Bull. Volcanol. Soc. Japan
Vol. /, (,**1) No. ,, pp. +*/ῌ++,LetterA Compact Ultraviolet Spectrometer System (COMPUSS) for Monitoring
Volcanic SO, Emission: Validation and Preliminary Observation.
Toshiya MORIῌ, Jun-ichi HIRABAYASHIῌῌ, Kohei KAZAHAYAῌῌῌ, Takehiko MORIῌῌῌ,
Michiko OHWADAῌῌῌ, Makoto MIYASHITAῌῌῌῌ, Hideki IINOῌῌῌῌῌ
and Yasuhiro NAKAHORIῌῌῌῌῌῌ
(Received, July +2, ,**0 ; Accepted, February 0, ,**1)
A compact and handy system for measuring SO, ﬂuxes from volcanoes is newly built using a miniature UV
spectrometer. This COMPact Ultraviolet Spectrometer System (Hereafter referred as “COMPUSS”) is muchsmaller and lighter than a correlation spectrometer (COSPEC), which has been used for SO, ﬂux measurementsfrom volcanoes over -* years. Validation of the COMPUSS was carried out comparing COMPUSS and COSPEC
instruments at Sakurajima and Miyakejima volcanoes by car and heliborne traverse method, respectively. Our
results show that agreement of the SO, ﬂux between the COSPEC and the COMPUSS data is vouched for traverse
measurements. The COMPUSS has much advantage compared to the COSPEC for volcanic SO, ﬂux measurements, because of its portability and possibility for further improvement of the system.
Key words : SO, ﬂux, DOAS, UV spectrometer and COSPEC+. Introduction
For the last -* years, correlation spectrometer (COSPEC) has been used as the only method for groundbased remote sensing to measure sulfur dioxide emission
rate from volcanoes and had a prominent contribution
on the progress in recent volcanology. The COSPEC
was ﬁrst designed for monitoring industrial emission ofSO, and NO, in +30*’s (Ho# and Millan, +32+) andlater used for monitoring volcanic SO, emissions by
various researchers (e.g., Mo#at and Millan,+31+ ; Stoiber
and Jepsen, +31-). Integrated COSPEC measurements
showed that correspondence of SO, ﬂux with magma
discharge rates at Unzen volcano, Japan (Hirabayashi etal., +33/) or with seismicity and ground deformation at
Soufriere Hills volcano, Montserrat (Watson et al., ,***).
In the beginning of this century, a remarkably small
miniature ultraviolet spectrometer was introduced andused successfully to volcanic SO, ﬂux measurements(Galle et al., ,**,). This compact and low-cost UVῌ Laboratory for Earthquake Chemistry, Graduate School

spectrometer system o#ered a chance to carry out various kinds of new monitoring methods. Owing to its
compactness and portability, the instrument enabled us
to make walking traverse measurements (McGonigle
et al., ,**, ; Oppenheimer et al., ,**.). At Soufriere
Hills volcano, continuous (day-time) measurements ofSO, ﬂux is carried out using two scanning systems withthe miniature UV spectrometer (Edmonds et al., ,**-).
Several groups developed new methods for measuring
the plume speed, which has been always a fundamental
problem for ﬂux estimation, by the use of the miniature
UV spectrometer (McGonigle et al., ,**/ ; William-Joneset al., ,**0). Under current situation, each researcher
builds their own systems using the spectrometer (Galleet al., ,**, ; Horton et al., ,**0 ; McGonigle et al., ,**,).
In ,**,, we started to develop a Japanese version of theminiature UV spectrometer system. The COMPUSS
and its operating software are specially designed focusing on easy handling for measurements and on availabil-

Japan.
of Science, University of Tokyo, Bunkyo-ku, Tokyo, ῌῌῌῌῌ Sapporo District Meteorological Observatory, Japan++-ῌ**--, Japan.
Meteorological Agency, Sapporo, Hokkaido, *0*ῌ***,,
ῌῌ Volcanic Fluid Research Center, Tokyo Institute ofJapan.
Technology, Meguro-ku, Tokyo, +/,ῌ2/++ , Japan. ῌῌῌῌῌῌ Chubu Aviation Weather Service Center, Japan Meteoroῌῌῌ Geological Survey of Japan, National Institute of Adlogical Agency, Tokoname, Aichi, .13ῌ*22+, Japan.
vanced Industrial Science and Technology, Tsukuba,
Ibaraki, -*/ῌ2/01, Japan.
Corresponding author : Toshiya Moriῌῌῌῌ Seismological and Volcanological Department, Japane-mail : mori@eqchem.s.u-tokyo.ac.jp
Meteorological Agency, Chiyoda-ku, Tokyo, +**ῌ2+,,,

