Experimental simulations of volcanic ash resuspension by wind under the effects of atmospheric humidity

Ash deposited during volcanic eruptions can be resuspended by wind and become hazardous for health and infrastructure hours to decades after an eruption. Accurate resuspension forecasting requires accurate modelling of the threshold friction velocity of the volcanic particles (U(th)*), which is the key parameter controlling volcanic ash detachment by wind. Using an environmental wind tunnel facility this study provides much needed experimental data on volcanic particle resuspension, with the first systematic parameterization of U(th)* for ash from the regions Campi Flegrei in Italy and also Eyjafjallajökull in Iceland. In this study atmospheric relative humidity (and related ash moisture content) was systematically varied, from <10% to >90%, which in the case of the Eyjafjallajökull fine ash (<63 μm) produced a twofold increase in U(th)*. Using the Campi Flegrei fine ash (<63 μm) an increase in U(th)* of only around a factor of 1.5 was observed. Reasonable agreement with force balance resuspension models was seen, which implied an increase in interparticle adhesion force of up to a factor of six due to high humidity. Our results imply that, contrary to dry conditions, one single modelling scheme may not satisfy the resuspension of volcanic ash from different eruptions under wet conditions.