MECHANISM OF ATMOSPHERIC OXIDATION OF 1,1,1,2-TETRAFLUOROETHANE (HFC-134A)

The chlorine-initiated photooxidation of hydrofluorocarbon 134a (CF3CH2F) has been studied in the temperature range 235-318 K and at 1 atm total pressure using UV absorption. Trifluoroacetyl fluoride [CF3C(O)F] and formyl fluoride [HC(O)F] were observed as the major products. IR analysis of the reaction mixture also showed carbonyl fluoride [C(O)F2] as a product. By measurement of the yields of HC(O)F from the photooxidation as a function of [O2] and temperature, the rate of the unimolecular decomposition of the oxy radical, CF3CHFO, reaction (5), was determined relative to its reaction with O2, reaction (4): CF3CHFO + O2 --> CF3C(O)F + HO2 (4) CF3CHFO --> CF3 + HO(O))F (5). The results were treated using both an arithmetic derivation and numerical integration with a detailed reaction scheme. Inclusion of other recently published kinetic data leads to the following recommended rate expression for reaction (5) at 1 atm k5 = 7.4 x 10(11) exp[(-4720 +/- 220)/T] s-1. The errors are 1sigma. The observation of enhanced product yields in the present work is attributed to the reaction of the CF3O radical with HFC 134a leading to further peroxy radical formation. The results have been incorporated into a 2D atmospheric model to assess the environmental implications of HFC 134a release in the troposphere.