The Executive Summary of the 2010 WMO/UNEP Ozone Assessment cites lifetime problems and problems associated with lifetimes:
“Evidence is emerging that lifetimes for some important ODSs (e.g., CFC-11) may be somewhat longer than reported in past assessments. In the absence of corroborative studies, however, the CFC-11 lifetime reported in this Assessment remains unchanged at 45 years. Revisions in the CFC-11 lifetime would affect estimates of its global emission derived from atmospheric changes and calculated values for Ozone Depletion Potentials (ODPs) and best-estimate lifetimes for some other halocarbons.”
“A stronger BDC would decrease the abundance of tropical lower stratospheric ozone, increase poleward transport of ozone, and could reduce the atmospheric lifetimes of long-lived ODSs and other trace gases.”
“Carbon tetrachloride (CCl4) tropospheric abundances have declined less rapidly than expected. Emissions derived from data reported to the United Nations Environment Programme (UNEP) are highly variable and on average appear smaller than those inferred from observed abundance trends. Although the size of this discrepancy is sensitive to uncertainties in our knowledge of how long CCl4 persists in the atmosphere (its “lifetime”), the variability cannot be explained by lifetime uncertainties.”
A re-evaluation is needed of the lifetimes of important halogen source gases (e.g., CFC-11, CCl4, Halons, HFCs, HCFCs, and related species), since evidence has emerged that in many cases the actual lifetimes may be considerably longer than those currently assumed in the 2010 WMO/UNEP Ozone Assessment (2010), and in the scenarios used to drive the Chemistry-Climate Models (CCMs). This represents a major uncertainty in reconciling top-down and bottom-up emission estimates, and in model projections.
This SPARC activity on ‘Lifetime of halogen source gases’ will provide a comprehensive review, and include
- an overview of the theory of estimating lifetimes using models and observations;
- an update of the kinetic data that determine lifetimes;
- lifetimes deduced from observed trace-gas distributions;
- and model estimates of lifetimes, which will require new CCM runs.
Groups that contribute to the validation of CCMs (see CCMVal) will therefore be critical participants in this initiative. The results are expected to be an important input to the next WMO/UNEP Ozone Assessment due to be published in 2014.
This SPARC activity, launched at the 2011 SPARC Scientific Steering Group meeting held in Pune, India, will result in a SPARC science report by spring 2013 to form the basis for the 2014 Ozone Assessment. The scope of the re-evaluation includes:
- estimating the numerical values for lifetimes,
- estimating the uncertainties for numerical values for lifetimes,
- assessing the influence/use of different lifetime definitions (e.g. steady-state /instantaneous lifetimes), and
- assessing lifetime changes associated with the changing climate.
Presentation with the outline for the SPARC assessment report (9.1 MB)
Langley Research Center, USA
NASA Goddard Space Flight Center, USA
Empa, ETH Zurich, Switzerland
Susan E. Strahan
Universities Space Research Association (USRA), USA
SPARC Science Report:
SPARC Report No. 6 (2013) Lifetimes of Stratospheric Ozone-Depleting Substances, Their Replacements, and Related Species. M.K.W. Ko, P.A. Newman, S. Reimann, S.E. Strahan
SPARC activity report:
SPARC Newsletter No. 42 (2014) p. 14: “The SPARC Activity: Lifetime of halogen source gases”