using the flash photolysis and laser photolysis method combined with laser induced fluorescence (LIF) detection technique in the temperature range 250–430 K.

H2 O2 + h (

360 nm)

Reaction mixtures consisting of a reference organic compound, the sample organic reactant and the radical precursor, diluted in pure air. These have been prepared in the reaction chamber and left to mix prior to photolysis for approximately 15 min. Measured amounts of the reagents were flushed into the reaction chamber by a stream of synthetic air and then filled with to atmospheric pressure. Typical photolysis times ranged from 15 to 20 min. In the presence of the oxidant X (OH radical or Cl atom) the HFO under investigation and the reference compound are consumed by the following reactions:

3. Results and discussion Relative kinetic studies, were performed at 1 atm total pressure on the reactions of OH/Cl with 2-fluoropropene, hexafluoroisobutylene and (E/Z)-1,2,3,3,3-pentafluoropropene at temperatures of 287, 291, 298, 303, and 313 K. Tables 1 and 2 lists the rate coefficient ratios obtained at each temperature from linear least-squares analyses of the kinetic data plotted according to equation I for the reactions initiated by OH and Cl, respectively. The absolute rate coefficients obtained for OH/Cl with the Hydrofluoroolefins derived from the rate ratios and the Arrhenius parameters obtained from the Arrhenius plots of the rate data shown in Figs. 1 and 2. The rate coefficient ratios are each from a minimum of at least three experiments. To put the rate coefficients for the reaction of OH/Cl with the Hydrofluoroolefins on absolute basis rate coefficient for the respective temperatures derived from the following Arrhenius expression (in cm3 molecule−1 s−1 units) were used: k (OH + 1-butene) 6.54 × 10−12 exp (468/T) [11], k (OH + propane) 1.29 × 10−11exp (−730/T) [12] and k (Cl + propene) 1.43 × 10−14 exp (2886/T) [13]. The errors given for the rate coefficient ratios in Table 1 are the two least-squares standard deviations (2σ) from the linear regression analyses plus an additional 20% to cover uncertainties associated with the values of the reference rate coefficients. For all the

(7)

Chlorine atoms were generated by photolysis of molecular chlorine: 191

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Table 1 Rate coefficients and Arrhenius Parameters for the gas phase reactions of OH radicals with Hydrofluoroolefins determined at different temperatures. HFOs

T (K)

kHFOs (cm3 molecule−1 s−1) −11

CH3CF]CH2

287 291 298 298 303 313

(1.94 (1.86 (1.70 (1.55 (1.56 (1.46

± ± ± ± ± ±

0.45) × 10 0.40) × 10−11 0.37) × 10−11 0.39) × 10−11 0.34) × 10−11 0.32) × 10−11

(CF3)2C]CH2

250 287 273 291 298 296 298 298 303 313 331 375 430

(8.02 (9.49 (8.03 (7.90 (6.55 (7.82 (6.58 (8.13 (5.06 (3.74 (8.15 (8.32 (8.28

± ± ± ± ± ± ± ± ± ± ± ± ±

0.27) × 10−13 2.10) × 10−13 0.30) × 10−13 1.79) × 10−13 1.53) × 10−13 0.55) × 10−13 2.25) × 10−13 0.30) × 10−13 1.24) × 10−13 1.10) × 10−13 0.34) × 10−13 0.34) × 10−13 0.31) × 10−13

(E/Z)-CF3CF]CHF

287 292 298 298 303 313

(3.82 (3.18 (2.85 (2.62 (2.75 (2.21

± ± ± ± ± ±

0.94) × 10−12 0.75) × 10−12 0.78) × 10−12 0.76) × 10−12 0.66) × 10−12 0.52) × 10−12

(Z)-CF3CF]CHF

206 221 240 253 274 296 322 342 362 380

(1.64 (1.54 (1.43 (1.38 (1.33 (1.22 (1.29 (1.28 (1.29 (1.31

± ± ± ± ± ± ± ± ± ±

0.02) × 10−12 0.01) × 10−12 0.01) × 10−12 0.01) × 10−12 0.01) × 10−12 0.14) × 10−12 0.01) × 10−12 0.01) × 10−12 0.03) × 10−12 0.02) × 10−12

296

(2.15 ± 0.23) × 10−12

(E)-CF3CF]CHF

A (cm3 molecule−1 s−1)

