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Atmos.Meas.Tech.,6,3225–3241,2013 Atmospheric O p www.atmos-meas-tech.net/6/3225/2013/ e n doi:10.5194/amt-6-3225-2013 Measurement A c c ©Author(s)2013.CCAttribution3.0License. Techniquese s s The ToF-ACSM: a portable aerosol chemical speciation monitor with TOFMS detection R.Fröhlich1,M.J.Cubison2,J.G.Slowik1,N.Bukowiecki1,A.S.H.Prévôt1,U.Baltensperger1,J.Schneider3, J.R.Kimmel2,4,M.Gonin2,U.Rohner2,D.R.Worsnop4,andJ.T.Jayne4 1LaboratoryofAtmosphericChemistry,PaulScherrerInstitute,Villigen,Switzerland 2TofwerkAG,Thun,Switzerland 3MaxPlanckInstituteforChemistry,Mainz,Germany 4AerodyneResearch,Inc.,Billerica,Massachusetts,USA Correspondenceto:A.S.H.Prévôt([email protected]) Received:4July2013–PublishedinAtmos.Meas.Tech.Discuss.:25July2013 Revised:21October2013–Accepted:24October2013–Published:26November2013 Abstract. We present a new instrument for monitoring accessibletotheQ-ACSM.Thisallowsforquantificationof + aerosolcomposition,thetime-of-flightaerosolchemicalspe- certain hydrocarbon and oxygenated fragments (e.g. C H 3 7 ciationmonitor(ToF-ACSM),combiningprecisionstate-of- andC H O+,bothoccurringatm/Q=43Th),aswellasim- 2 3 the-arttime-of-flightmassspectrometrywithstability,relia- provinginorganic/organicseparation. bility,andeasyhandling,whicharenecessitiesforlong-term monitoringoperationsonthescaleofmonthstoyears.Based onAerodyneaerosolmassspectrometer(AMS)technology, theToF-ACSMprovidescontinuousonlinemeasurementsof 1 Introduction chemicalcompositionandmassofnon-refractorysubmicron aerosolparticles.IncontrasttothelargerAMS,thecompact- Over the last decades, ongoing research efforts have solid- sized and lower-priced ToF-ACSM does not feature parti- ified the knowledge base about the significant role aerosols clesizing,similartothewidely-usedquadrupole-ACSM(Q- play in Earth’s ecosystem (Gu et al., 2003; Mercado et al., ACSM).ComparedtotheQ-ACSM,theToF-ACSMfeatures 2009;Mahowald,2011)andclimate(LohmannandFeichter, a better mass resolution of M =600 and better detection 2005;Forsteretal.,2007;Carslawetal.,2010).Furthermore, 1M limits on the order of <30ngm−3 for a time resolution of evidence for severe adverse effects of aerosols on human health has been reported (Seaton et al., 1995; Laden et al., 30min. With simple upgrades these limits can be brought down by another factor of ∼8. This allows for operation at 2000;Cohenetal.,2005;PopeandDockery,2006),though the mechanisms of action and effect of aerosol composi- higher time resolutions and in low concentration environ- tion remain largely unclear. To assess and address these is- ments. The associated software packages (single packages suesalargenumberofairqualitymonitoringendeavoursare forintegratedoperationandcalibrationandanalysis)provide needed.Essentialtothisareinstrumentscapableofgathering ahighdegreeofautomationandremoteaccess,minimising insituinformationaboutthechemicalpropertiesandcompo- theneedfortrainedpersonnelonsite.Intercomparisonswith sitionoftheambientparticlesonalong-termbasis.Suchin- Q-ACSM, C-ToF-AMS, nephelometer and scanning mobil- strumentscanprovidevaluableinsightsintomanyattributes ityparticlesizer(SMPS)measurements,performedduringa of the aerosol, e.g. source or toxicity, with higher time res- first long-term deployment (>10 months) on the Jungfrau- olution (minutes to hours) than conventional filter sampling jochmountainridge(3580ma.s.l.)intheSwissAlps,agree with subsequent post-processing. Effects on ecosystem and quantitatively.Additionally,themassresolutionoftheToF- climate mainly occur on large temporal and spatial scales, ACSM is sufficient for basic mass defect resolved peak fit- therefore it is similarly important to be able to collect these ting of the recorded spectra, providing a data stream not data over long-term periods. In addition, this facilitates the PublishedbyCopernicusPublicationsonbehalfoftheEuropeanGeosciencesUnion. 3226 R.Fröhlichetal.:TheToF-ACSM:aportableaerosolchemicalspeciationmonitor conductofepidemiologicalstudiesusefulinassessinglinks Jungfraujoch(JFJ,3580ma.s.l.),intheSwissAlps.Thisde- betweenhealthandaerosols. ployment demonstrated the instrument stability, sensitivity, The various types of the Aerodyne aerosol mass spec- andenabledquantitativecomparisonwithotheraerosolmass trometer (hereafter denoted AMS; quadrupole-AMS: Jayne spectrometersandparticleinstruments. etal.,2000,compacttime-of-flight(C-ToF)-AMS:Drewnick et al., 2005, high resolution time-of-flight (HR-ToF)-AMS: DeCarloetal.,2006)haveproventobeveryproductiveand 2 Apparatus powerful tools in terms of recording aerosol mass spectra with high sensitivity. An overview of numerous AMS cam- The components of the ToF-ACSM are mounted in a rect- paigns demonstrating the importance of organics in the to- angular rack with edge lengths of 65cm×51cm×60cm, tal ambient PM1 aerosol budget is shown in Canagaratna it weights 75kg and consumes approximately 330W etal.(2007),andJimenezetal.(2009)usedAMSdatatoun- when the inlet valve is open (to compare Q-ACSM: ravelthechemicalevolutionoforganicaerosolintheatmo- 48.3cm×53.3cm×83.8cm, 63.5kg, 300W; HR-ToF- sphere. However, the monetary and manpower investments AMS:104cm×61cm×135cm,170kg,600W).Thiscom- associatedwithAMSmeasurementsanddataanalysismake pact size facilitates transport and enables a simpler integra- thisinstrumentimpracticalforlong-term,widespread,semi- tion into existing monitoring stations or places where space autonomousmonitoringinitiatives. islimited,e.g.aeroplanes. The Aerodyne quadrupole aerosol chemical speciation Figure1showsaschemeoftheToF-ACSMwiththemain monitor (Q-ACSM, Ng et al., 2011) is built upon the same components. A primary difference between the ToF-ACSM samplinganddetectiontechnologyastheAMSbutwithre- and the Q-ACSM and AMS is the different vacuum system duced complexity (e.g. no particle size measurement) and design. The vacuum chamber, which has a total length of performance. The ACSM is specially