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Vol. 21 No. 4ļ¼2006ļ¼ ļ¼47ļ¼ 329
Research Paper J. Aerosol Res.ļ¼21ļ¼4ļ¼ļ¼329-340ļ¼2006ļ¼
Aerosol Size Distribution Measurement Using Multi-ChannelElectrical Mobility Sensor
Panich INTRAļ¼ and Nakorn TIPPAYAWONGļ¼
Received 2 December 2005Accepted 26 July 2006
Abstractļ¼Aerosol particles can be classified according to their electrical mobility. A multi-channel detector has been developed to classify electrically charged airborne particles under theinfluence of an electric field. The detector is capable of measuring the electrical mobility ofparticles in the sub-micrometer size range. It is compactly designed as an assembly of twoconcentrically-aligned cylindrical short electrodes with a fixed clearance. Sheath air and aerosolflows enter from one end, pass through the annulus and exit the other end. An electric field isapplied between the inner and outer electrodes. Particles with a given electrical mobility arecollected on a designated electrometer ring where electrical signals are measured to obtain sizedistributions. In this study, a mathematical model was developed to optimize the locations ofparticle detection sensors and the calculations of particle trajectories were performed for variousparticle sizes. A prototype detector constructed in the present work gave the particle sizes whichare in good agreement with those obtained by a scanning electron microscope. Signal currentfrom the detector was also analyzed to give number concentration of particles. Experimentalresults agreed well with the theoretical predictions. The proposed model was proven to be usefulin designing the detector and the prototype detector showed promising results for aerosol sizemeasurement.
Key Wordsļ¼ Aerosol, Electrical Mobility, Size Distribution, Spectrometry, Electron Microscopy.
1ļ¼Introduction
Aerosols are found in many human environmentsas well as processes in various industries such asfood, pharmaceutical and medical, electronic andsemiconductor industries. Governments and industrystress the importance of aerosol measurement, not onlyto protect the environment and health of their peopleas required by law, but also to improve industrialprocesses, increase productivity and gain competitiveadvantage. Information about the size spectrum ofthese fine particles is required if they are to be fully
understood and controlled. A widely used type ofdevice capable of measuring these ultra-fine particlesis an electrical mobility analyzer such as a differentialmobility analyzerļ¼DMAļ¼. A typical setup of theinstrument consists of a pair of parallel electrodesbetween which a potential is applied and a gas flows.An aerosol flow containing charged particles isintroduced adjacent to one of the electrodes. Aparticle-free sheath flow initially separates the aerosolflow from the opposite electrode. The electric fieldcauses charged particles to move toward the oppositeelectrode across the gap between the electrodes,according to their electrical mobility which is relatedto their size.
There have been numerous studies and developmentson the particle electrical mobility analyzer. Many
ļ¼ Department of Mechanical Engineering, Faculty of Engineering,Chiang Mai UniversityChiang Mai 50200, Thailand
330 ļ¼48ļ¼ ćØć¢ćć¾ć«ē ē©¶
previous studies concern single size channel detection,improvements in performance and expansion ofmeasurement range 1ļ½ 7ļ¼. Recent developments werereviewed by Flagan 8ļ¼. To measure particle size spectra,a DMA capable of single channel signal detection isconnected to a particle counter and operated in ascanning voltage mode, measuring a complete sizespectrum in about 30ļ½60 s. Similar instruments withmulti-channel signal detection capability have recentlybeen under development by several workers. Tammetet al. 9ļ¼designed and developed an electrical aerosolspectrometerļ¼EASļ¼which is able to classify particlesin similar fashion to, but faster than, a typical DMAdue to its multi-channel measurement capability.Graskow 10ļ¼developed a fast aerosol spectrometer tomeasure nanoparticles. Biskos et al. 11ļ¼later reporteda development of a differential mobility spectrometerļ¼DMSļ¼, derived from Graskowās concept. Kulon etal. 12ļ¼described a bipolar charge aerosol classifierwhich uses electrostatic technique with differentclassifying sections operating at a number of appliedvoltages in series. The system is able to simultaneouslymeasure particle charge as well as size. The presentstudy focuses on development of a multi-channel sizeanalyzer able to measure aerosol size distribution nearreal time, based on similar principle to previouslymentioned instruments. Nonetheless, there werecollective differences between the present spectrometerand each of the existing instruments, which were asfollows ;ļ¼iļ¼the concept of the present instrument was
based on a compact, inexpensive and portable unit.Short column classifier and a small number of detectionchannels were used to reduce diffusion effect of theparticle inside the classifier. Overall dimensions andweight were such that it was easy to handle and movearound ;ļ¼iiļ¼the instrument adopted a tangentialaerosol inlet upstream of the first electrode ring toensure uniform particle distribution across the annularaerosol entrance to the classifier column ;ļ¼iiiļ¼ratherthan diffusion charging, the instrument employedunipolar coronaļ¼diffusion and fieldļ¼chargingmethod ;ļ¼ivļ¼the applied voltage was set to maintainat low level, well below the corona onset voltage, toavoid unintentional charging of the particles inside theclassifier ; andļ¼vļ¼the incompressible Navier-Stokesequation and Maxwellās equations were numericallysolved in previous studies 13ļ¼to investigate flow andelectric fields inside the classifier and obtain appropriatedimensions, geometries and arrangement of the chargerand the electrostatic classifier in this instrument. Acomparison between the present instruments and theexisting instruments is shown in Table 1.
