An aerosol trajectory model (ATM), which couples TOMS aerosol index (AI) measurements with multiple-level parcel trajectories, is presented for determining the three-dimensional (3-D) distribution of a tropospheric aerosol cloud. The ATM is illustrated with an idealized 2-D (height-longitude) cloud in linear vertical shear. The half width of the vertical parcel distribution (an indicator of how well the cloud is resolved) is inversely proportional to time and to vertical shear. The degree to which a cloud can be resolved is limited by an uncertainty principle, whereby model precision improves with time, while accuracy degrades with time because of accumulating trajectory errors. ATM is applied to the ash cloud from the September 1992 eruption of Mount Spurr, Alaska. Disagreement in the predicted cloud structure occurs between 3-day ATM runs using United Kingdom Meteorological Office (UKMO) and National Centers for Environmental Prediction (NCEP) winds. This is due to significant differences in the UKMO and NCEP zonal wind speed near the tropopause, which cause large trajectory separations over 3 days. The UKMO-predicted cloud range (310--390 K) agrees well with radar and pilot observations of the ash cloud, while the NCEP-predicted range shows strong disagreement with observations in the region of the jet maximum. This indicates the potential (when independent observations are available) for using ATM to partially validate wind fields. ¿ 1999 American Geophysical Union