Mobility particle size spectrometers

20 Pages·2017·2.13 MB·English

by  A. Wiedensohler

Aerosol Science and Technology
ISSN: 0278-6826 (Print) 1521-7388 (Online) Journal homepage: http://www.tandfonline.com/loi/uast20
Mobility particle size spectrometers: Calibration
procedures and measurement uncertainties
A. Wiedensohler, A. Wiesner, K. Weinhold, W. Birmili, M. Hermann, M.
Merkel, T. Müller, S. Pfeifer, A. Schmidt, T. Tuch, F. Velarde, P. Quincey, S.
Seeger & A. Nowak
To cite this article: A. Wiedensohler, A. Wiesner, K. Weinhold, W. Birmili, M. Hermann, M. Merkel,
T. Müller, S. Pfeifer, A. Schmidt, T. Tuch, F. Velarde, P. Quincey, S. Seeger & A. Nowak (2017):
Mobility particle size spectrometers: Calibration procedures and measurement uncertainties,
Aerosol Science and Technology, DOI: 10.1080/02786826.2017.1387229
To link to this article: http://dx.doi.org/10.1080/02786826.2017.1387229
© 2017 The Author(s). Published
with license by Taylor & Francis© A.
Wiedensohler, A. Wiesner, K. Weinhold, W.
Birmili, M. Hermann, M. Merkel, T. Müller,
S. Pfeifer, A. Schmidt, T. Tuch, F. Velarde, P.
Quincey, S. Seeger, and A. Nowak
Accepted author version posted online: 11
Oct 2017.
Published online: 11 Oct 2017.
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AEROSOLSCIENCEANDTECHNOLOGY
https://doi.org/10.1080/02786826.2017.1387229
Mobility particle size spectrometers: Calibration procedures
and measurement uncertainties
A.Wiedensohlera,A.Wiesnera,K.Weinholda,W.Birmilia,e,M.Hermanna,M.Merkela,T.Mu€llera,S.Pfeifera,
A.Schmidta,T.Tucha,F.Velardea,f,P.Quinceyb,S.Seegerc,andA.Nowakd
aLeibnizInstituteforTroposphericResearch,Leipzig,Germany;bDepartmentofChemical,MedicalandEnvironmentalSciences,NationalPhysical
Laboratory,HamptonRoad,Teddington,Middlesex,UnitedKingdom;cMaterialsandAirPollutants,FederalInstituteforMaterialsResearchand
Testing,Berlin,Germany;dAerosolandParticleDiagnostic,Physikalisch-TechnischeBundesanstalt,Braunschweig,Germany;eGermanEnvironment
Agency,Berlin,Germany;fLaboratoryforAtmosphericPhysics,InstituteforPhysicsResearch,UniversidadMayordeSanAndres,LaPaz,Bolivia
7 ABSTRACT ARTICLEHISTORY
1
0 Mobilityparticlesizespectrometers(MPSS)belongtotheessentialinstrumentsinaerosolsciencethat Received6July2017
2
r determine the particle number size distribution (PNSD) in the submicrometer size range. Following Accepted26September2017
e
b calibration procedures and target uncertainties against standards and reference instruments are
m EDITOR
e suggestedforacompleteMPSSqualityassuranceprogram:(a)calibrationoftheCPCcountingefficiency PramodKulkarni
ov curve (within 5% for the plateau counting efficiency; within 1 nm for the 50% detection efficiency
N diameter), (b) sizing calibration of the MPSS, using a certified polystyrene latex (PSL) particle size
02 standardat203nm(within3%),(c)intercomparisonofthePNSDoftheMPSS(within10%and20%of
4 thedN/dlogDPconcentrationfortheparticlesizerange20–200and200–800nm,respectively),and(d)
0
7: intercomparison of the integral PNC of the MPSS (within 10%). Furthermore, following measurement
] at 0 umnactecrhtaiwntiitehsinha1v%ebeafetnerinsviezisntiggatceadli:b(raa)tioPnSLaptart2ic0l3esnizme.st(abn)daBridpsolianrthdeiffruasniognefcrohmarg1e0rs0tboa5se0d0nomn
y theradioactivenuclidesKr85,Am241,andNi63andanewionizerbasedoncoronadischargefollowthe
r
o
at recommended bipolar charge distribution, while soft X-ray-based charges may alter faster than
or expected.(c)Theuseofapositivehighvoltagesupplyshowa10%betterperformancethananegative
ab one.(d)TheintercomparisonoftheintegralPNCofanMPSSagainstthetotalnumberconcentrationis
L
al stillwithinthetargetuncertaintyatanambientpressureofapproximately500hPa.
c
si
y
h
P
al 1. Introduction a numerical inversion procedure (e.g., Hoppel 1978;
n
o
ati Mobility particle size spectrometers (MPSS) belong to Pfeiferetal.2014).