In this work, a simple model was developed topredict particle trajectories which influence theperformance of the detector and were used as a basisfor an aerosol size spectrometer system design. Theperformance of the spectrometer was examined usinga field emission-scanning electron microscopeobservation method with particle classification.Typical evaluation of number concentration and size
Table 1 Comparison between the present instrument and the existing spectrometers
EAS DMS BCAC EMSļ¼Tammet et al. 9ļ¼ļ¼ ļ¼Biskos et al. 11ļ¼ļ¼ ļ¼Kulon et al. 12ļ¼ļ¼ ļ¼This workļ¼
Measurement technique Electrical mobility Electrical mobility Electrical mobility Electrical mobilitySize range 10 to 10,000 nm 5 to 1,000 nm ļ¼ 3,000 nm 10 to 1,000 nmConcentration range 108ļ½1011 to 2Ć 104ļ½ 1Ć 109 to 4Ć 1013 n / a 1011 to 1013 particles / m3
5Ć 107 particles / m3 particles / m3
Time response ļ¼ 1 s 200ms 10 s 30 sCharger type Corona charger Corona charger Corona charger Corona chargerCharging method Unipolar diffusion and Unipolar diffusion charging Bipolar diffusion charging Unipolar diffusion and
field charging field chargingParticle detector Electrometers Electrometers Electrometers ElectrometersElectrometer channels 26 26 10 10Aerosol inlet technique n / a n / a n / a Tangential inletAerosol flow rate 12 L / min 5 L / min 2 L / min 1 L / minSheath air flow rate 36 L / min 35 L / min 58 L / min 10 L / minOperating pressure n / a 25.3 kPa n / a 34.5 kPaElectrode applied voltage 800 V 5 kV to 10 kV 5 kV 500 V to 3 kVClassifier length n / a 700mm 2,000mm 131mmInner electrode radius n / a 12.5mm 1.5mm 10mmOuter electrode radius n / a 26.5mm 25mm 25mm
n / a : information not available
Vol. 21 No. 4ļ¼2006ļ¼ ļ¼49ļ¼ 331
distribution were also presented.
2ļ¼Aerosol Transport under Electric Field
Aerosol size spectrometry in this research study willbe based on differential electrical mobility classification.The model described in the following sections isapplicable to aerosol transport under a strong electricfield inside a concentric cylinder classifier. Solutions tothe simplified governing equations are proposed andpresented. Particle motions are dependent on externalforces applied to the particles, such as gravitationalforce, drag force, diffusion force and electrical force.The particles move in both axial and radial directions.The axial motion was influenced by the fluid velocityprofile in the axial laminar flow. The radial motion isdue to the electric force which is the dominant force.When the particles are introduced into the classificationcolumn, any charged particle under the influence ofan electric field will have an electrical mobility, Zp,defined as the velocity per unit field. The electricalmobility of charged particle can be derived by equatingthe electric field force, Feļ¼ neE with the Stokes dragforce, Fdļ¼ 3 pĀµdpu / Cc and electrical mobility isgiven by :
ļ¼1ļ¼
where Āµ is air viscosity, u is the radial component ofparticle velocity, dp is the particle diameter, Cc isCunningham slip correction factors, e is charge ofelectronļ¼1.6Ć 10ļ¼19 Cļ¼and n is the number ofcharge on the particle, E is electric field strength inthe gap between the concentric cylinders. It is assumedthat the air flow is axisymmetric, laminar, fullydeveloped, and incompressible. If the end effects areneglected, then the electric field for the concentriccylindersļ¼the classifierļ¼will have only a radialcomponent with intensity given by
ļ¼2ļ¼
Here V is applied voltage, r is radius, r1 and r2 are radiiof the inner and outer cylinder, respectively. Particlesmay become charged due to field charging, diffusioncharging, bipolar charging or photo charging. In thisstudy, combined field and diffusion charging isassumed as appropriate for fine particles. The averagecharge ndiff caused by the diffusion charging in a timeperiod t by a particle diameter dp can be found fromthe Whiteās equation 14ļ¼:
ļ¼3ļ¼
where e is the elementary unit of charge, cļ¼i is the meanthermal speed of the ionsļ¼240 m / sļ¼, k is theBoltzmannās constantļ¼1.380658Ć 10ļ¼23 J / K, forairļ¼, T is the temperature, KE is the constant ofproportionality, Ni is the ion concentration, and t is theresidence time of the charger. For the corona-wirecharger 15ļ¼, an approximate expression for the Nitproduct can be derived :
ļ¼4ļ¼
where r1c and r2c are the radial position of the outerand inner charger cylinder, respectively, Ic is thecharging current, Zi is the mobility of ionļ¼equal to0.00014m2 / V.s for the positive ionļ¼, Ecļ¼rļ¼is thecharging electric field as a function of radial position,and Qc is the total flow rate through the charger. Thefraction charge per particle nfield caused by the fieldcharging in an electric field E derived by White 14ļ¼
and is given by
ļ¼5ļ¼
where e is the particle dielectric constant. Finally, bothfield and diffusion charging occur at the same time.This is known as continuum charging where particlecharge is the sum of the contributions from field anddiffusion charging. Fraction charged as a function ofthe particle diameter estimated from Whiteās theory isshown in Fig. 1. For the particle diameter less than
100
10
1
0.1
Mea
n ch
arge
per
par
ticle
Diffusion chargingField chargingCombined charging
Nitļ¼2.90Ć1014 ionsć»mļ¼3 sļ¼1
1 10 100 1000Particle diameterļ¼nmļ¼
Fig. 1 Fraction charged against particle diameter.
