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Environmental fate & pathways

Phototransformation in air

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Description of key information

Degradation rate constant with OH radicals: 2 × 10E-15 cm3/molecule/sec
GWP: 3400 (100 years time horizon)

Key value for chemical safety assessment

Additional information

The atmospheric degradation pathway for HFC-125 involves the initial reaction with hydroxyl radicals in the troposphere. The molecular breakdown proceeds via various free-radical intermediates to give the stable molecule C(O)F2and CF3O radical. C(O)F2is expected to be removed from the atmosphere by uptake into clouds, rain and the oceans and to be hydrolysed to HF and CO2within a few days to a few months. The environmental fate of CF3O radical involves the probable formation of CF3OH and the further degradation to CO2and HF, which are thus considered the main degradation end products of HFC-125. The proposed degradation pathway is supported by several studies (Tuazon E.C. and Atkinson R., J. Atmosph. Chem. 17: 179 -199, 1993; Edney E.O. and Driscoll D.J. International Journal of Chemical Kinetics2 4:1067-1081.1992; Franklin J. Chemosphere 27(8): 1565 -1601, 1993; and Hasson A.S. et al. J. Chem. Soc. Faraday Trans. 93(16): 2693 -99, 1997).

Talukdar et al. (1991) measured a rate constant of 1.9 ± 0.27 × 10-15cm3/molecule/sec for reaction of HFC-125 with hydroxyl radicals at 298 °K. A similar value of 2 × 10-15cm3/molecule/sec is listed in Evaluation Number 14 of JPL NASA (2003), as a result of a combined fit of different experimental data.

The global atmospheric lifetime of HFC-125 is listed in the IPCC third assessment (2001). An official value of 29 years was estimated considering the tropospheric lifetime, relative to CH3CCl3, calculated by means of an OH-mediated photodegradation rate proposed by JPL NASA (2003) at 272 °K, a default stratospheric loss of 8% of the tropospheric loss and a negligible oceanic lifetime of 10650 years.

A global warming potential of 3400 was calculated considering the estimated global atmospheric lifetime and a radiative efficiency of 0.23 Wm-2ppb-1.

Due to the low atmospheric degradation rate, HFC-125 potential to form ozone in the troposphere is considered negligible (Hayman and Derwent, 1997).