106

Toshiya MORI, Jun-ichi HIRABAYASHI, Kohei KAZAHAYA, Takehiko MORI,
Michiko OHWADA, Makoto MIYASHITA, Hideki IINO, Yasuhiro NAKAHORI

Fig. ,. A photo of the COMPUSS. (+) UV spectrometer (USB,***). (,) Telescope. (-) Opticalﬁber ; the other end should be connected to the
UV spectrometer. (.) Notebook computer. (/)
USB cable. (0) GPS receiver. (1) GPS antenna.
(2) Calibration Cell. Two calibration cells areinserted into the slots of the telescope. A
ballpoint pen is placed below the spectrometerfor size comparison.

Fig. +. (a) System conﬁguration of the COMPUSSfor traverse measurements. The system consistsof mainly four parts : a UV telescope, a UV
spectrometer (USB,***, Ocean Optics Inc.), a
Note PC and a GPS unit. (b) The rotating
mirror unit attached to the front part of thetelescope in the scanning measurements. The
stepper motor is connected to the controller,
which is controlled by a notebook PC.

ity for simple in-ﬁeld calibration of the system. We
believe that the COMPUSS and similar systems will
replace the COSPEC for volcanic SO, ﬂux measurements in the near future. The aim of this paper is tointroduce the instrumentation and the spectral analyzing method of the COMPUSS and to show the results ofcomparative measurements with the COSPEC.

,. Instrumentation
The compact UV spectrometer system (COMPUSS)
which we developed for SO, column amount measurements is based on USB,*** ultraviolet spectrometer(Ocean Optics Inc.) as described by McGonigle et al.
(,**,) and Galle et al. (,**,). The COMPUSS for
traverse measurements is basically composed of fourparts : the USB,*** spectrometer, a UV telescope, acomputer and a GPS unit (Fig. +a). The spectrometerfeatures a /* mm entrance slit and the wavelength range
is adjusted to ,/*ῌ.** nm, which is focused to measure
the SO, absorption at -*/ῌ-,* nm. The resulting
wavelength resolution is about *.0 nm for the whole
wavelength range. The lightweight (about *., kg) andcompact size (23ῌ0.ῌ-. mm) of the spectrometer enables the total system to be lighter and more compact
compared to the COSPEC system. The spectrometercan be powered by USB bus power from a notebook
computer and consumes ./* mW. The spectroscopic
performance of USB,*** spectrometers are described indetail in Galle et al. (,**,).
The system employs a telescope (0*ῌ0*ῌ+0/ mm)built with three quartz lenses (Figs. +a and ,). The
scattered UV light from the sky is collected by the ﬁrst
lens (/ cm diameter, focal length : 0* mm), and is thenA Compact Ultraviolet Spectrometer System (COMPUSS) for Monitoring Volcanic SO, Emission :Validation and Preliminary Observation.

collimated by the second lens (, cm diameter, focal
length : +/ mm). The third lens (+ cm diameter, focal
length : -* mm) focuses the light into an optical ﬁber
connected to the telescope. A UV ﬁlter designed topartially relieve the rapid increase in skylight intensityfrom -+* nm to --* nm is inserted between the second
and the third lenses (Figs. +a and -a). The ﬁeld-ofview (FOV) of the telescope is 0.1 mrad using the 2**-mm-diameter optical ﬁber (῎+./ m length), which connects the spectrometer and the telescope. Two slots
between the second and third lenses allow insertion of
one to two SO, calibration cells. This enables in-ﬁeld
calibration for SO, retrievals and conﬁrmation of thesystem operation. Existence of the two slots for thecalibration cells is one of the advantages of the
COMPUSS compared to other miniature UV spectrometer systems. By having n SO, calibration cells with
di#erent SO, column amounts, it is basically possible tohave n! di#erent SO, column amounts for the calibration. This not only ensures reliability of the calibration
but also enables calibration for wider column amountrange.
A notebook PC is used for spectral acquisition, realtime in-situ analyses and display of SO, column amount.
Software used for controlling the spectrometer andanalyzing the observed spectra is designed by our groupconsidering simple operation for measurements andcalibration in the ﬁeld. The position of the COMPUSS
can be logged and displayed simultaneously using GPS
and a freeware mapping program, Kashmir-D (http ://www.kashmir-d.com/index-e.html). The GPS’s coordinate data are used for the SO, ﬂux evaluations.
The total weight of the traverse system is less than + kg
without the weight of the PC. The COMPUSS canusually be operated for +./ῌ. hours with the notebook
computer’s battery.
Several additional equipments are needed for the scanning measurements : a rotating mirror unit, a stepper
motor controller (size : ,**ῒ,/*ῒ+** mm, weight :1** g) and an extra +,V battery. The rotating mirror
unit can be easily attached in front of the COMPUSS’s
telescope (Fig. +b). A mirror with ./῔ angle to thelight axis of the telescope is connected to the stepper
motor. This conﬁguration is similar to the scanning
conﬁguration reported by Edmonds et al. (,**-). The
rotating mirror unit enables the scanning measurementsof the plume by rotating the mirror with controlled
speed. The PC and the controller box are connected
via USB cable in order to link the rotation and thespectra acquisitions. The scanning system works morethan 0 hours using .Ah +,V battery (about , kg), thus,
the total weight of the system without the laptop computer adds up to ῎/ kg.

