Determination of the volcanic eruption source parameters—total grain-size distribution and vertical ash mass distribution (VMD) within the source—is carried out on a collection of measured-area samples and granulometry data. For this, the geophysical inverse methods and Hybrid Particle and Concentration Transport Model (HYPACT) driven by wind and turbulence fields simulated with the Regional Atmospheric Modeling System (RAMS) are used. A two-step inversion procedure is proposed to obtain approximate but physically meaningful solution when the total number of ashfall samples is small and it is not possible to make a good initial guess of the source parameters. First, a spectrum of particle fall velocities is estimated by selecting a best-fit subset of aerodynamically distinct subpopulations of free and aggregate particles from the trial set used to simulate a polycomponent ashfall. The singular value decomposition (SVD) analysis is then employed to identify spatial components of the ash emissions’ vertical distribution, as resolvable by the observations. Model validation experiments are conducted for the January 12, 2011, short-duration vulcanian explosion at Kizimen and paroxysmal phase of the December 24, 2006, sub-Plinian eruption at Bezymianny. The derived VMDs exhibit high variability in fine ash content (~ 60–100 wt%) as well as strong secondary maxima in the lower troposphere, likely reflecting the contribution of ash particles fallen out of co-pyroclastic flow ash clouds and partially collapsing eruption columns.