Journal of Natural Disaster Science, Volume 25, Number 2, 2003, pp85-91

Analysis of High Volcanic Gas Concentrations
at the Foot of Miyakejima Volcano, Japan

Naoko Iino
Faculty of Engineering, Kagoshima University
Kisei Kinoshita, Megumi Koyamada and Chikara Kanagaki
Faculty of Education, Kagoshima University
Akihiko Terada
Institute of Seismology and Volcanology, Hokkaido University(Received for 14. Jul., 2003 and in revised form 23 Jan., 2004)

ABSTRACT
Continuous measurement data of sulfur dioxide (SO2) in 2001 at the foot of Miyakejima volcano is discussed inconjunction with upper wind data at Hachijyojima, NOAA/AVHRR satellite imagery, and ground observation of
volcanic plumes from Mikurajima Island. The following results were obtained: (i) Fresh winds of more than 7
m/s produced high SO2 concentrations in the area downwind of the plumes. (ii) In winter, steady north-westerly
winds carried high SO2 concentrations for many hours to the monitoring station located southeast of the volcano.
(iii) In summer, winds were relatively calm due to the dominant Pacific high-pressure system over Japan, resulting in low SO2 concentrations at ground level on the island, except when typhoons passed. (iv) Travellinghigh/low pressure systems that move eastward are characteristic of spring and autumn weather, and high SO2 concentrations produced by strong, changeable winds were recorded at various stations as low pressure systems
accompanying cold fronts passed near the island.

1. INTRODUCTION
Eruptions of Miyakejima volcano, about 160 km south ofTokyo (Fig. 1), have occurred at intervals of about 20 years; 1940,
1962, 1983, and 2000. The latest activity started on 8 July, withthe ejection of enormous amounts of sulfur dioxide (SO2) and continues as of June 2003. In 2000, the SO2 flux, monitored by an airborne correlation spectrometer (COSPEC), averaged about 42000
tons/day, but has since decreased gradually. The flux had
decreased from 40000 to 10000 tons/day, with fluctuation, in 2001
but still exceeded 5000 tons/day in June 2003 (Kazahaya et al.,2003). Large eruptions, recorded on 10, 18, and 29 August 2000,
had respective cloud heights of about 8, 14, and 8 km (Japan
Weather Association, 2000). All Miyakejima inhabitants havebeen evacuated since 1 September 2000. Since then, no notable
eruptions have been recorded, and the volcanic clouds turned fromash-rich to white and vapour-rich since mid-September.
As volcanic gas tends to move together with the volcanic
plume, as found in previous studies of the Sakurajima and Asoplumes (Kinoshita et al., 2000; Kinoshita et al., 2001), advection of
the gas could be inferred from the plume images. Horizontal dispersion is seen in satellite images, and vertical plume movement inground observations.
The Tokyo Metropolitan Government started monitoring volcanic gas concentrations at the foot of Miyakejima volcano fromthe end of 2000. Here, we focus on periods of high SO2 concentra-

tions, defined as exceeding 1 ppm/hour, observed in 2001.
Analysis of SO2 data is discussed in conjunction with the upper
wind data at Hachijyojima, NOAA/AVHRR satellite imagery, andground observations of the volcanic plumes from Mikurajima
Island. Meteorological conditions when high SO2 concentrations

KEY WORDS: sulfur dioxide, mountain lee wave, upper wind, NOAA/AVHRR

Fig. 1

Locations of Miyakejima, Mikurajima and Hachijyojima
islands, and the upper air observatories (closed circles).