N the essential instruments in aerosol science that deter- Although these MPSSs are designed to operate autono-
[
y mine the particle number size distribution (PNSD) of mously with minimum attention, quality assurance meas-
b
ed the submicrometer aerosol particle population. Depend- ures are required on a regular basis to ensure the delivery
d
a ing on the setup, MPSSs are able to determine a PNSD of reliable data. Comparisons between different MPSS
o
nl fromtypicallyafewnanometerstoaroundonemicrom- instrument types and individuals have been reported in
w
Do eter in particle mobility diameter. The MPSS has been numerous studies (e.g., Dahmann et al. 2001; Khlystov
widely described in the literature (e.g., ten Brink et al. et al. 2001; Helsper et al. 2008; G(cid:2)omez-Moreno et al.
1983;Fissan etal. 1983;Kousakaetal.1985;Winklmayr 2015). Specific recommendationsfora complete systemof
et al. 1991; Wang and Flagan 1990; Chen et al. 1998; quality assurance measures, and well-founded estimates of
Wiedensohler et al. 2012), often under the name SMPS the measurement uncertainties have, however, been miss-
(ScanningMobilityParticleSizer)orDMPS(Differential ing in the literature. This article describes and discusses a
Mobility Particle Sizer). The primary product of an complete set of quality assurance measures for MPSS
MPSS measurement is an electrical particle mobility instruments based on bipolar diffusion charging and a
distribution, which needs to be converted to a PNSD by condensationparticlecounter(CPC)asthedetector.
CONTACT A.Wiedensohler [email protected] LeibnizInstituteforTroposphericResearch,Permoserstr.15,04318Leipzig,Germany.
Colorversionsofoneormoreofthefiguresinthearticlecanbefoundonlineatwww.tandfonline.com/uast.
©2017A.Wiedensohler,A.Wiesner,K.Weinhold,W.Birmili,M.Hermann,M.Merkel,T.M€uller,S.Pfeifer,A.Schmidt,T.Tuch,F.Velarde,P.Quincey,S.Seeger,andA.Nowakwithlicence
byTaylor&Francis
ThisisanOpenAccessarticledistributedunderthetermsoftheCreativeCommonsAttribution-NonCommercial-NoDerivativesLicense(http://creativecommons.org/licenses/by-nc-nd/
4.0/),whichpermitsnon-commercialre-use,distribution,andreproductioninanymedium,providedtheoriginalworkisproperlycited,andisnotaltered,transformed,orbuiltuponin
anyway.
2 A.WIEDENSOHLERETAL.
QualitystandardsforPNSDmeasurementsmaydiffer uncertainty. Producing a predictable bipolar charge dis-
for different applications such as atmospheric studies, tributionintheMPSSinstrumentisoneoftheprerequi-
laboratory investigations, workplace, or indoor exposure sitesforanaccuratedatainversion.
assessmentaswellasemissionsurveys.Theatmospheric To achieve the highest accuracy with an MPSS mea-
observationalinfrastructuresWMO-GAW(WorldMete- surement, it is also necessary to test the above-described
orologicalOrganization—GlobalAtmosphereWatch)and main technical components individually, i.e., (a) the
ACTRIS (Aerosols, Clouds, and Trace gases Research optional preimpactor (separating large particles), (b) the
InfraStructure), as two examples, have devised a quality bipolardiffusioncharger,(c)thedifferentialmobilityana-
assurance system that requires frequent instrumental lyzer (DMA), and (d) the CPC. To evaluate the perfor-
calibrations (WMO-GAW report 227; Wiedensohler mance, the following quality assurance procedures
et al. 2012). The European Center for Aerosol Calibra- (Figure 1) are proposed, based on our laboratory-based
tion (ECAC, http://www.actris-ecac.eu) and the World experienceattheECAC/WCCAP:
CalibrationCenterforAerosolPhysics(WCCAP,http:// a. calibration of the size-dependent counting effi-
wmo-gaw-wcc-aerosol-physics.org) offer such calibra- ciency curves of the reference CPC and candidate
tion services within WMO-GAW and ACTRIS. As a CPC(aspartofthecandidateMPSS)byatraceable
7 rule, an MPSS passes an ECAC/ WCCAP calibration if chain of particle number concentration to the SI
1
20 (a) the sizing uncertainty against a certified particle size (Syst(cid:3)eme international d’unit(cid:2)es) using a calibrated
ber standard of 203 nm is §3% or better, (b) the PNSD FCAE against primary standards for voltage, cur-
em should be within 10% against the reference MPSS in the rent,time,andmassflow
v
o sizerangefrom20to200nmand20%between200and b. calibration of the sizing of the DMA set-up by
N
2 800 nm, and (c) the integrated PNC agrees within 10% employingacertifiedparticlestandardsuchasPSL
0
4 with the PNC measured by an independently calibrated c. intercomparison of the candidate MPSS against a
0
7: referencecondensationparticlecounter(CPC). referenceMPSS
0
at The agreement of PNSDs measured by different d. intercomparison of the integrated PNC of the can-
]
y MPSSs has been found to be best for the particle size didate MPSS against the direct PNC measurement
r
o
at rangefrom20nmto200nminparticlemobilitydiame- ofareferenceCPC
r
bo ter (Wiedensohler et al. 2012). Outside this range, the UsingapreimpactorensuresthatthemeasuredPNSD
a
L deviations might be larger for a number of reasons: for will be physically limited at the cut-off diameter. This
cal particles < 20 nm, the fraction of charged particles in a improves the accuracy of the multiple charge inversion
si
y given bipolar charge equilibrium is low (Wiedensohler usedtoderivethePNSD.Theuseoftheimpactormight
h
al P 1988)andthetheoreticaldescriptionoflossesduetodif- notbeessentialinsituationswherethereisnosignificant
n fusionmightcauseelevateduncertainties.Aboveparticle PNC above the size range to be measured. In the atmo-
o
ati diameters of 200 nm, the larger fractions of multiply sphere, for example, the number concentration of par-
N
[ chargedparticlesandtheeffectofincorrectsizingonthe ticles larger than 800 nm is often very low, so a
y
b steep spectral slope possibly lead to an increased preimpactor might not be needed. However, the
d
e
d
a
o
nl
w
o
D
Figure1.TraceabilityofMPSSmeasurementstoasetofcertifiedstandardsandconventions.Tofacilitatethetime-efficientcalibration
ofcandidateinstruments,intermediatestandards(so-called“referenceinstruments”)areusedatthecalibrationcenter.
