The relative effectiveness of bromine compared to chlorine for destroying stratospheric ozone is explored. Two definitions previously used to quantify this relative effectiveness, typically referred to as α, are compared and a definition is presented to calculate α values applicable for column and global ozone loss. Calculations of α are made with a two-dimensional radiative/chemical/dynamical model and suggest that bromine is roughly 45 times more effective than chlorine for global ozone destruction. The physical processes underlying this result are probed, and sensitivity studies are presented that show that relatively large changes invoked in the modeled transport and heterogeneous chemistry lead to changes in this value of <15%. ¿ 1999 American Geophysical Unio