11:00 〜 13:00
[SVC28-P02] Temporal variations in sulfur dioxide flux during the August 2021 eruption at Fukutoku-Okanoba volcano, Japan
キーワード：火山ガス、二酸化硫黄放出率、TROPOMI、福徳岡ノ場火山
At Fukutoku-Okanoba volcano, Japan, a phreatoplinian eruption occurred in August 2021. Based on an observation using a satellite Himawari-8, the eruption started at around 21h00, August 12 (all dates and times in UTC). From 4h20 to 10h50, August 13, an umbrella cloud was formed clearly. Then the activity shifted from this phreatoplinian phase to a Surtseyan eruption at around 0h00, August 14. During these phases, pumice rafts floated to the northwestern side, and new islets were formed. The eruption ceased at 7h00, August 15. The eruption was recorded only by satellite observations and a few aerial surveys. To constrain the eruptive sequence and the scale of the eruption, we obtained temporal variations in sulfur dioxide (SO2) flux using a satellite remote sensing technique.
We estimated SO2 flux using TROPOspheric Monitoring Instrument (TROPOMI) onboard Sentinel-5 Precursor. Following the method by Theys et al. (2019, Sci. Rep.), hourly SO2 flux was analyzed using Level-2 data (Theys et al., 2017, Atmos. Meas. Tech.) assuming constant plume height and wind speed. The plume height and wind speed were estimated based on Himawari-8 images and a trajectory model HYSPLIT (Stein et al., 2015, Bull. Am. Meteorol. Soc.). As a constant plume height, we assumed multiple values (7, 12, and 15 km above sea level) because the SO2 cloud was transported in various directions due to vertical wind shear.
The SO2 flux from the beginning of the eruption to the phreatoplinian phase was estimated more than 10 kt/day (60 kt/day in maximum). In the phreatoplinian phase, the SO2 flux was not evaluated due to a thick umbrella cloud. During the transition phase from the phreatoplinian eruption to the Surtseyan eruption, the SO2 flux showed significant variations from 1 to 40 kt/day. In the last phase of the Surtseyan eruption, the SO2 flux decreased about an order of magnitude. The total amount of emitted SO2 excluding the phreatoplinian phase was estimated to be 20 kt. Assuming the flux during the phreatoplinian phase to be 50 kt/day, the total amount could be 40 kt. These values of the SO2 flux and the total amount may have huge errors due to errors in plume height and wind speed, underestimation by the umbrella cloud, and loss of SO2 in the atmosphere. Therefore, a detailed evaluation of these values is still needed.
We estimated SO2 flux using TROPOspheric Monitoring Instrument (TROPOMI) onboard Sentinel-5 Precursor. Following the method by Theys et al. (2019, Sci. Rep.), hourly SO2 flux was analyzed using Level-2 data (Theys et al., 2017, Atmos. Meas. Tech.) assuming constant plume height and wind speed. The plume height and wind speed were estimated based on Himawari-8 images and a trajectory model HYSPLIT (Stein et al., 2015, Bull. Am. Meteorol. Soc.). As a constant plume height, we assumed multiple values (7, 12, and 15 km above sea level) because the SO2 cloud was transported in various directions due to vertical wind shear.
The SO2 flux from the beginning of the eruption to the phreatoplinian phase was estimated more than 10 kt/day (60 kt/day in maximum). In the phreatoplinian phase, the SO2 flux was not evaluated due to a thick umbrella cloud. During the transition phase from the phreatoplinian eruption to the Surtseyan eruption, the SO2 flux showed significant variations from 1 to 40 kt/day. In the last phase of the Surtseyan eruption, the SO2 flux decreased about an order of magnitude. The total amount of emitted SO2 excluding the phreatoplinian phase was estimated to be 20 kt. Assuming the flux during the phreatoplinian phase to be 50 kt/day, the total amount could be 40 kt. These values of the SO2 flux and the total amount may have huge errors due to errors in plume height and wind speed, underestimation by the umbrella cloud, and loss of SO2 in the atmosphere. Therefore, a detailed evaluation of these values is still needed.