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Atmos.Meas.Tech.,6,3225– 3241,2013
www.atmos-meas-tech.net/6/3225/2013/
doi:10.5194/amt-6-3225-2013
©Author(s)2013.CCAttribution3.0License.
Atmospheric 
Measurement
Techniques
Open Access
The ToF-ACSM: a portable aerosol chemical speciation monitor
with TOFMS detection
R.Fröhlich
1
,M.J.Cubison
2
,J.G.Slowik
1
,N.Bukowiecki
1
,A.S.H.Prévôt
1
,U.Baltensperger
1
,J.Schneider
3
,
J.R.Kimmel
2,4
,M.Gonin
2
,U.Rohner
2
,D.R.Worsnop
4
,andJ.T.Jayne
4
1
LaboratoryofAtmosphericChemistry,PaulScherrerInstitute,Villigen,Switzerland
2
TofwerkAG,Thun,Switzerland
3
MaxPlanckInstituteforChemistry,Mainz,Germany
4
AerodyneResearch,Inc.,Billerica,Massachusetts,USA
Correspondenceto:A.S.H.Prévôt(andre.prevot@psi.ch)
Received:4July2013–PublishedinAtmos.Meas.Tech.Discuss.:25July2013
Revised:21October2013–Accepted:24October2013–Published:26November2013
Abstract. We present a a new instrument t for monitoring
aerosolcomposition,thetime-of-flightaerosolchemicalspe-
ciationmonitor(ToF-ACSM),combiningprecisionstate-of-
the-arttime-of-flightmassspectrometrywithstability,relia-
bility,andeasyhandling,whicharenecessitiesforlong-term
monitoringoperationsonthescaleofmonthstoyears.Based
onAerodyneaerosolmassspectrometer(AMS)technology,
theToF-ACSMprovidescontinuousonlinemeasurementsof
chemicalcompositionandmassofnon-refractorysubmicron
aerosolparticles.IncontrasttothelargerAMS,thecompact-
sized and lower-priced ToF-ACSM does not featureparti-
clesizing,similartothewidely-usedquadrupole-ACSM(Q-
ACSM).ComparedtotheQ-ACSM,theToF-ACSMfeatures
abetter mass resolution of
M
M
=600 and better detection
limitson theorderof
<
30ngm
3
foratimeresolutionof
30min. With simpleupgradestheselimitscan bebrought
downbyanotherfactorof
8.Thisallowsforoperationat
higher time resolutions and in low concentration environ-
ments.Theassociated softwarepackages(single packages
forintegratedoperationandcalibrationandanalysis)provide
ahighdegreeofautomationandremoteaccess,minimising
theneedfortrainedpersonnelonsite.Intercomparisonswith
Q-ACSM,C-ToF-AMS,nephelometerandscanningmobil-
ityparticlesizer(SMPS)measurements,performedduringa
firstlong-termdeployment(
>
10months)ontheJungfrau-
jochmountainridge(3580ma.s.l.)intheSwissAlps,agree
quantitatively.Additionally,themassresolutionoftheToF-
ACSMissufficientforbasicmassdefectresolvedpeakfit-
ting of the recorded spectra, providing a data stream not
accessibletotheQ-ACSM.Thisallowsforquantificationof
certain hydrocarbonandoxygenatedfragments(e.g.C
3
H
+
7
andC
2
H
3
O
+
,bothoccurringat
m/Q
=43Th),aswellasim-
provinginorganic/organicseparation.
1 Introduction
Overthelastdecades,ongoingresearcheffortshavesolid-
ifiedtheknowledgebaseaboutthesignificantroleaerosols
playinEarth’secosystem(Guetal.,2003;Mercado etal.,
2009;Mahowald,2011)andclimate(LohmannandFeichter,
2005;Forsteretal.,2007;Carslawetal.,2010).Furthermore,
evidencefor severe adverse effects of aerosols on human
healthhasbeenreported (Seatonetal.,1995; Laden et al.,
2000;Cohenetal.,2005;PopeandDockery,2006),though
the mechanisms of action and effect of aerosol composi-
tionremainlargelyunclear.Toassessandaddresstheseis-
suesalargenumberofairqualitymonitoringendeavoursare
needed.Essentialtothisareinstrumentscapableofgathering
insituinformationaboutthechemicalpropertiesandcompo-
sitionoftheambientparticlesonalong-termbasis.Suchin-
strumentscanprovidevaluableinsightsintomanyattributes
oftheaerosol,e.g.sourceortoxicity,withhighertimeres-
olution(minutestohours)thanconventionalfiltersampling
withsubsequentpost-processing.Effectsonecosystemand
climatemainly occuron largetemporal andspatial scales,
thereforeitissimilarlyimportanttobeabletocollectthese
dataoverlong-termperiods.In addition,thisfacilitatesthe
PublishedbyCopernicusPublicationsonbehalfoftheEuropeanGeosciencesUnion.
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3226
R.Fröhlichetal.:TheToF-ACSM:aportableaerosolchemicalspeciationmonitor
conductofepidemiologicalstudiesusefulinassessinglinks
betweenhealthandaerosols.
