Vertical distributions of particles (0.1 &mgr;m radius) and gases are derived for a simplified model of the stratosphere. A constant source acts at a specific range of heights, and the troposphere is a sink. Gravitational settling is shown to have a large effect on the distribution and residence time of the particles. This effect cannot be ignored in derivation of vertical eddy coefficients from tracer data, and is of the right magnitude to reconcile the results obtained from particulate and gaseous radioactivity.