332 ļ¼50ļ¼ ćØć¢ćć¾ć«ē ē©¶
1 Āµm, the electric field existed in the charger has anegligible effect on the charging process 3ļ¼.
The most commonly used configuration for anelectrical mobility analyzer employs concentricannular flows. A flat velocity profile was commonlyassumed, even through the actual velocity profile inthe instrumentās annular cavity was a non-uniformdeveloping profile. Liu et al. 16ļ¼reported that thevalue of the electrical mobility calculated on the basisof a uniform flow with a flat velocity profile in theclassifier was approximately 12ļ¼ higher than thevalue calculated on the basis of a fully developed,non-uniform velocity profile. The transfer function wasapplied by Biskos et al. 11ļ¼to a multi-channel DMS.They noted that the transfer function shape would beinfluenced by the gas velocity profile within theannulus. In Biskos et al. analysis, a simplified modelthat masked these effects was used. Therefore, in thepresent study, rather than the widely adopted transferfunction method, the trajectory method that takes intoaccount the flow velocity profile was employed. Thissimple model is able to predict the particle motionbehavior, mobility range, and size classification of themulti-channel spectrometer. Additionally, the trajectorymethod is capable of showing how particles moveinside the annular gap which is useful in comparisonwith flow visuals.
For efficient performance of any electrical mobilityanalyzers, operation at laminar flow conditions isrequired so that particle trajectories can be accuratelydetermined. Neglecting gravitational effect andBrownian motion, the non-diffusive particle trajectoryis described by the system of differential equations.
ļ¼6ļ¼
ļ¼7ļ¼
where ur and uz denote the radial and axialcomponents of the air flow velocity. Similarly, Er andEz denote the radial and axial components of theelectric field. For a fully developed parabolic velocityprofile in the concentric annular, urļ¼ 0, uzļ¼ uzļ¼rļ¼,Erļ¼rļ¼ļ¼V /ļ½r lnļ¼r2 / r1ļ¼ļ½, and Ezļ¼ 0. Substitutinginto Eqs.ļ¼6ļ¼andļ¼7ļ¼gives the following differentialequations describing the trajectory of an aerosolparticle in an annular flow :
ļ¼8ļ¼
ļ¼9ļ¼
where :
dp / dz denotes the constant pressure gradient. UsingEqs.ļ¼8ļ¼andļ¼9ļ¼, the trajectory of the particle in anannular flow is given by
ļ¼10ļ¼
Integrating Eq.ļ¼10ļ¼, the migration paths of the chargedparticles can be determined. Their landing locationdownstream of the aerosol inlet is given in terms oftheir electrical mobility, the mean velocity of the flow,and the electric field strength :
ļ¼11ļ¼
The particle entering the classifier at a radial positionof r1 has trajectory taking it to an axial position of z,which can be obtained as
ļ¼12ļ¼
and the expression for the trajectory of the particle thattake into account the flow velocity profile entering thespectrometer at a radial position of r1ļ¼ d which hastrajectory taking it to an axial position of z and isgiven by
ļ¼13ļ¼
Once, the particle trajectories inside the instrument are
Vol. 21 No. 4ļ¼2006ļ¼ ļ¼51ļ¼ 333
known, size classification of deposited particles canbe determined.