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N. IINO, K. KINOSHITA, M. KOYAMADA, C. KANAGAKI AND A. TERADA

occurred were assessed. Brief reports in Japanese on this topic are
given in Iino et al., 2002 and Koyamada et al., 2003.
2. DATA AND METHODS OF ANALYSIS
2.1. Continuous measurement of sulfur dioxide
The Tokyo Metropolitan Government began continuous monitoring of the SO 2 and H2S levels at three stations, the Miyake
Branch Office, the Airport and Ako, on Miyakejima, in December
2000. Three other locales, Ainohama, Akacooco and Igaya, wereadded in September 2001. In February 2002, the number of stations was extended to 10 by adding ones distributed around thefoot of the volcano. Figure 2 shows the 6 stations in operation in
2001. The SO2 and H2S concentrations respectively were measured by the ultraviolet fluorescent and controlled potential electrolysis methods.
The volcanic gas-monitoring stations provided two kinds of
temporal data; five-minute and one-hour averaged. Figure 3 shows
the variations in SO2 concentrations observed at the Branch Office
on 21-22 August 2001. Although the sharp peaks seen in the fiveminute averaged data are smeared in the one-hour averaged data,high SO2 concentration conditions are well represented by the latter. As the upper wind data used in the comparison with the SO2

Branch Office

Igaya

N

Ainohama

AirportAko

Akacooco

5km
Fig. 2 Locations of volcanic gas-monitoring stations on Miyakejima
in 2001(closed squares) and the vent (cross).

concentration is for six-hour intervals, we consider that the temporal resolution of the one-hour averaged data is sufficient; therefore,
only the one-hour values are discussed here.
The volcanic gas at Miyakejima is the high-temperature type,in which sulfur is mainly oxidized in the reaction H2S + 2 H2O
SO2 + 3 H2. Equilibrium depends on the ambient temperature andpressure. H2S is the principal gas at low-temperatures and surfacepressures, but not at high-temperatures. Therefore, only the SO2
concentrations are discussed. The highest SO2 concentration, 15.2
ppm, was recorded at the Airport station on 20 April 2001, whereas the highest H2S concentration was 2.9 ppm at the same station
on 28 January and 26 March 2001. Environmental standards for
the SO 2 concentration set by the Japanese Government as therespective desirable levels for the one-hour and daily averages are
below 0.1 and 0.04 ppm. The American Conference ofGovernmental Industrial Hygienists has designated a 2 ppm concentration of SO2 as the exposure threshold value for labourer. In
this study, we focused on high concentration events, in which theone-hour average value was 1 ppm, in order to clarify the basic
features of the phenomena.
2.2. Upper air data
The locations of the upper air observatories around
Miyakejima are shown in Fig. 1. Upper wind data were observed 4times a day, 03, 09, 15, and 21 JST (Japanese Standard Time =UTC + 9 hours), by radiosonde measurement at each station,
except Hamamatsu. The temperature and relative humidity atthese stations were measured only at 09 and 21 JST. The
Hamamatsu wind data were available only for 09 and 21 JST.
The upper wind around the Oyama summit (814 m a.s.l.) of
Miyakejima volcano has a dominant effect on the SO2 concentration at ground level. As shown in Fig. 1, the nearest upper air
observatory is at Hachijyojima, which has a similar geographicalsituation also being an isolated island. The Hachijyojima winddata at 925 hPa, corresponding to an altitude of about 830 m a.s.l.,close to the summit height of Miyakejima, was used. To investigate atmospheric stability conditions in detail, vertical profiles of
air temperature T [K] up to 3000 m and of the potential temperature θ [K] were plotted as functions of altitude z [m] (e. g., Fig.
10);Rd

Ê p ˆ Cpq ( z) = T ( z)Á 0 ˜ ,Ë p( z) ¯
where, p(z), p0, Rd, and Cp respectively are the air pressure[hPa], standard pressure at 1000 hPa, gas constant for dry air [J / K
kg], and specific heat at constant pressure [J / K kg]. In addition,
the Froude number Fr [dimensionless] was calculated from theequation
Fr =

U .
NH

Where U is the wind velocity [m/s] around the summit, H theheight of Miyakejima volcano, 814 [m], and N the Brunt-Vaisalafrequency, [rad /s], represented as
Fig. 3 Comparison of five-minute and one-hour SO2 concentration
values.

