The scientific community has defined two useful parameters to estimate the potential impact of controlled and replacement substances on ozone destruction: the Ozone Depletion Potential (ODP) and the Chlorine Loading Potential (CLP). Wuebbles first proposed ODP in the 1983 paper "Chlorocarbon Emission Scenarios" as a measure of the destructive potential of a particular substance relative to depletion caused by an equal amount of a reference substance. CFC-11 is typically defined as the standard reference compound and is assigned an ODP of 1.0. In similar fashion, CLP is defined as the total amount of chlorine contained in a particular compound reaching the stratosphere relative to an equivalent release of CFC-11. A standard value of 1.0 is again assigned as the CLP of CFC-11. Pyle et al. (1992) review the modeling and observational techniques used in evaluating ODPs and CLPs in the chapter "Ozone Depletion and Chlorine Loading Potentials" of the Scientific Assessment of Ozone.
ODP is usually defined as a single, steady-state measure of ozone depletion over the entire lifetime of the substance. Knowledge of tropospheric OH concentration is one of the limiting factors in determining lifetimes of alternative substances and accounts for differing ODP values calculated by various research groups. Fisher et al. (1990) address these uncertainties in "Model Calculations of the Relative Effects of CFCs and Their Replacements on Stratospheric Ozone." Prather and Watson (1990) discuss effects of ODP using several emission scenarios and phaseout schedules of CFCs and alternatives in "Stratospheric Ozone Depletion and Future Levels of Atmospheric Chlorine and Bromine."
Recent attention has been directed toward shorter term forecasts of ODP. In the 1992 paper "Time Dependent Ozone Depletion Potentials for Short and Long-Term Forecasts," Solomon and Albritton use an empirical, time-dependent approach to suggest that HCFCs may produce more significant short-term ozone loss than has been previously predicted by steady-state models.
Because HFCs contain no chlorine, their ODP has been assumed to be zero. But HFCs containing a CF3 group may be sufficiently stable to reach the stratosphere, where fluorine radicals can engage in catalytic destruction of ozone. Ravishankara et al. (1994) investigate such a possibility in "Do Hydrofluorocarbons Destroy Stratospheric Ozone?" They conclude that the ODP for HFCs containing a CF3 group is negligibly small and constitutes no more of a threat than HFCs and HCFCs that do not contain a CF3 group.
In the recent commentary "Better Protection of the Ozone Layer," Ko, Sze, and Prather (1994) discuss the need for an improved method of evaluating ODPs for ozone-depleting substances not addressed by the Montreal Protocol restrictions. They suggest that evaluating potential effects on ozone by substances such as solid rocket fuel from the space shuttle or emissions by stratospheric aircraft is qualitatively and quantitatively different than techniques used in addressing potential impacts of substances under regulation by the Protocol agreements.