designed for unat- 43cm (Q-ACSM: same; AMS: 59cm) is divided into four tended monitoring applications with minimal user interven- differentiallypumpedsections.APfeifferSF2704-stagetur- tion to close the gap between AMS and filter sampling. It bomolecular pump (www.pfeiffer-vacuum.com) is mounted is able to record mass spectra of ambient non-refractory directly to the vacuum chamber, and backed by the same submicron aerosol with unit mass resolution (UMR) up to VacuubrandMD1diaphragmpump(www.vacuubrand.com) mass to charge ratios of 200Th, although the region above utilised in all AMS systems. The analyser is evacuated 140Th is usually omitted because of its negligible contri- through a direct opening to the vacuum chamber and thus bution to aerosol mass and a decreasing transmission of doesnotrequireanadditionalpump.Thepressureisreduced the quadrupole. To date it has been used successfully by overthestagesfrom∼5×10−2mbarattheexitoftheaero- more than 40 research groups all over the world (cf. Sun dynamiclensto∼10−7mbarintheionisationchambercon- et al., 2013, 2012; Budisulistiorini et al., 2013; Seto et al., tainingvaporiserandioniser(foracloserdescriptionofinlet 2013; Takahama et al., 2013) and has inspired some very andvaporiser/ioniserseeSect.2.1).Theelectronicsrequired fruitful international cooperations like the ACSM subgroup foroperationofthesystem,theacquisitionPCandthedata of the European ACTRIS (Aerosols, Clouds, and Trace acquisition(DAQ)cardareallmountedwithintheinstrument gases Research Infrastructure Network) project (www.psi. rack. ch/acsm-stations).Besidestheindividualscientificoutputof every instrument, the unique databases produced with the 2.1 Operationalprinciple ACSM by such monitoring networks comprise a combina- tion of chemical information, high time resolution, long- Aerosol enters the instrument over the inlet system on the term measurements, and more, like the ability to measure frontal face of the vacuum chamber. This inlet system con- semivolatilenitrateandorganicswithoutfilterartefactsorof- sistsofanautomatic3-wayvalveswitchsystem,theaerody- flineanalysis.Thisprovidesinvaluableopportunitiesforthe namic lens and a critical orifice. Aerodynamic lens as well modellingcommunity. asthevaporiser/ionisersystemareidenticaltothoseusedin In this manuscript, we present a new instrument based both the Q-ACSM (Ng et al., 2011) and the research grade on AMS and ACSM technology, the time-of-flight ACSM AMS (Jayne et al., 2000) instruments, except that vaporiser (ToF-ACSM). This instrument retains the advantages of the and ioniser are divided into two parts to allow the filament Q-ACSMsuchascompactdesign,semi-autonomousopera- flangetoberemovedeasily. tion, and relatively low cost, while greatly improving mass With the valve switching system a filter is interposed pe- resolution and detection limits. It is equipped with a Tofw- riodically into the flow of ambient air to the instrument in erk ETOF (economy time-of-flight) ion mass spectrometer. order to measure the background signal. The particle signal Upgradesarepossiblesincethehardwareiscompatiblewith is then obtained by taking the difference between the total anyTofwerkTOFplatforms.Herewediscusstheoperation, signal measured without a filter (“sample mode”) and the testing, and initial deployment of a ToF-ACSM for a pe- backgroundsignalmeasuredwithfilter(“filtermode”).This riodof>10monthsatthehighaltituderesearchstation,the followstheprincipleappliedintheQ-ACSMandutilisesthe Atmos.Meas.Tech.,6,3225–3241,2013 www.atmos-meas-tech.net/6/3225/2013/ R.Fröhlichetal.:TheToF-ACSM:aportableaerosolchemicalspeciationmonitor 3227 4-stage split flow turbomolecular pump on existing AMS systems, may be directly removed from thevacuumchamber,allowingaquickandeasyreplacement of the filaments. The same principle applies to the detector flange. 100μm particle lens Numerous experiments with AMS and Q-ACSM have ioniser vaporiser shown that a fraction of the non-refractory particles does not flash vaporise at the vaporiser but bounces off the oven primary beam optics and subsequently is not detected (Canagaratna et al., 2007; TOF extractor Matthewetal.,2008).Acollectionefficiency(CE)factorwas Extraction pulser introducedtocorrectforthiseffect.Inthemajorityofcases theCEisapproximatelyCE=0.5.TheCEcausedbyparticle preamp ADC bounce increases with increasing particulate water content, detector PC acidity, and nitrate fraction (Middlebrook et al., 2012). Be- TOFMS causeitisnotpossibletoquantitativelydeterminethewater Fig.1.ToF-ACSMschematic.Ioniser/vaporiseraswellasdetector content,adryingsystem(e.g.Nafionmembranedriersfrom are easily accessible through two vacuum flanges at the backside PermaPureMD,www.permapure.com)istypicallyinstalled of the chambers. This enables an easy interchange of the ioniser onthesamplinginlet.Thecampaignandinstrumentspecific filamentsorbetweenthevarioussuitabledetectortypes. CEcanbeassessedforexamplebycorrelationtoco-located measurements (e.g. SMPS+aethalometer) or by theoretical considerationsusingthemeasuredchemicalcomposition,as same switching hardware. The sample flow into the instru- describedbyMiddlebrooketal.