AEROSOLSCIENCEANDTECHNOLOGY 3
preimpactorisstronglyrecommendedifthereisasignif- Calculations cover charging states from ¡10 to C10
icant source of course particles such as mineral dust or elementarycharges(includingzerocharge),thefractions
seaspray. stronglydependingtheexcessofairionsandonthepar-
Prior to their electrical mobility classification, the aero- ticlesize,especiallyiftheparticlesurfaceareaconcentra-
sol particle population has to undergo a bipolar charging tionishighduetoapronouncedaccumulationmode.
process,becausecarryingatleastonechargeisaprerequi- ADMA(see,e.g.,KnutsonandWhitby1975;Liuand
site for particles to be separated in an electric field. One Pui 1974; Flagan 1999; Stolzenburg and McMurry 2008)
class of bipolar diffusion chargers work on the basis of is usually designed as a cylindrical capacitor in which
theionizationofairmoleculesbyradioactivematerialthat charged aerosol is injected through an annular slit close
creates a cloud of both positive and negative ion clusters. to the outer electrode and merged with the particle-free
Commonly used radioactive sources are Kr85, Am241, anddriedsheathairflow.
Po210, or Ni63. Ideally, the entire particle population will ADMAseparatestheparticlesaccordingtotheirelec-
reach a bipolar charge distribution, which, for example, trical mobility Z , which is a function of particle charge
p
(cid:1)
can be calculated based on the charging theory by Fuchs (n e),particlediameterD ,thecorrespondingCunning-
e p
(1963) and assumptions on the mobility and mass of the hamslipcorrectionC andthegasviscosityh:
C
7 positive and negative ion clusters. Wiedensohler (1988)
1
ber 20 idnistrtroidbuuctieodn abapseadramoenterfiiztatetidonexopferitmhiesntbailpodlaarta.chIatrgies ZpD uEe Dne¢e¢3pC¢hC¢Dp
m
e theoretically independent of ambient pressure at room
v
o temperature and therefore assumed valid for PNSD meas- It is important to note that the Cunningham slip cor-
N
2 urements performed in a tempered room. The bipolar rection is based on empirical evidence and conventional
0
4 charge equilibrium is based on the ratio of the mobility of values for it are given in ISO 15900 (see also Wiedensoh-
0
7: positive and negative air ions, which would change in the ler et al. 2012). If the geometry and size of the DMA and
0
at same manner, if the pressure changes (Fuchs 1963). The its operationalparameters such as sheath flow and sample
y] bipolar charge distribution is widely used in commercial flow are known, one can calculate and adjust the voltage
r
o
at and custom-designed instrument softwares and recom- between the electrodes needed to let the fraction of
r
bo mendedbyconventionasacomputationbasisinthestan- charged particles with the target electrical mobility pass
a
L dard ISO 15900 [https://www.iso.org/standard/39573.html through the DMA from the entrance to the annular exit
cal ] for the entire submicrometer particle size range. There slitinthecentralrodofthecapacitor.The“DMAtransfer
ysi are also alternative bipolar diffusion chargers. The ion- function”(Birmilietal.1997;Collinsetal.2002;Hagwood
h
al P izer uses positive and negative ions produced by corona et al. 1999; Knutson and Whitby 1975; Russell et al. 1995;
n discharge,resultinginabipolarchargedistributionsimi- Stolzenburg 1988; Zhang and Flagan 1996) can be
o
ati lar to ISO 15900, according to the manufacturer. The described as a triangle selecting a range of electricalmobi-
N
[ bipolar charging by soft X-rays produces a different litieswithanaveragedtransferprobabilityof50%.Inreal-
y
b bipolar charge distribution, which was determined and ity, the transfer function differs slightly from a triangle
d
de parameterizedbyTiggesetal.(2015). andthetransferprobabilitydecreaseswith decreasingpar-
a
o Theadvantageofbipolardiffusionchargingisarelative ticle diameter. Particle losses within a DMA, including a
nl
w independence from the PNC and the aerosol flow rate, as nonideal transfer function, can however be accounted for
o
D longastheequilibriumnumberconcentrationofionpairs by using the method of “equivalent length”, as described
is sufficiently higher than the PNC as well as the aerosol in Wiedensohler et al. (2012). This particle loss correction
flow is limited to few l/min. Since the typical equilibrium then onlydependson the DMA type and the aerosolflow
ion pair concentration is approximately 107 cm¡3, the rate. Possible malfunctions of a DMA could occur due to
PNC should not exceed 106 cm¡3. The calculated bipolar internal aerosol particle deposition, flow or electric field
chargedistributionmayhaveareducedaccuracydueto disturbances, arcing, or mishandling during disassembly/
a. the very low charging efficiency for particles reassembly. This is especially a problem if these nonideal-
smaller than 20 nm, which have a large fraction of itiesleadtoabroadeningofthetransferfunction.
