Rate and Mechanism of the Atmospheric Degradation of 1,1,1,2‐Tetrafluoroethane (HFC‐134a)

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Abstract
The atmospheric chemical behaviour of 1,1,1,2‐tetrafluoroethane (CF3CFH2, HFC‐134a) with respect to its rate and mechanism of degradation in the troposphere has been investigated. The rate coefficient for the reaction of (1 a) CF3CFH2 + OH→CF3CFH + H2O has been determined in direct time‐resolved experiments using laser‐pulse initiation and laser long‐path absorption. A value of k1a = (4.6±0.5)×10−15 cm3/s at T = 295K has been found. The ratio of the rate coefficients for the reactions of the CF3CFHO‐radical with O2, (4) CF3CFHO + O2→CF3CFO + HO2, and C‐C bond fission, (5) CF3CFHO + M→CFHO + CF3 + M, for T = 295 K and ptotal = 50 mbar (O2) has been obtained to be k4/(k5 [M]) = 1.5×10−19 cm3, with the individual values being k4 = 2.7×10−15 cm3/s and k5 [M] = 1.8×104 s−1. The mechanism of the atmospheric degradation of HFC‐134a has been investigated in the temperature range 244‐295 K and ptotal = 1000 mbar using UV photolysis/FTIR product studies. From these experiments the Arrhenius expression k4/(k5 [M]) = 8.7±10−25 exp ((3240+990−610) K/T) cm3 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, O2 partial pressure and total pressure. For the reaction of CFHO with Cl atoms the Arrhenius expression k18 = 5.7×10−14exp (−(1130 ± 160) K/T) cm3/s was determined in the temperature range 244‐298 K.

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