Rapid sensing of volcanic SO₂ fluxes using a dual ultraviolet camera system: new techniques and measurements at Southern Italian volcanoes

7/25/2019 Rapid sensing of volcanic SO fluxes using a dual ultraviolet camera system: new techniques and measurements

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Universit degli studi di Palermo

Facolt di Scienze Matematiche Fisiche e Naturali

Dipartimento di Scienze della Terra e del Mare (DiSTeM)

Rapid sensing of volcanic SO fluxes using a dual

ultraviolet camera system: new techniques and

measurements at Southern Italian volcanoesGiancarlo Tamburello

Tutor: Coordinator:

Prof. . iuppa! Prof. F. Parello

Co-tutors:

Dr. . McGoni"le#n". G. Giudice

Reviewers:Patric$ llard% #nstitut de Ph&si'ue du Globe de Parislie *ppenheimer% Department of Geo"raph&% +niersit& of ambrid"eMatthe, -atson% Department of arth Science% +niersit& of /ristol

D*TT*0T*D#0#0#NG*1#M#22### #3*(*D. 445)

6N+078449 : DM/084;;


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Tale of Contents

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.1.5 Plume s"eed calculation.......................................................................................3

.1.( SO mass measurement ........................................................................................$

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The most eident epression of ma"matic de"assin" is the atmospheric

dispersion of "ases and particles emitted from a olcano summit. plume is a

hetero"eneous s&stem% includin" an airCdominated "as phase and aerosols

(*ppenheimer% 844=)% and it accounts for the maorit& of the ener"& and mass dail&

output of a 'uiescent olcano (llard% ;995J Shinohara% 844


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1. )ntroduction

(ia ,et and dr& deposition)% to "ie rise to olcanic air pollution% acidification of

soils% rain,aters and fresh,aters% and eentuall& anomalousl& hi"h S contents in

e"etation (see Delmelle% 844= for a recent reie,). Durin" lar"eCscale eruptions%

S*8is conerted in the stratosphere into radiatiel& actie sulphate aerosols% ,hich

ma& reflect sunli"ht bac$ to space (Graf et al. ;995). Sulphate aerosols from the Mt

Pinatubo plinian eruption% for instance% cooled the troposphere% offsettin" the

anthropo"enic "reenhouse effect for a limited duration (0oboc$% 8448). T*MS (Total

ozone mappin" spectrometer)% alon" ,ith the infrared T*BS (T#0*S *ptical Bertical

Sounder)% proided earl& estimates of the stratospheric sulphur &ield of this eruption

(;4 T" of SJ see *ppenheimer% 844= for a reie,). The Pinatubo eruption also

caused stratospheric heatin"% an alteration in "lobal stratospheric circulation pattern%

and an approimatel& 84I reduction in ntarctic ozone leels (Mcormic$ et al.

;99@). The Mt. Pinatubo eruption has been the most stri$in" recentl& obsered

demonstration of the dramatic increase in S atmospheric burden durin" a olcanic

eruption (see 0oboc$ and *ppenheimer% 844=).

#n li"ht of the aboe% remote sensin" techni'ues hae increasin"l& been used

oer the last decades in olcanic S*8 plume measurement% si"nificantl& broadenin"

the abilit& of olcanic "as "eochemistr& to contribute to olcanic hazard assessment

(*ppenheimer% 84;4). For instance% spectroscopicCsensin" of olcanic S*8 effectiel&

contributed to identif&in" PinatuboHs reCa,a$enin" in ;99; (Daa" et al. ;99?J 1off

;998)% therefore miti"atin" a"ainst the effects of the eruption. The "ro,in" need to

improe accurac& and time resolution of S*8 obserations at actie olcanoes has

encoura"ed the scientific communit& to refine traditional obseration techni'ues (e.".%

*SPJ Moffat and Milln ;95;)% and to eplore ne, horizons (a brief reie, is

"ien in c-a"ter2of this report). Technolo"ical pro"ress has boosted this process%

and% for instance% the preiousl& used correlation spectrometers (Moffat and Milln

;95;% Stoiber et al. ;9


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1. )ntroduction

This PhD report aims to contribute to proin" further the potential C in a

olcanolo"ical contet C of a noel techni'ue% recentl& deeloped for ima"in" S* 8

emissions! the +B camera. *er the last @ &ears% the +B camera has emer"ed as a $e&

tool to stud& transient "asCdrien olcanic phenomena% such as Strombolian (Mori and

/urton% 8449) and Bulcanian (7amamoto et al.% 844


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2. SO lu' measurements/ our!decade story

) SO flux measurements: four-decade story

hapter 8

S*8flu measurements!

fourCdecade stor&

)#$ I%TRO&'CTIO%

Bolcanic de"assin" occurs in ran"e of ,a&s! durin" olcanic dormanc&% and in

periods of either lo, intensit& effusie eruptions or hi"hl& ener"etic (1a,aiian to

Plinian) eplosie actiit&. Bolcanic "as measurements proide important information

on under"round ma"ma conditions% enablin" the definition of% for instance% the masses

of ma"matic bodies in the subsurface% ,hether ma"ma flo, rate to the surface is

escalatin" or decreasin"% ,hether a olcanic s&stem is open or sealed to "as flo,% and

,hether h&drothermal interactions are inoled. Such data are ital for olcano

5


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monitorin"% and hae si"nificantl& contributed to olcanic hazard assessment at a

number of tar"ets ,orld,ide (*ppenheimer% 844=)

Bolcanic "as "eochemistr& has been limited for oer a centur& to inCsitu

samplin" of fumaroles and plumes. Traditional collection methodolo"ies included

flas$s filled ,ith al$aline solutions (Gi""enbach% ;95@)% andEor bubblin" of ent "ases

throu"h al$aline solutions or baseCtreated filters% ,hich act as acid traps (S&monds et

al. ;99>). Despite the numerous "as species detected% samplin"% and transport to the

laborator& for subse'uent anal&sis re'uire da&s to be accomplished% resultin" in a

number of limitations! poor temporal resolution (,ith "as data traditionall& ta$en at

;4C@1z rate) relatie to "eoph&sical techni'ues (seismolo"& primaril&)% ,hich can

ac'uire data continuousl& and ,ith an hi"h temporal resolution ( ; 1z)J the inherent

ris$s for researchers ,ho approach the entsJ and the fact that olcanic summits are

often not ph&sicall& accessible% restrictin" samplin" to lo, temperature fumaroles%

,hich ma& be unrepresentatie of the olcano as a ,hole.

*er the last =4 &ears% "roundCbased remote measurement of S* 8 flu has

been pla&in" an increasin"l& important role in olcano monitorin". The clear

adanta"e is its capabilit& to ac'uire data far from the craters and areas at ris$% ,ith a

time resolution of minutes to hours (far hi"her than inCsitu samplin" methodolo"ies).

#n this chapter% # reie, briefl& the stateCofCtheCart of S*8flu methodolo"ies% and the

pro"resses made oer the last ten &ears (Fi". 8).

)#) "!SICT5.OR/

/roadl& spea$in"% remote "as sensin" techni'ues are based on the

measurements of the attenuation of li"ht (natural or artificial) b& a olcanic "as plume

(Fi". ;). bsorption inoles onl& particular spectral ,indo,s% dependin" on the

tar"et "as species% and is concentrationCdependent as described b& the 3ambertC/eer

3a,!

(;) )=)4e89


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,here):;).

The most ubi'uitousl& used spectral ,indo, for such measurements has been

the ultraiolet (Galle et al.% 844=)% and particularl& do,nCscattered solar radiation

sampled from the "round to assess olcanic S*8emission rates. 0esearch has focused

on S*8because of its stron" +B absorption features% lo, bac$"round atmospheric

concentrations ( ; ppb) and relatiel& hi"h abundance in olcanic plumes (K; ppm).

#n contrast% olcanic 18* and *8 are etremel& difficult to measure remotel&%

because of their hi"h concentrations in bac$"round (ambient) air.

fter more than =4 &ears of alued serice in Bolcanolo"&% the correlation

spectrometer (*SPJ Moffat and Milln ;95;% Stoiber et al. ;9


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replaced : ;4 &ears a"o C b& miniature +B +S/Cpo,ered "ratin" spectrometers%

,ith spectral anal&sis ia differential optical absorption spectroscop& (D*S)

(McGoni"le et al. 8448% Galle et al. 844=% 1orton et al. 844?% McGoni"le 8445%

Aantzas et al. 8449).

;4

Figure #: This timeline chart illustrates the advances in %& remote sensing of volcanic gas.

;9?4

;954

;9

844 ,ith the obectie of automaticall& measurin" S*8

flues from the summit craters. F3M detected eleated S*8flues durin" the 8445

eruption of Stromboli (/urton et al. 8449% Fi". @)% ,hich ,ere proposed to reflect a

chan"e in ma"matic conectie circulation in the olcanoOs upper conduit.

;=

Figure +: -#flu* time series from an rist/bal volcano on #' ovember #00# ("alle et al.,

#010).