unchargedparticles, Particles within a small electrical particle mobility
b. increasing fractions of multiply charged particles bandwidth pass through the DMA and their number
for the particle size ranges greater than 200 nm, concentrationismeasuredbytheCPC(see,e.g.,Agarwal
and andSem1980;StolzenburgandMcMurry1991;Wieden-
c. the charging state of the aerosol particles before sohleretal.1997;Hermannetal.2007;Tuchetal.2016).
enteringthecharger,forexample,anyextremeuni- Small particles with diameters below approximately
polarcharging. 100 nm need magnification to a size, which can be
4 A.WIEDENSOHLERETAL.
detectedopticallyintheCPC.Inaregularbutanol-based forparticlessmallerthanthecertifiedPSLparticlesizecal-
conductive cooling CPC, the aerosol flow is slightly ibration depends on the Cunningham slip correction and
heatedandthensaturatedwithbutanolvapor.Inacool- thus on its uncertainty. The intercomparisons of PNSDs
ingsection(alsocalledthecondenser),thebutanolvapor against each other, as well as the integrated PNC against
becomes supersaturated and condenses onto the par- the reference CPC are based on “black box” approaches,
ticles, forming droplets of approximately 10 mm. These because the bipolar charge distribution, Cunningham slip
droplets are led through a focusing nozzle and then correction, and diffusion loss correction used in the data
counted individually by a laser optic. From the droplet inversion are conventions and are not traceable. While
counting frequency, counting interval duration, and the the intercomparison of the PNSDs of the candidate and
aerosol flow rate, the time resolved PNC can be calcu- thereference MPSSsonly providesaqualitative result(the
lated with a time resolution of typically 1 s. The lower PNSD is principally not traceable), a quantitative perfor-
detectionefficiencydiameterisdeterminedbytheKelvin mance measure is given by the intercomparison of the
diameteroftheparticles,whichisafunctionofthephys- integratedPNCofthecandidateMPSSagainstthedirectly
icaldiameterandthechemicalaffinitybetweentheparti- measured PNC of the reference CPC. Preferably, the
cle and the butanol, and can be varied by changing the CPCsshouldhavethesameD .
P50
7 temperature difference between the saturator and the MPSS-based calibrations at the ECAC/WCCAP are
1
20 cooling section. The chemical affinity is dependent on usually done with atmospheric aerosols. This procedure
r
be theparticlessurfacepropertiesandhencetheCPCdetec- hastwomainreasons:
em tion efficiency is – especially at lower sizes—particle 1. The atmospheric aerosol is already naturally pre-
v
o material dependent. This material dependency is how- charged, but with an unknown nonequilibrium
N
2 ever rather small if butanol is used as working fluid and bipolar charge distribution. When using a bipolar
0
4 the temperature difference between saturator and con- diffusion charger, there is a high probability of
0
7: denserisoptimizedtoreachaspecificD . obtainingabipolarchargedistributioncorrespond-
0 P50
at Malfunctions may occur if (a) the saturation process ingclosetotheidealequilibrium.
]
y is not at optimum, (b) the temperature difference is not 2. A unipolar precharged laboratory-generated aero-
r
o
at stable, (c) droplets are lost by impaction on the edges of sol needs more interaction with gas ions to be
r
bo the focusing nozzle, (d) the particle beam is outside the brought into a bipolar charge equilibrium, e.g., by
La laser focus, or (e) the nominal flow rate is not reached. using a bipolar charger with a sufficient activity.
cal Manufacturers give uncertainties of up to 10% to the However,iftheequilibriumisnotreached,thecal-
si
y measured PNC. In reality, however, it is only a few per- culations of PNSDs and PNCsbased on the multi-
h
al P centiftheinstrumentistechnicallyperformingwell. ple charge corrections will be inaccurate. This
n wouldleadtoaninvalidintercomparison.
o
ati Results from several ECAC/WCCAP calibration
N 2. Calibrationprocedures
[ workshops are shown to demonstrate the different steps
y
b The calibration procedures mentioned above have been of the calibration procedure. These results are for illus-
d
de implemented at ECAC/WCCAP for instruments used trativepurposes,andtheresearchinstitutionsofthecan-
a
o in atmospheric aerosol measurements, and are here didateinstrumentshavebeenanonymizedinthelegends
nl
w describedindetail.Thecalibrationprocedures are,how- ofthedifferentfigures.