N=g dq ,T0 dz

ANALYSIS OF HIGH VOLCANIC GAS CONCENTRATIONS AT THE FOOT OF MIYAKEJIMA VOLCANO, JAPAN

Table 1 Total observation times at each monitoring station in 2001
and the number of times that the percentage of high SO2
concentrations exceeded 1 and 2 ppm. Note that the gasmonitoring stations at Ainohama, Akacooco, and Igaya
were installed in September 2001.

Fig. 4 Monthly average SO2 concentrations in 2001. Concentrations
at the Branch Office, Airport, and Ako are given on the left
and those at Ainohama, Akacooco, and Igaya on the right.

where, g is the gravitational acceleration constant [m/s 2], T0
the lowest layer temperature [K] except for the surface, and dθ/dz
the temperature gradient [K/m] between the lowest layer and summit.
2.3. Satellite data analysis
Since August 2000, eruption clouds and ash-rich/less plumes
have been detected at Miyakejima by the Kagoshima group basedon meteorological satellite data (Iino et al., 2001; Kinoshita et al.,2003). The resulting satellite images are displayed at
http://arist.edu.kagoshima-u.ac.jp/miyake/index-e.htm.
Since mid-September 2000, volcanic plumes were detected bytaking the difference between the visible and near infrared channels in the NOAA/AVHRR images. This method enhances thewhite and vapour-rich plumes because of the size difference
between sulfuric acid aerosols and meteorological clouds.
Although some types of meteorological clouds appeared very similar to volcanic plumes, we were able to identify the plumes bymaking color images of the short-infrared channel because thatchannel is more sensitive to droplet size than the visible one.
2.4. Ground and helicopter observations
The Earthquake Research Institute (ERI) of the University ofTokyo installed a web camera system at Mikurajima, about 20 km

Miyakejima plume observations: http://www.eri.u-tokyo.ac.jp/
terada/kansi/.
*2
GSJ Open File Reports: http://www.gsj.jp/GDB/openfile/ index_j.html(in Japanese).
*1

Table 287

Direction of the crater from each station and the averagevalues and standard deviations of wind direction andvelocity for each station when high SO 2 concentration
events occurred in 2001.

SSE of Miyakejima (Fig. 1), and analyzed the motion of volcanic
clouds from August 2000 to May 2002 (Terada and Ida, 2003).
The system recorded distant and wide view images every 10 seconds, a portion of which has been shown in the website (inJapanese) *1. The latter images were analyzed to determine thebehaviour and vertical structure of the plumes from the surface tothe air.
The Japan Meteorological Agency (JMA) installed high sensitivity cameras at the Airport station in December 2000 and at the
Kotekura-pylon station, 1.5 km NE of the Ako gas-monitoring station, in August 2001. These cameras mainly take close-up viewsof the crater and are able to take photos of plume flows, even atnight. During November - December 2001, we checked plumeflows by means of video-tapes, in particular when there was a discrepancy between the upper wind data and high SO2 concentrations.
Helicopter observations of Miyakejima volcano have been
made by researchers from the ERI, the Geological Survey of Japan(GSJ) of the National Institute of Advanced Industrial Science andTechnology, and other institutions, by arrangement with the JMA.
These observations provide precious information; e.g., on change
in the crater s structure and the flow of volcanic plumes. Photo
images taken from the helicopter and comments of the observers
are given in the GSJ website (in Japanese) *2. We used these photo
images to clarify plume flows in detail because they have a muchhigher spatial resolution than do satellite images.
3. HIGH CONCENTRATION EVENTS IN 2001
3.1. Annual tendency
Table 1 shows that the SO2 concentration rate exceeding 1
ppm at the Airport station in 2001 was 12.6 %. In addition, therate exceeding 2 ppm, the exposure threshold value designated by
the American Conference of Governmental Industrial Hygienists,
was 8.9 %. The rates exceeding these levels at the other stations
were a few percent or less. The monthly average SO2 concentration value at each station (Fig. 4) indicates that high concentration
events frequently occurred in winter at the Airport station locatedsoutheast of the volcano owing to prevailing north-westerly winds.
Table 2 shows the direction of the crater from each station,and the average values and standard deviation in wind direction
and velocity for each station when high SO2 concentration eventsoccurred. According to the idiomatic expression for meteorology,
the wind direction representations of 0, 90, 180 and 270 degrees
respectively are northerly, easterly, southerly, and westerly winds.
As stated previously, we used the 925 hPa wind data observed

