The processed excess 14C data are used to calibrate the cross-tropopause transport in a two-dimensional model. The model results are used to diagnose the mechanisms responsible for cross-tropopause transport in the model and the sensitivity of the transport rate to the changes in the transport parameter. Although the total flux across the tropopause is dominated by the eddy diffusion flux along the isentropic surface due to the large values assigned to the diffusion coefficient Kyy at the boundary, the cross-tropopause transport is more sensitive to changes in the circulation. Nitrogen oxides emitted by engines of high-speed civil transport could cause ozone depletion in the lower stratosphere. The expected depletion is proportional to the amount of oxides of nitrogen retained in the stratosphere. For a typical fleet, lowering the tropopause height by 1.2 km can cause a 14--22% increase in the amount of reactive nitrogen retained in the stratosphere. Thus it is necessary to have a fine enough vertical resolution near the tropopause in the two-dimensional model for determining the stratospheric residence time of the engine emissions deposited near the tropopause. ¿ American Geophysical Union 1993