Applications of quantitative thermal infrared hyperspectral imaging (8–14 μm): measuring volcanic SO2 mass flux and determining plume transport velocity using a single sensor

Abstract

Remote measurements of sulfur dioxide (SO2) path-concentration and emission rate (mass flux), released by passive volcanic degassing, are a key diagnostic of volcanic behavior. SO2 gas concentrations are also important for public health. Despite the significance of measuring SO2 mass flux at active volcanoes, an accurate method is still very difficult to obtain due largely to uncertainties of plume transport velocities and radiative transfer calculations. Here, we present a new infrared imaging method for deriving SO2 flux at active volcanoes with less than 20% plume transport velocity and radiative transfer uncertainties. The thermal hyperspectral imager (THI) was used for this study. THI is an uncooled remote sensing long-wave thermal infrared (TIR) imaging hyperspectral sensor that can obtain ~ 50 wavelength samples between 8 and 14 μm. Measurements of SO2 at the summit of Kīlauea volcano, collected using THI, are presented, where we obtained (for the periods of July 24–26, 2017, and February 5–6, 2018) a SO2 flux raging between 0 and 20 kg/s with occasional peaks higher than 40 kg/s. The spatial distribution of SO2 path-concentrations was obtained from the THI images by processing them using a newly developed SO2 retrieval algorithm, the SO2 amenable lookup table algorithm (SO2-ALTA). The sampling rate (30 Hz) of the microbolometer camera within THI means raw frames can be used for wind velocity derivation. Volcanic plume motion can be thus inferred by tracking plume features in sequential frames using spatial correlation techniques and knowing the camera angular orientation. Volcanic SO2 flux was estimated from the spatial distribution of SO2 and wind velocity. We evaluated temporal trends of the SO2 flux during the day and the night at the summit of Kīlauea volcano under both clear sky weather with northeasterly trade winds and cloudy sky conditions with low variable wind. These tests showed that the camera and techniques described in this paper provide an effective tool for monitoring SO2 fluxes remotely with less than 20% plume transport velocity and radiative transfer uncertainties and under a variety of background conditions (clear and cloudy sky, cold and hot backgrounds).