3ļ¼Electrical Mobility Spectrometry
3.1 Spectrometer DescriptionA schematic of the aerosol size spectrometer used
in this research study is depicted in Fig. 2. Thespectrometer has one short column which consists ofcoaxially cylindrical electrodes. The advantage ofcylindrical geometry is that distortion of electric fieldbetween electrodes is minimal due to the absence ofcorners and edges. Operation and performance of theinstrument depend upon aerosol transport under theinfluence of flow and electric fields. It was importantto ensure that both flow and electric fields were laminarand uniformly distributed inside the classifyingcolumn. There are two streams which are the aerosoland sheath air flows. The two flows are regulated andcontrolled via mass flow controllers. The inside of theannular was constructed in such a way that smoothwall and turbulence free merging of the two gas flowswere ensured. Nonetheless, the flow field inside theapparatus was further investigated by solvingnumerically the continuity and Navier-Stokes equationsusing a commercial computational fluid dynamicsoftware package, CFDRCTM, reported in 13ļ¼. It wasshown that a parabolic velocity profile was establisheda few hydraulic diameters downstream from inlets,while negligible disturbances occurred at the pointwhere the two flowsļ¼aerosol and sheath airļ¼merged.Flow simulation results showed similar trend to thoseby Chen and Pui 17ļ¼and Chen et al. 18ļ¼. For furtherchecking of the flow, the flow pattern inside theclassifier was examined using a similar flowvisualization techniques reported by Otani et al. 19ļ¼,Asai et al. 20ļ¼, and Myojo et al. 21ļ¼. It was possible toevaluate the extent of laminar and turbulent flowswithin the classifier. Flow visualization showed similarresults to those reported in the literature. The chargedparticles enter the analyzer column near the centralrod by a continuous flow of air. Since the central rodis kept at a positive high voltage, the charged particlesare deflected radially outward. They are collected onelectrically isolated electrometer rings positioned atthe inner surface of the outer electrode of the column.Electrometers connected to these electrodes measurecurrents corresponding to the number concentrationof particles of a given mobility which is related to theparticle size. Electrical current detection method wasconsidered to be easier and faster than direct particle
detection measurements. In addition, the applied highvoltage is maintained at a lower value than the coronaonset voltage to avoid unintentional charging of theparticles within the classifier. 3.2 Mobility and Size Classification
The theoretical background on aerosol transportpreviously described was used in designing thespectrometer system. In order to classify particlesize, the particle electrical mobility was considered.The design practice was similar to Kulon et al. 12ļ¼.The spectrometer was divided into a number ofwell-insulated virtual ground electrometer ringsļ¼Rļ¼ 1012Wļ¼. Fig. 3 shows the principle of themobility classification. Intra and Tippayawong 13ļ¼
reported that the region between the inner electrodeand the electrometer rings on the outer wall exhibiteda uniform distribution of electric field that similar trendto those by Chen and Pui 17ļ¼and Chen et al. 18ļ¼. It cangenerally be considered to be a function of radius only.However, there were small non-uniformities close tothe wall between each electrometer ring gap whichwere made of electrical insulator. The effect wasdependent on ring width to ring separation ratio, aswell as ring separation. In this work, the arrangementwas designed in such a way that the effect of existenceof the insulator between neighboring electrodes is notsignificant. Each outer electrometer ring of thespectrometer represents one size classification channel
Sheath air flowļ¼Qsļ¼
Isolation housing Electrometer rings
High voltage central electrode Outlet
Isolation rings
Charged particlesļ¼Qaļ¼
Outer Housing
Electrometers
i
ur
uz
Fig. 2 Schematic of an aerosol size spectrometer based onelectrical mobility technique.
Sheath airflow
zi ze, i
Zmaxćp, i Zmin
ćp, i
Aerosolflow
Fig. 3 Principle of mobility classification in the spectrometer.
334 ļ¼52ļ¼ ćØć¢ćć¾ć«ē ē©¶
and its axial location along the column depends on theparticle electrical mobility. Considering one particularelectrometer ring: the particle enters the spectrometerat the position immediately adjacent to the inner wall,and is deposited at the leading edge of the ring. In thiscase, the maximum mobility, Zmax
p, i , of particle to depositon this ring is
ļ¼14ļ¼
where zi is the axial position between the aerosol entrylocation and the leading edge of the electrometer ring.This equation can be used to determine the diameterof the particle where all the terms are known. UsingStokeās law to classically define the electric mobility,Zp which can be written in terms of dmax
p, i , the particlediameter with the maximum mobility is given by
ļ¼15ļ¼
Similarly, the minimum mobility, Zminp, i , of the particle
that enters the spectrometer at the outer most radialposition of the aerosol inletļ¼at rļ¼ r1ļ¼ dļ¼anddeposits at the trailing edge of the ring is
ļ¼16ļ¼
where ze, i is the width of the electrometer ring, and dis the width of the aerosol flow inlet. The particle size,dmin
p, i , with the minimum mobility is given by
ļ¼17ļ¼
Only particles with electrical mobility between thesetwo limits Zmin
p, iļ¼ Zp, iļ¼ Zmaxp, i will deposit on this ring
and contribute to the measured signal. The range ofelectrical mobility is a function of the voltage appliedto the central electrode, flow settings, geometricalfactors of the spectrometer and particle size. Assuminga uniformly distributed particle concentration at theentrance, a constant electrometer ring widthļ¼12mmļ¼,a given ring separationļ¼1mmļ¼and a fixed number ofelectrometer ringsļ¼10 ringsļ¼, relationships betweenthe predicted electrical mobility and each sizeclassification channel along the lengthļ¼131mmļ¼ofthe spectrometer column for a given operatingconditionļ¼1.0 L / min aerosol flow, 10.0 L / minsheath air flow, 1.0 kV central rod voltage, 34.5 kPaoperating pressureļ¼can be computed. Typical resultsare illustrated in Figs. 4 and 5 for mobility intervalsand their corresponding particle size bins, respectively.It was found that the mobility range for each channelwas not evenly distributed and there was a slightoverlap between adjacent channels in terms of theelectrical mobility predicted. For a better design of thespectrometer, these overlaps were minimized bymanipulation of the voltage applied to the centralelectrode, flow settings and the geometrical factors ofthe spectrometer.3.3 Number Concentration from Signal Current
MeasurementThe current from the deposition of charged particles
on each electrometer ring is magnified by the amplifierand measured by the sensitive electrometer. If themobility distribution function fļ¼Zpļ¼is defined as
10ļ¼5
10ļ¼6
10ļ¼7
Ele
ctri
cal m
obili
tyļ¼m2
Vļ¼1
sļ¼1 ļ¼
Electrometer ring number
1 2 3 4 5 6 7 8 9 10
Fig. 4 Predicted electrical mobility range at each electrometerringļ¼1.0 L / min aerosol flow, 10.0 L / min sheath airflow, 1.0 kV central rod voltage, 34.5 kPa operatingpressureļ¼.
