Several recent studies have quantified the air exchange rate between the tropics and midlatitudes in the lower stratosphere using airborne and satellite measurements of chemical species. It is found that the midlatitude air is mixed into the tropical lower stratosphere with a replacement timescale of 13.5 months (with 20% uncertainty) for the region from the tropopause to 21 km <Volk et al., 1996> and at least 18 months for the region of 20--28 km <Schoeberl et al., 1997>. These estimates are used to adjust the horizontal eddy diffusion coefficients, Kyy, in a two-dimensional chemistry transport model. The value of Kyy previously used to simulate the subtropical barrier, 0.03¿106 m2/s, generates an exchange time of about 4 years, and the model without subtropical barrier (Kyy=0.3¿106 m2/s) has an exchange time of 5 months. Adjusting the Kyy to 0.13¿106 m2/s from the tropopause to 21 km and 0.07¿106 m2/s above 21 km produces the exchange timescales which match the estimates deduced from the measurements. The subtropical barrier prevents the engine emissions of the high-speed civil transport (HSCT) aircraft from being transported into the tropics and subsequently lifted into the upper atmosphere or mixed into the southern hemisphere. The model results show that the calculated ozone response to HSCT aircraft emissions using the Kyy adjusted to observed mixing rates is substantially smaller than that simulated without the subtropical barrier. ¿ 1998 American Geophysical Union