The various types of the Aerodyne aerosol mass spec-
trometer(hereafterdenoted AMS; quadrupole-AMS:Jayne
etal.,2000,compacttime-of-flight(C-ToF)-AMS: Drewnick
etal.,2005,high resolutiontime-of-flight(HR-ToF)-AMS:
DeCarloetal.,2006)haveproventobeveryproductiveand
powerful toolsin terms of recording aerosol mass spectra
withhighsensitivity.AnoverviewofnumerousAMScam-
paignsdemonstratingtheimportanceoforganicsin theto-
tal ambient PM
1
aerosol budget is shown in Canagaratna
etal.(2007),andJimenezetal.(2009)usedAMSdatatoun-
ravelthechemicalevolutionoforganicaerosolintheatmo-
sphere.However,themonetaryandmanpower investments
associatedwithAMSmeasurementsanddataanalysismake
thisinstrumentimpracticalforlong-term,widespread,semi-
autonomousmonitoringinitiatives.
The Aerodyne quadrupole aerosol chemical speciation
monitor(Q-ACSM,Ngetal.,2011)isbuiltuponthesame
samplinganddetectiontechnologyastheAMSbutwithre-
duced complexity (e.g. no particlesizemeasurement) and
performance. The ACSM is specially designed for unat-
tendedmonitoringapplicationswithminimaluserinterven-
tion to closethegap between AMS and filter sampling. It
is able to record mass spectra of ambient non-refractory
submicron aerosolwith unit massresolution (UMR) up to
masstochargeratiosof200Th,althoughtheregionabove
140Th isusually omitted becauseof its negligiblecontri-
bution to aerosol mass and a decreasing transmission of
the quadrupole. To date it has been used successfully by
more than 40 research groupsall over the world (cf. Sun
etal.,2013,2012; Budisulistiorinietal.,2013;Seto etal.,
2013; Takahamaet al., 2013)and hasinspired somevery
fruitfulinternationalcooperationsliketheACSMsubgroup
of the European ACTRIS (Aerosols, Clouds, and Trace
gases Research InfrastructureNetwork) project (www.psi.
ch/acsm-stations).Besidestheindividualscientificoutputof
every instrument, theuniquedatabases produced with the
ACSM bysuchmonitoringnetworkscompriseacombina-
tion of chemical information, high time resolution, long-
term measurements,and more, liketheability to measure
semivolatilenitrateandorganicswithoutfilterartefactsorof-
flineanalysis.Thisprovidesinvaluableopportunitiesforthe
modellingcommunity.
In this manuscript, we present a new instrument based
onAMSand ACSM technology, thetime-of-flight ACSM
(ToF-ACSM).Thisinstrumentretainstheadvantagesofthe
Q-ACSMsuchascompactdesign,semi-autonomousopera-
tion,andrelativelylowcost,whilegreatlyimprovingmass
resolutionanddetectionlimits.It isequippedwithaTofw-
erk ETOF(economytime-of-flight)ion massspectrometer.
Upgradesarepossiblesincethehardwareiscompatiblewith
anyTofwerkTOFplatforms.Herewediscusstheoperation,
testing, and initial deployment of a ToF-ACSM for ape-
riodof
>
10monthsatthehighaltituderesearchstation,the
Jungfraujoch(JFJ,3580ma.s.l.),intheSwissAlps.Thisde-
ployment demonstratedtheinstrument stability,sensitivity,
andenabledquantitativecomparisonwithotheraerosolmass
spectrometersandparticleinstruments.
2 Apparatus
ThecomponentsoftheToF-ACSMaremounted in arect-
angular rack with edge lengthsof 65cm
×
51cm
×
60cm,
it weights 75kg and consumes approximately 330W
when the inlet valve is open (to compare Q-ACSM:
48.3cm
×
53.3cm
×
83.8cm, 63.5kg, 300W; HR-ToF-
AMS:104cm
×
61cm
×
135cm,170kg,600W).Thiscom-
pactsizefacilitatestransportandenablesasimplerintegra-
tionintoexistingmonitoringstationsorplaceswherespace
islimited,e.g.aeroplanes.
Figure1showsaschemeoftheToF-ACSMwiththemain
components.AprimarydifferencebetweentheToF-ACSM
andtheQ-ACSMandAMSisthedifferentvacuumsystem
design. Thevacuumchamber, which has atotal length of
43cm(Q-ACSM: same;AMS: 59cm)isdividedinto four
differentiallypumpedsections.APfeifferSF2704-stagetur-
bomolecularpump(www.pfeiffer-vacuum.com)ismounted
directly to the vacuum chamber, and backed by the same
VacuubrandMD1diaphragmpump(www.vacuubrand.com)
utilised in all AMS systems. The analyser is evacuated
through adirectopeningto thevacuum chamber and thus
doesnotrequireanadditionalpump.Thepressureisreduced
overthestagesfrom
5
×
10
2
mbarattheexitoftheaero-
dynamiclensto
10
7
mbarintheionisationchambercon-
tainingvaporiserandioniser(foracloserdescriptionofinlet
andvaporiser/ioniserseeSect.2.1).Theelectronicsrequired
foroperationofthesystem,theacquisitionPCandthedata
acquisition(DAQ)cardareallmountedwithintheinstrument
rack.