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#n ie, of these recent results% researchers hae focused their attention to,ards

the automation of S*8"as flu measurement s&stems. The N*B proect (Net,or$

for *bseration of Bolcanic and tmospheric han"e) started on *ctober 844@ ,ith

the obectie of establishin" a "lobal net,or$ of stations for S*8 flu measurements at

actie olcanoes. t present% N*B encompasses obserations at 84 olcanoes

includin" Mt. tna% one of the most actie and stron"est olcanic "as emitter in the

,orld. The net,or$ ma$es use of a scannin" dualCbeam MiniCD*S (Galle et al.%

844?)% ,hich represents another step for,ard in olcanic "as spectroscop& "ien its

capabilit& of measurin" S*8and /r* emissions from a olcano ,ith less than @

minutes time resolution durin" da&li"ht.

Fi"ure?sho,s one &ear of S*8flu measurements of the N*B station at

Tun"urahua% an andesiticCdacitic olcano in cuador. The dataset has been coupled

,ith the seismic si"nals monitored b& the #GPN net,or$. The comparison sho,s a

fair correlation onl& ,ithin a timeCscale of months.

;>

Figure $:verage daily -#flu* from tromboli, measured ith the F2!3 netor4. "rey barsindicates the #005 eruption (6urton et al., #007).


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2.3.3 Cylindrical lens DOAS

Scannin" and traersin" D*S modes derie the S*8molecules distribution

alon" a plane of the "as plume resoled ia pointCtoCpoint measurements. The

accurac& and duration of this assessments depend on the number of spectra collected.

Despite si"nificant pro"resses in improin" the samplin" fre'uenc& of D*S% from

half an hour to onl& a fe, minutes% the temporal resolution still remains ,ell belo,

that achieed b& "eoph&sical obserations% preentin" us from a full data comparison.

first attempt of spectroscopicall& capturin" S*8flues at hi"hCfre'uenc& has

been made b& McGoni"le et al. (8449) on Stromboli. This ,or$ is based on the use oft,o +S/8444 spectrometers% each coupled to telescopes ,ith c&lindrical lenses! these

hae 'uasiCrectan"ular fields of ie,% and allo, capture of an inte"rated column

amount directl& from a sin"le collected spectrum eer& ;C;.> s (Fi". 5) (a far more

rapid alternatie to traersin" or scannin"). The t,o fields of ie, ,ere separated b& a

small an"le% ,hich permitted trac$in" of plume hetero"eneities in the timeCstamped

datasets obtained from each spectrometer. mission rates ,ere thereb& inte"rated ,ith

"eoph&sical data% proidin" one of the first eamples of this nature% and demonstratin"

correlation bet,een the ma"nitudes of seismic and thermal si"nals and the S*8mass

;@

Figure 8: 9esults of measurements of the -& stations at Tungurahua. - #gas emission rate

is measured in tons per day by the -& instruments. ll the geophysical signals are uantifiedas the number of seismic events per day by a dense netor4 of seismometers around the volcano

("alle et al., #010).


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of Strombolian eplosions. This techni'ue presented some disadanta"es% ho,eer%

such as the permanent and inariable field of ie, of the lenses% and the difficulties in

$eepin" a "ood ali"nment bet,een the measured sections and plume location.

The same approach has also been deplo&ed on Mt. rebus in ntarctica

(/oichu et al. 84;4)% ,hich led to discrimination of S*8 fluctuations lin$ed to

atmospheric processes% from those resultin" from ma"matic actiit&. "as flu

periodicit& of ;;:8> minutes ,as obsered% leadin" to important findin"s re"ardin"

transport d&namics of olcanic "as in the conduit (see c-a"ter=).

)#4 SO'RC.SO*.RROR

#n recent &ears% a number of studies hae detailed the sources of error

associated ,ith S*8 flu measurements% ,hich hae led to attempts to improe

measurement accurac&. Follo,in" Galle et al. (84;4)% ,e reco"nize three important

sources of error! radiatie transfer% plumeCinstrument "eometr&% and plume speed.

;?

Figure 5: To telescopes ith cylindrical lenses scanning hori;ontal stripes of a vertically rising

plume on tromboli (left photo) and 3rebus (right photo).


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Ground based remote sensin" theor& is based on the assumption that radiation%

scattered b& molecules and particles in air behind the plume% passes throu"h the plume

,ith strai"ht paths% before bein" detected b& the instrument. #nstead% dependin" on the

opacit& of the plume (,hich contains ash% condensed ,ater and aerosols)% multiple

scatterin" can occur (Fi".


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These effects and their dependencies ,ere ,ell described b& Aern et al.

(8449)% ,ho simulated s$&li"ht optical paths in different scenarios. The concept of

Vair mass factorW (MF) ,as applied% defined as the ratio bet,een the measured slant

column densit& and the theoretical column densit& on a strai"ht line throu"h the

olcanic plume alon" the instrumentOs ie,in" direction. Therefore% an MF of ;

means that there is no error in the measurement% and hence that the effectie photon

path is e'ual to the len"th of a strai"ht line throu"h the plume alon" the instrument Os

ie,in" direction. 0esults sho, that MF depends si"nificantl& on plumeCinstrument

distance% and on S*8and aerosols concentrations in the plume. These dependencies

hae an affect on all "roundCbased remote measurements.

Scannin" methods re'uire $no,led"e of plume hei"ht and speed for S*8flu

calculation. The former% in conunction ,ith plume dispersion and imperfections in

orientation and leellin" of the instrument% can be a important sources of errors (up to

=4 I) (Salerno et al.% 8449J Galle et al.% 84;4). 0eliable plume speed deriation is

another ,ellC$no,n issue of these remote measurements (Stoiber et al.% ;9.;.@). This latter

method is% ho,eer% onl& useful ,hen at least part of the plume is oer the instrument.

;


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)#8 +IIT!TIO%S!%&O'T+OO;

The first hi"hCtime resolution measurements of the olcanic S* 8 flu hae

confirmed the nonCstationar& nature of 'uiescent olcanic emissions% and hae

hi"hli"hted the importance of stud&in" shortCperiod S*8flu fluctuations to improe

our understandin" of "as transport d&namics in the ma"maCconduits% and their control

on modes and timin" of a olcanic eruption.

t actie ents% "as ascent is essentiall& drien b& conection% due to a

temperature difference bet,een hot olcanic "ases and the cold atmosphere. fter the

"as has cooled do,n% the plume continues to rise due to buo&anc& until% hundreds of

meters from the ent% increasin" ,ind speed and collision ,ith denser atmospheric

la&ers turn its ertical rise into an horizontal flo,. side from the errors that are

common to all remote sensin" techni'ues% scannin" and traersin" methods are

performed $ilometres far a,a& from the olcano% loo$in" at a homo"eneous plume

that has lost its ori"inal features% in particular those temporal hetero"eneities that

ma$e up the lo,Cperiod components of olcanic "as emissions. Moreoer% the lo,temporal resolution (a fe, minutes at best) of D*S techni'ues ma$es them

unsuitable to characterise transient olcanic phenomena (e.".% eplosions) %

representin" the most ener"etic (and% therefore% potentiall& dan"erous) manifestations

of olcanic de"assin".

#n li"ht of the aboe% remote sensin" measurements from proimal sites ( ;

$m) could improe our $no,led"e of de"assin" processes% increasin" our spatial

resolution and preentin" chan"es in the plumeHs structure from bein" lost prior to

detection. Gien its fast samplin" rate% c&lindrical lens D*S could represent a "ood

tool for these purposes. This techni'ue is% ho,eer% hi"hl& dependent on plume ,idth

and direction% ,hich are more ariable ,hen obsered at short distances. #n the net

c-a"ter% # ,ill discuss the adanta"e of a noel methodolo"& of olcanic S*8ima"in"!

the +B camera.

;9


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3. U# camera and SO imaging

( '1 camera and SO imaging

hapter =

+B camera and S*8ima"in"

(#$ I%TRO&'CTIO%

This section briefl& reie,s the principles of operation% and the most recent

applications% of the +B camera techni'ue. The updated +B camera instrumental

setup% deeloped in this ,or$% is detailed on c-a"ter>.

(#) 2.%.R!+I%STR'.%T!TIO%

The use of an +B camera in Bolcanolo"& has been first reported b& Mori and

/urton% (844?) and /luth et al.% (8445). /roadl& spea$in"% the +B camera consists of

a main portable electronic bod& ,ith a thermoCelectricall& cooled D detector. The

D has t,o output amplifiers! one for lo, noise% and a second for eceptionall&

84


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hi"h d&namic ran"e. The eposure time of the camera can ran"e from ;4 milliseconds

to ;4 - at most.

The most commonl& used +B cameras (Fi". 9) as cited in the literature are!

;. po"ee lta +8?4% fitted ,ith a ;? bit @;8X@;8 piel Aoda$ AFC

48?; D detector (5444Y)J

8. po"ee lta ?% fitted ,ith a ;? bit ;48>X;48> piel Aoda$ AFC;44;C8 D detector (;8444Y)J

8;

Figure 7: The to most used %& camera model in volcanic

-#imaging (http:>>.ccd.com).