o
D ever, also suitable for other applications. Calibration of
an MPSS should ideally be done at a calibration center
2.1. Calibrationofcondensationparticlecounters
with international reputation, which meets the require-
mentsoftheISOstandardsoratmosphericobservational 2.1.1. Traceabilityofthereferenceparticlecounter
networks,especiallySI-traceability,andprovidingoneor TheCPCusedtomeasurethePNCiscalibratedagainsta
more regularly maintained and calibrated reference reference FCAE. A FCAE consists of an electrically con-
MPSSs and CPCs. However, calibrations can be per- ducting and grounded cup as a guard to cover the sens-
formed at measurement stations. These calibrations can ing element that includes a conducting high efficiency
includethePSLsizingandareferenceCPCandMPSSas aerosol filter (not necessarily conductive) to capture
comparison instruments, which have been calibrated aerosol particles, an electrical connection between the
beforeinthelaboratory. sensing element and an electrometer circuit, and a flow
The determination of size-dependent CPC counting meter. The capture efficiency for a FCAE is expected to
efficiencycurvesandthesizingoftheDMAsetup(atthe begreaterthan98%forparticlesabove5nmandsample
scale of the PSL particles) are traceable calibrations, flowsabove1l/min.The mainchallengeisthatparticles
because they can be traced back to SI units. The sizing might get lost by diffusion before entering the cup and
AEROSOLSCIENCEANDTECHNOLOGY 5
thenarenotdetected.FCAEsmeasureverysmallelectri- generation of the monodisperse calibration aerosol is
calcurrentsdowntothefemtoAmpererange,orrespec- briefly described in the following paragraph. Silver is
tively, electrical charge densities as small as 10¡15 evaporated in a N carrier gas flow through a tube fur-
2
Coulomb/cm3; and the signal to noise ratio is the limit- nace aerosol generator (Scheibel and Porstend€orfer
ing factor. FCAEs used for calibration purposes must— 1983).Inthecoolingsection,supersaturatedsilvervapor
according to ISO 27891:2015—have a stable zero base- nucleatestoprimaryparticlesafewnanometersindiam-
line, i.e., the zero-corrected absolute arithmetic mean eter.Thenucleationprocessandthesubsequentagglom-
electric current when no particles are present must be eration of the silver particles are then quenched by an
less than1 fA(femto-ampere) witha standard deviation additionalN flow.Afterward,thesilverparticleagglom-
2
(cid:3)
below 0.5 fA. If, for example, singly charged particles eratesareannealedat450 Cbypassingthroughasecond
¡
with a PNC of 1000 cm 3; are measured, the corre- tube furnace and solidify as nearly spherical particles at
sponding electrical current reading of an FCAE at 1 l/ theexit.Thesilverparticlesarethenchargedinabipolar
min sample flow would then be 2.67 fA with a relative diffusion charger. Monodisperse calibration aerosols in
uncertainty of at least § 20% (according to an absolute thesizerangeof3–40nmcanthenbeselectedbyusinga
uncertaintyof atleast§0.5fA).Acomprehensivecom- Nano-DMAsetup.ThesizeresolutioninaDMAisinflu-
7 parisonofFCAEmeasurementsfoundabout§5%rela- enced by the sheath to sample airflow ratio but also by
1
0
2 tive deviation (deviation of the means) among eight diffusionbroadening.Athighratios,e.g.,20:1,thetrans-
ber FCAEs with 1 l/min sample flow. The uncertainties in mitted particle fraction is highly monodisperse with a
em this test were up to 20% (Hogstr€om et al. 2014). typical geometric standard deviation (GSD) well below
v
o Although uncertainties are expected to increase at 1.1. This ratio should be high (a) to minimize the non-
N
2 smaller particle sizes and lower concentrations, the diffusive width of the transfer function and (b) to mini-
0
4 results provided experimental evidence that the require- mize the contribution of diffusional broadening to the
0
7: mentsoftheISOstandardcanpracticallybemet. transfer function. Finally, the test aerosol is diluted with
0
at The ECAC/WCCAP reference FCAE is SI-calibrated particle-free room air in a mixing chamber to adjust the
y] against a femto-Ampere source at the German national PNC and to provide sufficient flow rate for all
r
ato metrology institute (PTB—Physikalisch-Technische instruments.
r
bo Bundesanstalt, Braunschweig) The reference CPC of the Thelinearitycalibrationisdonewith40nmmonodis-
a
L ECAC/WCCAP is twice per year calibrated against the perseparticlesasshowninFigure3.A1:1slopedemon-
cal reference aerosol electrometer using singly charged strates an asymptotic counting efficiency of 100% (also
ysi monodispersesilverparticles. called plateau efficiency). We use a particle size of
h
nal P set-Fuigpurpere2vioshuoslwysdtehsecrEibCeAdCin/WTCuCchAPetCaPl.C(2c0a1li6b)r.atTiohne 4p0latnemau,bcoecuanutsinegtheeffiCciPeCncTy.SImodel3772operatesatthe
o
ati
N
[
y
b
d
e
d
a
o
nl
w
o
D
Figure2.CPCcalibrationsetupusedatECAC/WCCAP.
