Some tephra fallout deposits show an increase of mass and thickness at distances from thesource >100 km (areas of secondary mass maximum, ASMM) which demonstrates distal enhancedsedimentation from volcanic plumes. We explore development of the ASMMs during the 1992 August andSeptember Mount Spurr eruptions, USA, by combining fi eld data on the spatial distribution of mass and grainsize with (1) simulations of individual particle settling through a homogeneous and horizontally stratifiedatmosphere and (2) mesoscale models of the three-dimensional wind field that include the effect of theunderlying topography. The crosswind and downwind variations of deposit characteristics indicate that theincrease of sedimentation at the ASMMs is not formed solely because of preferential settling of small ashparticles (<125 μm), as commonly assumed in aggregation models. Instead, ASMM grain sizes correspond tothe fine modes of the bimodal total grain size distributions. There also appears to be a link between theASMM and the topography: the mass local minima occur across the windward flank of 2 km high mountainranges, while the ASMMs spread on the leeward flank. Mesoscale models of the three-dimensional wind fieldshow vertical oscillations in the wind over mountainous regions which may enhance mechanisms of enmasse sedimentation (aggregation, hydrometeor formation, and particle boundary layers), as well as strongspatial variations of the horizontal wind field in the lower troposphere. Our study demonstrates theimportance of using grain size, as well as mass, data to constrain the complex processes responsible forparticle sedimentation from volcanic plumes.