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Environmental fate & pathways

Phototransformation in air

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Endpoint:
phototransformation in air
Type of information:
other: experimental study and calculations
Adequacy of study:
supporting study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
accepted calculation method
Principles of method if other than guideline:
Vapour phase reactions of the test substance with OH, NO3, and O3 were studied at 298±4 K and 740±5 Torr using long-path FT-IR detection. Quantitative infrared cross-section of the test substance were obtained at 298K (24.85°C) in the region 100-2600 cm-1.
A chemistry transport model was used to calculate the atmospheric distribution and lifetime of the substance.
GLP compliance:
not specified
Specific details on test material used for the study:
99% purity
Light source:
other: UV/VIS lamps
Light spectrum: wavelength in nm:
>= 300
DT50:
ca. 1.9 d
Test condition:
Assumes reaction with OH only. No temperature dependence included in the model. OH-radical concentration: 1E6 molec.cm-3
Reaction with:
OH radicals
Rate constant:
0 cm³ molecule-1 s-1
Remarks on result:
other: reaction rate constant: 11.3 (+/-3.0) *E-12 cm³ molecule-1 s-1
Remarks:
OH concentration: 1E6 molec.cm-3
Reaction with:
ozone
Rate constant:
0 cm³ molecule-1 s-1
Remarks on result:
other: reaction rate constant: 6.5 (+/-0.2)*E-21 cm³ molecule-1 s-1
Remarks:
very slow reaction
Reaction with:
other: NO3
Remarks on result:
other: reaction rate constant upper limit: <3E-15 cm3.molecule-1.s-1
Transformation products:
yes
No.:
#1
No.:
#2
Results with reference substance:
CF2O and CF3CFO are reported to have atmospheric lifetimes of 5–10 days due to their incorporation into raindrops/aerosols in the atmosphere (De Bruyn et al., 1995).

Quantitative vapour-phase infrared spectra : 100 -2500 cm-1 at 298 K.

* The rate constants of the reactions of C4F6 with OH, NO3 and O3 were investigated.

The reaction rate constants for the reactions with OH and NO3 were determined using the relative rate method, by plotting ln(A0/A1) - kwA t versus ln(B0/B1)- kwB t. The slope is the ratio of rate constants kA/kB.

A0 and B0: initial concentrations

A1 and B1: concentrations at time t.

kwA and kwB are rate constants describing additional first-order processes (if any present) for A and B.

kw was measured to be 1.0 x 10E-5 s-1. (losses on the wall and/or photolysis)

kw was also measured for 3 reference compounds (C2H4: 1.2 x 1E-5 s-1; C3H6 : 1.1 x 1E-5 s-1 ; and cyclohexane: 5.7 x 1E-6 s-1). As none of the substances showed any significant loss in the absence of UV/VIS lamps on, kw was assumed to be <2 x 1E-6 s-1.

For reaction with O3, the pseudo-first-order method was used. Loss of C4F6 in an excess of O3 was measured to obtain a decay rate constant k' from the plot of ln[C4F6] versus time.

The values of the OH and NO3 reaction rate constants and their uncertainties were calculated in the following way: first, rate constants and σ values were obtained from four individual experiments for each of the reference compounds, then the uncertainty of the rate constant of the reference compound was incorporated by using standard propagation of error. Two or three reference compounds were used, and the value of the rate constant was calculated as the mean of different values weighted by their standard

deviations. The uncertainty is given as 2σ, where σ is the weighted average of the standard deviation obtained in the series of experiments. The uncertainty of the O3 rate constant was determined by taking to 2σ of the slope k’ versus [O3].

- rate constant at 298K for reaction of C4F6 with OH radicals: 11.3 + 3.0 x 1E-12 cm3.molecules-1.s-1.

- the reaction with NO3 were very slow, rate constant for reaction with NO3: < 3 x1E-15 cm3.molecules-1.s-1 (upper limit)

- the reactions with O3 were slow, rate constant : 6.5 + 0.2 x 1E-21 cm3.molecules-1.s-1

The main atmospheric degradation products were carbonyl fluoride and trifluoroacetyl fluoride.

* Integrated infrared cross section in the 900 -1850 cm-1 region: 2.18 + 0.22 x 1E-16 cm.molecules-1 (base e)

* Global and yearly averaged lifetime for C4F6 (assuming OH is the only removal agent): 1.9 day

The lifetime is strongly dependent on local and seasonal conditions (slower removal when OH is low, more efficient when OH is high, such as at low latitudes).

