Bull. Volcanol. Soc. Japan
Vol. 58 (2013) No. 1, pp. 183-189

ArticleSeparate Quantification of Volcanic Gas Fluxes from Showa andMinamidake Craters at Sakurajima Volcano, Japan
Ryunosuke KAZAHAYA＊,＊＊, Toshiya MORI＊ and Keigo YAMAMOTO＊＊＊(Received October 11, 2011 ; Accepted December 26, 2012)
We conducted SO2 flux measurements at Sakurajima volcano, Japan during 2007-2010 to ascertain differences inthe degassing activity at two craters of the volcano: Minamidake and Showa craters. Recent developments in SO2visualization techniques using a UV camera observation system enabled us to quantify the SO2 flux from each crater.
SO2 flux reflects conditions of the volcanic conduit. Therefore, a separate estimate of the gas fluxes from different
vents is useful to gain insight into conditions within the volcano. Sulfur dioxide flux from Showa crater ranged from a
few hundred to several thousand ton/day. In contrast, sulfur dioxide flux of 100-500 ton/day from Minamidake crater
remained at a lower level. These results suggest that degassing conditions (i.e. permeability of the conduit or theamount of degassing magma) of Minamidake crater have remained stable. In contrast, the degassing activity of Showa
crater is probably variable and sensitive to volcanic activities in the crater. The difference implies that drastic variationsof Sakurajima volcanoʼs SO2 flux data, observed using a conventional traverse method with a UV spectrometer, resulted
from the Showa craterʼs SO2 flux, not the Minamidake craterʼs SO2 flux.
Key words : volcanic gas, UV camera, SO2 flux, remote sensing

1．Introduction
Volatile components are dissolved in magma underpressure. As magma ascends to the shallow region of a
volcanic edifice, the magma begins to decompress, thereby
causing exsolution of volatiles, which provides a driving
force for explosive eruptions. Therefore, measurements of
volcanic gas emissions provide reliable information related
to the state of the magma and contribute to the monitoringof volcanic activity.
Sulfur dioxide (SO2), a major volcanic gas constituent
emitted from active volcanoes, can be detected using ultraviolet (UV) light absorption. Sulfur dioxide flux observations are used to monitor the variation of volcanic activity.
The instrument used most widely to measure SO2 flux is acorrelation spectrometer (COSPEC; Stoiber et al., 1983).
At the beginning of the 21st century, cheaper and morecompact UV spectrometer systems (e. g., mini-DOAS;
Galle et al., 2002; COMPUSS; Mori et al., 2007) weredeveloped to replace COSPEC. These advancements havefacilitated data collection and have increased the numberof observations. Although these developments have been
innovative, the production of high temporal resolution SO2flux measurements remains challenging as UV spectrometers scan volcanic plumes. Recently developed UV＊The University of Tokyo, Tokyo, Japan
Now at Institute of Seismology and Volcanology, Facultyof Sciences, Kyushu University.
＊＊
Sakurajima Volcano Research Center, Disaster Prevention Research Institute, Kyoto University, Kagoshimacamera observation systems (Bluth et al., 2007; Mori and
Burton, 2006) can provide images of the SO2 column
amount and measure SO2 flux at high frequencies (up to 2
Hz). Recent studies using this new tool have revealed
links between volcanic gas emissions and seismic signals(e.g., Kazahaya et al., 2011; Nadeau et al., 2011).
Sakurajima volcano is an active volcano in southern
Kyushu, Japan. This stratovolcano consists of two adjoining edifices: Kitadake and Minamidake. Previously,
the main active crater had been Minamidake crater, which
generated numerous eruptions since 1955. After the 1990s,
the volcanic activity subsided to moderate levels. Showa
crater, located east of Minamidake crater, awakened fromabout 60 year dormancy in June 2006. The main volcanic
activity shifted from Minamidake crater to Showa crater.
In contrast to the now active Showa crater, few explosions
have occurred at Minamidake crater since 2006 (JMA,
2007, 2008, 2009, 2010).
The two craters are the major degassing source of Sakurajima volcano since June 2006. Sulfur dioxide flux is controlled by the several degassing parameters. One candidate is permeability of a volcanic conduit (e.g. Edmonds etal., 2003). Because these two craters have different eruptive behaviors, as described above (e.g. number of erupJapan.
Corresponding author: Ryunosuke Kazahayae-mail: kazahaya@sevo.kyushu-u.ac.jp