107-. Spectrum acquisitions and evaluations
In order to obtain su$cient spectral intensity we needto select proper integration time of the spectra, or an
exposure time for a CCD detector of the USB,***.
Since the SO, peaks focusing on are in -*/ῌ-,* nm
range, integration time should be selected to maintainsu$cient spectral intensity (at least several hundred
counts intensity) at around -*/ nm without signal saturation at -,* nm. In normal conditions, +** ms (midday
in summer)῍+/** ms (late afternoon in winter) areused as the integration time depending on the weather,time and seasonal conditions. In the ﬁeld measurements, preferable time interval for obtaining SO,
column amounts is usually + to . seconds depending on
the required spatial resolution for the measurements.
Several spectra are averaged to reduce the noise of theobserved spectra before spectral analyses. The SO,
column amount in the plume can be obtained fromabsorbance of the plume, which can be expresses as
Aplume῏lῐΐῑlog῏῏Pplume῏lῐῑPdark῏lῐῐῌ῏Preference῏lῐῑPdark῏lῐῐῐῌ(+)

where Aplume(l) is the absorbance of the plume at wavelength l. Pplume, Preference and Pdark are spectral intensity
for the plume, reference and dark spectra, respectively.
The dark spectrum is background electrical current of
the CCD inside the spectrometer and the reference
spectrum is the spectrum obtained for no plume sky.
The dark spectrum is obtained by covering the telescope. The reference spectrum (Fig. -b) is obtainedby aiming the no plume sky with the telescope. Sinceboth the plume and reference spectra depend on the sky
light and the atmospheric attenuation, absorbance ofthe plume calculated above provides attenuation (absorption and scattering) by the plume. SO, column
amount can be calculated by means of di#erential optical absorption spectroscopy (DOAS) technique, which
is widely used in atmospheric UV spectroscopy (Platt,+33. ; Platt and Perner, +32-). The advantage of the
DOAS technique is that interference of scattering canbe reduced in the spectral analyses. McGonigle et al.
(,**,) and Galle et al. (,**,) calculated SO, column
amount from the spectra by ﬁtting a library di#erential
absorption spectrum of SO, to the observed di#erential
absorption spectrum, using least squares procedure. In
contrast, peak and trough height of ﬁve SO, peaks
between -*/ and -+2 nm (between the arrows in Fig. c) are used in our evaluation method for COMPUSS,
for simple and in-situ real-time calculation of the SO,
column amount. The peak heights for respective peaks
are calculated using the vertical scale di#erence between
peak top and a line segment bonding the troughs on theboth side of the peak. This approach of using peakheights for SO, evaluation is similar with FLYSPEC’s

108

Toshiya MORI, Jun-ichi HIRABAYASHI, Kohei KAZAHAYA, Takehiko MORI,
Michiko OHWADA, Makoto MIYASHITA, Hideki IINO, Yasuhiro NAKAHORI

evaluation method (Horton et al., ,**0). In COMPUSS’s
case, peak heights of / peaks are independently calculated
and then converted to SO, column amounts by applying
calibration parameters. The representative SO, column
amount is obtained by averaging the results of several
peaks. The operating software for the COMPUSS displays and records ﬁve SO, column amount values calculated using respective ﬁve peaks together with the representative value. This idea of dealing with ﬁve peaks
independently is a unique feature of the COMPUSS
operating software and can use as an indicator for
presence of scattering interference by atmosphere between the plume and the COMPUSS.
The noise level of SO, column amount is usually froma several ppmm to some tens of ppmm. The noise levelbasically depends on the spectroscopic intensity, andnumber of spectra to average to obtain the sample
spectra. Thus, by increasing the integration time andnumber of spectra to average, apparent SO, error can be
minimized. However, this will decrease the time resolution as well as spatial resolution. Conversion factors
to translate from the peak heights to SO, column
amounts were obtained using SO, calibration cells prior
to measurements. We also put the calibration cells
between the traverse measurements to check stability ofthe measurements.