2.1 Operationalprinciple
Aerosol enterstheinstrument over theinlet systemon the
frontalfaceofthevacuumchamber.Thisinletsystemcon-
sistsofanautomatic3-wayvalveswitchsystem,theaerody-
namiclensandacriticalorifice.Aerodynamiclensaswell
asthevaporiser/ionisersystemareidenticaltothoseusedin
boththeQ-ACSM(Ngetal.,2011)andtheresearchgrade
AMS(Jayneetal.,2000)instruments,exceptthatvaporiser
andioniseraredividedintotwopartstoallowthefilament
flangetoberemovedeasily.
Withthevalveswitchingsystemafilterisinterposedpe-
riodicallyinto theflow of ambientairtotheinstrumentin
ordertomeasurethebackgroundsignal.Theparticlesignal
isthenobtained by taking thedifferencebetweenthetotal
signal measured without a filter (“samplemode”) and the
backgroundsignalmeasuredwithfilter(“filtermode”).This
followstheprincipleappliedintheQ-ACSMandutilisesthe
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R.Fröhlichetal.:TheToF-ACSM:aportableaerosolchemicalspeciationmonitor
3227
100μm
particle lens
4-stage split ow turbomolecular pump
ioniser
vaporiser
primary beam optics
TOF extractor
Extraction
pulser
preamp            ADC
PC
detector
TOFMS
Fig.1.ToF-ACSMschematic.Ioniser/vaporiseraswellasdetector
areeasilyaccessiblethroughtwovacuumflangesatthebackside
ofthechambers.Thisenablesaneasyinterchangeoftheioniser
filamentsorbetweenthevarioussuitabledetectortypes.
sameswitchinghardware.Thesampleflowintotheinstru-
mentiscontrolledbyacriticalorifice.Theorificediameter
for operation under normal pressureconditions is 100µm,
admittingaflowof1.4cm
3
s
1
.
Theaerodynamiclens(Liuetal.,1995a,b;Liuetal.,2007;
Zhanget al.,2004),whichconsistsofaseriesofapertures
with decreasing diameter, focusesthesubmicron particles
intheaerosol inanarrowbeaminto thevacuum chamber
whilethegasesdiverge.Thelighterairmoleculesareprefer-
entiallystrippedfromtheaerosolbeamasitpassesthrough
skimmersseparatingthe4differentially-pumpedchambers.
Positionanddesignoftheskimmersareparticularlyimpor-
tant forACSM instrumentsbecauseof theshorter vacuum
chambercomparedtotheAMS.Highsignalscausedbyat-
mosphericgasesreducethelifetimeof thedetector signif-
icantlyand contribute to interferencesin theaerosol mass
spectrum.
At ambient pressure(1013mbar) the lens system hasa
closeto100%transmissionat vacuumaerodynamicdiam-
etersbetween
d
va
=
150nmand
d
va
=
450nmandanupper
cut-off(
<
15%transmission)around
d
va
=
1µm( Liuetal.,
2007).Thetransmissionforsmallerparticles(
<
100nm)is
somewhatreducedcomparedtotheQ-ACSM,whichisare-
sult of thedifferent vacuum systemsand pumping speeds
atthelensexitchamber.Arecentlydevelopedaerodynamic
lens extendstheparticletransmission to several microme-
tre(Williamsetal.,2013)andiscompatiblewiththeToF-
ACSM.
Attheendofthechamber,theparticlebeamimpactson
aresistively heated poroustungsten surface(
T≈
600
C).
Therethenon-refractoryconstituentsintheparticlesflashva-
poriseandaresubsequentlyionisedbyelectronimpact.The
electronsusedfortheionisation(
E
kin
=
70eV)areemitted
byatungstenfilamentarrangedperpendiculartotheparticle
beamin thevaporisation region.Theioniser flange,unlike
on existing AMS systems, may bedirectly removed from
thevacuumchamber,allowingaquickandeasyreplacement
ofthefilaments.Thesameprincipleappliestothedetector
flange.
Numerous experiments with AMS and Q-ACSM have
shown that afraction of thenon-refractory particles does
notflashvaporiseatthevaporiserbutbouncesofftheoven
andsubsequentlyisnotdetected(Canagaratnaetal.,2007;
Matthewetal.,2008).Acollectionefficiency(CE)factorwas
introducedtocorrectforthiseffect.Inthemajorityofcases
theCEisapproximatelyCE=0.5.TheCEcausedbyparticle
bounceincreaseswithincreasing particulatewater content,
acidity,andnitratefraction(Middlebrooketal.,2012).Be-
causeitisnotpossibletoquantitativelydeterminethewater
content,adryingsystem(e.g.Nafionmembranedriersfrom
PermaPureMD,www.permapure.com)istypicallyinstalled
onthesamplinginlet.Thecampaignandinstrumentspecific
CEcanbeassessedforexamplebycorrelationtoco-located
measurements(e.g.SMPS+aethalometer)orbytheoretical
considerationsusingthemeasuredchemicalcomposition,as
describedbyMiddlebrooketal.(2012).