−Ea/R (K)

−13

1029 ± 82

(5.38 ± 0.27) × 10

3210 ± 159

(1.31 ± 0.53) × 10−17

1765 ± 181

(7.85 ± 0.61) × 10−15

Hydrofluoroolefins studied, the reaction rate coefficients were found to be decrease slightly with increasing temperature in the range 287–313 K. The kHFOs values of Tables 1 and 2 are plotted in Arrhenius form in Figs. 1 and 2 for all reactions. The following Arrhenius

Pressure (Torr)

Reference

760 760 760 760 760 760

This work This work This work Tovar et al., 2014 [16] This work This work

20–60 760 20–60 760 760 760 700–760 5–200 760 760 20–60 20–60 20–60

Tokuhashi et al., 2018 [9] This work Tokuhashi et al., 2018 [9] This work This work Papadimitriou et al., 2015 [14] Tovar et al., 2014 [16] Tokuhashi et al., 2018 [9] This work This work Tokuhashi et al., 2018 [9] Tokuhashi et al., 2018 [9] Tokuhashi et al., 2018 [9]

760 760 760 760 760 760

This work This work This work Tovar et al., 2014 [16] This work This work

(100–300) 100 100 100 100 700 100 100 100 (30–300)

Papadimitriou et al., 2008 Papadimitriou et al., 2008 Papadimitriou et al., 2008 Papadimitriou et al., 2008 Papadimitriou et al., 2008 Hurley et al., 2007 [15] Papadimitriou et al., 2008 Papadimitriou et al., 2008 Papadimitriou et al., 2008 Papadimitriou et al., 2008

700

Hurley et al., 2007 [15]

[17] [17] [17] [17] [17] [17] [17] [17] [17]

expressions adequately describe the data in the temperature range 287–313 K with k (in cm3 molecule−1 s−1 units):

k (OH+ 2FP) = (5.38 ± 0.27) × 10

13 exp[(1029

± 82)/ T ]

Table 2 Rate coefficients and Arrhenius Parameters for the gas phase reactions of Cl atoms with Hydrofluoroolefins determined at different temperatures. HFOs

T (K)

kHFOs (cm3 molecule−1 s−1)

−Ea/R (K)

A (cm3 molecule−1 s−1)

Pressure (Torr)

Reference

CH3CF]CH2

287 292 298 298 303 313

(2.30 (1.91 (1.59 (1.64 (1.33 (1.07

± ± ± ± ± ±

0.53) × 10−10 0.41) × 10−10 0.34) × 10−10 0.26) × 10−10 0.31) × 10−10 0.24) × 10−10

2664 ± 131

(2.09 ± 0.44) × 10−14

760 760 760 760 760 760

This work This work This work Tovar et al., 2014 [16] This work This work

(CF3)2C]CH2

287 292 298 296 298 303 313

(5.00 (4.20 (3.50 (3.45 (3.50 (2.54 (2.51

± ± ± ± ± ± ±

1.24) × 10−11 0.98) × 10−11 0.83) × 10−11 0.24) × 10−11 0.85) × 10−11 0.57) × 10−11 0.53) × 10−11

3143 ± 378

(8.81 ± 1.27) × 10−16

760 760 760 700–760 760 760 760

This work This work This work Papadimitriou et al., 2015 [14] Tovar et al., 2014 [16] This work This work

(E/Z)-CF3CF]CHF

287 292 298 298 303 313

(8.99 (8.12 (6.44 (4.52 (6.24 (4.06

± ± ± ± ± ±

2.14) × 10−11 1.84) × 10−11 1.63) × 10−11 0.98) × 10−11 1.44) × 10−11 1.11) × 10−11

2694 ± 304

(7.84 ± 1.02) × 10−15

760 760 760 760 760 760

This work This work This work Tovar et al., 2014 [16] This work This work

(Z)-CF3CF]CHF

296

(4.36 ± 0.48) × 10−11

(E)-CF3CF]CHF

296

700

Hurley et al., 2007 [15]

(5.00 ± 0.56) × 10−11

700

Hurley et al., 2007 [15]

192

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C.B. Rivela et al.