(2012). ment is controlled by a critical orifice. The orifice diameter for operation under normal pressure conditions is 100µm, 2.2 Time-of-flightmassanalyser admittingaflowof1.4cm3s−1. The Tofwerk ETOF mass analyser is based on the CTOF Theaerodynamiclens(Liuetal.,1995a,b;Liuetal.,2007; analyser used by the first-generation ToF-AMS systems Zhang et al., 2004), which consists of a series of apertures (Drewnick et al., 2005), and shares the same housing and with decreasing diameter, focuses the submicron particles interface connections. However, the fine grids used to de- in the aerosol in a narrow beam into the vacuum chamber velop the electric fields which guide the ions are replaced whilethegasesdiverge.Thelighterairmoleculesareprefer- with metallic plates, providing a more robust, economical entiallystrippedfromtheaerosolbeamasitpassesthrough solution at the expense of resolving power and sensitivity. skimmers separating the 4 differentially-pumped chambers. Fewervoltages(drifttube:∼3000V,reflectorbackplaneand Positionanddesignoftheskimmersareparticularlyimpor- grid:200–800V,pulser:∼800V,detector:∼3500V)arere- tant for ACSM instruments because of the shorter vacuum quired to operate the analyser, reducing the complexity of chamber compared to the AMS. High signals caused by at- the power supply and potential for failure during long-term mospheric gases reduce the lifetime of the detector signif- operation. The ions are detected using an SGE dynode de- icantly and contribute to interferences in the aerosol mass tector(www.sge.com),whicharemuchmorerobustthanthe spectrum. micro-channelplates(MCP)usedinstandardAMSsystems At ambient pressure (1013mbar) the lens system has a and can handle exposure to atmospheric pressure and hu- close to 100% transmission at vacuum aerodynamic diam- etersbetweend =150nmandd =450nmandanupper midity. The detector lifetime of the instrument deployed at va va cut-off(<15%transmission)aroundd =1µm(Liuetal., the Jungfraujoch for this study was 1yr. Together, the use va of the ETOF+SGE system gives an 8-fold (4-fold SGE × 2007). The transmission for smaller particles (<100nm) is 2-fold ETOF) reduction in sensitivity as compared to the somewhatreducedcomparedtotheQ-ACSM,whichisare- CTOF+MCP(fordetectionlimitsseeSect.3.2),andamass sult of the different vacuum systems and pumping speeds resolvingpower( M )of∼600insteadof∼1000.Itisnoted atthelensexitchamber.Arecentlydevelopedaerodynamic 1M that a specially-designed flange, which bolts directly onto lens extends the particle transmission to several microme- the end of the vacuum chamber, is used to hold the detec- tre (Williams et al., 2013) and is compatible with the ToF- tor mount, providing unconstrained access and allowing for ACSM. fast replacement of an old detector or quick swap-out to a At the end of the chamber, the particle beam impacts on a resistively heated porous tungsten surface (T ≈600◦C). higher-sensitivity MCP detector for specific time periods of interest.ThisisincontrasttotheQ-ACSMsystemwherethe Therethenon-refractoryconstituentsintheparticlesflashva- quadrupole head has to be disassembled to replace the de- poriseandaresubsequentlyionisedbyelectronimpact.The electrons used for the ionisation (E =70eV) are emitted tector or the AMS where the entire mass analyser must be kin removed. byatungstenfilamentarrangedperpendiculartotheparticle beam in the vaporisation region. The ioniser flange, unlike www.atmos-meas-tech.net/6/3225/2013/ Atmos.Meas.Tech.,6,3225–3241,2013 3228 R.Fröhlichetal.:TheToF-ACSM:aportableaerosolchemicalspeciationmonitor 2.3 Dataacquisitionandanalysis start-upoperationisautomated,includingpumpingdownof thevacuumchambers,makingitsuitableforoperationinthe ThesignalsaredigitisedusingaUSB-basedacquisitioncard absenceoftrainedpersonnel. connected to a compact PC. The 14-bit analogue-to-digital For further, more detailed analysis of the data, the HDF converterdigitisestheanaloguesignalinto214 discretelev- filesmaybeanalysedusingtheIgor-based“Tofware”pack- elsat0.8GSs−1withamaximummassspectrumacquisition age, which offers the usual analysis features employed by rateof200Hzandmaximumaveragingdepthof65535ex- theatmosphericsciencecommunitysuchasmasscalibration, tractions/spectrum.The64-foldincreaseintheresolutionof peakintegrationandhigh-resolutionpeakfitting.Adedicated thedigitisationwithrespecttotheAcqirisAP240cardused plug-intotheTofwarebasepackageisemployedtodealwith intheAMSallowsforamuchmoreaccuratesettingofboth theapplicationofACSM-specificcorrections,filtersubtrac- theelectronicbaselineandnoisesuppressionthreshold.The tionandotherinstrument-specificrequirements. increaseddynamicrangealsoaffordshigherfidelityrecord- ing of single ion signals, and thus improved linearity in in- 3 Quantificationofaerosolmass strumentresponse. Data are recorded using the TofDaq data recorder from The ToF-ACSM provides mass spectra of non-refractory TofwerkAG,whicharefedintopresentationandanalysisus- submicronparticulatematerialthatvaporisesat∼600◦Cand ingtheTofwerk“IgorDAQ”packagerunningundertheIgor ∼10−7mbar. Further speciation (e.g. into nitrate, sulphate, environment(WaveMetricsInc.,OR,USA). chloride, ammonium, and organic compounds) is attained Thebackgroundandtotalairmeasurementsarealternated through analysis of fragmentation patterns (Allan et al., throughautomated,synchronisedswitchingofaninletvalve 2004). suchthattheresultingtimeseriesdataresemblethatof“fast- mode”MSoftheAMS(Kimmeletal.,2011).Inthismode 3.1 Calibrations of operation, the background is assumed stable over the timescale of a single data point (10min), and recorded for For the mass spectra to become quantitative, a number of only1minbeforeswitchingtototalairsamplingforthere- calibrations are necessary to relate raw detector signals to maining9min.Toignoretransientsintheinlet,thefirst20s quantitative mass spectra and to account for changes inside ofdataafteravalveswitchmustbediscarded. (detectorsignaldecay)oroutside(pressure)theinstrument. The software then interpolates and optionally smoothes a running background trend between the filter data points, 3.1.1 Inletflow which is then subtracted from the individual total measure- ments. The ionisation efficiency (IE), m/Q and flow cali- The dependency of the pressure measured after the critical brations,baselinesubtraction,sensitivitycorrectionandfrag- orifice on the flow has to be calibrated to detect and even- mentationpatterns(Jayneetal.,2000;Jimenezetal.,2003; tuallycorrectforchangesintheinletflowduringoperation. Allanetal.,2004;Drewnicketal.,2005)areapplied,yield- This can be done by connecting a sensitive needle valve to ing the integrated species time trends at a maximum 20s theinlet andrecording thepressure andflow whileopening temporalresolution.Thesaveddataproductsthusconsistof