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every 6 hours at Hachijyojima station. To utilize hourly SO2 concentration data, we interpolated the nearest time wind data for theintermediate time SO2 data. In calculating the average wind direction at Akacooco station, 360 degrees was added to the raw data if
less than 180 degrees to eliminate the numerical jump at 0 degree.
The results given in Table 2 are also shown as sectors in Fig. 5.
The radius and angle of each sector respectively show the averagewind velocity values and the wind directions range. Fig. 6 is ascatter diagram of wind velocity and SO2 concentration for eachstation at the wind observation time for a leeward direction ofwithin 10 degrees. It shows that high concentration events wererecorded at leeward stations in fresh wind exceeding approximately 7 m/s, which value might be a criterion for predicting high concentration events.
It should be noted that high concentrations were occasionallyobserved at Igaya and Ako stations when the wind velocity was
lower than 7 m/s and that the standard wind direction deviation for
Akacooco station is notably large. This may be because of local
wind variation at Hachijyojima is not representative of
Miyakejima, as understood from the meso-scale meteorologicalpatterns. Synoptic charts for days when high SO2 concentrations
were recorded at these stations, show the local weather to bestrongly affected by typhoons, low/high pressure systems, or a
nearby cold fronts. In those situations, spatial and temporal differences in the wind field between Hachijyojima and Miyakejima
may become large. In such cases, the upper air observations at sixhour intervals at Hachijyojima are not sufficient to follow changes
in the wind field around the summit of Miyakejima.
To examine those cases in which wind direction alone could
not explain high concentrations, Iino et al. (2003) studied theevents for each station in which the concentration exceeded 0.5
ppm when the leeward direction was not within 90 degrees. Most
events could be understood by examining the weather patterns, but
a few could not be, even though the volcanic plumes were seen to
be directed to the stations in ground observation videos recorded at
Miyakejima by the JMA. These events may have been caused by
local winds and have been affected by topography. In related
research, Sasaki et al. (2002) made numerical simulations for four
typical cases, taking into account detailed topographical and meteorological conditions, in order to understand the high surface SO2
concentrations recorded at Miyakejima. They concluded thatadvection is dominant over other factors and that a slight change inwind direction causes a marked variation in the SO2 concentration.
3.2. Seasonal characteristics(1) Winter and summer
In winter, as discussed in Sec. 3.1, steady north-westerly
winds brought high SO2 concentrations lasting many hours to themonitoring station located southeast of the volcano. These persisted for 15-20 hours at the Airport station; markedly longer than at
the other stations. In summer, winds are relatively calm due to thedominant Pacific high-pressure system, and the volcanic plumes
tend to rise. The SO2 concentrations on the island therefore arelow, except when typhoons pass near the island. We found, however, that downwind of the volcano, 100-400 km away on themainland, volcanic gases affected areas due to daytime convection(Nagai et al., 2001; Furuno et al., 2002; Kinoshita et al., 2003; Iinoet al., 2004).

