Consistent sets of atmospheric lifetimes and radiative forcings on climatefor CFC replacements: HCFCs and HFCs

Recognition of deleterious effects of chlorine and bromine on ozone and cli mate over the last several decades has resulted in international accords to halt the production of chlorine-containing chlorofluorocarbons (CFCs) and bromine-containing halons. It is well recognized, however, that these chemi cals have had important uses to society, particularly as refrigerants, as s olvents, as plastic blowing agents, as fire retardants and as aerosol prope llants. This has led to an extensive search far substitute chemicals with a ppropriate properties to be used in place of the CFCs and halons. The purpo se of this study is to evaluate in a consistent manner the atmospheric life time and radiative forcing on climate for a number of replacement compounds . The unique aspect of this study is its attempt to resolve inconsistencies in previous evaluations of atmospheric lifetimes and radiative forcings fo r these compounds by adopting a uniform approach. Using the latest version of our two-dimensional chemical-radiative-transport model of the global atm osphere, we have determined the atmospheric lifetimes of 28 hydrohalocarbon s (HCFCs and HFCs). Through the comparison of the model-calculated lifetime s with lifetimes derived using a simple scaling method, our study adds to e arlier findings that consideration of stratospheric losses is important in determining the lifetimes of gases, Discrepancies were found in the reporte d lifetimes of several replacement compounds reported in the international assessment of stratospheric ozone published by the World Meteorological Org anization [Granier et al., 1999] and have been resolved. We have also deriv ed the adjusted and instantaneous radiative forcings for CFC-11 and 20 othe r halocarbons using our radiative transfer model. The sensitivity of radiat ive forcings to the vertical distribution of these gases is investigated in this study and is shown to be significant. The difference in the global ra diative forcing arising from the assumption of a constant vertical profile for these gases is found to range from 0 to 36%, with higher difference for short-lived gases. Global Warming Potentials (GWPs) for the compounds are determined using the lifetimes and radiative forcings evaluated in this stu dy and are found to differ from values reported by Granier et al. [1999] ow ing to the differences in our calculated radiative forcings and lifetimes.