Ozone Depletion Potentials: A new approach

The Ozone Depletion Potential (ODP) is given by the time integrated global ozone loss of an ozone depleting substance (ODS) relative to a reference ODS (usually CFC-11). The ODP is used by the Montreal Protocol (and subsequent amendments) to inform policy decisions on the production of ODSs. Since the early 1990s, ODPs have primarily been estimated using an approximate formulism that utilizes the lifetime and the fractional release factor of the ODS. This has the advantage that it can use measured concentrations of the ODSs to estimate their fractional release factors. However, there are strong correlations between the stratospheric lifetimes and fractional release factors of ODSs and these can introduce uncertainties into ODP calculations when the terms are estimated independently. Instead, we show that the ODP is proportional to the average global ozone loss per equivalent chlorine molecule released in the stratosphere by the ODS loss process (which we call the Γ factor) and, importantly, this ratio varies only over a relatively small range ( 0.5-1.5) for ODPs with stratospheric lifetimes of 20 to more than 1,000 years. The chemistry-climate model CESM-WACCM and the NOCAR two-dimensional model are used to show that the Γ factor varies smoothly with stratospheric lifetime for ODSs with loss processes dominated by photolysis and is larger for long-lived species, while stratospheric OH loss processes produce relatively smaller Γ factors. The fractional release approach only approximately captures these relationships. The models are used to compare ODPs computed using ozone changes (i.e., Γ factors) and fractional release factors. We show that even though ODPs are similar using the two methods, the variation of the ODP with lifetime is often incorrect when using fractional release factors. We also show that while the absolute Γ's vary between WACCM and NOCAR models, much of the difference is removed for the Γ/Γ_CFC-11 ratio that is used in the ODP formula. Finally, recommendations are given to estimate ODPs when only lifetimes of the ODSs are known.