(2) Spring and autumn
Travelling high/low pressure systems that move eastward arecharacteristic of spring and autumn weather. High SO2 concentrations caused by strong changeable winds were recorded at various
stations as low-pressure systems that accompanied cold fronts
passed near the island.
Typical temporal variations in SO2 concentrations in autumn
for one-hour SO2 concentration values at six stations and 925 hPa
wind directions at Hachijyojima from 4 to 6 November 2001 areshown in Fig. 7. The SO2 concentration peaks observed in turn at
Akacooco, Ako, Igaya, and the Branch Office and Airport stations(clockwise) clearly correspond to changes in wind direction. The
peak occurrence is consistent with change in wind direction. The
weather patterns on 4 to 5 November were a mobile high-pressure
system that extended and followed behind a low which moved
from southwest to northeast near the southern coastline of theJapanese archipelago. After this mobile high, another low accompanied by a cold front passed over the Miyakejima volcano on 6
November. Again, peaks were observed at stations leeward of the
volcano. SO2 concentrations increased at stations downwind, anddecreased as the wind direction again changed. Throughout thisFig. 5 Graph of Table 2. The radius and angle of each sector respectively show the average wind velocity and wind direction
range values obtained from the average value +/- the standard
deviation. The center represents the crater of the volcano.
The relative location of each station from the crater is shownby a closed square.

Fig. 6 Scatter diagram of the wind velocity and SO2 concentration ateach station at wind observation time for a leeward direction
within 10 degrees.

ANALYSIS OF HIGH VOLCANIC GAS CONCENTRATIONS AT THE FOOT OF MIYAKEJIMA VOLCANO, JAPAN

period, wind velocities were about 10 m/s, except at 03 and 09 JST
on 6 November. The respective velocities at those times on thatday were 24 and 28 m/s, and high concentrations at Ainohama station were expected from the wind directions, but the concentrations there remained at 0.1 to 0.2 ppm. This may be attributed tovery strong, collimated wind that produced volcanic gas and plumeflow in a very narrow band. Under that condition, a slight devia-

Fig. 7 Temporal variations in the one-hour SO2 concentration values
at six stations (left) and 925 hPa wind directions at
Hachijyojima (right) from 4 to 6 November 2001. The firsttwo pairs of wind directions are the values for -360 degrees
subtracted from the raw data to maintain varying wind direction continuity.

Table 389tion would produce a large difference in SO 2 concentrations.
Similar events are plotted in Fig. 6. In section 4, some similar situations are discussed based on photo and satellite images.
4. RELATION BETWEEN HIGH CONCENTRATIONEVENTS AND PLUME IMAGES
The relation between visible plumes shown in ground andsatellite observations and high SO2 concentration events at the
ground surface was investigated based on the vertical structure of
the plumes seen in photo images taken by the Mikura-camera andthose from helicopters *2. The NOAA images show the advection
and dispersion of plumes over a wide area. Although cloudy
weather obstructed both types of observation, the NOAA difference images of the visible and near infrared bands discriminate theplumes containing sulfuric aerosols from the meteorologicalclouds. In the gray scale difference image, the plume is shown as abright object, as in Fig. 8, whereas meteorological clouds appear
dark. Some 176 scenes of plumes having various shapes weredetected in 2001, all of which are displayed on the website mentioned in 2.3.
The plume behaviour in these images, SO2 concentration, and925 hPa upper wind data at Hachijyojima are given in Table 3. In
the plume column, no rise means an almost horizontal plumeflow near the summit with no rising. Lee wave but miss indi-

Summary of plume behaviour, SO 2 concentration, and 925 hPa upper wind data in 2001 at
Hachijyojima for the days NOAA satellite images were available.

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N. IINO, K. KINOSHITA, M. KOYAMADA, C. KANAGAKI AND A. TERADA

cates that the photo image shows a mountain lee wave but that theplume does not cover the area around a gas-monitoring station.
High SO2 concentrations were often recorded when a mountain lee
wave formed in fresh wind exceeding 7 m/s and was blown to a
gas-monitoring station, whereas high SO2 levels were not recorded
when a plume missed a station.
Typical respective cases of fresh and light winds are shown in
Figs. 8a and b as seen on NOAA satellite images at 13:38 JST on 1
December and at 13:28 JST on 23 November. Corresponding
photo images from the Mikura-camera are shown in Figs. 9a and b.