Fig. -. (a) Transmittance of the UV ﬁlter inside
the telescope between -** nm and -0* nm. (b)Examples of the raw spectra obtained using the
COMPUSS. The spectra are obtained on a clear
winter day aiming zenith. The spectra (darkcorrected) from the top are sky spectrum without
the UV ﬁlter (dotted line), sky spectrum with
the UV ﬁlter (reference spectrum : solid boldline) and two sky spectra with +3. ppmm (graybold line) and .2* ppmm (thin solid line) calibration cells. (c) Absorbance of the two calibration
cells calculated using spectra in (b). Peak topsof the ﬁve peaks used in the quantiﬁcation wereindicated with peak numbers and arrows. The
absorbance between -** nm and -*- nm areslightly distorted due to low spectral intensity as
seen in (b). The inset in (c) shows a schematicfor calculating the peak height for the peak +.

.. Results and discussion
In order to verify the newly built system, SO, standard cells with various SO, column amount are tested in
the ﬁeld. We also show the COMPUSS’s results together with COSPEC’s results at two Japanese
volcanoes. Comparison of the results with COSPEC’s
results is an important issue to check and maintain thedata quality and continuity for accumulated ﬂux dataover several decades using the COSPEC.
.ῌ+ Response validation with SO, calibration
The response validation for the COMPUSS was
carried out on a clear day by putting the telescope on a
tripod aiming zenith. Using the two slots of the telescope (Fig. +a), variety of SO, column amounts are
obtained with ﬁve cells, which are calibrated in thelaboratory, by using one or two cells at each measurement. Figures .a and .b show correlations between
the peak height and column amount of SO, synthesizedwith the standard cells for SO, peaks at -*2.0 nm (Peak, in Fig. -c) and -+- nm (Peak . in Fig. -c), respectively. The linear or quadratic relations between the peakheight and the column amount are obtained for all the
peaks with correlation coe$cients (r,) over *.33 between * and ,1** ppmm of SO,. In practical use forvolcanic plume monitoring, the SO, column amountrange used here is basically enough and the results of thetest show the capability of the system at least up to ,1**
ppmm. This ﬁeld calibration shows the advantage ofA Compact Ultraviolet Spectrometer System (COMPUSS) for Monitoring Volcanic SO, Emission :Validation and Preliminary Observation.

109

Fig. .. Example of plots between the peak height and the column amount of SO, for absorbance peaks for (a)Peak , and (b) Peak . in Fig. -c. The regression curve and line are calculated by the least-squares-method.

the COMPUSS having two slots for the calibration cells.
.ῌ, Traverse measurements at Sakurajima Volcano
Sakurajima volcano locates in the southern part of
Kyushu Island, Japan. The volcano emits SO, with anaverage of ,*** ton/day for last several decades (Hirabayashi et al., +332). In October ,**-, a comparison
measurement using three COMPUSSes was carried out
at Sakurajima volcano. Three COMPUSSes (C+, C,
and C-) were mounted on three di#erent cars aiming
vertically up. Traverse measurements are made by taxiing cars under the plume in C+, C, and C- order. We
also put the COSPEC in the last car for comparison
with the C- COMPUSS. Figure /a shows a time
versus SO, column amount plot of the C- COMPUSS
together with that of the COSPEC. Unlike SO, output
by the COMPUSS, SO, variation obtained by the
COSPEC has many spike-like noises. These spike-likenoises are due to a brief covering of the FOV by trees in
the case of COSPEC’s output. In contrast, these obstructions in the optical path do not a#ect the SO,
output with the COMPUSS, as long as the duration of
the FOV covering is shorter than the integration time ofthe spectra acquisition which is usually a few hundred toa thousand milliseconds. When the traverse route is in
the sparse woodlands with some sky light through thetrees, COSPEC measurements are impossible due to thelarge noise. In contrast, the COMPUSS measurements
are still possible as long as there is certain amount of sky
light intensity at the SO, absorption wavelength range.
This fact expands the opportunity of measuring SO, ﬂux
with various conditions compared to the case of the
COSPEC.
Figure /b shows a time versus SO, column amountplot for three COMPUSSes. The three proﬁles basically have high similarity except at a few places. In thetraverse measurement, we tried to keep distance be-

