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Environmental fate & pathways

Phototransformation in air

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Description of key information

VDF undergoes reactions with hydroxyl radicals in the atmosphere, the calculated rate constant with is 2.27E-12 cm3/molecule.sec.

Key value for chemical safety assessment

Additional information

In the atmosphere, VDF is considered to be degraded mainly by reaction with OH-radicals in the troposphere. Atmospheric degradation rate constants ranged from 2.0 to 2.5E-12 cm3/mol. s. The calculated rate constant with AOPWINv1.92a (U.S. EPA, 2008) is 2.27E-12 cm3/molecule. ses, the half life of VDF, based on a 12-hr day with an OH radical concentration of 1.5E6 OH/cm3, is 4.7 days (see Annexes 2 and 3 for the QMRF and QPRF).

The degradation of VDF in the lower atmosphere will be initiated mainly by reaction with naturally occurring hydroxyl radicals. The rate constant (kOH) for this process was determined by Howard (1976) at low pressures, in the "fall-off" region (0.001 - 0.01 atm). Extrapolating Howard’s values to atmospheric pressure gives kOH = 2.4E–12 cm3/molecule.sec at 296 K. Krejci (1995) found a similar value, kOH = 2.5E–12 cm3/molecule.sec, at the same temperature.Baasandorj et al. (2010) measured a rate constant at room temperature (296 K) of 2.79 E-12 cm3/molecule.sec.

Assuming a mean tropospheric OH concentration of 1E06 molecules/cm3, the half-life of VDF is calculated to be approximately 3.3 days. However, this is a global annual average value and, for such a short-lived species as VDF, the actual atmospheric persistence will vary greatly (with latitude and season of year, even time of day), mainly on account of variations in the local OH concentration.

Becker et al (1974) determined the rate constant for reaction of VDF with ozone to be 8E–20 cm3/molecules at ambient temperature. Assuming the mean tropospheric O3 concentration to be 7.5E11 molecules/cm3, the half-life of VDF with respect to reaction with O3 is 134 days. This reaction will therefore make a negligible contribution to the atmospheric degradation of VDF.

The ultraviolet absorption of VDF falls off abruptly above 190 nm (Bélanger and Sandorfy, 1971; Sirkin and Pimentel, 1984) so that photolysis at wavelengths reaching the troposphere (> 290 nm) is not significant.

Carbonyl fluoride (COF2) and formaldehyde (HCHO) are predicted to be degradation products, irrespective of the regioselectivity of the addition of OH radicals to VDF (ECETOC, 2005). Formaldehyde occurs naturally in the atmosphere and its fate is not discussed further. Carbonyl fluoride will not be photolysed or undergo chemical reaction in the gas phase of the atmosphere, but will be removed by uptake into cloud droplets and subsequent hydrolysis to CO2 and HF, within a few weeks (Cox et al, 1995).