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[O11-P16] Observation of volcanic gases at Sakurajima with a simple alkaline filter paper method and proposal of gases emission model
Keywords: a simple alkaline filter paper method, volcanic gases
1. Introduction
Sakurajima constantly erupts many volcanic gases such as H2O, SO2, HCl, and HF. We measured the composition ratio of acid volcanic gases emitted from Sakurajima in order to understand the volcanic activity and to predict future eruptions. We compared the observed volcanic gases with the number of eruptions and amount of ash fall, and clarified the behavior of each volcanic gas. Based on these results and the eruption pattern of Sakurajima, we proposed volcanic gases emission model before and after the eruption.
2. methods
2-1Simple alkaline filter paper method
In the alkaline filter method, acidic volcanic gases are adsorbed on a base-impregnated filter paper by a neutralization reaction. The alkaline filter paper was prepared by washing the filter paper in a Na2CO3 solution by boiling it in distilled water, then drying it overnight. To protect the filter paper from ash fall and rainfall, a simple exposure stand (Fig.1) was made using plastic bottles and cups, and installed at 11 locations in the prefecture every month.
2-2 Self-made absorption spectrophotometer
The collected alkaline filter paper was boiled in distilled water at 70C for 30 minutes, and the filtrate was analyzed using a self-made absorbance spectrophotometer (Fig.2). The concentration of HCl, SO2, and HF was determined by the silver chloride turbidimetric method, the barium sulfate turbidimetric method, and the SPANDS method.( Fig. 4.
3. Results and discussion
3-1Cl-/SO2, F-/SO2, and F-/Cl- as indicators of volcanic activity and average daily eruption frequency
Hirabayashi reported in the 1980s that volcanic activity increases when the HCl/SO2 ratio increases, and that the peak in 1979, a month without an eruption, was a precursor to an eruption. In the 1980s, Hirabayashi reported that volcanic activity increases when the HCl/SO2 ratio increases, and that the peak of a non-eruptive month in 1979 was a precursor to an eruption. We also compared the number of eruptions during the period with the Cl-/SO2 ratio calculated from the volcanic gas collection at Akamizu, Arimura, and Yunohira, which are located within 3 km from the crater.
(Fig. 6) The correlation coefficient of F-/SO2 was the highest, indicating that F-/SO2 is more useful as an indicator than Cl-/SO2, which Hirabayashi used as an indicator of precursor phenomena.
3-2 Comparison of the ratio of each volcanic gas deposition to the number of explosions as an indicator of volcanic activity
Fig. 7 compares the amount of each volcanic gas deposition and the ratio of the number of explosions at Arimura, and shows that the correlation of F- is higher than those of other volcanic gas components.
3-3 Comparison of volcanic gas deposition and volcanic ash content as indicators of chemical eruption prediction and volcanic activity
In order to clarify the behavior of volcanic ash and volcanic gases during eruptions, we compared volcanic gases and the average daily ash fall per day.Fig.5 shows that the period when the amount of SO2 is large coincides with the period when the amount of volcanic ash is large, suggesting that SO2 and volcanic ash behave in almost the same manner.
3-4 Volcanic gas emission model of Sakurajima before and after the eruption
We developed a model of volcanic gas emission before and after the eruption. (Fig.8) Based on the observation results, the amount of HF that degas occurs at 800C is large, suggesting that the magma temperature is high and a large amount of HF is released at the present time when the activity is inactive and a cap is not formed. On the other hand, when the volcano is active or before an explosive eruption, a cap is formed on the crater, and H2O, HF, HCl, SO2, and other gases that have foamed from the magma accumulate in the crater, but some of the volcanic gases are released. During an eruption, the accumulated gases are released, entraining gases in the atmosphere and forming a volcanic plume. Since the maximum temperature at the time of explosion is reported to be 540C, it is thought that S that degasifies at 400C or higher sometimes degas and SO2 is generated.
4. Future plan
We want more observation sites closer to the crater and conducting continuous measurements using sensors.
And we would like to advance the prediction of eruptions by chemical indicators.
References
1) Hirabayashi,Annual Report of Disaster Prevention Research Institute, Kyoto University,24,B1,1981,11-20.