itstepwise. Tofwerk-format HDF files, containing the full mass spectra 3.1.2 Baselineanddetector and associated diagnostics, and of 10min and 1h timescale tab-delimited text files of mass loading time trends and im- IntheToF-ACSMtheretrievaloftheconversionfactorfrom portant diagnostics, and organic species spectrum matrices a signal amplitude at the detector measured in mV × ns to suitable for analysis by tools such as the multilinear en- ionss−1,theso-calledsingleioncalibration,isfullyautoma- gine ME-2 (Paatero, 1999; Canonaco et al., 2013) and pos- tised. The same applies to the determination of the spec- itive matrix factorisation (PMF; Paatero, 1997; Paatero and trum’sappropriatebaselineandthedetectorgain.Inregular Tapper, 1994). In addition, IgorDAQ is designed to oper- intervals the system checks and, if necessary, readjusts the atewiththeapplicationprogramminginterfacefromGoogle baselineandgainsettingsautonomously. Inc. (www.google.com), allowing for the automated upload Analogously to all AMS systems, variations of the nitro- ofdatatoadedicated,password-protectedlocationandsub- gensignalatm/Q=28Thwhichisassumedtobeconstant sequentparsinganddisplayofdataproductsinWebbrowsers due to its abundance in the atmosphere can be used to cor- and on mobile devices using publicly-available JavaScript rectforintrinsicchangesintheinstrumentlikeadecayofthe widgets. The DAQ may also be configured to read param- iondetectorsignalintheToFmoduleoccurringbetweenthe eters from the server before the start of each measurement automaticgainadjustments. cycle,forexampleifachangeinthefilamentemissioncur- rentweredesired.Thisalleviatestheneedforremote-desktop applicationsforwhichtheInternetconnectionmay,particu- larlyatremotesites,notbesufficient.Finally,theinstrument Atmos.Meas.Tech.,6,3225–3241,2013 www.atmos-meas-tech.net/6/3225/2013/ R.Fröhlichetal.:TheToF-ACSM:aportableaerosolchemicalspeciationmonitor 3229 3.1.3 m/Q 1010 28.N00267 O2 CO2 184W Easily identifiable ions from the chamber background are 109 usedtodeterminem/Qasafunctionofiontime-of-flight.As u.)108 highlightedinFig.2,forambientsampling,ionsselectedat al (a.107 thelowm/Qendofthespectrumtypicallyincludenitrogen Sign106 (N+: 28.0067Th), oxygen (O+: 31.9904Th), argon (Ar+: 105 2 2 39.9629Th) and carbon dioxide (CO+: 43.9904Th). Since 104 2 the calibration function is non-linear, one also needs a cali- brationpointintheheavyendofthespectrum.Aslongasone usestungstenfilamentsintheioniser,ionsofthefourstable Fig.2.RawmassspectrumoftheToF-ACSM(logarithmicscale). isotopesoftungsten182W+,183W+,184W+ and186W+ will The x axis is interrupted between m/Q=50Th and 175Th to show all peaks used in the mass-to-charge calibration. The cali- alwaysbevisibleandcanbeusedforthem/Qcalibration.In them/QcalibrationdepictedinFig.2,theisotopewiththe bration peaks are as follows: nitrogen (N2: 28.0067Th), oxygen nucleon number A=184 (184W+: 183.9509Th) was used. (4O3.29:93014.T99h0),4aTnhd)t,uanrggsotnen(A(1r8:4W39:.9168239.9T5h0)9,Tchar)b.on dioxide (CO2: The m/Q calibration is dynamically adjusted every 10min by the software to account for potential drifts in instrument performanceduringdeployment,e.g.inresponsetochanges For the mIE calibrations, ammonium nitrate (NH NO ) 4 3 inroomtemperature. particles of known size and concentration are needed, sim- ilar to the calibrations of the Q-ACSM (Ng et al., 2011). 3.1.4 Signal-to-mass Hence,thesamecalibrationequipmentisrequired.NH NO 4 3 is mainly used because it is easily accessible and atomised, To quantify the mass concentrations measured by the ToF- vaporiseswith100%efficiencytoionsfromtheammonium ACSM,thesignaltomassrelationofthedevicehastobede- andnitratespecies.NH NO isalsowellfocusedbytheaero- 4 3 termined.Amass-basedcalibrationmethodusingthemass- dynamiclensanddoesnotexperienceparticlebounceatthe based ionisation efficiency mIE (Onasch et al., 2011) given vaporiser. Particles of NH NO can be produced from an 4 3 in ions measured per picogram of aerosol particles entering aqueous solution by a nebuliser, size selected with a DMA theinstrumentisapplied.Equation(1)yieldsthemasscon- afterbeingdriedby,e.g.asilicageldrierandthenfedsimul- centrationγ ofaspeciesiderivedfromthemeasuredsignals i taneouslytotheToF-ACSMandaCPCforcounting. I ofitsmassfragmentsj. i,j Withtheequipmentdescribedabove,afixedamount(be- tween 300 and 1500cm−3) of NH NO calibration aerosol 1 X 4 3 γ = · I (1) particles with a uniform mobility diameter in the range be- i (mIE ·q ) i,j i V j tweendm=300–350nmareselectedwiththeDMAandsam- pled by the instrument. This diameter and concentration with γ in units of µgm−3, mIE in ions pg−1, I in ions range is recommended because there the lens still has unit i i i,j s−1 and the volumetric sample flow q in cm3s−1. As the transmission and the error caused by doubly charged parti- V mIE isdifferentforeveryambientspecies,itisconvenientto cles is minimised. Great care should be taken in the set-up i expressthedifferentmIE intermsofmIE (i.e.themIEof ofDMAandCPC.Uncertaintiesinnumberconcentrationor i NO3 thesumofthemainnitratefragments:NO+ atm/Q=30Th particle size will obviously reduce the accuracy of the mIE + and NO m/Q=46Th) determined in the calibration (see calibration. The software then automatically calculates the 2 Eq.3belowandJimenezetal.,2003).Tothisend,arelative mIE onlyfromthesignaloftheNO+andNO+fragments NO3 2 ionisationefficiencyRIEisdefined: ofthenitrateusingEq.