Real-time tephra-fallout accumulation rates and grain-size distributions using ASHER (ASH collector and sizER) disdrometers

ASHER, a new sensor for the characterization of tephra fallout in real time, was designed and developed for easy field deployment during volcanic eruptions. It can provide information on the accumulation rate of tephra fallout in real time as well as grain-size and settling velocity of falling particles. Particle detection is achieved with a laser barrier, with size and settling velocity being calculated from the amplitude and duration of obscuration peaks. The sampling rate (31,500 Hz), laser thickness (0.5 mm) and operation (ON/OFF state and dual acquisition mode) are adapted to minimize the noise level and allow detection of particles as small as ~100 μm. Additional measurements of weight and level of accumulated material within a removable collector allow broadening of the ASHER operation to accumulation rate from 10−2 to 103 g m-2s-1. Detailed calibration tests were performed in laboratory conditions on single grains of known shape and density along with a high-speed camera to test the capability to measure grain size and terminal velocity, and during two field campaigns at Stromboli and Etna volcanoes to test the operation in the field. Long-term field deployment has shown that combining the optical barrier with an automatic collector allows for a better characterization of tephra fallout, providing an estimate of density, and, therefore, it optimizes sensor operation and minimizes false alerts. Moreover, the low power requirements and onboard processing allows to operate the sensor remotely and solely on solar power in a remote location. Although technical improvements in sensor sensitivity and processing are still possible, the results presented suggest that ground sensors for real-time detection and analysis of tephra could potentially contribute to understanding the dynamics of explosive eruptions and could be successfully integrated into monitoring systems of active volcanoes.