Airborne measurements of volcanic gas composition during unrest at Kuchinoerabujima volcano, Japan

Abstract

Airborne measurements of volcanic gas composition using an unmanned aerial vehicle (UAV) and Cessna aircraft were conducted at Kuchinoerabujima volcano, Japan, between 2014 and 2016. Because eruptions occurred in August 2014, May 2015, and June 2015, access to the summit crater was limited because of the risk of sudden eruption such that airborne measurements were the only viable method to measure the volcanic gas composition. Multi-GAS and alkali-filter pack measurements were made on the leeward side of the crater and around the crater, using the Cessna and UAV, respectively. Observations using the UAV could measure the dense plume and quantify the gas species of H2O, CO2, SO2, H2S, H2, HCl, and HF, while the observations using the Cessna could measure only the diluted plume and quantify CO2, SO2, H2S, and H2. The seven airborne observations enabled us to monitor variations in the volcanic gas composition. Over the observation period, the SO2/H2S ratio decreased from 10 to 1.9. The H2O/SO2 ratio, H2/SO2 ratio, and apparent equilibrium temperatures (AET) estimated using the volcanic gas composition increased after the 2014 eruption. The decrease in the SO2/H2S ratio might be attributed to changes in the pressure of degassing magma and interactions with the hydrothermal system. The airborne methods presented here highlight the utility of using light aircrafts to safely conduct volcanic gas measurements during periods of volcanic unrest when traditional ground-based methods are not possible.