The concentration of ash in volcanic plumes, inferred from dispersal data

The mass concentration of pyroclasts of a particular grain size decreases as an exponential function of time in the body of the umbrella cloud that forms at the top of explosive volcanic-eruption plumes. Data on deposit-thickness variations and grain-size distributions of ash deposits can be used to infer particle concentrations in volcanic clouds using a simple physical model for sedimentation from umbrella clouds. The concentrations of ash in plumes that penetrate the operating heights of commercial aviation greatly exceed the concentrations that cause engine damage and failure (>2000 mg/m³) and dangerous concentrations (>50 mg/m³) can persist for hundreds of kilometers downwind of the vent. However, aggregation of volcanic ash in the atmosphere can lead to much more rapid decreases in concentration. -Authors