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The former% ,hich is clearl& less epensie% proides more than enou"h spatial

resolution and has considerabl& reduced readout times% enablin" hi"her ac'uisition

fre'uencies relatie to the ;48>X;48> piel camera.

Zuartz lenses do not affect ,aelen"ths in the near +B spectrum% their fied

focal len"ths ran"in" 8@C;4@ mm% and thus proidin" a field of ie, of 8>C;=

respectiel&. The focus is manual and the iris ran"es from F8.


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,here #P and #/ are plume and bac$"round ima"es respectiel&% after bein" dar$C

ima"e subtracted. Dar$ ima"e collection% obtained coerin" the camera lens% is

necessar& to accommodate for dar$ current and electronic offset. s these phenomena

are 'uasiCconstant% oer timescales of ; hour% follo,in" the ten minute s,itch on

thermal stabilisation% a sin"le dar$ ima"e% per camera% ,ill be enou"h for such an

ac'uisition duration. onersion from to ppmRm is achieed throu"h a calibration

procedure inolin" cells of $no,n S*8concentration (seesection>.;.=).

Sulphur dioide absorption ends at =84 nm (Bandaele et al.% ;99>)% and filters

[ and \ operate in the ran"e =44C=84 nm and =84C=>4 nm respectiel& (Fi". ;;). ll

interferences are supposed to be spectrall& flat oer this =44:=>4nm ran"e% and

therefore to hae no effect on measured absorbance. Filters can be manuall& s,itched

if the samplin" period is K= s (Mori and /urton% 844?)% or automaticall& usin" a +S/

controlled filter ,heel (e.". the po"ee F-@4C5S Filter -heel% Fi". ;8). The latter

allo,s one to ta$e t,o consecutie ima"es in > s.

8=

Figure 11: ulphur dio*ide absorption cross section (&andaele et al., 177+).


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lternatel&% a hi"her samplin" fre'uenc& can be achieed usin" a sin"le filter

in a spectral re"ion ,here S*8is absorbed (Mori and /urton% 8449J /luth et al.% 8445J

Dalton et al.% 8449)% in ,hich case is measured as!

(=) A=lo";4)P)@

The difficult& ,ith a sin"le filter% ho,eer% is that one is measurin" the plume

attenuation caused b& both "as and aerosols% ,ith no means of discriminatin" bet,een

the t,o (seesection>.;.;).

(#( R .C.%T!77+IC!TIO%S

0emote sensin" techni'ues hae been used in the last decades to measure the

S*8flu from olcanoes and therefore monitor their actiit& status. This has made it

possible to lin$ S*8 emission rates ,ith ma"matic bud"ets at seeral olcanoes

(llard et al.% ;99>J altabiano et al.% ;99>J 1iraba&ashi et al.% ;99@J llard% ;995J

-atson et al.% 8444J dmonds et al.% 844=% iuppa et al.% 84;4)% but oer time scales

of ,ee$s to &ears. 1i"hCrate S*8 obserations hae lon" been challen"in"% preentin"

8>

Figure 1#: pogee F?$0@5 filter ?heel mounted on a pogee %& camera.


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inesti"ation of fastCoccurrin" olcanic processes (e.".% eplosions)% and their

underl&in" de"assin" mechanisms. Seeral efforts hae been made in calculatin" S*8

flu from infrasonic data% but ,ithout an& tan"ible result (Bidal et al.% 844?).

*er the last fe, &ears% the +B camera technolo"& has represented an

important step for,ard in olcanic "as flu measurement% enablin" the capture of

consecutie ima"es of a olcanic plume at steps of ;s. This hi"hl& improed

temporal resolution has ultimatel& opened the ,a& to comparison of S* 8 flu time

series ,ith seismic and infrasonic datasets% thus contributin" to reduce the eistin"

"ap bet,een olcanoC"eochemistr& and "eoph&sics% and our better understandin" of

de"assin" process.

The adanta"e of the hi"h temporal and spatial resolution of the +B camera

has been demonstrated b& Mori and /urton (8449)% ,ho first 'uantified the "as mass

8@

Figure 1': Time seuence of -#column amount images of a single hornito eruption (!ori

and 6urton, #007).


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emitted durin" a sin"le (Strombolian) eplosion on Stromboli (cfr. section @.=).

1o,eer% the ascent speed of the eruption plume ,as fast relatie to the field of ie,

of the camera (Fi". ;=)% ma$in" s,itchin" bet,een t,o filters (manuall&% or ,ith a

filter ,heel) bet,een t,o consecutie ima"es impractical. To oercome this problem%

the authors used% for most of the stud&% a sin"le =;4 nm bandpass filterJ the&

additionall& obsered t,o further hornito eruptions C usin" both filters C to estimate

the aerosol effect on S*8eruptie mass.

The obsered S*8 mass amounts ran"ed from ;@C>4 $" in fie discrete

eplosions% ,hich led the authors to conclude that eplosie "as output contributes to

=:


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discrete eents. The obsered eplosions ,ere small% and oerall responsible for

ariations in the total S*8 flu in the order of ;4C84 $"Rs;. The results hae

corroborated the use of both acoustic (ecess pressure inte"ration) and +B camera as

independent techni'ues to retriee "as masses associated ,ith discrete de"assin"

eents (Fi". ;>). #n addition% the authors proided support for earlier theor& that the

dominant acoustic si"nals recorded at Paca&a are created b& Strombolian bubbleCburst

eents.

1i"h temporal resolution S*8emission rate data hae been collected ,ith a

sin"le +B camera at Santia"uito olcano (Guatemala) in 844< and 8449 b& 1olland et

al. (84;;). Santia"uito is an ideal case stud& to inesti"ate processes of de"assin"

durin" domeCformin" etrusion of an intermediate ma"ma (compositionall& er&

similar to products of the ;948 Plinian eruption of the parental Santa Mar]a olcano).

The dome has been characterised b& continued etrusion of dacite bloc$ laa flo,s

accompanied b& fre'uent (4.@:8 per hour) lo,Cintensit& eplosions% producin" ash

and "as plumes to hei"hts of 4.@:8 $m aboe the olcano (Sahetap&Cn"el et al.%

844C; $" s;% punctuated b&

small increases in emission rates (up to 8:= $"Rs;) durin" eplosions% as sho,n in

Fi". ;@

85

Figure 1$: Time series of -#emission rates and eruption periods (graybars). The pseudo colour graph shos the plume of antiaguito and the

section (blac4 line) used for flu* calculations (Bolland et al., #011).


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first inte"ration of +B camera and seismic measurements has been

performed b& Nadeau et al. (84;;) in 6anuar& 8449 at Fue"o% a basalticCandesite

stratoolcano (=


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Moreoer% the hi"h correlation bet,een tremor and S*8 flu (Fi". ;5) ,as

used to support the commonl& held ie, that rapid flo, of bubbles and bubbleCrich

ma"matic fluids are the main source of seismic tremor (houet% ;998J houet% ;99?).

Aazaha&a et al. (84;;) eamined the relationship bet,een B3P pulses and

olcanic "as emissions at Mt. sama (6apan)% an andesiticCdacitic olcano

characterised b& the presence of a small open ent on the crater bottom ( =44m

a.s.l.). fter a recent eruption on Februar& 8449% ashCfree de"assin" bursts from this

ent hae fre'uentl& been obsered% occurrin" ust after a B3P pulse. The authors

hae obtained hi"h temporal resolution S*8 flu data usin" t,o +B cameras%

follo,in" the methodolo"ies conceied b& Mori and /urton (844?)% /luth et al.

(8445)% and Aantzas et al. (84;4).

Aazaha&a et al. (84;;) reealed that the eruptie "as emissions at Mt. sama

are lar"er than those at of Stromboli b& one to three orders of ma"nitude% contributin"

to about ;? I of the bul$ S*8emission. The stron" correlation bet,een S*8mass and

the B3P displacement produced durin" "as burstin" (Fi". ;


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s this rapid reie, sho,s% applications of the +B camera s&stem hae been

increasin" in number and 'ualit& oer the last @ &ears% and demonstrate the enormous

potential of this methodolo"&.

(#4 .RRORS!%&+IIT!TIO%S

0emote sensin" techni'ues ,hich use scattered sunli"ht as a li"ht source are

all affected b& errors due to the radiatie transfer (see section 8.>). Sin"le or t,o

coupled +B cameras% in contrast to the D*S techni'ue% ,or$ usin" a small

,aelen"th ran"e% possibl& preentin" the assessment of the radiatie transfer

problem. 1ere belo, ,e ,ill attempt at 'uantif&in" +B cameras errors on the base of

the results reported b& Aern et al. (8449) for ,aelen"ths of =44 nm and =8@ nm

(close to those of the filters)% bein" that radiatie transfer is ,aelen"th dependent.

-eHll emplo& the concept of Vair mass factorW (see section 2.) to describe the error

ma"nitude.