by Tovar et al., 2014 [16] using the relative rate method and atmospheric pressure, the values are in cm3 molecule−1 s−1 (1.55 ± 0.39) × 10−11, (6.58 ± 2.25) × 10−13 and (2.62 ± 0.76) × 10−12 respectively, whereas that, the rate coefficient values for 2FP, HFIB and PFP with OH radicals in this study are (in cm3 molecule−1 s−1): (1.70 ± 0.37) × 10−11, (6.55 ± 1.53) × 10−13 and (2.85 ± 0.78) × 10−12, respectively. It can be observed that these values are in excellent agreement with the rate coefficient obtained in this work at the same experimental conditions. On the other hand, Papadimitriou et al., 2015 [14] have reported a value of (7.82 ± 0.55) × 10−13 cm3 molecule−1 s−1 for the rate coefficient of the HFIB + OH reaction at room temperature and atmospheric pressure employing the relative rate method and FTIR spectroscopy to monitor the reactants. It is possible to observe a good agreement, within experimental errors, with the value reported in this work. In addition, previous studies of the separate isomers (Z)CF3CF]CHF and (E)-CF3CF]CHF room temperature rate coefficients with OH radicals has been reported by Hurley et al., 2007 [15] using the relative rate method at 298 K and FTIR technique. The values of rate coefficients reported by these authors were: (1.22 ± 0.14) × 10−12 and (2.15 ± 0.23) × 10−12 for the separate isomers, respectively in cm3 molecule−1 s−1. Our reported value for the mixture (E/Z)CF3CF]CHF ((2.85 ± 0.78) × 10−12 (in cm3 molecule−1 s−1 units) is in close agreement with the value of the reaction of OH radicals with the (E)-CF3CF]CHF isomer.

-24

-26

-27

3

-1 -1

ln k (cm molecule s )

-25

-28

-29

-30 3.15

3.20

3.25

3.30

3.35

3.40

3.45

3.50

-1

1000/T(K ) Fig. 1. Arrhenius plots of the kinetic data obtained in this study between 287 and 313 K for the reactions of OH with 2-fluoropropene (CH3CF]CH2) (■); hexafluoroisobutylene ((CF3)2C]CH2) (▲) and (E/Z)-1,2,3,3,3-pentafluoropropene ((E/Z)-CF3CF]CHF) (●).

3.2. Cl reaction a 298 K

-21.5

As mentioned above, concerning the reactions of Cl atoms with HFOs studied in this work, room temperature reaction rate has been reported by Tovar et al., 2014 [16] using in situ FTIR combined with the relative rate method. The values of rate coefficient for the reaction of 2FP, HFIB and PFP with Cl atoms in cm3 molecule−1 s−1 were: (1.64 ± 0.26) × 10−10, (3.50 ± 0.85) × 10−11 and (4.52 ± 0.98) × 10−11, respectively. Furthermore, previous studies of the separate isomers (Z)-CF3CF]CHF and (E)-CF3CF]CHF room temperature rate coefficients with Cl atoms has been reported by Hurley et al., 2007 [15] using the relative rate method at 298 K and FTIR technique. The values of rate coefficients reported by these authors were: (4.36 ± 0.48) × 10−11 and (5.00 ± 0.56) × 10−11 for the separate isomers, respectively in cm3 molecule−1 s−1. Additionally, Papadimitriou et al., 2015 [14] have reported a value of (3.45 ± 0.24) × 10−11 cm3 molecule−1 s−1 for the reaction of HFIB with Cl atoms at room temperature and atmospheric pressure employing the relative rate method and FTIR spectroscopy to monitor the reactants. It can be seen an excellent agreement between this value and the room temperature rate coefficients of the reactions of Cl + HFIB reported in this work of (3.50 ± 0.85) × 10−11 cm3 molecule−1 s−1.

-22.0

3

-1

-1

ln k(cm molecule s )

-22.5 -23.0 -23.5 -24.0 -24.5 -25.0 -25.5 3.15

3.20

3.25

3.30

3.35

3.40

3.45

3.50

-1

1000/T(K ) Fig. 2. Arrhenius plots of the kinetic data obtained in this study between 287 and 313 K for the reactions of Cl with 2-fluoropropene (CH3CF]CH2) (■); hexafluoroisobutylene ((CF3)2C]CH2) (▲) and (E/Z)-1,2,3,3,3-pentafluoropropene ((E/Z)-CF3CF]CHF) (●).

