Effective equivalent stratospheric chlorine and the future of ozone depleting substances in the stratosphere

Effective equivalent stratospheric chlorine (EESC) has been used in the international ozone assessments to predict when ozone depleting substances (ODSs) would recover to levels observed in 1980. The WMO/UNEP Scientific Assessment of Ozone Depletion (2002) predicted that EESC would drop below 1980 levels in 2044. Over the last few years various groups have begun to use EESC as a "chlorine" proxy for the estimation of stratospheric ozone trends. The EESC used in these past assessments and trend studies is appropriate to the mid-latitude lower stratosphere only, because of the 3-year age of air assumed. Recently, we have revised the calculation of EESC to make it a better quantitative representation of inorganic halogen levels throughout the stratosphere. This revised EESC was used to predict that the Antarctic ozone hole would recover in 2068. In this presentation, we will illustrate that both the temporal evolution and magnitude of EESC is highly dependant upon the mean age-of-air. We will illustrate a number of uncertainties in the estimation of ODS recovery in the stratosphere. In particular, the recovery of EESC to 1980 levels is mainly dependant on mean age-of-air. We will also illustrate the pitfalls in using EESC as a proxy for ozone trend analysis studies.