100 101 102 103
Particle diameterļ¼nmļ¼
Ele
ctro
met
er r
ing
num
ber
10
9
8
7
6
5
4
3
2
1
Fig. 5 Predicted particle size range at each electrometer ringļ¼1.0 L / min aerosol flow, 10.0 L / min sheath air flow,1.0 kV central rod voltage, 34.5 kPa operating pressureļ¼.
Vol. 21 No. 4ļ¼2006ļ¼ ļ¼53ļ¼ 335
ļ¼18ļ¼
where dNp is the number of aerosols in the mobilityrange dZp. For the spectrometer with sheath air flow,the signal current of a singly charged particles, Ie, canbe derived making use of the well known result thatthe rate at which particles are collected on the outerelectrode of the spectrometer is given by :
ļ¼19ļ¼
where Q is the sample flow rate, e is the value of theelementary charge, Zp is the particle electricalmobilityļ¼i.e. particle drift velocity / field strengthļ¼,and fļ¼Zpļ¼dZp is the number concentration of aerosol,Np, with mobility between Zp and Zpļ¼ dZp. For thepresent spectrometer, considering the critical mobilityin the range of Zmin
p, i ļ¼ Zp, iļ¼ Zmaxp, i , all particle are
measured, the signal current of a singly chargedparticles collected on the electrometer ring in channeli become
ļ¼20ļ¼
where Ie, i is the electrometer signal current measuredat channel i, and Qa is the sample aerosol flow rate.Therefore, the currents have the simple form as
ļ¼21ļ¼
where Np, i is the particle number concentration atchannel i. The signal current was then used toevaluate particle number concentration. Thus, theparticle number concentration of particles, Np, i, in themobility range from Zmin
p, i ļ¼ Zp, iļ¼ Zmaxp, i is related to
the signal current, Ie, i , as follows :
ļ¼22ļ¼
In fact, the aerosol contains not only singly chargedparticles of the desired particle size, but also multiplycharged particles of a larger particle size. Thus, themultiply charged particles must be applied to Eq.ļ¼22ļ¼,and can be rewritten to give
ļ¼23ļ¼
where nļ¼dpļ¼is the average number of elementarycharges carried by particles with diameter dp. To
obtain size distribution, the geometric midpointdiameter dmid
p, i , is calculated as :
ļ¼24ļ¼
where dminp, i , is the particle diameter with minimum
mobility in channel i, and dmaxp, i , is the particle diameter
with maximum mobility in channel i. The geometricmidpoint particle number concentration in channel ican be approximated as :
ļ¼25ļ¼
The measured size channel i distribution resultscorrespond to the channel concentration Np, i dividedby the channel geometric width :
ļ¼26ļ¼
4ļ¼Experimental Evaluation
4.1 Experimental SetupA test facility to evaluate a prototype of the aerosol
size spectrometer was constructed, as shown in Fig. 6.The system consists of a diode type corona particlecharger, a spectrometer, a signal detection system, aflow arrangement and a computer controlled interfacesystem. The combustion aerosol generator was used togenerate a polydisperse carbonaceous diffusion flameaerosol for this experiment. An aerosol flow rate of1.0L / min was used and a pre-filtered sheath air flowwas supplied at 10.0 lpm, controlled by flowcontrollers. The flow was delivered by means of avacuum pump. The flow was conditioned by placing aperforated screen upstream to ensure uniform laminarflow. The classifying column is comprised of astainless steel tube with 25mm diameter, a centralelectrode rod of 10mm diameter and 131mm inlength. A potential of 1.0 kV applied to the centralelectrode was used. The spectrometer system wasmaintained at 34.5 kPa in order to increase mobilityresolution for particle with diameter greater than100 nm 11ļ¼. The signal current from deposited chargedparticles on each electrode ring was measured with aKeithley 6517A electrometer and a Keithley 6522scanner card, interfaced to a personal computer via anIEEE-488 interface.4.2 Collection and Analysis for SEM
In this study, the deposited particles inside thespectrometer at each electrometer ring were analyzed
336 ļ¼54ļ¼ ćØć¢ćć¾ć«ē ē©¶
for their size using a scanning electron microscopeļ¼SEMļ¼. The sampling was carried out using a 3mm