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Ryunosuke KAZAHAYA, Toshiya MORI and Keigo YAMAMOTO

tions), their degassing activities are expected to be different. However, it is difficult to measure the volcanic gas
flux from each crater because the two craters are close toeach other and volcanic plumes from the two craters mix
near the craters. Conventional observation techniques using
a UV spectrometer can only acquire a mixture of volcanic
plumes emitted from multiple craters.
To measure the SO2 flux from each of the three craters
at Mt. Etna, Italy, Aiuppa et al. (2008) performed walking
traverse measurements around the craters. However, thistechnique is not applicable to Sakurajima volcano becauseof its frequent explosive eruptions. High spatial resolution
of SO2 imaging data provided by UV camera observation
systems enable estimation of SO2 fluxes from multiple
sources as shown at a fumarolic field of Vulcano, Italy(Tamburello et al., 2011). The evaluation of SO2 flux
from each crater is crucial to investigate the similarity anddissimilarity of an individual craterʼs degassing activity,
which is quite important information for the understandingof eruptive mechanisms. The SO2 flux of each crater
enables us to compare the degassing activity and othergeophysical data related to the eruptive activity. This
report describes the observation of SO2 fluxes from thetwo active craters at Sakurajima volcano using the UV
camera observation system, and presents discussion of thedegassing activities of Showa and Minamidake craters.
2．Observations
We conducted SO2 flux measurements using a UV
camera observation system (Bluth et al., 2007; Mori and
Burton, 2006) at Sakurajima to measure the SO2 flux
separately from each crater. The observational site was
Arimura (Fig. 1). The distance between the craters andArimura was 2.7 km. Measurements were conducted on 7
June 2007, 7 May 2008, 1 December 2009, and 14 September 2010.
The measurement on 7 June 2007 was made with one
UV camera and two UV band pass filters (Mori and
Burton, 2006), which were switched alternately by manual
operation. The SO2 images were obtained every 3 s.
After 2008, measurements were conducted using two UVcameras and a UV spectrometer (USB2000/2000+; Ocean
Optics Inc.). The UV cameras were synchronized. The
time resolution of the system was 0.5 Hz. The UV scattering between a volcanic plume and an instrument engenders underestimation of the SO2 amount (Kern et al.,2010; Mori et al., 2006). The SO2 column amount on theimages was corrected by the SO2 column amount, which
was calculated using UV spectra of the same volcanic
plume observed using the UV spectrometer. A conventional traverse SO2 flux measurement using a compact UV
spectrometer system (COMPUSS; Mori et al., 2007) was
also made on 1 December 2009 simultaneously with the
UV camera observation to compare the SO2 flux dataobtained using the UV camera and those of the traverse

Fig. 1. Map of Sakurajima volcano. The black circle andsquare respectively show the locations of Minamidake
crater and Showa crater. The observation site, Arimura,is shown as the black cross.

method.
3．Separate SO2 flux quantification of multiple craters
3-1 SO2 flux calculation
Principle techniques for SO2 flux calculation of the UV
camera observation system were described in earlier
reports (Bluth et al., 2007; Mori and Burton, 2006). EachSO2 amount of a point on an image represents the SO2column amount, which is the integrated SO2 concentration
along the CCD line of sight. The sulfur dioxide column
amount profile on a line of an SO2 image that is perpendicular to plume-flow direction corresponds to the SO2column amount profile of a cross-section of the volcanic
plume. The SO2 amount within the cross-section is calculated by integrating the SO2 column amount on the line
(hereinafter the “flux calculation line”). Sulfur dioxide
flux is derived by multiplying the integrated SO2 column
amount within the cross-section by the plume speed. The
plume speed is obtained from the plume motion. For
practicality, a time lag among integrated SO2 column
amount time series of two parallel cross-sections of theplume perpendicular to the plume-flow direction is calculated. The plume speed is ascertained by dividing thetime lag from the distance between the cross-sections.
These two cross-sections correspond to a flux calculationline and an arbitrary line parallel to the flux calculationline on the image. The time lag is examined using thecross-correlation method (e.g. Mori and Burton, 2006).
To measure the SO2 flux for each crater separately, theflux calculation line is selected for each crater (Fig. 2a).
When plumes from multiple sources flow apart without
mixing, separate measurements of SO2 flux from thedifferent sources can be determined using UV image
analysis of the flux calculation lines. Each flux calculationline is used to measure each craterʼs flux. Even though
plumes from the two craters mix at a distance, sulfur