2.2 Time-of-flightmassanalyser
The Tofwerk ETOF mass analyseris based on theCTOF
analyser used by the first-generation ToF-AMS systems
(Drewnicket al., 2005),and sharesthesamehousing and
interfaceconnections. However, thefinegridsused to de-
velop the electricfieldswhich guidetheionsarereplaced
with metallicplates, providing amorerobust, economical
solution attheexpenseofresolving power and sensitivity.
Fewervoltages(drifttube:
3000V,reflectorbackplaneand
grid:200–800V,pulser:
800V,detector:
3500V)arere-
quired to operate theanalyser,reducing thecomplexity of
thepowersupplyandpotentialforfailureduringlong-term
operation.TheionsaredetectedusinganSGEdynodede-
tector(www.sge.com),whicharemuchmorerobustthanthe
micro-channelplates(MCP)usedinstandardAMSsystems
and can handleexposure to atmospheric pressure and hu-
midity.Thedetectorlifetimeoftheinstrumentdeployedat
theJungfraujochfor thisstudy was1yr.Together,theuse
oftheETOF+SGEsystemgivesan8-fold(4-fold SGE
×
2-fold ETOF) reduction in sensitivity as compared to the
CTOF+MCP(fordetectionlimitsseeSect.3.2),andamass
resolvingpower(
M
M
)of
600insteadof
1000.Itisnoted
that aspecially-designed flange, which bolts directly onto
theend ofthevacuumchamber,isusedtohold thedetec-
tormount,providingunconstrainedaccessandallowingfor
fast replacementofan old detectororquickswap-out toa
higher-sensitivityMCPdetectorforspecifictimeperiodsof
interest.ThisisincontrasttotheQ-ACSMsystemwherethe
quadrupoleheadhasto bedisassembled to replacethede-
tectorortheAMSwheretheentiremassanalysermustbe
removed.
www.atmos-meas-tech.net/6/3225/2013/
Atmos.Meas.Tech.,6,3225–3241,2013
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3228
R.Fröhlichetal.:TheToF-ACSM:aportableaerosolchemicalspeciationmonitor
2.3 Dataacquisitionandanalysis
ThesignalsaredigitisedusingaUSB-basedacquisitioncard
connected to acompactPC.The14-bitanalogue-to-digital
converterdigitisestheanaloguesignalinto2
14
discretelev-
elsat0.8GSs
1
withamaximummassspectrumacquisition
rateof200Hzandmaximumaveragingdepthof65535ex-
tractions/spectrum.The64-foldincreaseintheresolutionof
thedigitisationwithrespecttotheAcqirisAP240cardused
intheAMSallowsforamuchmoreaccuratesettingofboth
theelectronicbaselineandnoisesuppressionthreshold.The
increaseddynamicrangealsoaffordshigherfidelityrecord-
ingofsingleionsignals,andthusimprovedlinearityinin-
strumentresponse.
Data arerecorded using theTofDaq datarecorderfrom
TofwerkAG,whicharefedintopresentationandanalysisus-
ingtheTofwerk“IgorDAQ”packagerunningundertheIgor
environment(WaveMetricsInc.,OR,USA).
Thebackgroundandtotalairmeasurementsarealternated
throughautomated,synchronisedswitchingofaninletvalve
suchthattheresultingtimeseriesdataresemblethatof“fast-
mode”MSoftheAMS( Kimmeletal.,2011).Inthismode
of operation, the background is assumed stable over the
timescaleofasingledatapoint(10min),andrecorded for
only1minbeforeswitchingtototalairsamplingforthere-
maining9min.Toignoretransientsintheinlet,thefirst20s
ofdataafteravalveswitchmustbediscarded.
The software then interpolates and optionally smoothes
arunningbackground trendbetween thefilter datapoints,
whichisthensubtractedfromtheindividualtotalmeasure-
ments.Theionisation efficiency (IE),
m/Q
and flow cali-
brations,baselinesubtraction,sensitivitycorrectionandfrag-
mentationpatterns(Jayneetal.,2000;Jimenezetal.,2003;
Allanetal.,2004;Drewnicketal.,2005)areapplied,yield-
ing theintegrated species timetrendsat amaximum 20s
temporalresolution.Thesaveddataproductsthusconsistof
Tofwerk-formatHDFfiles,containingthefullmassspectra
andassociateddiagnostics,andof10minand1htimescale
tab-delimitedtextfilesofmassloadingtimetrendsandim-
portant diagnostics, and organicspecies spectrummatrices
suitable for analysis by tools such as the multilinear en-
gineME-2(Paatero,1999;Canonacoetal.,2013)andpos-
itivematrixfactorisation(PMF;Paatero,1997;Paateroand
Tapper, 1994). In addition, IgorDAQ is designed to oper-
atewiththeapplicationprogramminginterfacefromGoogle
Inc.(www.google.com),allowingfortheautomatedupload
ofdatatoadedicated,password-protectedlocationandsub-
sequentparsinganddisplayofdataproductsinWebbrowsers
and on mobiledevices using publicly-available JavaScript
widgets. TheDAQmayalso beconfiguredtoreadparam-
etersfromtheserverbeforethestart ofeachmeasurement
cycle,forexampleifachangeinthefilamentemissioncur-
rentweredesired.Thisalleviatestheneedforremote-desktop
applicationsforwhichtheInternetconnectionmay,particu-
larlyatremotesites,notbesufficient.Finally,theinstrument
start-upoperationisautomated,includingpumpingdownof
thevacuumchambers,makingitsuitableforoperationinthe
absenceoftrainedpersonnel.
