Sing Lau

Abstract:

Volcanic ash clouds are carefully monitored as they present a significant hazard to humans and aircraft. The primary tool for forecasting the transport of ash from a volcano is dispersion modelling. These models make a number of assumptions about the size, sphericity and density of the ash particles. Few studies have measured the density of ash particles or explored the impact that the assumption of ash density might have on the settling dynamics of ash particles. In this paper, the raw apparent density of 23 samples taken from 15 volcanoes are measured with gas pycnometry, and a negative linear relationship is found between the density and the silica content. For the basaltic ash samples, densities were measured for different particle sizes, showing that the density is approximately constant for particles smaller than 100 µm, beyond which it decreases with size. While this supports the current dispersion model used by the London Volcanic Ash Advisory Centre (VAAC), where the density is held at a constant (2.3 g cm-3), inputting the measured densities into a numerical simulation of settling velocity reveals a primary effect from the silica content changing this constant. The VAAC density overestimates ash removal times by up to 18 %. These density variations, including those varying with size beyond 100 µm, also impact short-range particle-size distribution (PSD) measurements and satellite retrievals of ash.

Abstract:

Unsieved ash density measurements from 23 unsieved raw ash samples originating from 15 volcanoes, including mass of sample measured with a digital scale and volume measured with a nitrogen gas pycnometer. Data measured in July-August 2022.