Table +. Correlation and proportional coe$cients of
the SO, column amounts obtained by the three
COMPUSS and the COSPEC.

tween the three cars to be +*ῌ/* m apart from eachother which corresponds to average of +ῌ/ seconds time
di#erences between the cars. However, the distanceoccasionally stretched over +** m, due to interference ofthe local tra$c. The time di#erence likely caused theslight disagreement in the three measurements. The
SO, ﬂuxes measured with the three COMPUSSes were
11,, 031 and 1++ ton/day for C+, C,, and C-, respectively. On ,+ Oct. ,**-, further evaluation measurements
were carried out with three COMPUSSes (Fig. /c).
We drove more carefully than the previous day to keep
the distance between cars more constant. The SO,
ﬂuxes measured with the three COMPUSSes were /-0,/-2 and /.* ton/day for C+, C,, and C-, respectively.
Table + shows correlation and proportional coe$cients between outputs of the three COMPUSSes andthe COSPEC for ,* and ,+ October results. The correlation and proportional coe$cients between the three
COMPUSSes (Table +) are calculated by correcting
the time di#erences of a few seconds between thecars. For the COSPEC data, the spike-like noises areprecluded from the COSPEC chart. High correlation(r,ῌ*.32-) with proportional coe$cient of *.33* is110Toshiya MORI, Jun-ichi HIRABAYASHI, Kohei KAZAHAYA, Takehiko MORI,
Michiko OHWADA, Makoto MIYASHITA, Hideki IINO, Yasuhiro NAKAHORI
Fig. /. Plots of SO, column amounts versus time
obtained by the car traverse measurements at
Sakurajima. (a) COMPUSS (C- : green line) and
COSPEC (black line) data obtained on ,* Oct.
,**-. (b) Comparison of three COMPUSS on ,*Oct. ,**-. The three COMPUSSes were on thedi#erent car taxiing with a few seconds di#erencein the order of C+ (red line), C, (blue line) and C- (green line). (c) Comparison of three COMPUSSes on ,+ Oct. ,**-.

Fig. 0. Plots of SO, column amounts versus time
obtained by the helicopter traverse measurementswith COSPEC (thin red line) and the COMPUSS(bold green line) carried out at Miyakejima
volcano, Japan on (a) ,, July ,**- and (b) 2June ,**.. The COSPEC’s SO, column amountdata circled in blue line correspond to output of+.-2 ppmm SO, calibration cell. The COMPUSS
proﬁle is piled on the COSPEC proﬁle with thesame time and SO, column amounts level so thatthe calibration cell values are equal to the scale of
the COMPUSS data. The COSPEC zero leveland gain (the output of the standard cell) was
drifted with time on 2 June ,**.. The COMPUSS
data are scaled with the ﬁrst three data of thestandard cell.

A Compact Ultraviolet Spectrometer System (COMPUSS) for Monitoring Volcanic SO, Emission :Validation and Preliminary Observation.

111

Table ,. Comparison of SO, ﬂuxes obtained using the COMPUSS andCOSPEC at Miyakejima volcano.

obtained between the COSPEC and C- COMPUSS
data. This good agreement between the two instruments is quite signiﬁcant considering the di#erence of
the FOVs between the COSPEC (1ῌ,+ mrad) and the
COMPUSS. The disagreement in proportional coe$cient from unity observed on ,* October is up to 0./ῌand this disagreement is mainly due to rapid time
ﬂuctuation of the plume and/or to minor misalignmentof the telescope from zenith. Disagreement of the
proportional factor has improved to less than ,ῌ for all
the pairs on ,+ October. The results of three COMPUSSes show high reliability and reproducibility of the
COMPUSS measurements. The one-to-one correspondence between the COMPUSS and the COSPEC outputs
show that the COMPUSS can secure the data continuity
with accumulated COSPEC data and can replace the
COSPEC without any problem.
.ῌ- Heliborne traverse measurements at Miyakejima volcano
Miyakejima volcano is a basaltic island on the IzuMariana arc and is located about +2* km south ofTokyo. The volcano has been intensively discharging
volcanic gas since August ,*** after the summit caldera
formation. Sulfur dioxide ﬂux from the volcano has
been monitored repeatedly by heliborne COSPEC measurements until ,**/. Extremely high monthly averaged
SO, ﬂux of /1*** ton/day has been recorded in Dec.
,*** and the monthly averaged ﬂux still has 1*** ton/
day at least until December ,**- (Kazahaya et al.,,**.). Since July ,**-, we carried out parallel heliborne SO, ﬂux measurements using the COMPUSS andthe COSPEC measurements for seven times. In the
heliborne measurements, the COMPUSS’s telescope was
directly attached to COSPEC’s telescope and the UV
spectrometer was ﬁxed on COSPEC’s body. The proﬁles
obtained by the two instruments (Fig. 0) show simultaneous and similar temporal variation patterns of SO,
column amounts and accordingly similar SO, ﬂux
values. The average SO, ﬂuxes obtained by the COMPUSS and COSPEC are commonly in good agreement
within +*ῌ disagreement (Table ,). However, there