+B cameras re'uire to be placed at a lateral distance from the plume. Such

distance depends on plume dimension and lens focal len"th. #n the last three &ears% #

used a t,o coupled +B camera s&stem (see section .1.1) in different conditions

(from small "as sources such as a fumarolic field% to lar"e "as plumes issuin" from

open ents)% and operatin" at distances of 4.= : > $m a,a& from the "as. Fi"ure 84

=4

Figure 17: To orst case scenarios for remote -#measurements: e*plosions ith ash andclouds or fog.


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sho,s the dependence of MF on distance at =44 nm and =8@ nm and fora S*8

concentration of ;.??R;4C=ppm (correspondin" to 84 ppmRm alon" a strai"ht path) and

no aerosols present. s distance to the plume increases% dilution (see section 2.)

pro"ressiel& "ro,s and the MFs drop to alues considerabl& belo, ;. This

su""ests that% be&ond = $m and under the plume conditions listed aboe% ,e measured

less than


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the plume (here supposed to be the ,orst case% much hi"her than the maimum

ambient considered) clearl& stands out! this ma& lead to measure >4I of the

real S*8slant column densit& at =44 nm in a surroundin" ambient aerosolCfree (

^ 4). The hi"her the ambient % the more radiation is scattered b& aerosols into the

instrumentOs field of ie, bet,een the plume and the instrument% thus dilutin" the

absorption si"nal (up to 84I of the real S*8slant column densit& at =44 nm).

Such results sho, that +B camera measurements ma& potentiall& under"o

considerable underestimation of the S*8slant column densit& (up to 84 I) due to the

cumulatie effect of lateral distance and aerosol (li"ht dilution). ccordin"l&% nearC

ent "as measurements ( 8 $m) of an aerosolCfree plume (no ,ater condensation

must occur) ma& reduce the error to =4 I . +nfortunatel&% olcano summits often

under"o condensation of ,aterCrich air masses% and ma& easil& be coered b& clouds%,hich preent measurements. #n addition% a nearCent plume is 'uite often too

condensed and "asCaerosol concentrated to be measured.

=8

Figure #1: ir !ass Factor at different aerosol e*tinction coefficients (3) in the planet

boundary layer (A62) at '00 nm and '#$ nm.


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. T-e U# camera/ A ne, -ard,are set!u" and sot,are

4 The '1 camera: ! new hardware set-up and software

hapter >

The +B camera!

ne, hard,are setCup and soft,are

4#$ '1 C!.R!

.1.1 Set!u" and retrieval

0ecent +B camera applications (Mori and /urton% 844?J Dalton et al.% 8449J

1olland et al.% 84;;J Nadeau et al.% 84;;) hae used a sin"le passCband filtered deice

in order to oercome the dela&s due to filter s,itchin"% thus achiein" a hi"her

samplin" fre'uenc& (; 1z). This techni'ue is% ho,eer% potentiall& subect to an

oerestimation of the S*8column amount% due to combined li"ht absorption b& both

"as and aerosols.

==


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1ere% # propose an alternatie to preious +B camera settin"s% consistin" of

t,o +B cameras operatin" simultaneousl& from the same position. T,o po"ee

#nstruments lta +8?4 cameras ,ere mounted on a steel rail atop a tripod% each

coupled to a Penta /8@8

Figure ##: ulphur dio*ide absorption cross section (&andaele et al., 177+) and trasmittance of the

to filters E6A'10 and E6A''0.


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The retrieal 'uation = allo,s the calculation of absorbance A per piel

ta$in" an ima"e of S*8Cfree s$& as the reference. The +B camera should be oriented

to at least 94 to aoid issues associated ,ith stron"l& inhomo"eneous illumination

across the field of ie,J for flu computation% clear s$& should be contained in the

ima"e either side of the plume. To oercome the inherent dela& of ta$in" a reference

ima"e% 'uation = has been rearran"ed and simplified into the components!

(>) A=lo";4)P)Plo";4)@)@

simpl& ta$in" the lo"arithm of the ratio of the t,o i"netteCcorrected (see section

>.;.8)% dar$Csubtracted filter ima"es of the plume. ssumin" that the second term is

constant across the ima"e% ,e can recast it as!

(@) A=lo";4)P)P6

,here the alue 6 is a scalar subtracted to the resultin" ima"e to set S*8

concentrations e'ual to zero in the absence of olcanic "as (clear s$&). t least 5@

piels of clear s$& should ideall& be present in both sides of an& ro,Ecolumn of

interest for accurate computation of concentrations therein% and i"nette correction is

re'uired.

=@

Figure #': The complete %& camera system adopted in this or4.


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The use of t,o distinct +B cameras raises the 'uestion of ,hether to adoptin"

different inte"ration times for each filter% "ien that the bac$"round s$&li"ht

illumination is far more intense at ==4 nm than at =;4 nm. 1ere% ,e used the same

inte"ration time for both cameras. This ,as chosen to almost saturate the ==4 nm

filtered camera (?4444 oer a maimum of ?@@=?) to maimise the si"nalCtoCnoise

ratioJ thus the measured li"ht intensit& in the =;4 nm ,as "enerall&


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time (K ; s)% and resultin" in increased noise. -e therefore recommend the use of

M"F8,indo,s% ,hich hae @ I reflectance

s mentioned aboe% man& recent ,or$s report S*8flu data collected ,ith a

sin"le +B camera% drien b& the need to achiee hi"h temporal resolution ,ith a

sin"le deice. This ma$es it difficult to discriminate bet,een aerosol and "as

absorbances. The t,o filtered +B camera techni'ue% instead% assumes that the aerosol

contribution to the attenuation of li"ht at =;4 nm and ==4 nm is the same% and its

effects is therefore remoed b& calculatin" the ratio% as aboe.

The effect of the use of either one or t,o filters is discussed in Fi"ure 8@. The

fi"ure reports results of a plume cross section dra,n from an S*8ima"e collected on

Stromboli on 84 Ma& 84;;. -e hae calculated a concentration profile usin" the

retrieal e'uation for one (D purple line in Fi". 8@) and t,o ( "reen line in Fi". 8@)

filtered cameras from '. = and @. The "as emitted from the central craters ,as

aerosolCrich relatie to the "as emitted from the nearb& hornito% possibl& resultin" in a

S*8flu oerestimation of the former usin" a sin"le filtered camera (D purple line in

=5

Figure #$: Alume cross@section of to central craters and an hornito close to each other ta4en

on tromboli on #0 !ay #011.The lines and 6 represent the light intensity profiles reported onthe '10 nm and the ''0 nm filtered cameras respectively. It is clear an attenuation of light

intensity in due to -#and aerosolD the 6 profile highlights that the aerosol effect is stronglypresent in the entral plume ($0 G of the total absorption) and is absent in the hornito plume

(probably resulting from lo concentrations in general). The lines and represent instead thecalculated column amount profile ith the to filters and one filter methods respectively. It is

evident an overestimation of the detected -#in the entral plume (relative to the Bornito one)in hich the aerosol effect has been uantified ith the ''0 nm filtered camera.


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Fi". 8@). The aerosol contribution to the total absorbance is sometimes ne"li"ible % as

in the case for "as emitted from the hornito% in ,hich the one and t,o filtered camera

methods "ie the same result.

.1.2 #ignetting correction

#n this stud&% efforts hae been made to correct for the effect of i"nettin" on

the +B camera ima"es. #n photo"raph&% i"nettin" is an optical effect causin" the

corners of an ima"e to be dar$er than its centre. There are seeral causes for

i"nettin"!

;. Mechanical i"nettin" results from the outer rim of the filter placed in

front of the lenses% ,hich partiall& bloc$ the li"ht beams% and usuall&

produce a er& steep bri"htness attenuationJ

8. *ptical i"nettin" is an intrinsic feature of multiple element lenses and it

can be completel& remoed b& settin" a small iris aperture. This is

because% b& usin" a smaller aperture% the different paths li"ht passin"

throu"h the diaphra"m and reachin" each piel on the D are "ettin"

narro,er :and e'uall& distributed in the ,hole D areaJ ,hile usin" a

bi" aperture the incident li"ht on the central re"ions of the D is lar"er

than on the peripher& (proportional to the different paths).

=. Natural i"nettin" (""ar,al et al.% 844;) is a problem of ener"& loss of

the li"ht beam passin" throu"h the lens ,ith a nonCperpendicular

traector&. The falloffcaused b& the natural i"nettin" can be modelled

as!

(?) 5v=cos>

,here \ is the an"le bet,een the normal to the lens ais and the

direction of the li"ht beam.

=


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>. Piel i"nettin" is caused b& the an"ularCdependence of the D sensors%

,hich produces a stron"er si"nal from incident photons ,ith a

perpendicular traector&.

n eample of i"nettin" is sho,n in Fi". 8?% correspondin" to ima"in" of a

portion of clear s$& ,ith a =;4 and ==4 nm passCband filtered +B camera. #t should be

noted that neither is the i"nettin" function s&mmetrical across the ima"e (Fi". 8?c) C

as one mi"ht epect C nor do the t,o lenses hae the same i"nettin" function.