SEM copper tape placed on each inner surface of theelectrometer ring, as shown in Fig. 7. Aerosol sampleswere collected electrostatically for at least 30min ofoperation. Particle imaging was carried out using aJEOL JSM-6335F Field Emission Scanning ElectronMicroscope. During the SEM analysis, agglomerateswere selected and imaged randomly to minimize bias.Magnifications between 9,500 X and 80,000 X weretypically used, giving three to six particles per image.The SEM projected surface area distribution wasobtained first, by thresholding the original SEM imageand next, by calculating the projected surface area ofeach particle. Image processing was carried out usingImageJ, a public domain image analysis softwarewhich was developed at the National Institutes ofHealthļ¼http://rsb.info.nih.gov/ij/ļ¼. A portion of theparticles were found to be non-spherical andcoagulated. It was known that the fraction ofcoagulated particles increased with an increase incollection time of the particles. In the procedures,highly coagulated particles were excluded to avoid theconfusion between the coagulation taking place duringparticle growth in the gas phase, or on the samplingplate during particle collection. For each data point,about 18ļ½ 40 particle fields of view distributedthroughout the sampling plate were used for SEM
analysis to estimate particle surface area anddetermine the corresponding geometric mean projectedarea diameter. The equivalent mean projected areadiameterļ¼dPAļ¼can be calculated from the projectedsurface areaļ¼Aļ¼and is given by
ļ¼27ļ¼
It should be noted that in the free molecular regimeļ¼Knā« 1ļ¼, aerosol surface area is equivalent to
geometric surface area for spherical particles. Becauseparticle mobility and molecule attachment rate aregoverned by particle-molecule collisions, it is thereforetheoretically possible to use the mobility analysistechnique to measure the aerosol surface area 24ļ¼.Rogak et al. 25ļ¼demonstrated that for mobilitydiameters smaller than 400 nmļ¼extending well intothe transition regimeļ¼, the equivalent sphere projectedarea diameterļ¼the diameter of a sphere having thesame projected areaļ¼of particles scaled with the
Outer electrode
Inner electrode
3 mm SEM copper tape
Fig. 7 Schematic of the SEM sampler constructed.
Flowmeter
Corona chargerFlowmeter
Impactor Combustionaerosol
generator
External computer,data logging,user interface
Positive highvoltage supply
Positive highvoltage supply
Filter
Filter
Sheath airE
lect
rom
eter
rin
gs
Hig
h vo
ltage
ele
ctro
de
Kei
thle
y el
ectr
omet
er 6517A
ļ¼Sc
anne
r ca
rd 6522
Excess air
Vacuumpump
E1 E2 E3 E4 E5 E6 E7 E8 E9 E10
Fig. 6 Experimental setup for the measurement system.
Vol. 21 No. 4ļ¼2006ļ¼ ļ¼55ļ¼ 337
particle mobility diameter for fractal-like particles. Itwas noted that the overall trend of the experimentalmeasurements showed close agreement with thatpredicted theoretically. Similar methods of particlesize comparison were conducted and reported byCamata et al. 26ļ¼, Hummes et al. 27ļ¼, Kuga et al. 28ļ¼,Seol et al. 29ļ¼, Ku and Maynard 24ļ¼. In free molecularregime, the electrical mobility of the charged particlebecomes
ļ¼28ļ¼
where l is the mean free path of the carrier gas. Thus,combining Eqs.ļ¼15ļ¼andļ¼28ļ¼, it can be seen that inthe free molecular regime, the particle diameter withmaximum mobility is given by
ļ¼29ļ¼
Similarly, combining Eqs.ļ¼17ļ¼andļ¼28ļ¼, the particlediameter with minimum mobility is given by
ļ¼30ļ¼
5ļ¼Results and Discussion
5.1 Size Comparison with SEM ResultsFig. 8 shows typical SEM images of the particles
collected on selected electrometer rings. Theclassification sizes wereļ¼aļ¼177 nm,ļ¼bļ¼191 nm,ļ¼cļ¼262 nm,ļ¼dļ¼314 nm,ļ¼eļ¼363 nm, andļ¼fļ¼470 nmwith the inner electrode voltage of 1.0 kV, aerosol flowrate of 1.0 L / min, sheath air flow rate of 10.0 L / minand operating pressure of 34.5 kPa. The SEM data foreach individual data point is shown in Table 2. As canbe seen from Table 2 that after classification theparticles appeared to approximate monodisperse sizesystem, with calculated geometric standard deviations
of about 1.03ļ½ 1.13. Fig. 9 provides comparison ofpredictedļ¼geometric midpointļ¼mobility diameterwith average measuredļ¼geometric meanļ¼equivalentsphere projected area diameter at each electrometerring in the spectrometer. The data represents particlesin the size range between 100ļ½ 450 nm. From theresults obtained in this investigation, it was found thatthe diameter derived from projected surface area ofagglomerates analyzed by SEM agreed well withprediction from mobility analysis. The largestdifference observed was about 15ļ¼ at 150 nm. Atother sizes, the differences were within 5ļ¼ . Theoverestimation by the present spectrometer was insimilar magnitude to that reported by Camata et al. 26ļ¼
