Estimating emission flux of H2S from fumarolic fields using vertical sensor array system

Abstract

The emission flux of volatiles from each fumarolic field in volcanic and geothermal areas can be used to evaluate the current state of magmatic activity and predict its future trends. The emission flux of SO2 has been quantified in many fumarolic fields using remote sensing techniques, such as differential optical absorption spectroscopy (DOAS). However, most of these remote sensing techniques are inapplicable to fumarolic fields emitting volatiles depleted in SO2 to which most of the geothermal fields are classified. In this study, we developed a vertical sensor array system to quantify the emission flux of H2S from each fumarolic field by integrating the cross-sectional distributions of H2S concentrations in the volcanic plume using the vertical sensor array system. In Iwo-yama of the Kirishima volcanic complex, the cross-sectional distribution of H2S concentrations was determined using the walking traverse method by moving the vertical sensor array system in the plume perpendicular to the direction of plume transport. The emission flux of SO2 (2.2 ± 0.4 ton SO2/day) was estimated from that of H2S using the walking traverse method (2.6 ± 0.5 ton H2S/day) and the molar ratio of the plume (SO2/H2S=0.45) corresponds well with that estimated optically by JMA. We concluded that the emission flux quantified using the vertical sensor array system was reliable. In the Oyunuma pond in the Kuttara volcano, the emission flux of H2S was quantified as 2.0 ton H2S/day through the fixed point method, wherein the vertical sensor array system was fixed in one point, whereas the cross sectional distribution of H2S in the plume was estimated using the natural variation in wind direction. The topography is often irregular and wind direction is variable in most fumarolic fields; thus, in general, the fixed point method should be more suitable to determine the emission flux of H2S from fumarolic fields, wherein H2S occupies a major portion of the total sulfur emission.