-hilst i"nettin" can be ascertained from ima"es containin" the olcanic

plume% it is better to aoid this perturbin" influence upon some of the piels% in order

to hae the lar"est possible number of data points for the anal&sis. The simplest ,a&

to do this is to point the camera a,a& from the sun at a cloudless s$&% at >@ zenith

an"le at around ;4!44 or ;?!44 ,hen the sun is neither at zenith nor the horizon% so

the ima"ed s$& is uniforml& illuminated (Fi". 85).

=9

Figure #8: &ignetting effect in '10 nm (a) and ''0 nm (b) filtered %&

cameras and the relative intensity profiles at hori;ontal half@height.


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#ma"es should be collected accordin" to the approimate li"ht intensit& alues

of the plume ac'uisitions themseles% "ien the sli"ht nonClinearit& in the camera

response e.".% this ,ould normall& correspond to near saturatin" the ==4 nm filter% and

around ;4I of the maimum counts for the =;4 nm case (usin" the same eposure

time). *nce one ima"e is collected per filter% this is normalised ,ith respect to the

maimum piel intensit& to create a mas$% ,hich all subse'uentl& ac'uired ima"es are

diided b&% to correct for i"nettin". 1ere ,e su""est to collect as man& i"nettin"

mas$s as possible ,ith different inte"ration times durin" the first ac'uisitions%

especiall& if li"ht conditions ar&% in order to derie a sort of librar& to be used to

correct for i"nettin" in successie ac'uisitions.

.1.3 Cali%ration

+B cameras measure the absorbance in the presence of S*8molecules in the

optical path of each piel. onersion from (dimensionless) to S*8column densit&

(ppmRm) is "enerall& achieed usin" 'uartz S*8cells of $no,n concentration.

possible calibration strate"& is illustrated in Fi". 84

Figure #5: vignetting correction mas4 has been derived from a clear@s4yimage and applied to (a) to get (b).


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enou"h clear s$& as bac$"round to ima"e the cells and the olcanic plume

simultaneousl&.

#n this stud&% ,e propose an alternatie method for +B camera calibration%

,hich re'uires placin" the cells one b& one immediatel& in front the lenses in order to

calculate the absorbance as!

(5) A=lo";4

[)C)@

)C

)@],here # is the li"ht intensit& due to S* 8cell absorption% and #/ is li"ht intensit& of

the clear s$& (bac$"round). This method allo,s the calculation of the absorbance in a

lar"er number of piels (in our case% an aera"e ta$en oer the central 844844 piel

section)% and i"nettin" does not need to be corrected because the ratio minimizes the

dar$enin" of the ed"es. For consistenc&% ,e used cells of e'ual diameter (8@ mmJ0esonance 3td.) of the filters% placed immediatel& in front of the filters in the inC

>;

Figure #


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houseCbuilt mounts. The S*8concentration for each cell is then plotted a"ainst %

measured as aboe% and the "radient of the bestCfit re"ression line is ta$en as the

calibration ratio (Fi". 89). The latter is then multiplied b& subse'uentl& ac'uired

plume ima"e piel alues to conert to ppmRm.

.1. Pi'el dimension

S*8 flu measurements performed b& traersin" underneath the plume ta$e

adanta"e of a GPS unit for "eoCreferencin"% ,hich allo,s one to calculate the

traelled distance for each spectral ac'uisition and processin"% and finall& inte"rated

column amounts alon" the route. The +B camera ,or$s instead from a fied position%

and # is deried b& inte"ratin" S*8column densit& alon" a line of piels (a section

throu"h an S*8concentration ima"e). The dimension"% in meters% of a piel that is

loo$in" an obect (the plume) placed at distance dfrom the camera can be calculated

tri"onometricall& as follo,s!

(


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,here EO#is the field of ie, of the lens in de"rees. 1o,eer%" also depends on the

slope of the section in the ima"e as illustrated in Fi". =4% ,here the number of piels

that connects points / and D ("ra& piels in Fi". =4)% multiplied b& the dimension

"does not correspond to the real len"th of the t,o lines. correction factor can be

calculated as!

(9)

5=nv

8n-8

n0A7

"r="5

,here"ris the real piel dimension% nvand n-are the number of piels of the ertical

and horizontal component of the section% respectiel&J and nma'is the number of piel

of the bi""est component. The correction factor ran"es from ; to ;.>;@.

.1.5 Plume s"eed calculation

+B spectroscopic scannin" s&stems t&picall& calculate "as flu b& multipl&in"

the inte"rated amount of S*8distributed alon" a section% the soCcalled inte"rated

>=

Figure '0: ifference beteen the number of pi*elsconnecting to points and the real distance beteen them.

-n the right, to cases, 6 and , and their respectivecorrection factors.


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column amount (#)% b& an aera"e plume speed. The calculation must thus rel&

upon accurate $no,led"e of plume elocit&. s a matter of fact% a common

assumption made is that the olcanic plume moes at the same speed as the ,ind. The

latter is often measured ,ith anemometers on the crater rim or directl& on the

measurement site. 0ecentl&% ,ind speed has also been calculated b& means of

mathematical and meteorolo"ical models (/urton et al.% 8449). en so% anemometerC

deried plume speed data can be affected b& uncertainties of ;4C>4 I (Stoiber et al.%

;9)% or multiple spectrometer

methods (-illiams et al.% 844?J McGoni"le et al.% 8449J /oichu et al.% 84;4).

Generall&% these methods consist in calculatin" a time la" ` la"% ,hich corresponds to

the time for a cloud of olcanic "as to trael from a start to an end point% separated b&

a distance 2. `la",ill be dependent on the distance 2% and the elocit& of the

plume p!

(;4) lag=7

v"

The +B camera represents an important step for,ard in the ealuation of the

plume speed inasmuch as its spatial resolution% in tandem ,ith its hi"h temporal

resolution (up to ; 1z)% ma$e it possible to capture numerous crossCsection of the

plume at a "ien time (Fi". =;a)% and to calculate the speed of rapid emissions and

bursts (e.". Strombolian eplosions% "as puffin"). 1ere% ,e propose three different

methods for calculatin" the speed of a olcanic "as plume!

;. # timeCseries crossCcorrelationJ

8. #nCplume crossCcorrelationJ

=. aluation frame b& frameJ

>>


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The # timeCseries crossCcorrelation (McGoni"le et al.% 8449J /oichu et al.%

84;4) consists in calculatin" the ariations oer time of inte"rated column amounts

related to t,o parallel plume crossCsections% separated b& a distance 2. Thus% the t,o

timeCseries% ,hich are similar to each other% ,ill ehibit a dela& `la" as mentioned

aboe. #n order to calculate the plume speed% the timeCla" can be calculated ,ith

crossCcorrelation methods.

rossCcorrelation is the mathematical method of estimatin" the similarit& of

t,o si"nals as a function of a timeCla" applied to one of them. #t is defined b&

continuous functions hain" the form!

(;;) dt=

gtd

>@

Figure '1: (a) -# column amount image of the 3 crater (3tna) and to cross@sectionsperpendicular to the plume and parallel to each other (light and dar4 grey lines). (b) The columnamount profiles and the calculated I time series (c) relative to the to previous cross sections. (d)

cross correlation function resulting from the to I time series, the calculated time@lag is 5.7 s.


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that is simpl& a moin" dot product alon" time. :>g


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n alternatie method ,e used consists in the direct estimation of the "as

displacement throu"h the anal&sis of a se'uence of absorbance profiles parallel to the

plume transport direction% as sho,n in Fi". =8(a% b). -e hae tested this method in

the fumarolic s&stem of Bulcano (eolian islands% #tal&)% ,here the lo, flues of

indiidual fumaroles% and their relatiel& rapid dispersion% did not allo, the

calculation of t,o separated # timeCseries ,ith shared inhomo"eneities% as sho,n

in the tnean eample aboe. #n Fi". =8% ,e sho, a ertical risin" plume case in

,hich ,e can identif& some features in the first absorbance profile (li"ht "ra& line in

Fi". =8b)% ,hich is presered in the consecutie frame (blac$ line in Fi". =8b) ta$en

after ;.< s. This condition allo,s the calculation of 2 b& means of crossCcorrelation.

These crossCcorrelation techni'ues are no lon"er effectie ,hen plume speed

chan"es rapidl& oer a ,ide ran"e (from K84 to @ mRs C;)% as in the case of "as bursts

>5

Figure '': (a) Time variations of a normali;ed -#column amount profiles (multi@colour lines).

The grey line shos the different positions of the e*plosion front . (b) The frame@to@frame

calculated e*plosion front speed. (c) -# column amount image and the location of the cross@section.


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and Strombolian eplosions% in ,hich een a samplin" fre'uenc& of ;.8 1z is

insufficient. onse'uentl&% in such cases% "as speed ariations hae been calculated

manuall& loo$in" the moements of the eplosion front oer inCplume sections (Fi".

==)

.1.( SO mass measurement

NonCpassie de"assin" is a recurrent mechanism of ener"& dissipation at openC

ent olcanoes. #t consists of the emission of oerCpressurized "as poc$ets% in either

bursts or puffs (1arris and 0ipepe% 8445). This rapid emission creates a cloud of

concentrated "as that is clearl& detached from the ent (Fi". =>)% and rises faster then

the surroundin" "as. fter a fe, seconds or minutes% dependin" on the concentration

of the "as% this "as poc$et min"les ,ith the surroundin" plume(s)% and disperses in the

atmosphere.