Separate Quantification of Volcanic Gas Fluxes from Showa and Minamidake Craters at Sakurajima Volcano, Japan

185

Fig. 2. Sulfur dioxide visualization image and calculated SO2 flux time series of the two craters. (a) Sulfur dioxide
visualization image taken by the UV camera observation system on 1 December 2009. The colors present the SO2column amount as shown in the color bar set on the right. The image was taken from Arimura. Sulfur dioxide flux
calculation lines are set above the two craters to calculate the SO2 flux of each crater. The green line above Minamidake
crater is used to calculate Minamidake craterʼs SO2 flux. The black slanted line is for Showa craterʼs SO2 flux. (b)Variation of SO2 fluxes of two craters on 1 December 2009. The black and green curves respectively show the SO2
fluxes of Showa and Minamidake craters. Periods filled with yellow correspond to occurrences of eruptions.

dioxide flux of each crater can be measured before mixing
occurs. The mixed plumeʼ s flux can also be estimatedusing flux calculation lines on the image by setting the flux
calculation line to get a cross-section of the mixed plume.
When SO2 flux from one crater cannot be measured
directly because of the mixing and the other can bemeasured, the missed flux can be derived by subtracting
SO2 flux of other crater from the mixed plumeʼs flux.
Wind direction affects SO2 flux measurements. When
images are taken from a leeward location, the plumemoves upward on the images. For that reason, distinguishing fluxes from multiple craters is easy. In contrast,
when images are taken from a windward location, theplume remains above the source in the images, making it
difficult to measure SO2 flux. When some part of a plume
misses the field of view (FOV) of an instrument (e. g.,being behind a volcanic body), the SO2 flux might beunderestimated or it might not be measurable.
With a weak wind from the northwest, from Arimura,the plumes from Showa crater and Minamidake crater are
visible as rising separately. Under such conditions, theSO2 flux from each crater can be quantified separately(Figure 2a). With strong winds, the volcanic plume moves
laterally and SO2 flux estimation is impossible because
parts of the volcanic plume are outside the FOV of the
instrument.
3-2 Elimination of volcanic ash effect for SO2
amount quantification
Volcanic ash causes underestimation of the SO2 flux
(Andres and Schmid, 2001) because the ash does not let
UV light penetrate through. Even during passive degassing stages, volcanic plumes sometimes contain ash, which
affects the estimation of the SO2 flux in the column. Thiseffect lessens with distance because dispersion and precipitation of ash lowers the ash concentration. To eliminate the ash effect, we quantified the SO2 flux at several
distances from the crater by elaborating the flux calculation lines on SO2 images (Fig. 3). We set the five flux
calculation lines on the UV image (Fig. 3b) and compared
the calculated flux on each line (Fig. 3a). Figure 3a shows
the temporal fluctuation of SO2 flux of the flux calculationlines. The absolute values of the timelines differ becauseof the ash effect. The lower lines show lower SO2 fluxes.
Each timeline has a time lag because of the positions of thelines. The upper two lines show similar SO2 fluxes to thetime averaged values to within a 5 % difference (Fig. 3a).
Therefore, we concluded that the ash effect is cancelled forthe two lines. To compare the SO2 flux timeline witheruptive activity and other geophysical data, the calculation line should be set up as close to the crater as possible.
However, if the flux calculation line is set up close to thecrater, then the amount of ash is higher in the volcanic
plume near the crater, which means that the calculation of
SO2 column amounts will be an underestimate. We
examined the effect as described above and selected theaqua flux calculation line, which is unaffected by the ash,
but which is the nearest to the crater (Fig. 3).
Accuracy of the SO2 flux estimated using this procedure
was examined by comparison with the SO2 flux obtainedusing the conventional traverse method. Figure 4 presents
a comparison of the traverse method with the UV camera
observation. Results show that the SO2 fluxes obtainedusing the two measurement techniques agree well except
for data obtained during eruptions (Fig. 4). Underestimation of SO2 flux caused by the volcanic ash is cancelled out
by elaboration of the flux calculation lines.

