A GLOBAL 3-DIMENSIONAL STUDY OF THE FATE OF HCFCS AND HFC-134A IN THE TROPOSPHERE

We present the first attempt to use a global three-dimensional model of the troposphere to study the degradation chemistry of the alternative chlorofluoro- and fluorohydrocarbons HCFC-22, HCFC-123, HCFC-124, HFC-134a, HCFC-141b, and HCFC-142b and the main removal processes from the troposphere of these halogenated hydrocarbons and their oxidation products. Lifetimes of the parent hydrochlorofluorocarbons (HCFCs) and HFC-134a range from 1.3 to 20 years, with oxidation by the OH radical being the dominant removal process. Using the emission scenarios of McCulloch (1993), we calculate that HCFC-22 volume mixing ratios will reach a maximum of 190 parts per trillion by volume (pptv) around the year 2005, the mixing ratios of all other HCFCs remaining below 100 pptv. The chlorine loading potentials (CLP) of these HCFCs range from 0.014 for HCFC-123 to 0.166 for HCFC-142b, whereas HFC-134a has a zero CLP. Based on recent kinetic information, it is calculated that none of the relatively stable intermediate oxidation products such as COF2, COFCl, CF3COF, and CF3COCl will substantially build up in the atmosphere, the global abundances of these compounds being generally less than 1% those of the parent compounds. A possible exception is HCOF for which the heterogeneous removal parameters have not been experimentally determined. Hydrolysis in clouds, and to a lesser extent in seawater, is the process efficiently removing these species. Organic nitrates from all HCFCs studied will not reach substantial concentrations and will not contribute to chlorine delivery to the stratosphere. The deposition fluxes of the very stable trifluoroacetic acid (TFA), derived from the oxidation of HFC-134a and HCFCs-123 and 124, will reach in the year 2020 maximum levels of 2 mu mol/m(2)/yr (about 1 nmol/L rainwater). Maximum deposition occurs in tropical regions, associated with high oxidation rates of the parent compounds, and high rainfall. These predicted concentrations and deposition fluxes are orders of magnitude smaller than what is thought to be toxic for humans, for the fauna, and for the flora.