6 A.WIEDENSOHLERETAL.
For the CPC calibration of the detection efficiency
curve, the particle number concentration of the mono-
disperse silver particles should be in the range of 1000–
5000 cm¡3 to avoid coincidence in the measuring
volumeoftheCPCoptics,andtoreachasufficientnum-
ber concentration for the aerosol electrometer measure-
ment. At WCCAP/ECAC, we use an electrometer flow
rate of 4 l/min, decreasing the lower detection limit to
approximately200cm¡3foracceptableaccuracyandsig-
nal to noise ratio of the current. Monodisperse particles
are generated in the range 5–40 nm and the counting
efficiencyiscalculated,takingintoaccount:
a) themeasuredaerosolflowrate
b) the number of particle counts at the digital CPC
pulseout
7 c) thecountingtime
1
0
2 Figure 3.Calibration of the ECAC/WCCAP reference CPC model d) thePNCderivedfromtheelectrometer
r
be 3772 (coincidence-corrected) against the reference aerosol elec- To evaluate the calibration results, following targets
em trometermodelTSImodel3068B.Dataareplottedatatimereso- areconsidered.
ov lutionof1s. a) The plateau efficiency should not deviate more
N
2 than 5% from the reference CPC (manufacturers
0
4 normallyindicateanuncertaintyupto10%).
0
7: 2.1.2. Calibrationofcandidatecondensation b) Thelowercounting efficiency diameterof theCPC
0
at particlecounters 3772 for silver particles normally ranges from 7 to
y] AsuccessfulCPCcalibrationintermsoftheCPCcount- 9nmasshowninTuchetal.(2016)(themanufac-
r
ato ing efficiency curve and the DP50 is a prerequisite for turergivesthevalueof10nm).
r
bo evaluating the performance of an MPSS. The following The performance of a CPC can be evaluated by using
La steps have to be considered, taking a regular CPC TSI abestfittoafunctionthatdescribesthesteeppartofthe
cal model3772asanexample: detectionefficiencycurve.Asanexample,Figure4shows
ysi 1. Initial status check without any maintenance to theCPCcountingefficiencycurvesofaninitialandfinal
h
nal P oab.tMaineatshuerisntagtutshoeftehxeacctanCdPidCateaeCrPoCso.l flow rate, acanldibMratciMonu.rTryhe(1t9h9e1o)reitsicaallsofitgfiuvennctiinonthfreofimguSrteolaznendbwurags
o
ati which is allowed to deviate up to 3% from the used for data interpretation. The fit parameters are the
[N nominal 1 l/min. The exact flow rate should be plateau efficiency A [dimensionless],the lowerdetection
by usedinthecalculationofthecountingefficiency limit B [nm] and D , C [nm]; the particle diameter is
ded b. Checking the CPC counting efficiency curve. If denotedbyx. P50
a
o necessary, the candidate instrument will then In this example, the CPC was cleaned after the initial
nl
w gothroughmaintenance check. Note that the counting efficiency curve and the
o
D 2. MaintenanceofthecandidateCPC: lowerdetectionefficiencydiameterofthereferenceCPC
a. Cleaning of the saturator wick or, alternatively, are determined during the calibration workshops in
replacingitwithanewone(thiscanbedoneby parallel.
theuseronaregularschedule)
b. Cleaning of the aerosol nozzle that focuses the 2.1.3. UncertaintyinCPCefficiencycalibration
dropletflowintotheoptics(thiscanbedoneby For the determination of the CPC detection efficiency, a
theuseronaregularschedule) detailed methodology is described in ISO 27891:2015.
c. Cleaning of the critical orifice that ensures a The main difference to the setup in Figure 2 is that the
constant aerosol flow rate (this can be done by monodisperse test aerosol is split into only two equiva-
theuseronaregularschedule) lentpartialflowsandthePNCismeasuredinparallelby
d. Cleaning the optics, if necessary (this can be only one candidate CPC and a reference FCAE. While
onlydonebyanexperiencedperson) methodology and outcome are in principle the same in
e. Measuringtheactualflowrateagain both cases, the ISO standard has an elaborated concept
3. Final calibration after maintenance of the candi- for the estimation of uncertainties in the determination
dateCPC of the candidate CPC’s detection efficiency. Due to the
AEROSOLSCIENCEANDTECHNOLOGY 7
7
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al Figure4. ExampleofCPCcountingefficiencycurvesoftheinitialandfinalcalibrationaftermaintenance.Theuppertwofiguresshow
sic theinitial(a)andfinal(b)calibrationofacasewhenDP50wasnotwithinthetargetrangeof7–9nm,whilethelowertwofiguresshow
y anexamplewhenthetargetplateauefficiencywasnotreachedduringtheinitialcalibration(c)butwasreachedaftermaintenanceand
h
al P successfuladjustment(d).(ancountingefficiencyof1correspondsto100%).
n
o
ati
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[ complexityoftheexperimentalsetupandthecalibration and can be determined semiempirically in the pla-
y
b procedureitself,straightforwarderrorpropagationisnot teau region. The uncertainty is in all probability
d
de feasible and the combined relative standard uncertainty few % depending on the selected monodisperse
oa in the detection efficiency is the result of several type A diameter.