Forfurther,moredetailed analysisofthedata,theHDF
filesmaybeanalysedusingtheIgor-based“Tofware”pack-
age, whichofferstheusual analysisfeaturesemployed by
theatmosphericsciencecommunitysuchasmasscalibration,
peakintegrationandhigh-resolutionpeakfitting.Adedicated
plug-intotheTofwarebasepackageisemployedtodealwith
theapplicationofACSM-specificcorrections,filtersubtrac-
tionandotherinstrument-specificrequirements.
3 Quantificationofaerosolmass
The ToF-ACSM provides mass spectra of non-refractory
submicronparticulatematerialthatvaporisesat
600
Cand
10
7
mbar.Furtherspeciation(e.g.into nitrate,sulphate,
chloride, ammonium, and organic compounds) is attained
through analysis of fragmentation patterns (Allan et al.,
2004).
3.1 Calibrations
Forthemass spectra to becomequantitative, anumber of
calibrationsarenecessary to relate rawdetectorsignals to
quantitativemassspectraandtoaccountforchangesinside
(detectorsignaldecay)oroutside(pressure)theinstrument.
3.1.1 Inletflow
Thedependencyofthepressuremeasuredafterthecritical
orificeontheflowhasto becalibratedtodetectandeven-
tuallycorrectforchangesintheinletflowduringoperation.
Thiscanbedonebyconnectingasensitiveneedlevalveto
theinletandrecordingthepressureandflowwhileopening
itstepwise.
3.1.2 Baselineanddetector
IntheToF-ACSMtheretrievaloftheconversionfactorfrom
asignalamplitudeatthedetectormeasuredinmV
×
nsto
ionss
1
,theso-calledsingleioncalibration,isfullyautoma-
tised. Thesameapplies to the determination of the spec-
trum’sappropriatebaselineandthedetectorgain.Inregular
intervalsthesystemchecksand,ifnecessary,readjuststhe
baselineandgainsettingsautonomously.
AnalogouslytoallAMSsystems,variationsofthenitro-
gensignalat
m/Q=
28Thwhichisassumedtobeconstant
duetoitsabundanceintheatmospherecan beusedtocor-
rectforintrinsicchangesintheinstrumentlikeadecayofthe
iondetectorsignalintheToFmoduleoccurringbetweenthe
automaticgainadjustments.
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R.Fröhlichetal.:TheToF-ACSM:aportableaerosolchemicalspeciationmonitor
3229
3.1.3
m/Q
Easily identifiable ions from the chamber background are
usedtodetermine
m/Q
asafunctionofiontime-of-flight.As
highlightedinFig.2,forambientsampling,ionsselectedat
thelow
m/Q
endofthespectrumtypicallyincludenitrogen
(N
+2
:28.0067Th), oxygen (O
+2
: 31.9904Th), argon (Ar
+
:
39.9629Th)andcarbondioxide(CO
+2
:43.9904Th).Since
thecalibrationfunctionisnon-linear,onealsoneedsacali-
brationpointintheheavyendofthespectrum.Aslongasone
usestungstenfilamentsintheioniser,ionsofthefourstable
isotopesoftungsten
182
W
+
,
183
W
+
,
184
W
+
and
186
W
+
will
alwaysbevisibleandcanbeusedforthe
m/Q
calibration.In
the
m/Q
calibrationdepictedinFig.2,theisotopewiththe
nucleon number
A=
184 (
184
W
+
:183.9509Th)wasused.
The
m/Q
calibrationisdynamically adjusted every10min
bythesoftwaretoaccountforpotentialdriftsininstrument
performanceduringdeployment,e.g.inresponsetochanges
inroomtemperature.
3.1.4 Signal-to-mass
ToquantifythemassconcentrationsmeasuredbytheToF-
ACSM,thesignaltomassrelationofthedevicehastobede-
termined.Amass-basedcalibrationmethodusingthemass-
basedionisationefficiencymIE( Onaschetal.,2011)given
inionsmeasuredperpicogramofaerosolparticlesentering
theinstrumentisapplied.Equation(1)yieldsthemasscon-
centration
γ
i
ofaspecies
i
derivedfromthemeasuredsignals
I
i,j
ofitsmassfragments
j
.