* Global warming potential:

Radiative forcing calculations were performed based on the measured infrared absorption cross sections and the Chemical Transport Model calculated atmospheric distributions.

For calculations using CO2 as a reference gas, the expression given in IPCC (1990) with updated constant in Myhre at al. (Geophysical Res. Letters 25:2715 -2718, 1998)

 C4F6  20 years 100 years  500 years
relative to CO2  0.091  0.027 0.0084 
 relative to CFC-11  1.4 x 1E-5  5.8 x 1E-5  5.2 x 1E-5
Conclusions:
A very short lifetime of 1.9 day was determined for hexafluorobutadiene. Because of the high reactivity with OH-radicals, the lifetime is strongly dependent on local and seasonal conditions.
The average 100-year GWP estimate was estimated to be 0.027 compared to CO2. Thus it is not expected to contribute to ozone depletion.
Carbonyl fluoride and trifluoroacetyl fluoride were identified as the main atmospheric degradation products. When incorporated in rain drops/aerosols in the atmosphere (likely within a few days), carbonyl fluoride is eventually converted to HF and CO2, and trifluoroacetyl fluoride is eventually converted to HF and TFA.
Endpoint:
phototransformation in air
Type of information:
calculation (if not (Q)SAR)
Adequacy of study:
supporting study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
accepted calculation method
Principles of method if other than guideline:
Smog chamber/FTIR techniques were used to provide information on the properties for the substance:
- kinetics of reactions with chlorine atoms,
- kinetics of reactions with hydroxyl radicals,
- atmospheric lifetimes, and global warming potentials
GLP compliance:
not specified
Light source:
other: fluorescent blacklamps
Details on test conditions:
Photolysis experiments were conducted in a 140 L Pyrex reactor interfaced to a FTIR spectrometer. The reactor was surrounded by 22 fluorescent blacklamps used to photochemically initiate the experiments.
- chlorine atoms and OH radicals were produced by photolysis.
- relative rate experiments were conducted in the presence of chlorine atoms or OH-radicals, for the test substance and reference compounds.

pressure: 700 torr total pressure of N2 or air diluent
temperature: 296K

DT50:
1.1 d
Test condition:
OH-radical concentration: 1E6 molec.cm-3
Reaction with:
other: chlorine
Rate constant:
0 cm³ molecule-1 s-1
Remarks on result:
other: rate constant: 7.28 +/- 0.99 x 1E-11 cm3.molecule-1.s-1
Reaction with:
OH radicals
Rate constant:
0 cm³ molecule-1 s-1
Remarks on result:
other: rate constant: 9.64 +/- 1.76 x 1E-12 cm3.molecule-1.s-1

* rate constant of the reaction of hexafluorobutadiene with chlorine

the observed loss of hexafluorobutadiene was plotted versus those of the reference compounds C2H2 (acetylene) and C2H4 (ethylene).

The averaged rate constant for hexafluorobutadiene was determined at 7.28 + 0.99 x 1E-11 cm3.molecule-1.s-1

* rate constant of the reaction of hexafluorobutadiene with OH (at 295K)

the observed loss of hexafluorobutadiene was plotted versus those of the reference compounds C2H2 (acetylene) and C2H4 (ethylene).

The averaged rate constant for hexafluorobutadiene was determined at 9.64 + 1.76 x 1E-12 cm3.molecule-1.s-1

* Global average atmospheric lifetime for hexafluorobutadiene (based on OH removal): 1.1 day

Rate constants were determined at 295K. However the appropriate temperature for atmospheric lifetime determination is 272 K (-1.15°C). Assuming an increase of 10% in the rate constant between 295 and 272K, and using a global average OH-radical concentration of 1E-6 cm-3.

The average daily concentration of OH radicals in the atmosphere is dependent on location and season.

* Global warming potential:

Because of the very short atmospheric lifetime (1.1 day), there is no significant contribution to radiative forcing of climate change and the estimation of the GWP was not considered meaningful (not calculated in the publication).

Conclusions:
A very short atmospheric lifetime of 1.1 day was determined for hexafluorobutadiene. Because of the high reactivity with OH-radicals, the lifetime is strongly dependent on local and seasonal conditions.

Description of key information

Key value for chemical safety assessment

Half-life in air:
1.9 d
Degradation rate constant with OH radicals:
0 cm³ molecule-1 s-1

Additional information