(3): mIE RIEi = mIENOi3. (2) mIENO3 = INO3,m/nQ·=ρ3V0+·fIN·Oq3,m/Q=46. (3) V The RIE of a species i with respect to the mass-based ion- i HereI aretheionsignalsinionss−1,nisthenumberof isation efficiency of NO is unitless. Commonly used RIE i,j 3 particles measured with the CPC in cm−3, ρ the density of values are 1.4 for organics and 1.3 for chloride. The RIE NH NO ingcm−3,V thevolumeofoneNH NO particle values of NH and SO should be calibrated at the begin- 4 3 4 3 4 4 incm3,f thefractionofNH NO thatisnitrateandq the ning of each deployment and then be reviewed on a regular 4 3 V sampling flow in cm3s−1. At the same time the RIE is basis.Typicallytheyliebetween2.5–5and0.6–1.2,respec- NH4 determined from the signal of the ammonium fragments at tively. During the long-term measurement at the Jungfrau- joch, the ToF-ACSM RIEs were RIE =3.23±0.42 and m/Q=15, 16 and 17Th via the mass fraction of NH4 in RIE =0.65±0.05. NH4 NH4NO3. SO4 www.atmos-meas-tech.net/6/3225/2013/ Atmos.Meas.Tech.,6,3225–3241,2013 3230 R.Fröhlichetal.:TheToF-ACSM:aportableaerosolchemicalspeciationmonitor Figure 3 shows the summed ToF-ACSM signal of nitrate 3 8x10 (blue,leftaxis)andofammonium(orange,rightaxis)atsev- 3 6x10 eralmassconcentrationsoftherespectivespeciessampledby s) 6 Slope = 773.7 ± 5.11 N ttchoeenmtTraoastFsio-cAnosCn.ScNeMnot.treFaottihroabntotithsheoibohsnigesrh,veaerldianomevamerroraensiwupmoidnesseirgaonnfagtleh(eodsfeiscgponintae-l O ions (ions/3ToF-ACSM 42 R2 = 0.999825 RSl2ope== 205.989.7928 7±9 1.16 42MSCA-FoT4snoi( snHoi lessmass)resultsfromthehighRIENH4 (2.99). N RIENH4 = 2.99 ± 0.03 )s/ The final RIENO3, which later is applied to the ambient 0 0 data, is slightly higher than one (RIENO3,ambient=1.1) be- 0 5 10 15 20 25 30 causethetwofragmentsofnitrateusedinthecalibrationonly NO or NH mass (µg/m3) 3 4 accountforabout90%ofthetotalnitratesignal.Thepartof nitratethatfragmentsintoseparatenitrogenoroxygenatoms Fig. 3. NO3 (blue) and NH4 (orange) ionic signals from the mIE isnotmonitoredinthecalibrationbecauseofthelowsignal calibrationplottedagainstthemassoftherespectivespecies,calcu- latedfromtheoutputoftheCPC.Thedashedlinesrepresentlinear tobackgroundratioatthecorrespondingm/Qratios. fitstotheNH4(red)andNO3signals(green).Thecorresponding Once RIE has been measured, the RIE of sulphate NH4 slopesandcoefficientsofdeterminationaregivenintheboxes(red: (RIE )caneasilybedeterminedbysampling(NH ) SO particSlOe4sandadjustingRIE toyieldionbalanceb4et2ween4 NH4; green: NO3). The RIENH4 was calculated from the ratio of SO4 thedeterminedslopes. ammoniumandsulphate. Itisrecommendedtorepeatthesignal-to-masscalibration atleastevery8weeksduringnormaloperationandwithin- creasedfrequencyfollowingaventingofthevacuumcham- ammoniumandorganicsignalsareevident,yieldingvariabil- ber. ityof±0.2µgm−3,whilenitrate,sulphate,andchloridevary only±0.025µgm−3. 3.2 Detectionlimits The table in Fig. 4b compares published 3-σ detection limits of each species in ngm−3 for the Q-ACSM (Ng ThechemicalspeciesdetectablewiththeToF-ACSM(organ- etal.,2011;Sunetal.,2012)andToF-AMS(DeCarloetal., ics,ammonium,nitrate,sulphateandchloride)areretrieved 2006) instruments, scaled by Eq. (4), with those for the by a recombination of ionic signals of the single fragments ToF-ACSM equipped with the SGE detector. At 1min time thatthespeciesbreakdowntoduringionisationandvapori- resolution they are 182ngm−3 for ammonium, 198ngm−3 sation(Allanetal.,2004).Inthefollowing,detectionlimits for organics, 18ngm−3 for sulphate, 21ngm−3 for nitrate, aredefinedasthreetimesthestandarddeviation(3-σ)from and11ngm−3 forchloride.Inadditionwereportthedetec- zeroata1mintimeresolution.Theycanbescaledtodiffer- tion limits for a Q-ACSM measured simultaneously at the entaveragingtimesusingtheformula Jungfraujoch(formoreinformationwerefertoSect.4.1)to r those reported for the ToF-ACSM. In this context it is im- 60s DL =DL · , (4) portant to note that the detection limits are not absolute but t 1min t willvaryasafunctionofthebackgroundmassspectralsignal withthedetectionlimitof1minDL andDL beingthe (Drewnicketal.,2009).Aimoffutureworkistofurtherre- 1min t detectionlimitforagivenaveragingtimet.Detectionlimits ducetheimpactoftheairbeamontherecordedsignals.First arelargelygovernedbytheextenttowhichbackgroundsig- tests have demonstrated that an order of magnitude reduc- nal originating from ions of ambient gases (mainly O , N , tionintheair-to-aerosolsignalratiocomparedtothecurrent 2 2 Ar,H O,andCO )interfereswithmeasurementofaselected configuration presented in this manuscript is feasible while 2 2 species. Species with significant signal at m/Q affected by keepingthe totalaerosol throughput highenough. Thiscor- suchbackgroundsignalhavehigherdetectionlimits. responds to improvements of ∼3 times in organic species To measure the detection limits, an additional filter was sensitivity. placed on the ToF-ACSM inlet before the standard fil- An upgrade from the currently used SGE detector to the tered/unfiltered switching valve, yielding a constant stream less-robustandshorter-livedMCPswilldecreasetheshown of particle-free air. Figure 4a shows the time series of the detectionlimitsofallspeciesbyanotherfactorofabout4. difference signal (difference between the signal at the two With the two modifications described above, the ToF- positions of the switching valve) of this particle-free air. ACSM would advance into the domain of the C-ToF-AMS This signal is centred at zero by definition, with the ob- whosedetectionlimitsarecurrentlystilllowerbyfactorsbe- servedfluctuationsdeterminingspecies-dependentdetection tween 10 and 20, except for the chloride whose detection limits. Each data point in Fig. 4 was obtained from 20s of limitwith11ngm−3alreadyliesatasimilarlylowlevellike averaging, with the background signal recorded every 360s in the C-ToF-AMS (4ngm−3) and in the V-mode HR-ToF- (visible as gaps in the data stream). The higher interfer- AMS (12ngm−3). The V-mode HR-ToF-AMS has a better encesofwatervapourandotheratmosphericgaseswiththe massresolutionbutalsoslightlyhigherdetectionlimitsthan Atmos.Meas.Tech.,6,3225–3241,2013 www.atmos-meas-tech.net/6/3225/2013/ R.Fröhlichetal.:TheToF-ACSM:aportableaerosolchemicalspeciationmonitor 3231 ThelowerdetectionlimitsoftheToF-ACSMallowforthe µg/m)3 00..24 organic nitrate sulphate ammonium chloride One datapoint every 20s operationathighertimeresolutions(multipledatapointsper (onc. -00..20 minute) while achieving detection limits needed for ambi- mass c -0.4 0(a) 1000 2000 3000 4000 5000 6000 7000 