A three-dimensional global chemical/dynamical model is used to study ozone mass exchange between the stratosphere and the troposphere. The ozone budget is calculated in the northern and southern extratropical lowermost stratosphere and in the tropical upper troposphere during both perturbed and background aerosol conditions. The stratospheric ozone mass flux into the troposphere is highest during the spring months in both hemispheres and higher in the northern hemisphere than in the southern hemisphere. During background aerosol conditions the net modeled stratospheric flux of ozone to the troposphere is 792 Tg/yr. The results suggest that the heterogeneous chemical reactions occurring on the surface of sulfate aerosols can have a significant impact on the ozone mass flux across the tropopause, leading to a possible 15% reduction in the annual global ozone mass transported from the stratosphere to the troposphere after a large volcanic eruption. Most of the reduction in ozone mass flux occurs in the northern hemisphere. In the southern hemisphere winter and spring the ozone concentration is already significantly perturbed by the heterogeneous reactions occurring on the surface of polar stratospheric clouds over Antarctica. The further increase in heterogeneous conversion due to the enhanced sulfate aerosol produces a smaller increase in active chlorine (ClO) than in the northern hemisphere, leading to a smaller increase in ozone destruction. ¿ 1997 American Geophysical Union