Interrelationship of Stratospheric Age of Air, Age Spectrum and Constituent Lifetimes

Assessments of the response of ozone to changes in composition rely on mixing ratio boundary conditions for chlorofluorcarbons and halons. These boundary conditions are specified from measurements for years up to the present. Future boundary conditions are calculated from a model, using estimates of atmospheric release and the atmospheric lifetime for each species. We have calculated the fluxes of CFC-12 (CF2Cl2) and CFC-11 (CFCl3) necessary to maintain the mixing ratio boundary conditions imposed on our chemistry climate model for a simulation of the years 1960 through 2005. These fluxes were found to match industrial release estimates over that time period. The agreement has prompted investigation of the interrelationships among mean age of air, age spectrum, constituent lifetime and the distribution of total inorganic chlorine. Experiments with the GSFC 2D model have shown that the atmospheric lifetime calculated from the integrated total amount of a CFC divided by the integrated loss rate is relatively insensitive to changes in the circulation. In these experiments, which involved large changes to the circulation, the mean age of air and the implied surface fluxes changed by a factor of two while the deduced lifetime changed by only 25%. We use the results from our coupled 3D CCM calculations to estimate the uncertainty in projections of the mixing ratio boundary conditions for CFCs into the future. We estimate the impacts of the speeding up of the residual circulation and the decrease in the mean age of air that is predicted by chemistry climate models on future scenario calculations.