efansttscahmanpgliensg.inTchoisncmeanktreastiiotnwleikllessuhitoerdt-tfeorrmsiptuluatmioenssawreheexre- Elapsed me (seconds) (b) QQ--AACCSSMM QQ--AACCSSMM QQ--AACCSSMM QQ--AACCSSMM TTooFF-- VV--mTooFd-AeMS CC--TTooFF--AAMMSS pected.TheToF-ACSMalsoprovidessignificantadvantages ((JNagy neet )al., ((SSuunn et ((TPhSi Is Mstuadyy ((TPhSi Is Astuugd y AACCSSMM (HDReTCoaFr-lAoMS ((DDeeCCaarrlloo et 2011) 2al0.,1 200)12) 2P0S1I 2L)ab 2JF0J122012) ((TPhSi Is Astuugd y 2(D0e0C6a)rlo et 2al0.,0 260)06) in measurement locations with slower temporal variability 2012) 2JF0J1220)12) al., 2006) but lower concentrations due to its lower detection limits. AAmmmmoonniiuumm 21655566 11336699 1704572 342530 117802 3388 1166 OOrrggaanniiccss 1831800 22995588 22143667 11250145 119988 2222 1199 For example, the ToF-ACSM detection limit for organics at SSuullpphhaattee 213301 332299 11026334 1908661 1178 55 22 a15mintimeresolutionis51.1ngm−3.Forsuchalowcon- NNiittrraattee 16063 338833 223660 17031 1291 33 11 CChhlloorriiddee 9670 116644 112304 17030 191 1122 44 centrationtofallabovetheQ-ACSMdetectionlimit,approx- *DL are scaled to 1 min by√tmeas/60s imately 9h of integration would be required (based on the Fig.4.(a)Signalsoffilteredair(inµgm−3)recordedwithanaver- JungfraujochdetectionlimitsscaledbyEq.4). agingtimeof20s.Duringthegapsinthetimeseriesthebackground signalwasmonitored.Green:organics,red:sulphate,blue:nitrate, orange:ammonium,pink:chloride.(b)Overviewoverthe1minde- 4 Firstdatasetandintercomparisons tectionlimitsinngm−3(3-σ)oftheToF-ACSMequippedwiththe SGE detector (red box), Q-ACSM, HR-ToF-AMS in V-mode and The initial deployment of the ToF-ACSM took place at the C-ToF-AMS. Sphinx Observatory of the High Altitude Research Station Jungfraujoch. The (ongoing) measurements proved the in- the C-ToF-AMS. Compared to the ToF-ACSM, the species strument’s stability and suitability for unattended long-term dependentlimitsoftheV-modeHR-ToF-AMSarelowerby monitoringoperation.Todateithasproducedmorethanten factorsbetween4and9. months of quantitative mass spectra of the alpine ambient TheToF-ACSMimprovesonthedetectionlimitsreported aerosol. Here we discuss ToF-ACSM performance during fortheQ-ACSMbyNgetal.(2011)byfactorsof8.5(NH ), thisinitialdeploymentandintercomparisonswithco-located 4 4.1 (organics), 7.3 (SO ), 3.1 (NO ), and 5.5 (Cl). Com- instrumentation. 4 3 paredtotheQ-ACSMdetectionlimitsreportedbySunetal. (2012), the ToF-ACSM shows improvement by a species- 4.1 Generalinformation dependent factor of 7.5 to 18.3. The difference between Ng TheSphinxObservatoryoftheHighAltitudeResearchSta- et al. (2011) and Sun et al. (2012) is probably due to the tionJungfraujochislocatedinthehighAlpsoftheBernese different environments the instruments were situated in, i.e. Oberland(Switzerland)ontopofanexposedrockformation higherpollutionlevelsalsocausehigherbackgroundsignals. (3580ma.s.l.) in the ridge between the mountains Jungfrau Thesamedifferencecanbeobservedinthedetectionlimits andMönch(coordinates:07◦5900200E,46◦3205300N).Itisac- measured with Q-ACSM used for the intercomparison pre- cessiblebycograilroadyear-roundandconnectedtotheIn- sented in Sect. 4.2.1. The average ambient mass concentra- ternet, which facilitates the unattended operation of instru- tionsandrelativehumidityaremuchhigheratPSIthanatthe mentsbyenablingremoteaccess. highalpinestationontheJungfraujoch.Thevaluesobserved While the temperature inside the laboratory is kept more in thePSI laboratory arecomparable to the values recorded orlessconstantabove20◦Candtheinletisconstantlyheated in Beijing by Sun et al. (2012), while the results from the to 25◦C, the monthly average outside temperature on the JungfraujocharecomparabletoNgetal.(2011).Thisistrue Jungfraujochstaysbelowthefreezingpointofwaterallyear for the organics, the nitrate and the chloride. The ammo- (between−1.2◦CinJulyandAugustand−14.2◦CinFebru- niumandsulphatedetectionlimits,however,areverydiffer- ary). The temperature difference between inside and out- ent in our instrument. A possible explanation is that the Q- side and the generally low relative humidity at that altitude ACSMsensitivitytothesespeciesdependsmorestronglyon guaranteedryaerosolwithouttheneedforadditionaldrying massspectrometertuningandthereforeismoreinstrument- equipment.Becauseofthelowatmosphericpressure(onav- specific. The side by side comparison of the Q-ACSM and erage around 650mbar), a 130µm critical orifice was used theToF-ACSMontheJungfraujochshowanimprovementof in place of the standard 100µm orifice to retain the normal thedetectionlimitswiththelatterbyfactorsof1.8(ammo- samplemassflowrate.Tominimiselossesinthe5mmcop- nium), 6.1 (organics), 58.9 (sulphate), 3.5 (nitrate), and 6.6 pertubingofthe∼2minletline,anadditionalflowofabout (chloride).Theenhancementislowestforammonium,which 50cm3s−1(sampleflow∼2.4cm3s−1)isused. ismostaffectedbyairandwaterinterference,andhighestfor The aerosol concentrations at the Jungfraujoch are low sulphate,havingasurprisinglyhighlimitofdetectioninour (average PM concentration in 2012: ∼3µgm−3; BAFU, Q-ACSMsystem. 10 2012), approximately 25% of which is mineral dust (Col- laud Coen et al., 2004) originating to a considerable part www.atmos-meas-tech.net/6/3225/2013/ Atmos.Meas.Tech.,6,3225–3241,2013 3232 R.Fröhlichetal.:TheToF-ACSM:aportableaerosolchemicalspeciationmonitor from occasional Saharan dust events. These refractory par- (a) ticles are not detectable with the ACSM. These low con- 1 centrations occur because the station lies within the clean 2 continental free troposphere most of the year, especially in 3 winter,whileinsummeraerosolgeneratedatloweraltitudes reachestheJungfraujochduetoverticalexchangeprocesses (Lugaueretal.,1998;Henneetal.,2004).TheJungfraujoch siteisafamoustouristdestinationandtheSphinxObserva- torycanalsobevisitedbytourists,leadingtooccasionallo- cal emission plumes from, e.g. helicopters, snow crawlers, restaurantsorcigarettesmoke. (b) 3 6 ammonium Local time on the Jungfraujoch is Central European chloride Time(UTC+01:00),usingdaylightsavingtimeinsummer 4 nitrate (UTC+02:00).Alldatareportedinthismanuscriptisgiven organic inUTC. 