γ
i
=
1
(
mIE
i
·q
V
)
·
j
I
i,j
(1)
with
γ
i
inunitsof µgm
3
,mIE
i
inionspg
1
,
I
i,j
inions
s
1
andthevolumetricsampleflow
q
V
incm
3
s
1
. Asthe
mIE
i
isdifferentforeveryambientspecies,itisconvenientto
expressthedifferentmIE
i
intermsofmIE
NO
3
(i.e.themIEof
thesumofthemainnitratefragments:NO
+
at
m/Q
=30Th
andNO
+2
m/Q
=46Th) determinedin thecalibration (see
Eq.3belowandJimenezetal.,2003).Tothisend,arelative
ionisationefficiencyRIEisdefined:
RIE
i
=
mIE
i
mIE
NO
3
.
(2)
TheRIE
i
ofaspecies
i
withrespecttothemass-basedion-
isationefficiency of NO
3
isunitless. Commonly used RIE
values are 1.4 for organics and 1.3 forchloride. The RIE
values of NH
4
and SO
4
should becalibrated atthebegin-
ningofeachdeploymentandthenbereviewedonaregular
basis.Typicallytheyliebetween2.5–5and0.6–1.2,respec-
tively.During thelong-term measurementat theJungfrau-
joch,theToF-ACSMRIEswereRIE
NH
4
=
3
.
23
±
0
.
42and
RIE
SO
4
=
0
.
65
±
0
.
05.
10
4
10
5
10
6
10
7
10
8
10
9
Signal (a.u.)
10
10
N
2
28.0067
O
2
CO
2
184W
Fig.2.RawmassspectrumoftheToF-ACSM(logarithmicscale).
The
x
axis is interruptedbetween
m/Q=
50Thand 175Th to
show allpeaks usedinthemass-to-chargecalibration.The cali-
brationpeaks areas follows: nitrogen(N
2
:28.0067Th),oxygen
(O
2
:31.9904Th),argon(Ar:39.9629Th),carbondioxide(CO
2
:
43.9904Th),andtungsten(
184
W:183.9509Th).
For themIEcalibrations, ammonium nitrate(NH
4
NO
3
)
particlesofknownsizeandconcentrationareneeded,sim-
ilar to the calibrationsof the Q-ACSM (Ng et al., 2011).
Hence,thesamecalibrationequipmentisrequired.NH
4
NO
3
ismainlyusedbecauseitiseasilyaccessibleandatomised,
vaporiseswith100%efficiencytoionsfromtheammonium
andnitratespecies.NH
4
NO
3
isalsowellfocusedbytheaero-
dynamiclensanddoesnotexperienceparticlebounceatthe
vaporiser. Particles of NH
4
NO
3
can beproduced froman
aqueoussolutionbyanebuliser,sizeselectedwith aDMA
afterbeingdriedby,e.g.asilicageldrierandthenfedsimul-
taneouslytotheToF-ACSMandaCPCforcounting.
Withtheequipmentdescribedabove,afixedamount(be-
tween300and1500cm
3
)ofNH
4
NO
3
calibration aerosol
particleswithauniformmobilitydiameterin therangebe-
tween
d
m
=300–350nmareselectedwiththeDMAandsam-
pled by the instrument. This diameter and concentration
rangeisrecommended becausetherethelensstill hasunit
transmissionandtheerrorcausedbydoublychargedparti-
clesisminimised.Greatcareshouldbetakenintheset-up
ofDMAandCPC.Uncertaintiesinnumberconcentrationor
particlesizewillobviouslyreducetheaccuracyofthemIE
calibration.Thesoftwarethen automatically calculates the
mIE
NO
3
onlyfromthesignaloftheNO
+
andNO
+2
fragments
ofthenitrateusingEq.(3):
mIE
NO
3
=
I
NO
3
,m/Q=
30
+I
NO
3
,m/Q=
46
n·ρV·f·q
V
.
(3)
Here
I
i,j
aretheionsignalsinionss
1
,
n
isthenumberof
particlesmeasuredwiththeCPCincm
3
,
ρ
thedensityof
NH
4
NO
3
ingcm
3
,
V
thevolumeofoneNH
4
NO
3
particle
incm
3
,
f
thefractionofNH
4
NO
3
thatisnitrateand
q
V
the
samplingflowincm
3
s
1
.AtthesametimetheRIE
NH
4
is
determinedfromthesignal of theammoniumfragmentsat
m/Q=
15, 16 and17Th viathemassfractionof NH
4
in
NH
4
NO
3
.
www.atmos-meas-tech.net/6/3225/2013/
Atmos.Meas.Tech.,6,3225–3241,2013
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3230
R.Fröhlichetal.:TheToF-ACSM:aportableaerosolchemicalspeciationmonitor
Figure3showsthesummedToF-ACSMsignalofnitrate
(blue,leftaxis)andofammonium(orange,rightaxis)atsev-
eralmassconcentrationsoftherespectivespeciessampledby
theToF-ACSM.Forbothions,alinearresponseofthesignal
tomassconcentrationisobservedoverawiderangeofcon-
centrations.Notethatthehigherammoniumsignal(despite
lessmass)resultsfromthehighRIE
NH
4
(2.99).
ThefinalRIE
NO
3
,which later is applied to theambient
data,isslightly higherthan one(RIE
NO
3
,
ambient
=
1
.
