We develop a model to describe the formation of aggregates in a volcanic eruption column. The model combines a description of the rate of collision and sticking of particles with a model of the vertical transport in the eruption column. We thereby determine the evolution of the grain size distribution as a function of height in the eruption column. We consider aggregates in which liquid water provides the binding agent. For sufficiently large eruption columns we find that this limits the vertical extent of the zone where aggregates may form since near the source, all water is in the vapor form, while in the upper part of the column the mixture becomes very cold and freezes. However, in many cases, we find that the particles spend sufficient time in the central region, where the water is in the liquid form, that a substantial amount of aggregation occurs. Furthermore, we predict that owing to the reduction in the binding efficiency of water as particle size increases <Gilbert and Lane, 1994>, there is a relatively narrow size distribution of aggregates at the plume top. We also model the airfall deposits associated with this aggregate-rich distribution of particles which is injected to the top of the eruption column. We show that the relatively narrow size distribution of particles at the top of the column, coupled with the gravitational settling and transport by ambient winds, may lead to enhanced deposition close to the source and in some cases secondary thickening of the deposit. The relatively near-source deposition of fine ash in these deposits is associated with the fallout of aggregates. As a simple application, we show that the present dynamical aggregation model is consistent with the secondary thickening of the deposit from the May 18, 1980, eruption of Mount St. He¿lens. ¿ 2001 American Geophysical Union