The ealuation of the "as content in eplosie bursts or "usis challen"in"% in

ie, of the short timescale of the process. #n fact% the standard scannin" spectroscopic

methods hae time resolution inade'uate to resole these phenomena. 0ecentl&% the

deelopment of c&lindrical lenses D*S and +B cameras has opened the ,a& to

measurement of S*8flu at hi"hCfre'uenc& (up to ; 1z). The resultin" timeCseries can

>9

Figure '$: -#flu* increased after a trombolian e*plosion. The shaded area corresponds to thetotal -#mass emitted during the event (=a;ahaya et al., #011).


53/148


The ariations of the "as mass calculated inside a fied area durin" passie

de"assin" are ne"li"ible if compared to the ariations of the "as mass related to an

eplosion. To calculate the "as mass eplosiel& released durin" a "ien burst% the

procedure thus re'uires identification of a bac$"round "as mass. This represents a

baseline S*8mass that has to subtracted from the total measured "as mass (Fi". =?) to

obtain the eplosie contribution. Notabl&% this techni'ue does not re'uire $no,led"e

of plume speed% thus oercomin" the difficulties encountered in preious studies.

4#) 1'+C!.R!: !'S.R-*RI.%&+/7RO2R!*ORSO I!2I%2

The po"ee lta +B camera is a +S/ deice% "enerall& controlled ,ith a

commercial pro"ram called Ma#M D3 (Diffraction 3td% http!EE,,,.c&ano"en.com).

This code has been built for laborator& use% and has numerous limitations for field use.

@4

Figure '8: -#column amount image seuence during a trombolian e*plosion and the relative

variation of the -#mass, obtained from the integration inside the dotted suare. The final - #mass value is derived by subtracting from the ma*imum value of the -#amount profile (+

thframe

in the seuence) the corresponding interpolated bac4ground (orange dashed line).


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#n this stud&% # hae deeloped #ulcamera (Fi". =5)% a standCalone% userC

friendl& code for measurin" olcanic S*8flues ,ith +B cameras. The code consists

of t,o elements! #ulcameraFa and #ulcameraF"ost% ,hich mana"e ima"e

ac'uisition and postCprocessin"% respectiel&. Bulcamera is freel& do,nloadable from

https!EEsites."oo"le.comEsiteE"iancarlotamburelloE% ,here detailed instructions are also

"ien. #ulcamera,ill ,or$ ,ith po"ee #nstruments +8?4 and ? unitsJ ho,eer%

,e recommend use of the +8?4% "ien its hi"her si"nalCtoCnoise ratio and faster data

transfer. #ulcamera is desi"ned to operate ,ith t,o cameras% simultaneousl&% ,ith

bandpass filters centred on =;4 nm and ==4 nm. #t is imperatie to use t,o filters in

these obserations% to compensate for aerosol attenuationEbac$scatterin"% and to

minimize temporal mismatches associated ,ith filter chan"es on a sin"le camera

(Aantzas et al. 84;4).

#n particular% the functions of the code include! characterization of i"nettin"

ia the collection of clear s$& ima"es to compensate for an"ular dependenc& of piel

illuminationJ determination of calibration relationships bet,een absorbance and S* 8

cell concentrations% to enable conersion of the measured field ima"es into ppmRm

concentration ima"esJ use of simultaneousl& ac'uired spectroscopic S*8flu data to

calibrate the ima"esJ and% finall&% feedin" bac$ of all of these operations to the main

pa"e of #ulcameraF"ost% ,hich leads to the calculation of S*8flu time series andEor

"as masses associated ,ith eplosions. #ulcamerahas been etensiel& field tested at

southern #talian olcanoes% and it is hoped that others ,ill find it useful to contribute

to realizin" the si"nificant olcanolo"ical potential of +B camera technolo"&.

@;


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@8

Figure '5: screenshot of &ulcameraJs post processing main menu. K"raph L and K"raph 6L blue@scale images sho the '10 n m and ''0 nm filtered %& camera images already vignetting corrected.The pseudo@colour graph onthe right shos the -#column amount image calculated ith the euation $. The Kections graphL plot shos the light intensity values of the '10 nm and ''0 nm camera of a cross@section connecting the to yello cursors (2

and 9) in K"raph L. K ection ppm m>bsorbanceL graph shos the calculated - #column amount profile.


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4#( &O!S-'1 C!.R!CO7!RISO%

s a means of further 'uantitatie ealuation of the camera performance% ,e

eecuted an adChoc desi"ned field sure& on tna olcano% ,here ,e compared our

camera obserations ,ith those from a scannin" spectrometer s&stem% of ;.


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locations ; $m and ;8 $m from the "as. #n all cases the plume ,as ie,ed

approimatel& ortho"onall& to its propa"ation ector. Some thirt& such profile interC

comparisons ,ere performed% demonstratin" broad a"reement (t&picall& ,ithin ;@I)

in all cases% such as in the sample plots sho,n in Fi". =)% spatiall& filtered to match the ;8 ertical ,idth plume. Pairs of the

@>

Figure '7: Time series of average concentrations across the plume profile for

scanning spectroscopic observations (suare points), versus -# columns from arectangular field of vie spectrometer (higher time resolution grey data series) set to

match the plume idth. The observed agreement provides additional confidence inthe latter approach as an alternate to the %& cameras for acuisition of high time

resolution -# flu*es, albeit demanding more comple* alignment (=ant;as et al.,#010).


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latter hae been applied to "enerate inte"rated column amounts% ,ith multiplication of

the outputted concentrations% ta$en as proies for the mean column amount across the

field of ie,% b& the field of ie, len"th at the plume distance. Such data hae been

used to "enerate hi"h time resolution S*8 flues% b& analo"& to the camera

methodolo"& mentioned insection8.=.=(McGoni"le et al.% 8449J /oichu et al.% 84;4).

#n our eperiments% "ood correspondence ,as found bet,een the mean scanner

concentrations and those from the rectan"ular field of ie, instrument% e.".% as sho,n

in Fi". =9.

4#4 '1 C!.R!C!+I"R!TIO%'SI%2&O!S

#n section >.;.=% ,e hae discussed t,o methodolo"ies for +B camera

calibration usin" 'uartz S*8cells of $no,n concentration. +nless there is enou"h

clear s$& as bac$"round to be able to ima"e both the plume and a ,hole set of

calibration cells (Fi". 8


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1ere% ,e illustrate the use of an alternatie calibration techni'ue% based on the

simultaneous use of +B camera and D*S (first reported b& S. #llin" et al.% Bolcanic

plume obserations ,ith a S*8camera at Popocatepetl and olima% MeicoJ poster

presented at #B# ;;th "as ,or$shop% Aamchat$a% 0ussia% 84;;). The method

ma$es use of a telescope pointin" at the "as plume% in order to obtain S* 8 column

amount time series (Fi". >4). The telescope should be located bet,een the t,o +B

camera to ensure identical ali"nment (usuall& located on the centre of the ima"e). The

ac'uired spectral data from D*S are later processed usin" the +Bolc code (Aantzas

et al.% 84;;)% to obtain a series of spectra si() each associated ,ith a specific S*8

column amount ci(in ppmRm) (calculated as outlined b& Aantzas et al.% 84;;). Then%

the procedure inoles multipl&in" the t,o +B cameraHs filter functions =;4() and

==4() b& each spectrum function (Fi". >;a) to "et t,o scaled filter functions G=;4%i()

andG==4%i() (Fi". >;b). Finall&% one can calculate the absorbanceAas!

(;=)Ai=lo";4 5 G=;4%i d 5 G==4%i d

The deriedS*8cell column amounts ciare plotted a"ainst absorbanceAi as

sho,n in Fi"ure >;% and the "radient of the bestCfit re"ression line is ta$en as the

calibration ratio. The eample sho,n in Fi"ure >;c% supports fair a"reement bet,een

D*SCderied and 'uartz S*8cellCderied calibration coefficients.

This methodolo"& opens the possibilities of continuous +B camera ac'uisition

durin" da&li"ht . Nonetheless% some linearit& problems in D*S calibration hae

been reported durin" our measurements% ,hich ,ill re'uire further attention.

@?


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.

@5

Figure +1: (a) Transmittance of the to filters adopted in our %& camera system and a %&spectrum obtained ith a - system in presence of -#. (b) The result of the product

beteen the filter functions and the - spectrum, the negative logarithm of the ratiobeteen the integrals of the to functions represents an absorbance. (c) Alot and fit line

beteen the calculated absorbances and the -#column amount ith the - system(blue diamonds) compared ith to the calculated uart; - #cells, calibration line of the

%& camera system.


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5. A""lications to active volcanoes

8 !pplications to active volcanoes

hapter @

pplications to actie olcanoes

8#$ 1'+C!%O< STRO"O+I!%&.T%!