1)be-
causethetwofragmentsofnitrateusedinthecalibrationonly
accountforabout90%ofthetotalnitratesignal.Thepartof
nitratethatfragmentsintoseparatenitrogenoroxygenatoms
isnotmonitoredinthecalibrationbecauseofthelowsignal
tobackgroundratioatthecorresponding
m/Q
ratios.
Once RIE
NH
4
has been measured, the RIE of sulphate
(RIE
SO
4
)caneasilybedeterminedbysampling(NH
4
)
2
SO
4
particlesandadjustingRIE
SO
4
toyieldionbalancebetween
ammoniumandsulphate.
Itisrecommendedtorepeatthesignal-to-masscalibration
atleastevery8weeksduringnormaloperationandwithin-
creasedfrequencyfollowingaventingofthevacuumcham-
ber.
3.2 Detectionlimits
ThechemicalspeciesdetectablewiththeToF-ACSM(organ-
ics,ammonium,nitrate,sulphateandchloride)areretrieved
byarecombinationofionicsignalsofthesinglefragments
thatthespeciesbreakdowntoduringionisationandvapori-
sation(Allanetal.,2004).Inthefollowing,detectionlimits
aredefinedasthreetimesthestandarddeviation(3-
σ
)from
zeroata1mintimeresolution.Theycanbescaledtodiffer-
entaveragingtimesusingtheformula
DL
t
=
DL
1min
·
60s
t
,
(4)
withthedetectionlimitof1minDL
1min
andDL
t
beingthe
detectionlimitforagivenaveragingtime
t
.Detectionlimits
arelargelygovernedbytheextenttowhichbackgroundsig-
naloriginatingfromionsofambientgases(mainlyO
2
,N
2
,
Ar,H
2
O,andCO
2
)interfereswithmeasurementofaselected
species.Specieswithsignificantsignal at
m/Q
affectedby
suchbackgroundsignalhavehigherdetectionlimits.
To measurethedetectionlimits,anadditional filterwas
placed on the ToF-ACSM inlet before the standard fil-
tered/unfilteredswitching valve,yieldingaconstantstream
ofparticle-freeair. Figure4ashowsthetimeseriesof the
differencesignal(differencebetween thesignal at thetwo
positions of the switching valve) of this particle-free air.
This signal is centred at zero by definition, with the ob-
servedfluctuationsdeterminingspecies-dependentdetection
limits.Eachdatapoint in Fig.4wasobtained from20sof
averaging,withthebackgroundsignalrecordedevery360s
(visible as gaps in the data stream). The higher interfer-
encesofwatervapourandotheratmosphericgaseswiththe
8x10
3
6
4
2
0
N
O
3
i
o
n
s
(
i
o
n
s
/
s
)
30
25
20
15
10
5
0
NO
or NH
mass (µg/m
3
)
6x10
3
4
2
0
N
H
4
i
o
n
s
(
i
o
n
s
/
s
)
Slope =
773.7 ± 5.11
R
2
=
0.999825
Slope=
258.72 ± 1.16
R
2
=
0.999879
RIE
NH
4
= 2.99 ± 0.03
M
S
C
A
-
F
o
T
M
S
C
A
-
F
o
T
Fig.3.NO
3
(blue)andNH
4
(orange)ionicsignalsfromthemIE
calibrationplottedagainstthemassoftherespectivespecies,calcu-
latedfromtheoutputoftheCPC.Thedashedlinesrepresentlinear
fitstotheNH4(red)andNO3signals(green).Thecorresponding
slopesandcoefficientsofdeterminationaregivenintheboxes(red:
NH
4
;green:NO
3
).TheRIE
NH
4
wascalculatedfromtheratioof
thedeterminedslopes.
ammoniumandorganicsignalsareevident,yieldingvariabil-
ityof
±
0
.
2µgm
3
,whilenitrate,sulphate,andchloridevary
only
±
0
.
025µgm
3
.
Thetable in Fig. 4b compares published 3-
σ
detection
limits of each species in ngm
3
for the Q-ACSM (Ng
etal.,2011;Sunetal.,2012)andToF-AMS(DeCarloetal.,
2006) instruments, scaled by Eq. (4), with those for the
ToF-ACSMequippedwiththeSGEdetector.At1mintime
resolutiontheyare182ngm
3
forammonium,198ngm
3
fororganics,18ngm
3
forsulphate,21ngm
3
fornitrate,
and11ngm
3
forchloride.Inadditionwereportthedetec-
tionlimitsforaQ-ACSM measured simultaneously at the
Jungfraujoch(formoreinformationwerefertoSect.4.1)to
thosereportedfortheToF-ACSM.Inthiscontext it isim-
portanttonotethatthedetectionlimitsarenotabsolutebut
willvaryasafunctionofthebackgroundmassspectralsignal
(Drewnicketal.,2009).Aimoffutureworkistofurtherre-
ducetheimpactoftheairbeamontherecordedsignals.First
testshavedemonstratedthatan orderofmagnitudereduc-
tionintheair-to-aerosolsignalratiocomparedtothecurrent
configurationpresentedinthismanuscriptisfeasiblewhile
keepingthetotalaerosolthroughputhighenough.Thiscor-
respondstoimprovementsof
3timesin organicspecies
sensitivity.