nl
w (repetition and statistical treatment) and type B (other d. Splitter bias: A case-by-case evaluation quantifies a
o
D means) uncertainty components. The following uncer- possiblemisbalanceofthePNCsinthepartialflows
taintycomponentsareconsidered: by repeated measurements. The bias factor should
a. FCAEdetectionefficiency:Thecorrespondingtype bewithinthe0.95–1.05interval.Therelativeuncer-
B uncertainty is usually evaluated by a national taintyisusuallyverysmall(below1%).
metrology institute on a regular basis. Values are e. Repeatability of the efficiency determination: This
expectedtobebelow1%. component is evaluated case-by-case through
b. FCAE and CPC flow rates and their uncertainties short-term repetitions. Values below 1% are
are evaluated case-by-case by repeated comparison expected.
toacalibratedSI-traceablemassflowmeter.Uncer- f. Uncertainty contribution from test particle size
taintiesaremostlikelybelow2%. uncertainty: This component has an effect in the
c. Uncertaintyinmultiplechargecorrection:Depend- steep region where the CPC detection efficiency is
ingonthetestaerosolgenerationandthesizeofthe significantlysizedependent.Itistheproductofthe
particles, the FCAE raw data have to be corrected relative size uncertainty and the slope of the effi-
for multiply charged particles before conversion ciencycurve,whichitselfisthetargetofthecalibra-
intoPNC.Acase-by-caseevaluationisunavoidable tion. The uncertainty of the DMA-selected particle
8 A.WIEDENSOHLERETAL.
size results from the calibration with certified concentration of particles larger than 800 nm is negligible
monodispersetestparticlestandardoraDMAcali- in ambient air at ECAC/WCCAP, which represents an
brated according to ISO 15900:2009, trusting the urbanbackgroundaerosolpopulation.
Cunningham correction factor. The width of the Inthenextsections,thedifferentcalibrationstepsare
monodisperse particle size bin is then dependent explainedindetail,showingexamplesofresults.
on the aerosolflowto sheath flowrateand the dif-
fusionbroadening. 2.2.1. Sizingcalibrationandadjustment
Accordingtotheseestimations,thecombinedrelative At the ECAC/WCCAP, PSL (polystyrene latex) particles
standard uncertainty in the detection efficiency of CPCs (spheres) with a certified diameter of 203 nm are used
seemstobebelow10%inmostcases. forthesizingcalibration.Thereasonsare:
a) Thenumberconcentrationof203nmPSLparticles
is still sufficiently high in a dilute suspension (one
2.2. Calibrationofmobilityparticlesize
“drop” of PSL particle solution (1% by volume) in
spectrometers
150mlpurifiedwater)tomeasureastatisticallyrel-
A complete calibration of a candidate MPSS is complex evantnumberconcentrationpeak.
7 andrequiresthefollowingsteps: b) The layer of residual material from the aqueous
1
20 a) Set-upofthecandidateMPSStotheconfigurationas solutiononthePSLparticlesafterdryingisnotsig-
ber itisusuallyoperatedattheobservatoryorlaboratory nificantforPSLparticleslargerthen100nm.
m
e b) Set-up of the reference MPSS and CPC parallel to We consider the test at a single PSL size (203 nm) as
v
o thecandidateMPSS sufficient.Experimentalteststhatjustifythisdecisionare
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2 c) Initial intercomparison run of the candidate MPSS supplied later in Sect. 3.1.1. The sizing calibration con-
0
4 andthe reference MPSS andthe reference CPC for tainsthefollowingsteps:
0
7: the total particle number concentration for at least a) The initialcalibrationis done withPSL particles of
0
at 8htoobtainstatisticallyrelevantresults 203nmnominalsize.The initialcalibrationissuc-
]
y d) Evaluation of the results of the intercomparison of cessful,ifthegeometricmeandiameterofthemain
r
o
at thePNSDsandtheintercomparisonofPNCs peak recorded by the candidate MPSS operated as
bor e) Calibration of the candidate CPC of the MPSS normal, is within 3% of the certified PSL particle
a
L against the reference aerosol electrometer or refer- size(197to209nm).
cal enceCPCasdescribedabove b) The initial calibration is not successful, if the mea-
si
y f) Calibration of the sizing of the candidate MPSS suredpeakdiameterdeviatesmorethan3%fromthe
h
al P with certified particle PSL size standards with a nominal PSL particle size. According to our experi-
n highsizeresolution ence, in this case the best practice is to adjust the
o
ati g) Ifnecessary,solvingtechnicalproblemsofthecan- sheath air so that the correct size classification is
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[ didate MPSS, which cause deviations from the ref- achieved.Themostlikelycauseoftheerroristhatthe
y
b erence instruments that are larger than the target sheath airflow rate has changed with time due to a
d
de uncertaintyvalues shift of the flow meter. An adjustment of the “effec-
oa h) Final intercomparison run of the candidate MPSS tive length” in the calculation of the DMA-voltage is
nl
w and the reference MPSS against the reference CPC not considered suitable, since the geometry of the
o
D for the total particle number concentration for at DMAisratheraccurateandwellknown.However,if
least8h.IntercomparisonofthePNSDs(candidate the deviation ishigher than 10%, other causes of the
MPSS vs. reference MPSS) and PNC (integrated errorshouldbeconsidered.
candidateMPSSvs.referenceCPC) c) In the final calibration, it is again checked whether
i) Confirmation of a successful or a non-successful themainpeakiswithin3%ofthenominalPSLpar-
calibrationintermsofadetailedreport ticlesizeof203nm.
