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The impact of explosive volcanic plumes on climate and on air traffic strongly depends on the concentration and grain-size distribution (GSD) of pyroclastic fragments injected into the atmosphere. Accurate and robust modelling of the evolution of GSD during pyroclast transport from the vent to the ash cloud is therefore crucial for the assessment of major volcanic hazards. Analysis of field deposits from various recent Plinian eruptions shows that their total GSD is well described by a power law, as expected from the physics of magma fragmentation, with an exponent (D) ranging from 3.0 to 3.9. By incorporating these measured GSD into the initial conditions of a steady-state 1D model of explosive eruption columns, we show that they have a first-order impact on the dynamical behaviour of explosive eruption columns. Starting from an initial value of D, the model tracks the evolution of GSD in the column and calculates …