# report belo, the results of application of the dual +B camera to the southern

#talian olcanoes in arious olcanolo"ical contets! Bulcano and Stromboli (aeolian

islands) and tna. This allo,ed us to test the +B cameras potential and to eplore innoel ,a&s the de"assin" mechanisms and d&namics of these olcanoes.

@


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8#) .!S'R..%TSO**'!RO+.*I.+&&.2!SSI%2=+!*OSS!CR!T.R< 1'+C!%O$

5.2.1 )ntroduction

The rate of S*8 release from actie olcanoes (S*8 flu) is an important

parameter for olcano monitorin"% actin" as a pro& for under"round ma"ma ascent

rate% thus stron"l& correlatin" ,ith eruptie actiit& (altabiano et al.% ;99>). Since the

;954s% *SP (altabiano et al.% 844>) and more recentl& D*S (McGoni"le%

8445) hae been the most ,idel& used techni'ues for "roundCbased S*8 flu

monitorin". These are applied b& scannin" across the plume cross section%

(McGoni"le et al.% 844=)% or traersin" beneath the plume ,ith a ehicle% boat%

aircraft% or an unmanned aerial ehicle (McGoni"le et al.% 844

@9


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1ere ,e au"ment the increasin" usa"e of the +B camera to derie bul$ plume

S*8flu data% ,ith% to the best of our $no,led"e% the first application of the camera to

inesti"ate the comple structure of fumarolic s&stems.

#n this stud&% ,e follo, the protocol for +B camera measurement described in

c-a"ter > to ima"e the structure of the fumarolic field of 3a Fossa crater (Bulcano

island% eolian islands)% a small (=


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hae focused on t,o aspects of this! the dilution effect and multiple scatterin" in the

plume. The former is eacerbated at lar"e plume:instrument distances and the latter

b& eleated plume aerosol concentrations. #n our stud&% +B camera measurements

,ere made onl& =44 m from the "as and the thin plume ,as hi"hl& transparent%

hence ,e consider errors here to be minimal. s the plume transport speed is dened

accuratel&% usin" cross correlation% ,e estimate each flu to suffer errors of onl&

g;@I% therefore. The calibration ,as performed immediatel& prior to each ac'uisition

ie,in" S*8Cfree s$& at >@ zenith an"le adacent and a,a& from the fumarolic eld%

from the measurement location. #n both campai"ns% the calibration t line 08 ,as

4.999.

The applied in house built MultiCGS (iuppa et al.% 844@) unit combined an

infrared spectrometer for *8 determination (Gascard ##% calibration ran"e 4 C >444

ppmJ accurac& g8IJ resolution% 4.< ppm)% electrochemical sensors for S*8 (it&

Technolo"&% sensor t&pe =STEF% calibration ran"e% 4 C 844 ppm% accurac&% g8I%

resolution% 4.@ ppm) and 18S (Sensori% sensor t&pe 8% calibration ran"e% 4 C @4

ppm% accurac&% g@I% resolution% 4.5 ppm)% and temperature and relatie humidit&

sensors (Galltec% measurin" ran"e% 4 : ;44 I 0h% accurac&% g8I). The readin"s from

the latter t,o ,ere used to calculate ,ater concentration (in ppm)% assumin" a

constant standard pressure% ,ith the follo,in" e'uation!

(;>) +8* [""mv]=4.?;=@?e ;5.@48T8>4.95T-;44

Durin" the measurements% the fumarolic "as ,as continuousl& pumped into the

sensors at a flo, rate of ; lpm throu"h a PTF tube. dataClo""er board captured

si"nals from the sensors eer& t,o seconds % ,hile a handCheld GPS proided "eoC

referencin" of each datum. The MultiCGS ,as po,ered b& a ;8 B% 5 h lead batter&%

and housed inside a ,aterproof bo (=4 X 84 X ;@ cm).

?;


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MultiCGS measurements ,ere performed traersin" b& foot throu"h the

fumarolic field (Fi". >8) ,ith the inlet tube =4 C @4 cm from the "round. +sin" this

methodolo"&% ,e deried the 18SES*8% *8ES*8and 18*ES*8 molar ratios for each

fumarole from the "radient of the bestCfit re"ression lines in scatter plots . For more

details on this techni'ue see iuppa et al. (844@).

5.2.3 esults

@.8.=.; +B M0MS+0MNTS

The fumarolic field of 3a Fossa crater is 4.4>@ $m8 ,ide. Therefore% the best

,a& to ima"e it entirel& ia the +B camera% ,hile distin"uishin" indiidual fumaroles

?8

Figure +#: 4etch map of 2a Fossa crater and its fumarolic field. The blac4 dot represents thesite of %& camera observations. The dashed gray line mar4s the al4ing path through the

fumarolic field ta4en during the !ulti@" survey.


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and usin" s$& as bac$"round% is b& obserin" from the southern inner craterOs rid"e%

=44 m a,a& from the main ehalin" area (Fi"s. >8and >=).

Durin" the t,o field campai"ns% ,e se'uentiall& collected ima"es of the four

main fumaroles (F*% F% F@T% F;;J Fi"s. >8%>=) from the measurement position b&

pro"ressiel& rotatin" east,ard the +B camera. #n both da&s% ima"in" of the entire

fumarolic s&stem (from F4 to F;; in a - to cross section) ,as completed in 84

minutes% and each fumarole ,as obsered for periods of ; to ? minutes ta$in" an

ima"e eer& 8s.

The ,ind speed (measured ,ith a handCheld anemometer) ran"ed from 4 to

4.= mRsC;on Noember% and from =.@ to ? mRsC;on Februar&. The stron"er ,ind on

Februar& partiall& hampered resolin" bet,een the different fumaroles (especiall& F4

and F% Fi". >=a)% ,hose atmospheric dispersions ,ere some,hat oerlappin". *f the

84? couples of ima"es of the F4 Q F area ta$en on Februar&% onl& ;4 ,ere affected

b& de"assin" of the F4 fumarole% ,hile a mied F4 Q F plume ,as captured in the

remainin" cases. #n Noember% ,hen the ,ind ,as instead blo,in" more "entl&% the

?=

Figure +': (a) %& -#slant column density (ppmHm) image and (b) visible image of the fumarolicfield.


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"as ,as risin" erticall& (Fi". >=% >>a)% ma$in" the fumaroles more easil&

distin"uishable.

Durin" both sure&s% ,e calculated S*8 #s b& inte"ratin" piel

concentrations alon" representatie lines ,ithin the +B ima"e perpendicularl&

oriented to the plume transport direction% and a fe, meters from the fumarole in

'uestion in order to minimize interference from adacent sources (Fi". >>). The

summation ,as performed horizontall& for the Noember data% ,hen ,ind speeds

,ere lo, (4 to 4.= mRsC;) so the plumes rose erticall&J and erticall& for the Februar&

data% ,hen the ,ind ble, from the southCeast and rather more stron"l& (=.@ to ? mRs C;)

so that the "ases propa"ated 'uasiChorizontall&.

?>

Figure ++: (a) %& image ('10 nm filtered) of the F0NF area in ovember (upper image) and

February (bottom image). The '10 nm filtered image (in hich - #absorption occurs) as usedfor cross@correlation operation (and thus plume speed derivation), because it displays a larger

pi*el@to@pi*el intensity variation than the ratio image. The yello line shos the section of the

image along hich the cross@correlation is performed. (b) The plots sho (for ovember andFebruary survey from top to bottom) pi*el@to@pi*el intensity variations along the yello sectionsta4en in to consecutive '10 nm filtered images. From the pi*el shift (E) beteen the gray and

blac4 line (this ta4en along the same yello section but after an interval T of 1.


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n eample # record for the Noember sure& is "ien in Fi". >@%sho,in"

t&pical smooth fluctuations in "as emission oer timescales of ;4C84s. Durin" the

Februar& sure&% more irre"ular temporal # trends ,ere obsered! as the ,ind

speed increased% the "as plume eentuall& "rounded or ,ent out of the field of ie,%

precludin" an& retrieal (no # data ,ere calculated in such hi"h ,ind conditions).

Plume transport speed ,as calculated b& the inCplume crossCcorrelation method (Fi".

>>b% >>c)J this minimizes the error due to the plume speed uncertaint&% especiall& if

,e derie the shift usin" multiple sections (the &ello, line in Fi". >>a) in an ima"e.

The soCcalculated plume speed ran"ed from 8 mRsC; (Noember) to ? mRsC;

(Februar&). Finall&% the S*8flu ,as calculated combinin" # and plume speed% as

sho,n in Fi"ure >@! this allo,s deriation% for each main ent of the fumarolic s&stem

and ,ith hi"h timeCresolution% a record of subtle S*8flu ariations.

?@

Figure +$: n e*ample I record for the ovember survey (F0 fumarole). The -#flu* (in tHd@1)

as obtained by multiplying the plume speed (mHs@1) by the Integrated olumn mount (4gHm@1).


