Rate and mechanism of the atmospheric degradation of 1,1,1,2-tetrafluoroethane (HFC-134a)

The atmospheric chemical behaviour of 1,1,1,2-tetrafluoroethane (CF3CFH3, HFC-134a) with respect to its rate and mechanism of degradation in the troposphere has been investigated. The rate coefficient for the reaction of (1a) CF3CFH2 + OH-->CF3CFH + H2O has been determined in direct time-resolved experiments using laser-pulse initiation and laser long-path absorption. A value of k(1a) = (4.6+/-0.5)x10(-15) cm(3)/s at T= 295 K has been found. The ratio of the rate coefficients for the reactions of the CF3CFHO-radical with O-2, (4) CF3CFHO + O-2-->CF3CFO + HO2, and C-C bond fission, (5) CF3CFHO + M-->CFHO + CF3 + M, for T = 295 K and p(total) = 50 mbar (O-2) has been obtained to be k(4)/(k(5)[M]) = 1.5x10(-19) cm(3), with the individual values being k(4) = 2.7x10(-15) cm(3)/s and k(5)[M] = 1.8x10(4) s(-1). The mechanism of the atmospheric degradation of HFC-134a has been investigated in the temperature range 244-295 K and p(total) = 1000 mbar using UV photolysis/FTIR product studies. From these experiments the Arrhenius expression k(4)/(k(5)[M]) = 8.7x10(-25) exp ((3240(-610)(+990)) K/T) cm(3) was determined. Moreover, it is concluded that the major carbonyl products are CF3CFO, CFHO and CF2O which result from the two possible reactions of the oxy radical CF3CFHO. As a consequence the yield of these carbonyl products is dependent on temperature, O-2 partial pressure and total pressure. For the reaction of CFHO with Cl atoms the Arrhenius expression k(18) = 5.7x10(-14) exp (-(1130+/-160) K/T) cm(3)/s was determined in the temperature range 244-298 K.