Observations of Mount Pinatubo aerosol as measured by the ISAMS instrument on UARS are presented for December 1991. These observations show a tropical reservoir of high aerosol with steep gradients across the subtropics. On the 450 K isentrope there is a ring of high aerosol in midlatitudes which is separated from the tropical reservoir by an approximately zonal band of lower aerosol values. The ISAMS aerosol data are compared with output from the off-line three-dimensional transport model SLIMCAT. Integrations are performed with isentropic transport only, diabatic transport only, isentropic and diabatic transport, and isentropic, diabatic and sedimentary transport of aerosol. Integrations using winds and temperatures from UKMO and ECMWF are compared. It is shown that even in the lower stratosphere, cross-isentropic transport over the timescale of a month can have a significant effect on the distribution of aerosol. The SLIMCAT integrations with isentropic transport only do not form a ring of high aerosol in midlatitudes on the 450 K isentrope. Inclusion of diabatic transport in the SLIMCAT integration shows that this ring forms from diabatic descent of air containing higher aerosol concentrations from aloft. Isentropic transport of high-aerosol air from the tropics into midlatitudes on the 500 K isentrope, followed by diabatic descent, further increases the values in the midlatitude ring on the 450 K isentrope. Inclusion of the sedimentation of aerosol in SLIMCAT integrations significantly offsets the diabatic ascent in the tropics. The SLIMCAT integrations, particularly those run using UKMO analyses, produce too much transport of midlatitude air into the tropics. The implications for accurate modeling of the tropical reservoir are discussed. ¿ 1999 American Geophysical Union