AnupgradefromthecurrentlyusedSGEdetectortothe
less-robustandshorter-livedMCPswilldecreasetheshown
detectionlimitsofallspeciesbyanotherfactorofabout4.
With the two modifications described above, the ToF-
ACSMwouldadvanceintothedomainoftheC-ToF-AMS
whosedetectionlimitsarecurrentlystilllowerbyfactorsbe-
tween 10 and 20, except for thechloridewhosedetection
limitwith11ngm
3
alreadyliesatasimilarlylowlevellike
intheC-ToF-AMS(4ngm
3
)andintheV-modeHR-ToF-
AMS(12ngm
3
).TheV-modeHR-ToF-AMShasabetter
massresolutionbutalsoslightlyhigherdetectionlimitsthan
Atmos.Meas.Tech.,6,3225–3241,2013
www.atmos-meas-tech.net/6/3225/2013/
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R.Fröhlichetal.:TheToF-ACSM:aportableaerosolchemicalspeciationmonitor
3231
4
12
9
100
120
164
97
Chloride
1
3
19
101
236
383
103
Nitrate
2
5
17
986
1063
329
230
Sulphate
19
22
198
1515
2136
2958
1380
Organics
16
38
170
450
1052
1369
2656
Ammonium
C-ToF-AMS
(DeCarlo
2006)
V-ToF-AMS
(DeCarlo
2006)
ToF-
ACSM 
(PSI Aug 
2012)
Q-ACSM 
(PSI Aug 
2012
Q-ACSM 
(PSI May 
2012)
Q-ACSM 
(Sun 
2010)
Q-ACSM 
(Jayne)
4
12
11
73
134
164
60
Chloride
1
3
21
73
260
383
66
Nitrate
2
5
18
1061
1234
329
131
Sulphate
19
22
198
1204
2467
2958
810
Organics
16
38
182
323
747
1369
1556
Ammonium
C-ToF-AMS
(DeCarlo et
al., 2006)
V-mode
HRToF-AMS
(DeCarlo et
al., 2006)
ToF-
ACSM 
(This   study
2012)
Q-ACSM 
(
2012)
Q-ACSM 
(
Q-ACSM 
(Sun et 
al., 2012)
Q-ACSM 
(Ng et al.,
2011)
*DL are scaled to 1 min by
t
meas
/60s
-0.4
-0.2
0.0
0.2
0.4
7000
6000
5000
4000
3000
2000
1000
0
Elapsed 
fromoccasionalSaharan dustevents.Theserefractorypar-
icles are not detectable with the ACSM. Theselow con-
centrations occur becausethe station lieswithin theclean
continental freetropospheremost oftheyear,especiallyin
winter,whileinsummeraerosolgeneratedatloweraltitudes
reachestheJungfraujochduetoverticalexchangeprocesses
(Lugaueretal.,1998;Henneetal.,2004).TheJungfraujoch
siteisafamoustouristdestinationandtheSphinxObserva-
orycanalsobevisitedbytourists,leadingtooccasionallo-
calemission plumesfrom,e.g.helicopters, snowcrawlers,
restaurantsorcigarettesmoke.
Local time on the Jungfraujoch is Central European
1
2
3
6
4
2
0
09:30
10.08.2012
09:40
09:50
UTC
ammonium
chloride
nitrate
organic
sulphate
the quality of theQ-ACSM’s mass spectrum deteriorates.
Thequalityofthemassspectrumiscrucialfortheapplication
ofstatisticalsourceapportionmentmethodslikepositivema-
trixfactorisation(PMF;Paatero,1997;PaateroandTapper,
1994)andthemultilinearengine(ME-2;Paatero,1999).
Theorganicstickspectrumofcase3shownatthebottom
ofFig.6lookssignificantlydifferenttothetwootherrather
similarToF-ACSMspectraincases1and2.Thepronounced
signalsatthemass-to-chargeratios41Th,55Th,57Thand
69Th point towards asignificant fraction of hydrocarbon-
likeorganics(HOA)intheaerosol.Acomparisontoanambi-
entmassspectrumofHOAfromacampaigninParis(Crippa
etal., 2013) yieldsagood correlationwith an
R
2
of 0.78
forcase3,whiletheToF-ACSM spectrain cases1 and2
do not correlate with the HOA spectrum (
R
2
1
=
0
.
42 and
R
2
2
=
0
.
22).Thisresult illustratestheutilityofhighlytime
resolvedmeasurementsevenformonitoringapplications,as
herethelocalactivityofmachinerycanbeidentifiedanddis-
tinguishedfromtherestofthedataset.
2
ONLY ORGANICS SIGNAL
ONLY ORGANICS SIGNAL
ONLY ORGANICS SIGNAL
ONLY ORGANICS SIGNAL
ONLY ORGANICS SIGNAL
TOTAL SIGNAL
TOTAL SIGNAL
TOTAL SIGNAL
TOTAL SIGNAL
R2
HOA
= 0.78
1    
1    
Quad
   
ToF
   
ToF
   
Quad
ToF
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