using radial DMA, Hummes et al. 27ļ¼using short-type DMA, Kuga et al. 28ļ¼using low pressure DMA,Seol et al. 29ļ¼using very low pressure DMA, andDeppert et al. 30ļ¼using Vienna DMA. It should be
Table 2 Measured diameter data for each data point
Data Minimum Maximum Geometric mean Geometric standardpoint diameterļ¼nmļ¼diameterļ¼nmļ¼ diameterļ¼nmļ¼ deviation
1 148.69 207.80 177.61 1.052 178.44 213.15 191.91 1.033 198.02 349.00 262.89 1.084 198.67 432.62 314.08 1.135 273.73 481.38 363.00 1.076 332.54 499.55 407.58 1.06
ļ¼aļ¼dgļ¼177.61 nm ļ¼bļ¼dgļ¼191.91 nm
ļ¼cļ¼dgļ¼262.89 nm ļ¼dļ¼dgļ¼314.08 nm
ļ¼eļ¼dgļ¼363.00 nm ļ¼fļ¼dgļ¼407.58 nm
Fig. 8 Typical particle morphologies of agglomerates collectedļ¼1.0 L / min aerosol flow, 10.0 L / min sheath air flow,1.0 kV central rod voltage, 34.5 kPa operating pressureļ¼.
338 ļ¼56ļ¼ ćØć¢ćć¾ć«ē ē©¶
noted that the difference between the size obtained bythese DMAs and the SEM observation from literaturewere slightly larger than that found in this work. Thepossible reasons for the difference in measurementsfrom SEM and the spectrometer were considered to bedue mainly to the irregularity of the particle shape andthe multiply-charged particles 26, 29ļ¼. It was known thatthe SEM-measured particle size was consistent withthe spectrometer-measured one in the case of sphericalparticles 27ļ¼. For the effect of the multiply chargedparticle, the multiply charged aerosols have the sameelectrical mobility diameter, and may therefore becollected on the same electrometer ring. Consequently,the electrical signal current measured at any givenelectrometer ring will be due to particles of differentphysical sizes. In other words, if the EMS is set toextract singly charged particle of 20 nm in diameter,then doubly charged particle of 29 nm in diameter, andtriply charged particle of 36 nm in diameter would beextracted because the mobilities of these particles arethe same. Another reason for the underestimation ofSEM may be attributed to the simplification of theSEM size measurements which were the lack of highquality focusingļ¼which probably had 5ļ½ 10ļ¼ measurement uncertaintyļ¼, changes in particle sizesduring sampling, calibration errors, all the coagulatedparticles shown in these figure were formed on the SEMsampling plate, and difficulties in size determination.In case of coagulated particles, because the mobilityof a sphere having volume equivalent to such acoagulated particle was slightly greater than that ofthe coagulated particle 31ļ¼, the size of the coagulatedparticle classified by the spectrometer was slightlysmaller than the predicted size determined from thespectrometer. The detailed reasons for these differencesshould be investigated further. Overall, taking intoaccount the fact that the classification performance ofour EMS approximately followed the theoreticalprediction, the 15ļ¼ difference was considered tobe acceptable. It was thereby confirmed that thespectrometer was capable of correctly determiningparticle mobility diameter.5.2 Mobility and Size Distribution
Preliminary results were obtained and one typicalresult is depicted in Fig. 10. Signal current for thedistribution of the test aerosol size spectrum for eachelectrode was clearly shown, their values of the signalcurrent was in similar order of magnitude to thosereported by Graskow 10ļ¼. The signal current was thenused to evaluate number concentration and size
Part
icle
siz
e di
stri
butio
n dN
/ dl
ogļ¼d pļ¼ļ¼
part
icle
/ m3 ļ¼
3.5Ć1012
3.0Ć1012
2.5Ć1012
2.0Ć1012
1.5Ć1012
1.0Ć1012
5.0Ć1011
0.0
Particle diameterļ¼nmļ¼ 10 100 1000
Fig. 11 Measured size distribution of aerosolļ¼1.0 L / minaerosol flow, 10.0 L / min sheath air flow, 1.0 kVcentral rod voltage, 34.5 kPa operating pressureļ¼.
Electrometer ring number
Ele
ctro
met
er c
urre
ntļ¼pAļ¼
30
25
20
15
10
5
01 2 3 4 5 6 7 8 9 10
Fig. 10 Measured electrical signals from the spectrometerļ¼1.0L / min aerosol flow, 10.0L / min sheath air flow,1.0 kV central rod voltage, 34.5 kPa operating pressureļ¼.