k (OH+ HFIB) = (1.31 ± 0.53) × 10 k (OH+ PFP) = (7.85 ± 0.61) × 10 k (Cl + 2FP) = (2.09 ± 0.44) × 10

k (Cl + HFIB) = (8.81 ± 1.27) × 10 k (Cl + PFP) = (7.84 ± 1.02) × 10

17 exp[(3210 15 exp[(1765

14 exp[(2664

Papadimitriou et al., 2008 [17] have investigated the temperaturedependence of the reaction of OH radicals with the separate isomer (Z)CF3CF]CHF in the range 206–380 K using the PLP-LIF technique and a pressure range of (30–600) Torr reporting the following non-Arrhenius expression, k((Z)-CF3CF=CHF + OH) = (1.6 ± 0.2) × 10−18 T2 exp [(655 ± 50)/T] cm3 molecule−1 s−1. On the other hand, the temperature-dependence of the reaction with OH radicals with (CF3)2C]CH2 has been studied recently by Tokuhashi et al., 2018 [9] in the range 250–430 K using the PLP-LIF technique and in the pressure range of (5–200) Torr, the reaction rates observed found to be independent of pressure. The Arrhenius expression reported by the authors is k((CF3)2C=CH2 + OH) = (8.75 ± 0.23) × 10−13 exp [−(20 ± 10)/T] cm3 molecule−1 s−1. There are no prior experimental determinations of the Arrhenius parameters for the reactions of OH radicals with 2FP and the mixture (E/Z)-PFP. Furthermore, there are no previous kinetic data for the reactions of Cl atoms with 2FP, HFIB and PFP in function of temperature.

± 159)/ T ]

± 181)/T ]

± 131)/T ]

16 exp[(3143 15 exp[(2694

3.3. Temperature dependences

± 378)/ T ]

± 304)/ T ]

The errors in the activation term and the pre-exponential factor are the 2σ random statistical errors from fits to the data presented in Tables 1 and 2 and plotted in Figs. 1 and 2. 3.1. OH reaction at 298 K For the reactions of OH radicals with 2FP, HFIB and PFP, room temperature reaction rate could be compared with previous kinetic data reported 193

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C.B. Rivela et al. 12

240 244

10

248 252

Altitude (Km)

8

256 260

6

264 268

4

T (K)

Consequently, this work reports the first temperature dependence of the reactions of OH radicals with 2FP and the mixture (E/Z)-PFP and for the reactions of Cl atoms with the title HFOs. The negative temperature dependence of these reactions can be rationalized on a qualitative basis assuming that the lifetime of the excited bimolecular complex, formed between the OH radical/Cl atom and the fluoroalkene, with respect to decomposition back to the reactants decreases as the temperature increases. Therefore, the probability of the excited adduct being stabilized by collision with a third body falls with increasing temperature.

272 276

3.4. Estimation of the fluoroalkene loss rate by OH and Cl reaction

2

Using the Arrhenius expressions reported in this work, we have calculated the atmospheric lifetimes of the hydrofluoroolefins studied for different temperatures and their variations with altitude. At a given temperature (T) corresponding to a given altitude in the troposphere, the rate of loss of the HFOs is defined as the product between the OH rate coefficient (T) and [OH] at this altitude. Temperature profiles between 0 and 10 km using a 12 h daytime average global tropospheric OH radical concentration of [OH] = 2 × 106 radicals cm−3 [18] and [Cl] = 1 × 104 atoms cm−3 (24 h average) [19], the Arrhenius parameters reported in this work and considering a lapse rate in the troposphere of −6.5 K/km [20]. Table 3 shows the OH and Cl removal rates for the HFOs studied as a function of altitude in the troposphere assuming a temperature of 298.15 K at 0 km. The results shown in Table 3 are plotted in Figs. 3 and 4 for the reactions initiated by OH radicals and Cl atoms, respectively. The loss OH rates (in s−1) of the HFOs at sea level (0 km) are 3.39 × 10−5, 1.24 × 10−6 and 5.85 × 10−6 and near to the tropopause (∼10 km) around 8.88 × 10−5, 2.50 × 10−5 and 3.05 × 10−5 for 2FP, HFIB and PFP, respectively. In the case of the reactions initiated by Cl atoms, the loss rates (in s−1) of the HFOs at sea level (0 km) are 1.58 × 10−6, 3.34 × 10−7 and 6.58 × 10−7 and near to the tropopause (∼10 km) around 1.92 × 10−5, 6.30 × 10−6 and 8.18 × 10−6 for 2FP, HFIB and PFP, respectively.

