TROPOSPHERIC DEGRADATION CHEMISTRY OF HCFC-123 (CF3CHCL2) - A PROPOSED REPLACEMENT CHLOROFLUOROCARBON

HCFC-123 has been proposed as a replacement for some of the fully halo genated chlorofluorocarbons and other chlorinated hydrocarbons, which are being phased out under the Montreal Protocol. This paper reports l aboratory studies which were undertaken to determine kinetic and mecha nistic parameters of reactions involved in the atmospheric degradation of HCFC-123 and the use of these parameters in a 2D global model of t he troposphere to evaluate the yields of products formed in the degrad ation. The experimental studies have made use of the laser flash photo lysis technique with time-resolved ultra-violet absorption spectroscop y for the kinetic measurements and broad-band ultra-violet absorption spectroscopy for product characterization. Rate coefficients have been determined for the self-reaction of CF3CCl2O2 as (3.6+/-0.5) x 10(-12 ) cm3 mol-1 s-1 and for its reactions with HO2 and NO as (1.9+/-0.7) x 10(-12) cm3 mol-1 s-1 and (1.5-2.0) x 10(-11) cm3 mol-1 s-1, respecti vely, at room temperature. Kinetic data have also been obtained for th e reaction of CF3CCl2O2 with C2H5O2 and two channels have been identif ied; CF3CCl2O2 + C2H5O2 --> CF3CCl2O + C2H5O + O2, k = (9(-5)+9) x 10( -13) cm3 mol-1 s-1 and CF3CCl2O2 + C2H5O2 --> CF3CCl2OH + CH3CHO + O2, k = (3.6+/-0.5) x 10(-12) cm3 mol-1 s-1. Studies undertaken using the Cl-initiated oxidation of HCFC-123 suggest that trifluoroacetyl chlor ide, CF3COCl, is the major product of the gas-phase degradation. The k inetic and mechanistic data have been used to formulate a chemical mod ule of the degradation of HCFC-123 in the troposphere. The module has been incorporated into a 2D model of the global troposphere so that th e potential atmospheric impact of using HCFC-123 can be assessed.