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Ryunosuke KAZAHAYA, Toshiya MORI and Keigo YAMAMOTO

Fig. 3. Difference of SO2 fluxes obtained with different five SO2 flux calculation lines on the UV images. (a) Variation of
SO2 fluxes of multiple SO2 flux calculation lines above Showa crater. (b) A SO2 image with five SO2 flux calculationlines above Showa crater. The colors of the lines on the SO2 image correspond to those of the lines on the left graph. In
this case, we calculated the SO2 flux using the aqua line, which is the second highest one.

Table 1.

Fig. 4. Sulfur dioxide fluxes obtained using the UV camera
observation system and the traverse method. Variationof SO2 flux time series obtained using the UV camera
observation (solid line) and the traverse measurement
using the COMPUSS (squares and circles). Filled regions
correspond to the period of an eruption during which wecannot measure the SO2 flux because of the greatamount of ash with the UV camera observation. The
circles show data points including uncertainty becauseof volcanic ash emitted by explosive eruptions and direction change of eruptive plume. Horizontal error bars of
the traverse data points show the observational period ofeach traverse measurement. The UV camera observation
system was used to observe the plumes just above thetwo craters. The traverse measurement observed theplume 2.4-5 km away from the craters. A time difference prevailed between SO2 timeline of the traverse
method and the UV camera observation system. We corrected the time lag of the traverse measurement using asimple calculation with the wind speed and the average
distance between the crater and the route of traversal.

Observation conditions for each day.

3-3 Observational conditions of each day
The observational conditions of each day are presented
in Table 1. On 7 June 2007 and on 7 May 2008, the plumeemitted from Showa crater flowed westward and mixed
with the plume from Minamidake crater. In this case, wecannot calculate the SO2 flux from Minamidake crater
separately because of the mixing. Two lines were set on
the images to calculate the flux from Showa crater and thetotal flux. We calculated Minamidake craterʼ s flux by
subtracting Showa craterʼs flux from the total flux. The
volcanic plume did not contain a substantial amount of ash.
It was not necessary to have an arrangement of flux
calculation lines.
On 1 December 2009, volcanic plumes from Minami-Separate Quantification of Volcanic Gas Fluxes from Showa and Minamidake Craters at Sakurajima Volcano, Japan
Table 2.

187

Sulfur dioxide flux of each crater at Sakurajima volcano, Japan, 2007-2010.

dake and Showa craters rose vertically. We were able tomeasure the fluxes separately. In this case, we set two flux
calculation lines above the respective craters on the UV
image (Fig. 2). Volcanic ash in the plume was abundanton this day. Therefore, the flux calculation lines werearranged to reduce the ash effect.
On 14 September 2010, the ash-free volcanic plume rosevertically. Because of slow movement of the plume andthe very low SO2 column amount, it was difficult toestimate the speed of the plume from Minamidake crater
using either cross-correlation analysis (Mori and Burton,
2006) or analysis of the plume movement viewing video
image. We assumed that Minamidake craterʼ s plume
speed was equal to that of Showa craterʼs plume. The SO2flux of each crater was quantified separately.
4．Results and discussion
The SO2 flux was measured from each crater on four
separate days (Table 2). The variation of Showa craterʼs
SO2 flux was considerable (100-2000 ton/day on average).
In contrast, the SO2 flux of Minamidake crater was constant at around 300 ton/day. Large SO2 flux was observed
when the eruption frequency was high on 1 December
2009; the number of eruptions at Showa crater might berelated to the SO2 flux measured at Showa crater (Table 2).
Minamidake craterʼs SO2 flux data were stable. Therefore,
the total flux from the two craters is expected to correlatewith the number of eruptions at Showa crater. In thisregard, Mori et al. (2008) conducted traverse measurements intensively after 2003 and reported that the total
SO2 flux increased with the Showa crater activity. This
study further supports that inference. Mori et al. (2008)
were unable to distinguish the SO2 fluxes from the two
craters, but measured their sum.
The large variation of SO2 flux of Showa crater (1002000 ton/day average) implies a considerable change in theamount of degassing magma or variation of degassing
mechanism under Showa crater. In contrast, the average
SO2 flux of Minamidake crater was constant at 100-500
ton/day. Those values are similar to the flux values
reported for Minamidake crater during 2003-2006, before
reactivation of Showa crater (Mori et al., 2008). This factsuggests that the degassing conditions of the conduit (e.g.
changes of conduit radius or permeability) under Minamidake crater did not change even after the extensive eruptions at Showa crater.
5．Concluding remarks
Sulfur dioxide flux measurements were conducted at
Sakurajima Volcano during 2007-2010 using the UV camera
observation system to quantify SO2 fluxes separately fromthe two craters: Showa crater and Minamidake crater.
Sulfur dioxide flux of Showa crater fluctuated with theeruptive activity of 100-2000 ton/day. In contrast, theSO2 flux of Minamidake crater was 100-500 ton/day.
Data were similar during 2007-2010. These values also
resembled the data of 2003-2006, when Sakurajima volcano was in the more quiescent period before the activityof the Showa crater. This fact implies that the long-term
SO2 flux change after 2006 at this volcano results fromShowa craterʼs activity. Correlation was found between
the SO2 flux and the number of eruptions at Showa crater,suggesting that the eruptions changed the amount of degassing magma and/or degassing conditions under Showa
crater as described later. In contrast, the constant SO2 fluxof Minamidake crater implies that the conditions of
Minamidake crater are stable, with few explosions at thiscrater.
When assuming degassing of a convecting magma column(Shinohara, 2008), the gas flux change is explainable bychanges of the conduit radius, convection speed, change ofmagma column height, and the amount of volatiles in the

