The results of airborne measurement campaigns and two-dimensional (2-D) model calculations show compact relationships between certain pairs of long-lived species. Such correlations have been used in the present study to reduce the number of transported species used in the Commonwealth Scientific and Industrial Research Organisation (CSIRO) two-dimensional chemical transport model (2-D CTM) when calculating ozone changes from proposed fleets of supersonic aircraft or changing halocarbon concentrations. Differences to both the composition of the background atmosphere and to the predicted ozone changes are examined. It is shown that the use of correlations has only a small effect on the predicted ozone changes. Larger differences are found for the changing halocarbon scenario than for the supersonic aircraft scenario. These differences are due to difficulties associated with correctly accounting for the time variation of the source gas mixing ratios. Further work is required to reduce this source of error. The results of these studies should be applicable to 3-D CTMs, where large savings in computational time can be achieved through the reduction in the number of transported species. ¿ 1998 American Geophysical Union