The 925 hPa wind velocities at Hachijyojima were 16, 9, 8, and 8
m/s at 03, 09, 15, and 21 JST on 1 December. The atmosphere at
09 JST on 1 December was neutrally stratified to 1000 m and atwhich height there was a 0.6 K temperature inversion (Fig. 10a).
The Froude number between z = 196- 800 m was 1.9. A mountainlee wave was formed (Fig. 9a) and high SO2 concentrations weremaintained at the Airport station for the whole day, but concentrations were not so high at Ainohama station slightly north of theformer station (Fig. 11). This suggests that only the narrow
regions downstream of the volcano are strongly affected by volcanic gases, little being dispersed during steady, fresh winds.
In the case of the mild winds on 23 November 2001, therespective velocities were 3, 2, and 3 m/s at 03, 15, and 21 JST (09JST data was missing), the atmosphere was stable with a 0.3 K
temperature inversion around 1500 m (Fig. 10b). The Froudenumber between 192-800 m, based on 15 JST wind data, was 0.5.
Figure 9b shows plume rise with no lee wave formation. SO2 concentrations from 01 to 17 JST were less than 0.1 ppm at all the stations on the volcanic island.

Fig. 8 NOAA images. (a) 1 December 2001 at 13:38 JST. (b) 23
November 2001at 13:28 JST.

Fig. 10 Atmospheric conditions at 09 JST on 1 December and 23
November 2001.

Fig. 9Mikura-camera images. (a) 1 December 2001at 10:36 JST.
(b) 23 November 2001at 15:37 JST.

Fig. 11

Temporal variations in SO2 concentration observed at the
Airport and Ainohama stations on 1 December 2001.

ANALYSIS OF HIGH VOLCANIC GAS CONCENTRATIONS AT THE FOOT OF MIYAKEJIMA VOLCANO, JAPAN

These findings suggest that high SO2 concentrations caused by
fresh winds occur in narrow regions downwind and that slightdeviation produces a large concentration change. For instance, on
4 and 6 July (Table 3) wind velocities respectively were 13 and 17
m/s, and wind direction seems to have brought high concentrations
to the Airport station. Helicopter observations, which have muchfiner spatial resolution than NOAA images, well explain the low
SO2 concentrations at the Airport station (less than 0.1 ppm),
because the plumes flowed only in a narrow north-side area andmissed the station. In contrast, light winds allowed the plume torise without producing high concentrations at ground level. The
behaviour of volcanic gas resulting in high SO 2 concentrations
therefore is confirmed from the flow of the visible plume.
5.

CONCLUDING REMARKS

Miyakejima volcano has been ejecting a tremendous amountof volcanic gas since mid-August 2000. Continuous measurement
data on SO2 levels at the foot of the volcano in 2001 in conjunction
with upper wind data, satellite imagery of NOAA/AVHRR, andground observations of plumes from Mikurajima Island were
investigated with the following results:(i) High SO2 concentrations in the volcano were caused by
fresh upper winds around the summit height, and a narrow area
downwind of the plume was in danger of experiencing excessively
high SO2 concentrations. In particular, for many hours in winter,steady north-westerly winds brought high SO2 concentrations tomonitoring stations located southeast of the volcano.
(ii) When there was a calm wind, as in summer covered with
the Pacific high-pressure system, SO2 concentrations on the island
were low.
(iii) During changeable weather patterns caused by a mobile
high-pressure system or a low-pressure system accompanied by acold front, high SO2 concentrations were recorded at various stations downwind of the plume due to change in the wind direction.
(iv) We confirmed that the behaviour of volcanic gas emitted
from the crater at Miyakejima, which is of the high-temperature
type, can be inferred from the behaviour of the visible volcanic
plume.
ACKNOWLEDGEMENTS
We are grateful to the Tokyo Metropolitan Government forproviding the SO2 measurement data for Miyakejima and to theJapan Meteorological Agency for the video-tapes of the groundobservations at Miyakejima. We also thank the administrative
committee of the Satellite Data Station, Kagoshima University forproviding the NOAA/AVHRR data and Mr. A. Tupper, guest
researcher at Kagoshima University, from the AustralianCommonwealth Bureau of Meteorology, for his helpful discussions and careful reading of the manuscript.
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