2 sulphate 0 4.2 Intercomparisons 09:30 09:40 09:50 The High Altitude Research Station Jungfraujoch is inte- 10.08.2012 UTC gratedintoseveralmonitoringnetworks,includingtheSwiss Fig. 5. (a) Organic mass concentrations measured on 10 Au- national air pollution monitoring network NABEL (Na- gust 2012 with the ToF-ACSM (red line, time resolution: 10min) tionales Beobachtungsnetz für Luftfremdstoffe), the GAW and the Q-ACSM (black line, time resolution: 30min). The aver- (GlobalAtmosphereWatch)programmeoftheWMO(World agespectrarecordedintheregionsmarkedinyellow(14:00–16:00, Meteorological Organization) and the European ACTRIS case 1, higher concentrations), green (04:00–06:00, case 2, lower (Aerosols, Clouds, and Trace gases Research InfraStruc- concentrations)andblue(10:00,case3,short-termpeakonlyseen ture Network) network. As such, a large number of co- byToF-ACSM)arediscussedinFig.6.(b)Timeseriesduringpe- located long-term measurements of both gas- and aerosol- riod3withthehighestavailabletimeresolutionof20s.Thegaps species are conducted. For example, continuous long-term inthedataindicateperiodstheinstrumentwasacquiringtheback- measurements of the aerosol scattering coefficient (with groundconcentrations. a nephelometer) and the particle number size distribution (with a scanning mobility particle sizer, SMPS) are per- formed within the framework of the GAW program, and a (green,γ ≈0.35µgm−3),and(3)duringoneofthepeaks org Q-ACSM was operated at the JFJ as part of the ACTRIS at10:00(blue). monitoring project during the second half of 2012. In ad- The Q-ACSM (black line) operated with a time resolu- dition, during the intensive INUIT-JFJ/CLACE (Ice Nuclei tion of 30min and the ToF-ACSM (red line) with a 10min Research Unit/CLoud and Aerosol Characterization Exper- time resolution. The two time traces follow the same trend iment) campaign of February 2013, additional instruments andshowgoodagreementintermsofabsoluteconcentration were deployed at the Jungfraujoch site, including a C-ToF- buttheToF-ACSMobservesseveralshort-termsignalpeaks. AMS.Datafromthesewereusedtointercomparewiththose A closer look at one of the peaks (3, Fig. 5b) in the ToF- recordedbytheToF-ACSM. ACSM at 20s time resolution demonstrates that this plume actuallyconsistsoftwoplumes,eachoflessthan3mindura- 4.2.1 Q-ACSM tion.TherearetworeasonswhytheQ-ACSMdoesnotmea- suretheseplumes.First,thetimeresolutionoftheQ-ACSM The Q-ACSM was operated for five months alongside the measurements(30min)decreasesabilityoftheinstrumentto ToF-ACSM at the Jungfraujoch site, utilising a shared inlet resolve rapidly changing concentrations. More importantly, line. afundamentaldifferenceexistsbetweenQ-ACSMandToF- In Fig. 5a, the mass concentration of the organic frac- ACSMmassspectralacquisition,inthattheToF-ACSMac- tion measured by the two instruments over the course of quires the entire mass spectrum simultaneously (i.e. with the 10 August 2012 is shown. This day provides the op- eachextraction,seeFig.1),whiletheQ-ACSMsequentially portunity to study the output mass spectra at three different scans m/Q, with each scan cycle lasting several minutes. interesting situations circled in the figure. These situations Thus,iftheQ-ACSMisnotscanningtherelevantpartofthe are (1) concentrations well above the detection limit of the massspectrumduringtheplume,suchaplumemaygounde- Q-ACSM between 14:00 and 16:00 (yellow, organic mass tectedoryieldonlyapartialmassspectrum,biasingthemea- concentration γ ≈1.90µgm−3), (2) concentrations close surement.SuchabiasisevidentinFig.5ainthataveraging org to the Q-ACSM detection limit between 04:00 and 06:00 theToF-ACSMdatatothe30mintimescaleoftheQ-ACSM Atmos.Meas.Tech.,6,3225–3241,2013 www.atmos-meas-tech.net/6/3225/2013/ R.Fröhlichetal.:TheToF-ACSM:aportableaerosolchemicalspeciationmonitor 3233 does not cause the instruments to agree, with the aerosol the quality of the Q-ACSM’s mass spectrum deteriorates. massheresignificantlyunderestimatedbytheQ-ACSM.The Thequalityofthemassspectrumiscrucialfortheapplication veracity of the short duration peaks measured by the ToF- ofstatisticalsourceapportionmentmethodslikepositivema- ACSM was verified by comparison with nephelometer data trix factorisation (PMF; Paatero, 1997; Paatero and Tapper, collectedat5mintimeresolution(seeSect.4.2.3andFig.8). 1994)andthemultilinearengine(ME-2;Paatero,1999). Figure 6 shows Q-ACSM and ToF-ACSM mass spectra Theorganicstickspectrumofcase3shownatthebottom averaged over the periods identified in Fig. 5a, arranged as ofFig.6lookssignificantlydifferenttothetwootherrather follows. The top panel shows the raw difference spectrum similarToF-ACSMspectraincases1and2.Thepronounced (calculatedfromthedifferencebetweentheunfilteredandfil- signalsatthemass-to-chargeratios41Th,55Th,57Thand teredrawspectrum)fortheQ-ACSMduringperiod1,while 69Th point towards a significant fraction of hydrocarbon- thesecondpanelshowstheorganicfractionofthisspectrum likeorganics(HOA)intheaerosol.Acomparisontoanambi- integratedtointegerm/Q(“organicstickspectrum”).Panels entmassspectrumofHOAfromacampaigninParis(Crippa 3and4showthecorrespondingrawandorganicstickspectra et al., 2013) yields a good correlation with an R2 of 0.78 fortheToF-ACSM.Thesameplotsaredisplayedforperiod2 for case 3, while the ToF-ACSM spectra in cases 1 and 2 in panels 5 through 8, while panel 9 gives the organic stick do not correlate with the HOA spectrum (R2=0.42 and 1 spectrum for period 3. To make the spectral patterns more R2=0.22). This result illustrates the utility of highly time 2 visible,onlytheregionbetweenm/Q=40Thand100This resolvedmeasurementsevenformonitoringapplications,as shownandinsomeplotstheyaxisisinterruptedtoshowthe herethelocalactivityofmachinerycanbeidentifiedanddis- highsignalsatm/Q=43Thandm/Q=44Th.Coefficients tinguishedfromtherestofthedataset. of