0

280 284 288 1E-6

1E-5

1E-4 -1

HFOs loss (s )

Fig.3. Hydroxyl radical (OH) reaction loss rates for 2-fluoropropene (CH3CF]CH2) (▲), hexafluoroisobutylene ((CF3)2C]CH2) (■) and (E/Z)1,2,3,3,3-pentafluoropropene ((E/Z)-CF3CF]CHF) (●) as a function of altitude (km). 12

240 244 248 252

8

256 260

T(K)

Altitude (Km)

10

6

264 268 272

4

276 280

2

284 288

0 1E-6

1E-5 -1

3.5. Atmospheric implications

HFOs loss (s )

Fig. 4. Chlorine atom (Cl) reaction loss rates for 2-fluoropropene (CH3CF]CH2) (▲), hexafluoroisobutylene ((CF3)2C]CH2) (■) and (E/Z)1,2,3,3,3-pentafluoropropene ((E/Z)-CF3CF]CHF) (●) as a function of altitude (km).

Tropospheric lifetimes, τx, of the hydrofluoroolefins studied in this work were calculated through the expression: τx = 1/kx [X] with X = OH/Cl. Table 4 lists the atmospheric lifetimes with respect to the reaction with OH radicals and Cl atoms for the compounds studied in this work. Additionally, we have presented the atmospheric lifetimes with OH radicals and Cl atoms obtained previously [16]. These calculations were performed considering a 12 h daytime average global tropospheric OH radical concentration of 1 × 106 radicals cm−3 [21] and Cl atoms concentration of 5 × 103 atoms cm−3 [22,23]. They can

observe that the lifetimes of the reactions of hydrofluoroolefins with OH radicals are between 0.8 and 18 days. Regarding to reactions with Cl atoms, the atmospheric lifetimes are between 14 and 66 h where it will probably have important impact in coastal areas [24].

Table 3 Removal rates of the HFOs studied as a function of altitude in the troposphere. Altitude (km)

0 1 2 3 4 5 6 7 8 9 10

T (K)

298.15 291.65 285.15 278.65 272.15 265.65 259.15 252.65 246.15 239.65 233.15

kX+2FP(T)[X] (s−1)

kX+HFIB(T)[X] (s−1)

kX+PFP(T)[X] (s−1)

X = OH

X = Cl

X = OH

X = Cl

X = OH

X = Cl

3.39 × 10−5 3.66 × 10−5 3.97 × 10−5 4.32 × 10−5 4.72 × 10−5 5.18 × 10−5 5.70 × 10−4 6.32 × 10−5 7.04 × 10−5 7.88 × 10−5 8.88 × 10−5

1.58 × 10−6 1.94 × 10−6 2.39 × 10−6 2.97 × 10−6 3.73 × 10−6 4.74 × 10−6 6.09 × 10−6 7.93 × 10−6 1.05 × 10−5 1.41 × 10−5 1.92 × 10−5

1.24 × 10−6 1.58 × 10−6 2.02 × 10−6 2.64 × 10−6 3.47 × 10−6 4.64 × 10−6 6.28 × 10−6 8.63 × 10−6 1.21 × 10−5 1.72 × 10−5 2.50 × 10−5

3.34 × 10−7 4.22 × 10−7 5.39 × 10−7 6.98 × 10−7 9.13 × 10−7 1.21 × 10−6 1.63 × 10−6 2.23 × 10−6 3.09 × 10−6 4.37 × 10−6 6.30 × 10−6

5.85 × 10−6 6.67 × 10−6 7.66 × 10−6 8.85 × 10−6 1.03 × 10−5 1.21 × 10−5 1.42 × 10−5 1.70 × 10−5 2.04 × 10−5 2.48 × 10−5 3.04 × 10−5

6.58 × 10−7 8.05 × 10−7 9.94 × 10−7 1.24 × 10−6 1.56 × 10−6 1.99 × 10−6 2.56 × 10−6 3.35 × 10−6 4.44 × 10−6 5.98 × 10−6 8.18 × 10−6

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contemporary water and air samples, suggesting the existence of one or more large unknown sources. The exact nature and yields of the products from the OH and Cl initiated photooxidation of the hydrofluoroolefins studied here, however, still remains to be elucidated.

