Atmospheric chemistry of halogenated compounds

Using pulse radiolysis combined with transient UV absorption spectroscopy the kinetics of the atmospheric degradation of a series of C3-HFCs, one HCFC and 3 chloroethanes was studied. The UV absorption spectra and the kinetics of reactions for R=CF3CF2CFH, CF3CF(·)CF3, CF3CH(·)CF3, CH3CF2CH2, CF3CF2CF2, CF3CClH, CH2CLCClH, CCl3CCl2, and CCl3CClH radicals were investigated. The kinetics of the reactions of CF3CFHO2 radicals with HO2 and NO2 radicals was studied. The rate constants were determined to be (5±1.5) x 10-12 cm3 molecule-1 s-1 and (5.0±0.5) x 10-2 cm molecule-1 s-1, respectively. The kinetics of the gas-phase reaction of CF3C(O)OH with OH radicals was studied, the rate constant was found to be (1.7±0.5) x 10-13 cm3 molecule-1 s-1. The reaction with OH competes with incorporation into rain-water droplets and removes 10-20% of the CF3C(O)OH from the atmosphere. An FTIR smog chamber technique was used to determine the atmospheric products for HFC227ca (CF3CF2CF2H) and CCl3CH2Cl. For CF3CF2CF2O radicals the atmospheric fate is C-C decomposition while 24% of the CCl3CClHO radicals decompose via C-C bond scission, the rest eliminates HCl. The atmospheric fate of CF3CFHO radicals is either C-C decomposition or reaction with O2. The FTIR smog chamber technique and pulse radiolysis were used to study the branching ratio between C-C bond scission and reaction with O2. The fate of CF3C(O)F formed when CF3CFHO radicals react with O2 is hydrolysis to CF3C(O)OH. Since it is only the thermalized CF3CFHO radicals that may react with O2 the previous estimates of CF3C(O)OH formation from HFC-134a should be revised from 30% to 7-19%. (EG) 22 refs