Trospospheric degradation chemistry of HCFC-123 (CF 3CHCl 2): A proposed replacement chlorofluorocarbon

Abstract

HCFC-123 has been proposed as a replacement for some of the fully halogenated chlorofluorocarbons and other chlorinated hydrocarbons, which are being phased out under the Montreal Protocol. This paper reports laboratory studies which were undertaken to determine kinetic and mechanistic parameters of reactions involved in the atmospheric degradation of HCFC-123 and the use of these parameters in a 2D global model of the troposphere to evaluate the yields of products formed in the degradation. The experimental studies have made use of the laser flash photolysis technique with time-resolved ultra-violet absorption spectroscopy for the kinetic measurements and broad-band ultra-violet absorption spectroscopy for product characterization. Rate coefficients have been determined for the self-reaction of CF 3CCl 2O 2 as (3.6±0.5)×10 −12 cm 3 mol −1 s −1 and for its reactions with HO 2 and NO as (1.9±0.7)×10 −12 cm 3 mol −1 s −1 and (1.5–2.0)×10 −11 cm 3 mol −1 s −1, respectively, at room temperature. Kinetic data have also been obtained for the reaction of CF 3CCl 2O 2 with C 2H 5O 2 and two channels have been identified; CF 3CCl 2+O 2+C 2H 5O 2→CF 3CCl 2O+C 2H 5O+ C 2H 5O+O 2, k=(9 +9 −5)×10 13 cm 3 mol −1 s −1 and CF 3CCl 2O 2+C 2H 5O 2→CF 3CCl 2OH+CH 3CHO+O 2, k=(3.6±0.5)× 10 −12 cm 3 mol −1 s −1. Studies undertaken using the Cl-initiated oxidation of HCFC-123 suggest that trifluoroacetyl chloride, CF 3COCl, is the major product of the gas-phase degradation. The kinetic and mechanistic data have been used to formulate a chemical module of the degradation of HCFC-123 in the trophosphere. The module has been incorporated into a 2D model of the global troposphere so that the potential atmospheric impact of using HCFC-123 can be assessed.