The ECAC/WCCAP reference MPSS was described in Forthesizingcalibration,wescantheparticlesizerange
Wiedensohleretal.(2012).ECAC/WCCAPcurrentlypro- from 100 to 400 nm with a high size resolution (up and
videsfivesuchreferenceinstrumentsforcalibrations.Their down scans are typically approximately 4 min each) to be
main components are a bipolar diffusion charger, using a abletoquantifythepeakdiameter.Theupanddownscan
Kr85 370 MBq radioactive source, a Hauke-type DMA havetooverlapcorrectly,otherwisethedelaytimeiswrong
(Winklmayretal.1991)witha28cmlongelectrode,anda forofanMPSSinscanningmodeoperation.
CPC model TSI 3772. The measured PNSD covers the Figure5showsanexampleofthePSLparticlecalibra-
rangefrom10to800nminmobilitydiameter.Nopreim- tionofacandidateMPSS.Thecurvesshowfourmaxima,
pactorisusedduringthecalibrationexperiments,sincethe whichare:
AEROSOLSCIENCEANDTECHNOLOGY 9
a) the main peak at the nominal PSL particle size of distributions have been calculated using the bipolar
203nmforsinglychargedparticles charge distribution given in ISO15900, based on the
b) thefirstpeaktotheleftofthenominaldiameter(at parameterizationinWiedensohler(1988).
131 nm) represents doubly charged particles of For a successful intercomparison, the PNSD of the
203nm candidateMPSSneedstodivergenomorethanC/¡10%
c) thesecondpeaktotheleftofthenominaldiameter(at against the reference MPSS over the size range 20–
103nm)representstriplychargedparticlesof203nm 200nm.AnexamplePNSDintercomparisonisshownin
d) the low, broad peak to the right of the nominal Figure 6. The candidate MPSS is represented by the red
diameter represents doublets or triplets (agglomer- curveandthereferenceinstrumentbytheblackcurve.
ationoftwoorthreePSLparticles) Inthesecondstepofasuccessfulcalibration,thePNC
The red curve is the initial PSL calibration, while the of the reference CPC is intercompared against the inte-
blackcurveshowsthefinalsizingcalibrationafterasuc- grated PNC of the reference and candidate MPSS as
cessfuladjustmentofthesheathflowrate. shown in the scatter plots of Figure 7a and Figure 7b.
Because the majority of commercial and custom- Theslopeisclosetooneinbothcasesandisthuswithin
designed MPSSs are operated in scanning mode, we sug- thetargetrangefrom0.9to1.1.
7 gest to measure the electrical particle mobility distribution
1
0
2 with a high size resolution to obtain more than 20 data
r 2.2.3. ExampleofascheduledECAC/WCCAPMPSS
be points for the main PSL peak. The evaluation of the peak
em diameter can then be done by fitting a normal function intercomparisonworkshop
v In this section, we illustrate how MPSSs of atmospheric
o through the data points (see also Figure 5 solid line). This
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observatories generally perform during a scheduled
2 assuresahighprecisionofthesizingcalibration.
4 0 ECAC/WCCAP MPSS intercomparison workshop. As
7:0 mentionedabove,itisimportanttoscheduleregularcali-
0 2.2.2. Intercomparisonofparticlenumber
] at concentrationsandsizedistributions bknraotwionnsunocferMtaPinStSiess.to assure a high data quality with
y
or During the intercomparison of the candidate MPSS Figure 8a and Figure 8b show the PNSDs of all candi-
orat against the reference MPSS and reference CPC, the date MPSSs against the reference MPSS (black line) of the
ab instrumentsareconnectedto acommonmanifold,sam- initialandfinalcalibrationrun(eachapproximately8hin
L
al pling ambient aerosol. To obtain a sufficient counting total), respectively. For this workshop, it can be concluded
sic statistic, the initial and final intercomparison runs are that the candidate MPSSs performed generally rather well
y
Ph done for at least 8 h, respectively. To avoid misinterpre- during the initial calibration run (Figure 8a). Only one
al tations, periods with a clear nucleation mode are MPSS was outside the target uncertainty range of
n
atio excluded from the analysis. The particle number size C/¡10% (dashed lines) compared to the reference MPSS.
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Figure 5. Calibrations of the sizing of the MPSS DMA using PSL
particleswithanominaldiameterof203nm.Theexampleshows Figure 6. Successful intercomparison of the PNSD of the candi-
theinitialandfinalPSLcalibrationofacandidateMPSS.Thered date (red curve) against the reference MPSS (black curve). The
curve is the initial PSL calibration, while the black curve shows PNSDofthecandidateMPSSiswithin10%againstthereference
the final sizing calibration after a successful adjustment of the over a wide size range. The comparison was done for periods
sheathflowrate. whennonucleationmodehavebeenpresent.