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Ryunosuke KAZAHAYA, Toshiya MORI and Keigo YAMAMOTO

magma. However, the change of the amount of volatiles
in the magma is unlikely because a change of that parameter might engender variation of Minamidake craterʼ s
activity as well as the Showa craterʼs activity if assumingthat magma convections within the two conduits are driven
by the same magma. It is possible that the variation of
SO2 flux of Showa crater shows changes of the conduit
radius and/or magma column height of that crater.
The separate quantification techniques used to ascertain
SO2 flux from multiple craters enables the evaluation ofdifferent degassing activities at multiple craters at one
volcano. In the case of Sakurajima volcano, because
Showa crater has undergone many eruptions, this separate
quantification technique will be useful to examine the SO2flux fluctuation occurring before and after eruptions at thiscrater using high temporal SO2 flux measurements.
Comparison of the SO2 flux of Showa crater with othergeophysical data is another challenge that lies ahead.
Acknowledgments
The authors appreciate the efforts of many SVO members for giving us the opportunity to conduct measurements at Sakurajima volcano. Many thanks are extended
also to Hiroshi Shinohara and an anonymous reviewer for
valuable comments. The authors acknowledge Dan Uehira
for invaluable comments. We gratefully acknowledge a
Grant-in-Aid for JSPS Fellows from the Japan Society for
the Promotion of Science (JSPS).
References
Aiuppa, A., Giudice, G., Gurrieri, S., Liuzzo, M., Burton, M.,
Caltabiano, T., McGonigle, A.J.S., Salerno, G., Shinohara,
H. and Valenza, M. (2008) Total volatile flux from Mount
Etna. Geophys. Res. Lett., 35, L24302.
Andres, R.J. and Schmid, J.W. (2001) The effects of volcanic
ash on COSPEC measurements. J. Volcanol. Geotherm.
Res.,108, 237-244.
Bluth, G.J.S., Shannon, J.M., Watson, I.M., Prata, A.J. andRealmuto, V.J. (2007) Development of an ultraviolet digital
camera for volcanic SO2 imaging. J. Volcanol. Geotherm.
Res., 161, 47-56.
Edmonds, M., Oppenheimer, C., Pyle, D.M., Herd, R.A. andThompson, G. (2003) SO2 emissions from Soufrieʼre Hills
Volcano and their relationship to conduit permeability,hydrothermal interaction and degassing regime. J.
Volcanol. Geotherm. Res., 124, 23-43.
Galle, B., Oppenheimer, C., Geyer, A., McGonigle, A.J.S.,
Edmonds, M. and Horrocks, L. (2002) A miniaturised
ultraviolet spectrometer for remote sensing of SO2 fluxes: a
new tool for volcano surveillance. J. Volcanol. Geotherm.

