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 (CF 3 CFH 2 , 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) CF 3 CFH 2 +OH→CF 3 CFH+H 2 O 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 CF 3 CFHO-radical with O 2 , (4) CF 3 CFHO+O 2 →CF 3 CFO+HO 2 , and C-C bond fission, (5) CF 3 CFHO+M→CFHO+CF 3 +M, for T = 295 K and p total = 50 mbar (O 2 ) has been obtained to be k 4 /(k 5 [M]) = 1.5 x 10 -19 cm 3 , with the individual values being k 4 = 2.7 x 10 -15 cm 3 /s and k 5 [M] = 1.8 x 10 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]) = 8.7 x 10 -25 exp ((3240 -610 +990 ) K/T) cm 3 was determined. Moreover, it is concluded that the major carbonyl products are CF 3 CFO, CFHO and CF 2 O which result from the two possible reactions of the oxy radical CF 3 CFHO. 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 = 5.7 x 10 -14 exp(-(1130 ± 160) K/T) cm 3 /s was determined in the temperature range 244-298 K.