ON THE EVALUATION OF OZONE DEPLETION POTENTIALS

Observations of methane, CFC-11, and ozone losses are used along with insights from models and observations regarding interrelationships between tracers to develop a semi-empirical framework for evaluating global ozone depletion potentials. Direct measurements of some hydrochlorofluorocarbons including HCFC-22 in the Arctic lower stratosphere are also used to evaluate the local ozone depletion potentials there. This approach assumes that all of the observed ozone destruction in the contemporary atmosphere is due to chlorine and that the depletion is proportional to the local relative chlorine release. It is shown that the global ozone depletion potentials for compounds with relatively long stratospheric lifetimes such as HCFC-22 and HCFC-142b are likely to be larger than those generally predicted by gas phase chemical models, due largely to the importance of lower stratospheric ozone losses that are not simulated in gas phase studies. The analysis presented suggests that the globally averaged efficiency for ozone depletion by HCFC-22 is as much as a factor of 2 larger than some gas phase model estimates. For compounds with short stratospheric lifetimes such as CCl4 and CH3CCl3, on the other hand, gas phase models likely overestimates the ozone depletion potentials for the present-day stratosphere. Observations of polar ozone loss and reactive halogen radical abundances also imply that the globally averaged ozone depletion potentials for brominated species for the contemporary stratosphere could be as much as 1.5-3 times greater than some gas phase model predictions, depending upon lower stratospheric loss processes.