In order both to measure and to model the time dependence of atmospheric trace species, it is possible to make use of the correlations that occur between the stratospheric mixing ratios for a range of atmospheric constituents. If the correlation is to have any generality, it is necessary to take into account the time dependence of the release rate from the surface of the Earth. Previous studies have done this by using the age of the air &Ggr;. For compounds with stratospheric loss processes and rapidly changing tropospheric mixing ratios, the use of &Ggr; when normalizing correlations is inappropriate. The correlation derived in this way may be time-dependent and therefore of limited usefulness. In the work described here a two-dimensional (2-D) chemical transport model has been used to calculate &Ggr;*, the mean arrival time for a pulsed emission of a chemically active species released from the surface of the Earth. Because chemical reactions preferentially remove molecules from the tail of the distribution, &Ggr;* is always shorter than &Ggr;. The tropospheric concentrations of both CH3CCl3 (methylchloroform) and CH3CFCl2 (HCFC-141b) are currently undergoing rapid change. The results of time-dependent 2-D model calculations show that the use of &Ggr;* results in unique correlations for CH3CCl3 with CFCl3 and for CH3CFCl2 with N2O. Furthermore, empirical methods are described which allow &Ggr;* to be estimated if &Ggr; and the mean stratospheric lifetime of the species are known. This permits the methods described here to be applied to the measurements of atmospheric constituents taken at different times if estimates of the age of the air are available. ¿ 1999 American Geophysical Union