are also a few cases where the large discrepancy of ῍,*ῌ is found. Since same plume direction and speed
are used in the ﬂux analyses for both instruments, thedisagreement should be purely instrumental.
COSPEC’s SO, charts (Figs. 0a and 0b) are very
noisy and show a several variety of noise patterns. The
noise in the COSPEC chart is due to interference between rotating correlation disc in the COSPEC androtating rotor blades which are in the view of the
instruments (Daag et al., +330 ; Galle et al., ,**,). In
contrast, the COMPUSS’s output (Fig. 0a and 0b) is
not a#ected by rotating rotor blades, since the covering
duration of the view by the rotor blades are signiﬁcantly
shorter than the integration time of the UV spectrometer. The absence of the rotor blade noise for the UV
spectrometer system is also mentioned in Galle et al.
(,**,). For ﬂights on ,, Jul. ,**- (Fig. 0a), ,, Jan.
,**., +1 Feb. ,**. and +1 Aug. ,**., the noise has
relatively small and regular oscillation noise in SO,
column amounts. In these ﬂights, the results of the
COSPEC and the COMPUSS have good agreement.
For ﬂights on +0 Dec. ,**- and ,. Jul. ,**., beatinglike noise level is sometimes larger than the SO, signalsof the plume. In ﬂights on 2 Jun. ,**., the COSPEC
chart shows somehow random and peaky noise patterns(Fig. 0b). In these three ﬂights, the noise is deﬁnitely
a#ecting the ﬂux estimations for the COSPEC data, andthe large discrepancies between the two ﬂuxes are morethan ,*ῌ. The absence of the rotor blade noise is theclear advantages of the new system over the COSPEC.
This advantage of the COMPUSS will be more important and critical, when carrying out heliborne SO, ﬂux
measurements for plumes with much lower SO, column
amounts than those of Miyakejima.
/. Concluding remarks
Newly developed COMPUSS, using a commercial
compact UV spectrometer, is lighter and more portablecompared with the COSPEC. From the relation between peak heights and SO, column amounts, the
COMPUSS can readily measure at least up to ,1**

112

Toshiya MORI, Jun-ichi HIRABAYASHI, Kohei KAZAHAYA, Takehiko MORI,
Michiko OHWADA, Makoto MIYASHITA, Hideki IINO, Yasuhiro NAKAHORI

ppmm of SO,. The ﬁeld experiment demonstrats theability of the COMPUSS for reliable calibration of thesystem for wide column amount range. Simultaneous
measurements of the COMPUSS and the COSPEC
show good agreement. The new system can keep thecontinuity of the past COSPEC data obtained by traverse method. The COSPEC measurements are usually
inﬂuenced and give signiﬁcant noises with tree branches
or rotor blades in the view of the telescope. These
noises are absent in the case of COMPUSS. These
evidences expand the opportunity of conducting the ﬂux
measurements in various conditions. The COMPUSSand similar systems will probably replace the COSPEC
in near future in the volcanological community andcontribute to further understanding of the volcanic
degassing system.
Acknowledgements
We would like to thank all the member of the
Sakurajima Volcanological Observatory, DPRI, Kyoto
University for assistance for observation at Sakurajima
Volcano. The authors are grateful to Tokyo Fire Department and Tokyo Metropolitan Police Department
for providing helicopter ﬂights for observations at
Miyakejima Volcano. We also acknowledge Dr. H.
Shinohara for his constructive advice for building up
the system. Dr. T. Hashimoto and an anonymous
reviewer are gratefully acknowledged for improvementof the manuscript.
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ῌEditorial handlingTakeshi Ohba῍