Res., 119, 241-254.
Japan Meteorological Agency (2007) Bulletins on volcanic
activity for Sakurajima volcano. Nov., 2007.
Japan Meteorological Agency (2008) Bulletins on volcanic
activity for Sakurajima volcano. Nov., 2008.
Japan Meteorological Agency (2009) Bulletins on volcanic
activity for Sakurajima volcano. Nov., 2009.
Japan Meteorological Agency (2010) Bulletins on Volcanic
Activity for Sakurajima volcano. Nov., 2010.
Kazahaya, R., Mori, T., Takeo, M., Ohminato, T., Urabe, T.
and Maeda, Y. (2011) Relation between single very-longperiod pulses and volcanic gas emissions at Mt. Asama,
Japan. Geophys. Res. Lett., 38, L11307.
Kern, C., Deutschmann, T., Vogel, L., Wohrbach, M.,
Wagner, T. and Platt, U. (2010) Radiative transfer corrections for accurate spectroscopic measurements of volcanic gas emissions. Bull. Volcanol., 72, 233-247, doi: 10.
1007/s00445-009-0313-7.
Nadeau, P.A., Palma, J.L. and Waite, G.P. (2011) Linking
volcanic tremor, degassing, and eruption dynamics via SO2
imaging. Geophys. Res. Lett., 38, L01304.
Mori, T. and Burton, M. (2006) The SO2 camera: a simple,
fast and cheap method for ground-based imaging of SO2 involcanic plumes. Geophys. Res. Lett., 33, L24804.
Mori, T., Hirabayashi, J., Kazahaya, K., Mori, T., Ohwada,
M., Miyashita, M., Iino, H. and Nakahori, Y. (2007) A
Compact Ultraviolet Spectrometer System (COMPUSS) for
monitoring volcanic SO2emission: validation and preliminary
observation. Bull. Volcanol. Soc. Japan, 52, 2, 105-112.
Mori, T., Kagesawa, H., Hirabayashi, J., Yamamoto, K., Miki,
D., Yokoo, A., Kazahaya, K., Mori, T., Ohwada, M.,
Shinohara, H., Saito, G. and Yasuhara, M. (2008) Sulfur
dioxide flux from Sakurajima volcano for last ten years. In
The Tenth Joint Observations of Sakurajima Volcano, Kyoto
University, 137-142 (in Japanese).
Mori, T., Mori, T., Kazahaya, K., Ohwada, M., Hirabayashi, J.
and Yoshikawa, S. (2006) Effect of UV scattering on SO2emission rate measurements. Geophys. Res. Lett., 33,
L17315.
Shinohara, H. (2008) Excess degassing from volcanoes and its
role on eruptive and intrusive activity. Rev. Geophys., 46,
RG4005.
Stoiber, R.E., Malinconico, L.L. and Williams, S.N. (1983)
Use of the correlation spectrometer at volcanoes, In Forecasting Volcanic Events (Tazieff, H. and Sabroux, J.C. eds),
425-444.
Tamburello, G., Kantzas, E.P., McGonigle, A.J.S., Aiuppa, A.
and Guidice, G. (2011) UV camera measurements offumarole field degassing (La Fossa crater, Vulcano Island).
J. Volcanol. Geotherm. Res., 199, 47-52.

(Editorial handling Takeshi Nishimura)Separate Quantification of Volcanic Gas Fluxes from Showa and Minamidake Craters at Sakurajima Volcano, Japan

桜島における南岳・昭和火口からの二酸化硫黄放出率の分離
風早竜之介・森俊哉・山本圭吾

桜島火山にて紫外線カメラ観測装置を用いてガス観測を行い，南岳火口及び昭和火口から放出される二酸
化硫黄放出率の分離を行った．二酸化硫黄放出率は各火口の状態を反映していると考えられるため，複数火口が存在する火山でそれらの放出率を個別に測定する事は噴火活動を理解する上で非常に有用である．観測は 2007 年から 2010 年に行われた．その結果，昭和火口の放出率は数百トンから数千トンレベルで大きく変動したが，南岳火口の放出率は 100-500 トン程度でほぼ一定だった．これより，観測期間中，南岳の火道の状態は安定したままほとんど変化していないと考えられる．また，昭和火口の火道状態は昭和火口の噴火
活動・回数に応じて大きく変動している事が示唆される．桜島火山において長期的に行われている
COMPUSS を用いた二酸化硫黄放出率観測データは二つの火口の放出量の合計である．南岳のガス放出率がほとんど変わっていないことから，COMPUSS を用いたガス放出率観測で見出されている放出率変動は主に昭和火口による寄与が大きく，南岳の放出率変化はほとんど寄与していないと考えられる．

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