Table 4 Estimated tropospheric lifetimes of the HFOs studied in this work with different troposphere oxidants. HFOs

τOHa (days)

τOHb (days)

τCla (days)

τClb (days)

CH3CF]CH2 (CF3)2C]CH2 (E/Z)-CF3CF]CHF

0.8 18 4

0.3 9 2

14 66 51

7 33 18

a b

4. Conclusions This work constitutes the first temperature dependence study of OH radicals and Cl atoms with 2-fluoropropene and (E/Z)-1,2,3,3,3-pentafluoropropene and for the reaction of Cl with hexafluoroisobutylene. Negative temperature dependences were observed in all reactions studied. Estimation of the fluoroalkene loss rate by OH and Cl reaction were performed obtaining that the estimated lifetimes of the HFOs studied with respect to reaction with OH and Cl decrease with altitude, from sea level to near the tropopause. The photochemical ozone creations potentials (POCPs) and the global warming potentials for these hydrofluoolefins are negligible which makes them possible replacement for CFCs since they have high gas-phase reactivity.

Tovar et al. 2014 [16]. This work.

On the other hand, for this work the calculations were performed, considering a 12 h daytime average global tropospheric OH radical concentration of 2 × 106 radicals cm−3 [18], and Cl atoms concentration of 1 × 104 atoms cm−3 [19]. The lifetime of the reactions of HFOs with OH radicals are between 0.3 and 9 days and the corresponding lifetimes for the reactions with Cl atoms are between 7 and 33 days. Moreover, Papadimitriou et al., 2015 [14] has calculated the lifetime of the reaction of HFIB with OH radicals of 14.8 days, 33.5 days with Cl atoms and 41 years with O3 molecules. Furthermore, these authors demonstrated that the O3 chemistry has a negligible impact on atmospheric lifetimes of HFIB. From the results of the estimation of the fluoroalkene loss rate by OH and Cl reaction, we have obtained that the estimated lifetimes of these HFOs with respect to reaction with OH and Cl decrease with altitude (from sea level to near the tropopause) by around 38% (with lifetimes from 8.17 to 3.13 h) and 8% (with lifetimes from 7.28 to 0.6 days) for 2FP, 5% (with lifetimes from 9.33 to 0.46 days) and 5% (with lifetimes from 34.7 to 1.84 days) for HFIB, and 19% (with lifetimes from 47.6 to 9.14 h) and 5% (with lifetimes from 34.65 to 1.84 days) for PFP, respectively. This behavior reflects the decrease of temperature with altitude and associated decrease in reaction rates of the reactions. The temperature dependent rate coefficients determined in this study will allow a better representation in three-dimensional climate models where temperature variations in reactions are considered [25]. With the results obtained it is expected the OH chemistry of the HFOs studied to be predominant in the gas phase degradation, also the Cl chemistry could be included in the model calculations, particularly in regions with elevated levels of Cl atoms, such as coastal and polluted urban areas, where HFOs emissions may be high. Additionally, the photochemical ozone creation potential (POCPs) have been calculated using the method outline by Derwent et al., 1998 and Jenkin, 1998 [26,27]. This method is based only on the molecular properties of the considered compounds and their reactivity towards OH radicals, for that reason, when atmospheric lifetimes are rather short, it is expected that it will contribute significantly to the formation of ozone and other photooxidants in the atmosphere near their emission source. POCPs could be used to estimate the potential of ozone creation of VOCs relative to that of ethene which is given the value 100 [26,27]. This estimated method gives values of POCPs for 2FP, HFIB and PFP around, 13, 3 and 11 respectively. We conclude that, in relation to ethane as reference compound these HFOs will not make a significant contribution to tropospheric ozone formation. The global warming potential of (Z)-CF3CF]CHF and (CF3)2C]CH2 for the 100 year time horizon of < 5 for both compounds according to Papadimitriou et al., 2008 and Kasuaki et al., 2018 [17,9] respectively, therefore the ozone-depletion potentials for these compounds are nearly zero and it is expected to have a small climatic impact. The possible products formed in the reactions studied as CF3C(O)F, CH3C(O) F could be incorporated into rain-cloud-seawater [28], followed by hydrolysis to give trifluoroacetic acid (CF3C(O)OH, TFA) and acetic acid, respectively. TFA is a ubiquitous contaminant of the hydrosphere and high concentrations of this carboxylic acid have been observed in

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