determination (R2) given in the following are calculated The correlation in terms of absolute concentrations mea- onlyforthepartoftheorganicstickspectrabetweenm/Q= suredwiththeToF-andQ-ACSMisillustratedinFig.7.On 45Thand100Th;otherwisethecorrelationisdominatedby thetop,thetimeseriesoforganics,nitrate,sulphate,ammo- thehighsignalsat,e.g.m/Q=28Thorm/Q=44Th. nium,andchloridemeasuredwiththeToF-ACSMduringthe A comparison of both total raw difference spectrum and first six months of the deployment are shown together with stick organic spectrum between Q-ACSM and ToF-ACSM thecorrespondinginletpressuresandairbeamsignal.These forthecasewiththehigherconcentration(1)showsreason- two parameters are used to account for changes in flow or ableagreement(R2=0.62)exceptatthemasstochargera- detectorgainasdescribedinSect.3.1.Itisnotedthatmuch tiosofnaphthalenefragments(m/Q=61–64Th,m/Q=74– of the variation in the airbeam signal observed in this time 78Th, m/Q=86–89Th and m/Q=98–102Th). This is ex- seriesarisesfrompurposefulchangesinsettingsfortheopti- pected because the Q-ACSM (for calibration reasons) has misationofsignal.Thegapsinthetimeserieshavedifferent a source of naphthalene (C10H8) incorporated inside the reasons: planned interruptions to install hardware upgrades, vacuum chamber (Ng et al., 2011), constantly releasing software development, extended calibration and testing, or moleculesandtherewithcausingalargerbackgroundsignal unplanned replacement of failed prototype power modules. at the corresponding m/Q ratios. Additional differences in The prototype stage has now concluded and the system has the spectra originate from two short-lived plumes observed beenrunningcontinuouslyforseveralmonthswithoutinter- during the averaging period (cf. Fig. 5a), not being cap- ruption. turedbytheQ-ACSMduetothescanningofthequadrupole Figure 7c shows a two-week period (3 to 15 August) se- and lower temporal resolution. The correlation between Q- lectedforfurtheranalysisoftheToF-ACSM/Q-ACSMinter- ACSMandToF-ACSMbecomesmuchworsewhenthelow comparison. The organic mass concentrations measured by concentration case 2 is compared. While the ToF-ACSM the Q-ACSM (black line) and by the ToF-ACSM (red line, spectrumofcase2remainssimilartotheToF-ACSMspec- averaged to 30min time resolution) are plotted on the same trumofcase1(R2=0.81),theQ-ACSMspectrumofcase2 axis. Both instruments report approximately the same con- nowlooksverydifferentandevenhassomenegativevalues centrations and exhibit good temporal correlation. The gap atm/Q’sinfluencedbynaphthalenefragments.AR2of0.19 in this case was caused by a temporary interruption of the betweenQ-andToF-ACSMatcase2confirmsthisobserva- ionisercurrentduetoaresolvedhardwareissue. tion. Looking at the raw difference spectra, one can easily AtthebottomleftofFig.7,scatterplotsofthemasscon- recognise that while the ToF-ACSM spectrum still exhibits centrations of the four main species (chloride is not plotted nicelyseparated,Gaussian-shapedsignals,theQ-ACSMraw becausemeasuredconcentrationsalwayswerebelowdetec- differencespectrumisnoisy.Thepeaksarenotaseasilysep- tionlimitattheJungfraujoch)areshown.Thedatarecorded arable as in the raw difference spectrum of the Q-ACSM in with the ToF-ACSM are drawn on the y axis and the data case 1. In summary these observations confirm that when from the Q-ACSM on the x axis. Slope and coefficient of sampling from the same inlet, the mass spectra of both in- determination retrieved with a least orthogonal distance fit strumentsshow a goodagreement aslong as theconcentra- to the scatter data for each species are given in the plots. tionsarewellabovethedetectionlimitoftheQ-ACSM.Ap- There is a very good correlation for organics (R2=0.95, proaching the detection limit (DL =0.136µgm−3), org,30min www.atmos-meas-tech.net/6/3225/2013/ Atmos.Meas.Tech.,6,3225–3241,2013 3234 R.Fröhlichetal.:TheToF-ACSM:aportableaerosolchemicalspeciationmonitor TOTAL SIGNAL 1 ONLY ORGANICS SIGNAL Quad TOTAL SIGNAL 1 ONLY ORGANICS SIGNAL ToF TOTAL SIGNAL 2 Quad ONLY ORGANICS SIGNAL TOTAL SIGNAL 2 ToF ONLY ORGANICS SIGNAL 3 ONLY ORGANICS SIGNAL ToF R22 = 0.78 HOA Fig.6.AveragespectraoftheregionshighlightedinFig.5.Thespectraaregroupedintofourpairsofrawdifferencespectrum(calculated from the difference between the unfiltered and filtered raw spectrum) and the corresponding integrated UMR stick spectrum. The raw differencespectrashowthetotalsignalwhiletheUMRstickspectraonlyshowtheorganicfraction.SpectrarecordedwiththeQ-ACSM areshowninblueandToF-ACSMspectrainred.Thefirsttwopairsarefromtheafternoonperiodwiththehigherconcentrations(1),the followingtwopairsarefromthemorningperiodwiththelowerconcentrations,(2)andthelast(9th)spectrumisfromthepeak,whichwas onlyseenwiththeToF-ACSM(3).Coefficientsofcorrelationwerecalculatedforthespectralregionbetweenm/Q=45Thand100Th. BetweenToF-ACSMspectrumandQ-ACSMspectrumtheyareR2=0.62forcase1andR2=0.19forcase2.TheR2giveninthelastplot showsthecorrelationtoareferenceHOAspectrumfrom(Crippaetal.,2013). slope=1.06), nitrate (R2=0.94, slope=0.95) and sulphate measures higher values. This may be caused by additional (R2=0.87, slope=1.16) between ToF- and Q-ACSM. The noise in the scatter plot due to the low ammonium concen- largest difference is found for ammonium, which still has trations(maximum0.5µgm−3)typicalofhighaltitudesites a good correlation (R2=0.74, slope=1.30) but compared (Beig and Brasseur, 2000) and/or to the sensitivity of am- to the other species the scatter is larger and the ToF-ACSM moniumRIEtoinstrumenttuning,wheresmallerrorsinthe Atmos.Meas.Tech.,6,3225–3241,2013 www.atmos-meas-tech.net/6/3225/2013/

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Nov 26, 2013 Received: 4 July 2013 – Published in Atmos. Meas. Tech. Discuss. velop the electric fields which guide the ions are replaced with metallic is designed to oper- ate with the application programming interface from Google.
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