2) Yamada et al,Analytical Chemistry 61(2012)No.4,391-326
3) Mitsunaga Harumi et al,Analytical Chemistry 67(2018)12,743-747
4) Kagoshima District Meteorological Observatory, Sakurajima Eruption Observation Table
Sakurajima constantly erupts many volcanic gases such as H2O, SO2, HCl, and HF. We measured the composition ratio of acid volcanic gases emitted from Sakurajima in order to understand the volcanic activity and to predict future eruptions. We compared the observed volcanic gases with the number of eruptions and amount of ash fall, and clarified the behavior of each volcanic gas. Based on these results and the eruption pattern of Sakurajima, we proposed volcanic gases emission model before and after the eruption.
2. methods
2-1Simple alkaline filter paper method
In the alkaline filter method, acidic volcanic gases are adsorbed on a base-impregnated filter paper by a neutralization reaction. The alkaline filter paper was prepared by washing the filter paper in a Na2CO3 solution by boiling it in distilled water, then drying it overnight. To protect the filter paper from ash fall and rainfall, a simple exposure stand (Fig.1) was made using plastic bottles and cups, and installed at 11 locations in the prefecture every month.
2-2 Self-made absorption spectrophotometer
The collected alkaline filter paper was boiled in distilled water at 70C for 30 minutes, and the filtrate was analyzed using a self-made absorbance spectrophotometer (Fig.2). The concentration of HCl, SO2, and HF was determined by the silver chloride turbidimetric method, the barium sulfate turbidimetric method, and the SPANDS method.( Fig. 4.
3. Results and discussion
3-1Cl-/SO2, F-/SO2, and F-/Cl- as indicators of volcanic activity and average daily eruption frequency
Hirabayashi reported in the 1980s that volcanic activity increases when the HCl/SO2 ratio increases, and that the peak in 1979, a month without an eruption, was a precursor to an eruption. In the 1980s, Hirabayashi reported that volcanic activity increases when the HCl/SO2 ratio increases, and that the peak of a non-eruptive month in 1979 was a precursor to an eruption. We also compared the number of eruptions during the period with the Cl-/SO2 ratio calculated from the volcanic gas collection at Akamizu, Arimura, and Yunohira, which are located within 3 km from the crater.
(Fig. 6) The correlation coefficient of F-/SO2 was the highest, indicating that F-/SO2 is more useful as an indicator than Cl-/SO2, which Hirabayashi used as an indicator of precursor phenomena.
3-2 Comparison of the ratio of each volcanic gas deposition to the number of explosions as an indicator of volcanic activity
Fig. 7 compares the amount of each volcanic gas deposition and the ratio of the number of explosions at Arimura, and shows that the correlation of F- is higher than those of other volcanic gas components.
3-3 Comparison of volcanic gas deposition and volcanic ash content as indicators of chemical eruption prediction and volcanic activity
In order to clarify the behavior of volcanic ash and volcanic gases during eruptions, we compared volcanic gases and the average daily ash fall per day.Fig.5 shows that the period when the amount of SO2 is large coincides with the period when the amount of volcanic ash is large, suggesting that SO2 and volcanic ash behave in almost the same manner.
3-4 Volcanic gas emission model of Sakurajima before and after the eruption
We developed a model of volcanic gas emission before and after the eruption. (Fig.8) Based on the observation results, the amount of HF that degas occurs at 800C is large, suggesting that the magma temperature is high and a large amount of HF is released at the present time when the activity is inactive and a cap is not formed. On the other hand, when the volcano is active or before an explosive eruption, a cap is formed on the crater, and H2O, HF, HCl, SO2, and other gases that have foamed from the magma accumulate in the crater, but some of the volcanic gases are released. During an eruption, the accumulated gases are released, entraining gases in the atmosphere and forming a volcanic plume. Since the maximum temperature at the time of explosion is reported to be 540C, it is thought that S that degasifies at 400C or higher sometimes degas and SO2 is generated.
4. Future plan
We want more observation sites closer to the crater and conducting continuous measurements using sensors.
And we would like to advance the prediction of eruptions by chemical indicators.
References
1) Hirabayashi,Annual Report of Disaster Prevention Research Institute, Kyoto University,24,B1,1981,11-20.
2) Yamada et al,Analytical Chemistry 61(2012)No.4,391-326
3) Mitsunaga Harumi et al,Analytical Chemistry 67(2018)12,743-747
4) Kagoshima District Meteorological Observatory, Sakurajima Eruption Observation Table