Theoretical predictionSEM measurement
Predicted electrical mobility diameterļ¼nmļ¼
Mea
sure
d eq
uiva
lent
sph
ere
proj
ecte
d ar
ea d
iam
eterļ¼
nmļ¼
500
400
300
200
100
00 100 200 300 400 500
Fig. 9 Predicted versus measured particle size in the classifierļ¼1.0 L / min aerosol flow, 10.0 L / min sheath air flow,1.0 kV central rod voltage, 34.5 kPa operating pressureļ¼.
Vol. 21 No. 4ļ¼2006ļ¼ ļ¼57ļ¼ 339
distribution. An example of data, representing sizedistribution of combustion aerosol measured by thespectrometer was shown in Fig. 11. The log normalnature of distribution was clearly illustrated.
6ļ¼Concluding Remarks
The operation of an aerosol size spectrometer designbased on electrical mobility determination techniquehas been studied. The instrument design concept isbased on electrical mobility analysis and multiple sizedetection channels. The system was developed withthe aim of measuring aerosol particles in the size range10ļ½ 1,000 nm, and consisted of two coaxial cylindersas two opposite polar electrodes. Mathematicalmodeling for particle trajectory has been carried out.The operating parameters as well as geometricalfactors of the classifier section were designed basedon these mathematical investigation results togetherwith previous numerical modeling of flow and electricfields. A prototype of the spectrometer has been builtand tested. Its performance in evaluating particle sizeusing signal current to evaluate the numberconcentration of aerosol was compared with SEMresults. Experimental results were found to be in goodagreement with theoretical predictions and thespectrometer can be used successfully in obtainingaerosol size distributions. Nonetheless, the classificationperformance of the present instrument needs beexamined further with a system of monodisperseparticles from standard sized particles or generatedfrom a setup involving a tandem DMA. Futureverification experiments of the present instrumentalong side a standard DMA are planned.
Acknowledgments
This work was supported by the National Electronic andComputer Technology Center, National Science and TechnologyDevelopment Agency, Thailand.
Nomenclature
A : surface area ļ¼m2ļ¼cļ¼i : mean thermal speed of ions ļ¼m / sļ¼Cc : Cunningham slip correction factor ļ¼ļ¼ļ¼dp : particle diameter ļ¼mļ¼dmax
p, i : particle diameter with maximum mobility ļ¼mļ¼dmid
p, i : midpoint particle diameter ļ¼mļ¼dmin
p, i : particle diameter with minimum mobility ļ¼mļ¼dPA : equivalent projected surface area diameter ļ¼mļ¼e : value of elementary charge on an electron ļ¼Cļ¼E : electric field strength ļ¼V / mļ¼Ec : charging electric field strength ļ¼V / mļ¼Er : radial components of the electric field ļ¼V / mļ¼
Ez : axial components of the electric field ļ¼V / mļ¼Ic : charging current ļ¼Aļ¼Ie, i : electrometer current ļ¼Aļ¼k : Boltzmannās constant ļ¼J / Kļ¼KE : translational kinetic energy ļ¼N.m2 / C2ļ¼n : average number of elementary charges on the particle ļ¼ļ¼ļ¼ndiff : average charge of diffusion charging ļ¼ļ¼ļ¼nfield : average charge of field charging ļ¼ļ¼ļ¼Ni : ion concentration ļ¼ions / m3ļ¼Np, i : particle number concentration ļ¼particles / m3ļ¼Q : volumetric flow rate of gas ļ¼L / minļ¼Qa : aerosol flow rate ļ¼L / minļ¼Qc : total flow rate through the charger ļ¼L / minļ¼Qs : sheath air flow rate ļ¼L / minļ¼r : radial coordinate ļ¼mļ¼r1 : inner radius of the annulus ļ¼mļ¼r2 : outer radius of the annulus ļ¼mļ¼T : absolute temperature ļ¼Kļ¼u : flow velocity ļ¼m / sļ¼ur : radial components of the flow velocity ļ¼m / sļ¼uz : axial components of the flow velocity ļ¼m / sļ¼V : potential ļ¼Vļ¼zi : axial position ļ¼mļ¼ze, i : electrometer ring width ļ¼mļ¼Zi : ion electrical mobility ļ¼m2 / V.sļ¼Zp : particle electrical mobility ļ¼m2 / V.sļ¼Zmax
p : maximum electrical mobility of particle ļ¼m2 / V.sļ¼Zmin
p : minimum electrical mobility of particle ļ¼m2 / V.sļ¼
Greek Symbols
d : air density ļ¼kg / m3ļ¼e : dielectric constant ļ¼F / mļ¼l : mean free path ļ¼mļ¼Āµ : air viscosity ļ¼Pa.sļ¼
Subscripts
a : aerosol
i : channel number
r : radial direction
s : sheath air
z : axial direction
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