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S*8 flu results are summarized in the bo plot of Fi"ure >?. This clearl&

hi"hli"hts that% in both sure&s% the F fumarole ,as the main de"assin" area%

accountin" for @4I of the bul$ S*8de"assin". The total S*8flu from the olcano%

calculated b& summin" contributions from the > ehalin" areas aried from 8; tRdC;

in Noember (dotted bo) to ;8 tRdC; in Februar& (filled bo)% a factor 8 lar"er in

the former case% durin" the de"assin"Eheatin" eent% than in the latter% ,hen the

fumaroles ,ere coolin" do,n (#stituto Nazionale di Geofisica e Bulcanolo"ia% Sezione

di Palermo% unpublished data% 8449). These results are 'ualitatiel& similar to those

presented b& iuppa et al. (844?a)% ,ho% ,hilst usin" a er& different S*8 flu

??

Figure +8: 6o*@plot shoing -#, B#, -#and B#- flu*es from individualfumaroles and for the entire field in ovember #007 (dotted bo*) and

February #010 (solid bo*). n overall decrease in gas flu*es beteen the tosurvey is evident.


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retrieal techni'ue (,al$in" traerses ,ith a zenithCpointed +B spectrometer)%

deried S*8flues of == tRdC;and =@ tRdC;durin" the t,o unrests in the 844> :844?

period% and factors of 8C= lo,er flues (from 8 to ;< tRdC;

) for the periods in bet,een.

@.8.=.8 M+3T#CGS MS+0MNTS

Durin" each campai"n% ,e also made MultiGS traerses to derie the spatial

distribution of plume 18SES*8% *8ES*8and 18*ES*8 molar ratios oer the fumarolic

field (Fi". >5). #n a"reement ,ith iuppa et al. (844@)% ,e obsered contrastin"

compositions for the four representatie fumarolic areas (F4% F@T% F;;% F). Fi"ure

>5sho,s such results% illustratin" that the 18*ES*8and 18SES*8molar ratios aried

mostl& ,idel&% ran"in" from =4 to 884% and from 4.? to =.>% respectiel& across the

field. The F fumarole% ,hich is the hottest ent and t&picall& displa&s the most

ma"matic chemical si"nature (Nuccio and Paonita% 844;)% ,as characterized b& lo,er

18SES*8% and hi"her *8ES*8and 18*ES*8 molar ratios% than the rim fumaroles (e.".%

F4)% ,here h&drothermal "as contribution becomes more important% so 1 8S preailsoer S*8 (as is t&pical of h&drothermall&Cbuffered "ases). s preiousl& noted

?5

Figure +5: &ariation of B#>-#, -#>-# and B#->-# molar ratios along the !ulti@gas

acuisition path (data for the February survey are shon in this e*ample). E>- #molar ratios

ere derived from best fit regression lines in E vs. - #scatter plots (9#

values ere O 0.8). Thecentral data gap is due to the absence of fumaroles beteen the F11 and the F.


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(iuppa et al.% 844@a)% the F;; fumarole has 1 8SES*8and *8ES*8ratios intermediate

bet,een F4 and F.

5.2. Discussion and conclusions

Since ;9


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Fi"ure >? sho,s that the maor components 18* and *8 are mainl&

contributed b& the F fumarole% and b& the F@T fumarole to a lesser etent. #n

contrast% the F area onl& mar"inall& contributes to the olcanoOs 18S bud"et% ,hich is

instead dominated b& the rim fumaroles (F;; and F@T in particular).

*ur results also clearl& sho, that there ,as a factor 8 increase in *8

de"assin" durin" la Fossa crater de"assin"Eheatin" unrest (the deried *8 flues

,ere ?


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8#( STRO"O+I: I%SI25TSO%7!SSI1.SO 2!S.ISSIO%< STRO"O+I!%.R'7TIO%S< !%&

7'**I%2)

5.3.1 )ntroduction

ctie olcanoes dissipate their ma"maCtransported ener"& in a ariet& of

,a&s% amon" ,hich olcanic de"assin" is often dominant (*ppenheimer% 844=).

Passivede"assin" is fre'uentl& the onl& isible surface epression of the actiit& of

'uiescent olcanoes% and also ma$es up a lar"e fraction of the dail& massEener"&

release from persistent basaltic olcanoes (Shinohara% 844)% and ,hilst this is thou"ht to represent

onl& a small part of the total de"assin" bud"et of a olcano (llard et al.% ;99>)% it

carries important information on eplosion tri""erin" mechanisms% and as such on the

olcanic behaiour in "eneral (e.".% llard et al.% 844>J /urton et al.% 8445a).

*er the last fe, decades% remote sensin" techni'ues ,or$in" in the +B

re"ion of the electroma"netic spectrum hae increasin"l& been applied to eplore the

lon"Cterm (&ears) to mediumCterm (da&s) trends in the rate of S*8release from actie

subaerial olcanoes (McGoni"le and *ppenheimer% 844=J *ppenheimer et al.% 84;;J

Tamburello et al.% 84;;a). From these obserations% the time!averagedbud"ets of S*8

release for a number of indiidual olcanoes hae been obtained% and the "lobal

olcanic S*8flu has been ealuated ,ith reasonable accurac& (ndres and Aas"noc%

;99


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The recent adent of the +B camera represents a step for,ard in olcanic "as

remote sensin" (Mori and /urton% 844?J /luth et al.% 8445)% allo,in" S*8

measurement to be carried out ,ith unprecedented spatial and temporal resolution.

This techni'ue has rapidl& found ,idespread usa"e in tar"etin" preiousl&

unresolable aspects of olcanic de"assin"% such as the S* 8 bud"ets of indiidual

Strombolian (Mori et al.% 8449J Dalton et al.% 8449) and Bulcanian (1olland et al.%

84;;J Nadeau et al.% 84;;) eplosions% and spatial anal&sis of hetero"eneous "as

sources such as fumarolic s&stems (Tamburello et al.% 84;;b).

1ere% ,e use the dualC+B camera techni'ue detailed in Aantzas et al.% (8449)

and Tamburello et al.% (84;;) to proide a comprehensie and simultaneous

characterisation of the three main forms of S*8"as release from Stromboli olcano!

passie de"assin"% eplosie de"assin"% and puffin". Stromboli% in the eolian islands

(Fi". >


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S*8(or @4I of the total de"assin" bud"et) (1arris and 0ipepe% 8445J 0ipepe et al.%

844


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5.3.2 +ard,are and tec-niue

-e report here on results of an +B camera sure& carried out on Stromboli

from the ;4thto the ;?thof 6ul& 84;4. dditional data collected on the 84thMa& 84;;

are also included. *erall% ,e recorded ;?h of ima"e se'uences% and obsered ;=4

eplosions throu"hout. Most measurements ,ere carried out from the S0 site% on

the northern rim of the Sciara del Fuoco (Fi". >9a) a fe, meters aboe the ents% for ,hich ,e obtained S*8concentration profiles%

oer the piels% similar to that sho,n in Fi"ure >9b% and% b& inte"ration% the #.

Then% in order to calculate the plume transport speed% ,e calculated # time series

for t,o parallel cross sections of the plume in the S* 8column amount ima"e (Fi".

>9a) as discussed in thesection>.;.@. mean plume speed has been calculated usin"

cross correlation of these t,o # time series% for eer& episode of passin" de"assin"%

occurrin" bet,een eplosions. The eplosions themseles are too fast ho,eer% to

allo, a measurable dela& T% therefore a elocit& profile has been calculated manuall&

b& isuall& obserin" the pro"ression of the eruption cloud across the field of ie,% in

an ima"e se'uence. This process is epedited b& the mar$ed contrast bet,een the hi"h

S*8concentration in the eplosie "as plume a"ainst the far ,ea$er 'uiescent plume.

5=


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+B cameras also proide $no,led"e of the concentration distribution alon"

t,o dimensions C('%y)J b& inte"ratin" a second time alon" the dimension

perpendicular that that of the inte"rated column amount% one can calculate the

inte"rated olume amount (#B)% e.".% the cumulatie amount of S*8 ,ithin a

ima"inar& prism of infinite thic$ness (see "ra& dotted s'uare in Fi". >9a)!

(;>) =5

i

5

i

y

y

'

'

d'dyCg)#A b

This calculation does not re'uire $no,led"e of the plume speed% thus

oercomes the inherent difficulties in measurin" the ascent rate of the eplosion "as

thrust (see aboe). This techni'ue has been used here to calculate the S* 8 mass

contributed b& indiidual eplosie bursts (results in section @.=.=.8)% and b&

indiidual puffs (section @.=.=.=). The inte"ration ,indo, ran"ed bet,een ;?4X884

piels for the eplosions to =4X=4 piels for puffs. The former ,as more or less

constant for most eents% althou"h sli"htl& bi""er ,indo,s ,ere re'uired for the

lar"est eplosionsJ in the latter case this piel ran"e ,as al,a&s used as puff olumes

are rather constant.

5>

Figure +7: (a) -#column amount image obtained from 9 observation site. The blac4 and red

dotted lines represent the to sections ta4en for - #flu* and plume speed calculation (see te*t),the

Documents

Rapid sensing of volcanic SO₂ fluxes using a dual ultraviolet camera system: new techniques and measurements at Southern Italian volcanoes