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Environmental fate & pathways

Phototransformation in air

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Description of key information

Using the rate constant measured by Acerboni et al (1999) and an average °OH concentration of 10E6 molecule/cm3 (Prinn et al, 1995), the corresponding atmospheric lifetime is calculated to be approximately 1 day and the half life 0.69 day. Acerboni et al (2001), using the same rate constant in a 3 -dimensional chemical transport model (representing more closely the average behaviour of TFE in the atmosphere), predicted an °OH related lifetime of 1.9 days for TFE. Main atmospheric degradation product carbonyl fluoride hydrolyses in atmospheric water to form carbon dioxide (CO2) and hydrogen fluoride (HF) as end products, the HF being removed by rain (wash out).

Key value for chemical safety assessment

Half-life in air:
1.32 d

Additional information

The physico-chemical properties of TFE, i.e. its high vapour pressure, indicate that it should remain essentially in the gas phase.

TFE can react with hydoxyl radical (°OH) through addition on the double bond. Other atmospheric species can also react with TFE, in particular ozone (O3) and the nitrate radical (NO3°). The value of the rate constant for the reaction of TFE with °OH has been measured (Orkin et al, 1997; Acerboni et al, 1999) For comparison purposes Table A lists these rate constants.

Table A: °OH rate constants of TFE.

 Compound  Formula  KOH (10E-12 cm3/molecule/s)  Reference
 TFE  C2F4  11.3 ± 3.3  Acerboni et al 1999
 TFE  C2F4  10.2  Orkin et al 1997

Using the rate constant measured by Acerboni et al (1999) and an average °OH concentration of 10E6 molecule/cm3 (Prinn et al, 1995), the corresponding atmospheric lifetime is calculated to be approximately 1 day and the half life 0.69 day. Acerboni et al (2001), using the same rate constant in a 3 -dimensional chemical transport model (representing more closely the average behaviour of TFE in the atmosphere), predicted an °OH related lifetime of 1.9 days for TFE. However, whilst the use of a mean tropospheric

°OH concentration of 10E6 molecule/cm3 is scientifically justified, the EU TGD (Part 2, Page 51) recommends a vaule of 5 x 10E5 molecule/cm3. If the latter value were adopted, this would double the °OH + TFE lifetime to 3.8 days and increase the relative significance of the O3 + TFE sink.

TFE can equally react with ozone (O3). Several authors have reported values of the rate constant and lifetime for this pathway.

Table B: Reaction of TFE with O3

 KO3 (10E-21 cm3/molecule/s)  Lifetimea  Reference
 4.80 +/- 0.62  9y  Acerboni et al, 1999
 92  179d  Adeniji et al, 1981
 498  33d  Heicklen, 1966b
 28.4 1.59y   Toby and Toby, 1976b

a Assuming an O3 concentration of 7x 10E11 molecules/cm3 26ppbv

b As cited by Acerboni et al, 1999

Thus, the O3 -related lifetime of TFE may range from 33 days to 9 years. The lifetime is calculated from the O3 + TFE rate constant, assuming an ozone concentration of 26 ppbv (which is reasonable for the atmospheric background level). This rate constant should depend only on temperature (and indeed only slightly). The large discrepancy, from one study to another, probably indicates that what some authors were observing was not (only) the reaction of O3 with TFE. Acerboni et al (1999) measured the lowest rate constant, when performing their determination in the presence of cyclohexane, added to scavenge °OH and other radicals. In the absence of this additive, they found a rate constant about a factor of 10 higher. They suggest that secondary chemistry may have contributed to the loss of TFE in previous investigations, resulting in higher apparent rate constants. This seems quite likely. Importantly, however, even if the fastest rate constant, i.e. that of Heicklen (1966), is adopted, the resulting estimated lifetime of 33 days with respect to the O3 + TFE reaction implies that this sink is a very minor one compared to the °OH + TFE reaction (lifetime < 2 days, or < 4 days if an °OH concentration of 5 x 10E5 molecule/cm3 is adopted).

Acerboni et all (1999) also studied the possible reaction of TFE with NO3°. Her model calculations suggest that, due to the lifetime of >156 days associated with this reaction, only a small part of the TFE would be converted in this manner.

In all, the average atmospheric lifetime of TFE is considered to be < 2 days (t1/2 = 1.32 days for lifetime of 1.9 days Acerboni 2001).

The main oxidation pathway for TFE in the atmosphere, reaction with °OH, yields carbonyl fluoride C(=O)F2 as the main degradation product. Carbonyl fluoride hydrolyses in atmospheric water to form carbon dioxide (CO2) and hydrogen fluoride (HF) as end products, the HF being removed by rain (wash out). The lifetime of this process for carbonyl fluoride is expected to be of the order of 5 to 10 days. However, in a global 3 -dimensional modelling study, Kanakidou et al (1995) have calculated a tropospheric lifetime for C(=O)F2 with respect to uptake and hydrolysis in clouds of 3.9 - 7.1 days.

Acerboni G, Jensen NR, Rindone B, Hjorth J. 1999. Kinetics and products formation of the gas-phase reactions of tetrafluoroethylene with OH and NO3 radicals and ozone. Chemical Physics Letters 309: 364-368.

Acerboni G, Beukes JA, Jensen NR, Hjorth J, Myhre G, Nielsen CJ, Sundet JK. 2001. Atmospheric degradation and global warming potentials of three perfluoroalkenes. Atmospheric Environment 35: 4113-4123.

Prinn RG, Weiss RF, Miller BR, Huang J, Alyea FN, Cunnold DM, Fraser PJ, Hartley DE, Simmonds PG. 1995. Atmospheric trends and lifetime of CH3Cl3 and global OH concentrations. Science 269: 187 – 192.

Orkin VL, Huie RE, Kurylo MJ. 1997. Rate constants for the reactions of OH with HFC 245cb (CH3CF2CF3) and some fluoroalkenes (CH2CHCF3, CH2CFCF3, CF2CFCF3, and CF2CF2), J Phys Chem A: 101, pp. 9118 – 912.

Adeniji SA, Kerr JA, Williams MR. 1981. Rate constants for ozone-alkene reactions under atmospheric conditions. Int J Chem Kinet 13: 209-217.

Heicklen J. 1966. J Phys Chem 70: 477 with correction sheet added by the author to page 480 (as cited by Acerboni et al, 1999.)

Toby FS, Toby S. 1976. J Phys Chem 80: 2313 (as cited by Acerboni et al, 1999.)

Kanakidou M, Dentener FJ, Crutzen PJ. 1995. A global three-dimensional study of the fate of HCFCs and HFC-134a in the troposphere. Journal of Geophysical Research 100 (D9), 18781-18801