INSTRUMENTS AND METHODS FOR CHARACTERIZING RADIOACTIVE AEROSOLS

Otto G. Raabe

Lovelace Foundation for Medical Education and ResearchAlbuquerque, New Mexico U.S.A.

SUMMARY

The importance of characterizing aerosol proper-ties in evaluating the behavior and potential hazards ofradioactive aerosols is emphasized. The instrumentsand techniques for studying the properties of radioac-tive aerosols including special properties such aschemical form, solubility, and state of electrostaticcharge are discussed. The uses of electrostatic andthermal precipitators for electron-microscopic obser-vations and measurements, and of various aerodynam-ic samplers such as impactors, centripeters, cy-clones, and aerosol centrifuges for study of size dis-tribution, particle density and shape factors, andrelative specific activity are reviewed. The conven-tional terminology for describing particle size andsize distributions is described with emphasis on therelationships to the factors associated with the poten-tial hazards of inhaling radioactive aerosols.

INTRODUCTION

Characterization of radioactive aerosols involvesnot only size analysis and determination of activity andparticle concentration, but also the ascertainment ofinformation concerning all aspects of their physicaland chemical properties, including chemical form,particle specific activity distribution, particle densitydistribution, and state of electrostatic charge with re-spect to size. Other factors that may be important todetermine include solubility (as in body fluids after in-halation deposition) and especially aerodynamic char-acter as related to falling speed or diffusional proper-ties. Radioactive aerosols, that may be released orformed under various circumstances, may have a widevariety of unpredictable physical and chemical char-acteristics.

Some important properties of accidentally pro-duced plutonium aerosols which may affect their fateafter inhalation deposition include aerodynamic sizedistribution, physical density, extent of sintering ortemperature history, chemical composition, solubility,and degree of heterogeneity. The aerosols of interestmay be a combination of materials with plutonium onlya small fraction of the makeup of the particles so thatrelative specific activity with respect to size may bevariable.

AEROSOL SIZE AND INTERPRETATIONS

Aerosol particle size analysis including the math-ematical consideration of the statistical factors havebeen the subject of many symposia and conferencesl-4and publications. 5-10 A particle size distribution iscommonly treated by dividing a sample of the aerosolparticles into a number of distinct size classes. Whensizing has been completed, the observed step-like dis-tribution is a rough version of the more smooth distri-bution of the large population from which the samplewas taken. For this reason, the observed data arebest presented in the form of a histogram showingclearly their piecemeal character. The raw data his-togram may be standardized with respect to intervalsize by dividing the number of particles or amount ofradioactivity in each interval by the size of that inter-val. The interval sizes are often not equal. Greatergeneralization of the histogram is accomplished bydividing these standardized values by the total numberof particles or radioactivity in the sample to yield anormalized histogram, as shown in Figure 1. The

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normalized histogram is a step-like depiction of aparticle size distribution function (probability density),C (D):

C(D) = dF(D)C( dD (1)(which has units of fraction of the distribution per unitinterval size) in which D is the particle diameter andF(D) is the cumulative fraction of particles or radio-activity associated with diameters less than size D.When C(D) or the normalized histogram are integratedwith respect to size over all sizes the result is unity.A distribution function commonly employed to describeaerosol size distributions is the log-normal distribu-tion given by:

(lnD- lnM

C(D) = D I1n e 2D(na ) [0 < D (2)g g

with ln the natural logarithm, D the particle diameter,MD the median diameter of the distribution (geometricmean), and ag the geometric standard deviation of thedistribution. Hatch and Choate, 11 and Raabe, 12demonstrate that any characteristic of the particles ina population which is proportional to the q-th power ofthe diameter can also be described by a log-normaldistribution* with the same geometric standard devi-ation as the size distribution and with a median diam-eter, Dq, given by:

lnD = lnM + q(lncr )2q D q(na (3)

For example, the volume distribution of sphericalparticles with a number distribution that is log-normalwith a median diameter CMD (count median diameter)is also log-normal with median Dq given by equation 3with q = 3. A representative log-normal distributionis shown in Figure 2 for a distribution with CMD equalto 1. 0 Fim and geometric standard deviation equal to2. 0. Also included in Figure 2 are the positions ofvarious other characteristics of the distribution.

Since aerosol particles of unknown shape andproperties are studied with many different instrumentsand techniques, the "sizes" reported or studied maynot be easily relatable even though they may all becalled diameters. Unknown particle shapes will espe-cially lead to various interpretations, since a simplemeasure of diameter implies sphericity and lacks thesuitable shape factors necessary to provide accuratedescriptions of the properties of irregular particles.13-17 In optical microscopy, many conventions are usedto decide upon a linear measure for irregular particles.A useful convention has been to measure the diameterof a circular area that is equal to the observed two-dimensional projected area. These projected areadiameters can be shown by Cauchy's Theorem to havemean values that are directly relatable to the surfaceareas of the particles assuming the particles are ran-domly oriented. 18 Clearly, this measure of diameteris not the same as that used if the particles are mea-sured by electrostatic means, by light scattering, or

tPrepared under U. S. Atomic Energy CommissionContract No. AT(29-2)-1013.

-'It has been customary in particle size analysis toexpress all such distributions as a function of particlesize. This can be the source of some confusion. Forexample, a mass distribution is expressed in units offraction of the mass per unit particle size interval.

by sedimentation or inertial separation. All of thesemethods may yield very different values for a singleparticle and all may be called the particle diameter.

To this often confusing situation must be added thedifferent mean or median diameters used to describedistributions of sizes. These include, among others,the volume median diameter (VMD), the diameter ofaverage volume (Dv) (not to be confused with the meanof the volume distribution), the mass median diameter(MMD), the diameter of average mass (Dm) (not to beconfused with the mean of the mass distribution), thesurface area median diameter (AMD), the diameter ofaverage surface area (Da) (not to be confused with themean of the surface area distribution), the countmedian diameter (CMD), the mean diameter (D), themodal diameter (Dm), the mass median aerodynamicdiameter (MMAD), and the activity median aerodynam-ic diameter (AMAD) which is the common parameterfor describing radioactive aerosols.

There are at least two "aerodynamic diameters"commonly used to describe particle size. The first,Daerl, is the "diameter of a unit density sphere withthe same settling velocity as the particle in question"and was recommended by the Task Group on LungDynamics. 19 This definition is mathematically de-scribed by the following equation for spherical parti-cles:

aerl C(Daerl) = Dreal X (Dreal) (4)

with Dreal the physical diameter of the sphericalparticle, P the physical density of the particle,C(Dreal) the slip correction for the particle, Daerl theaerodynamic diameter and C(Daerl) the slip correctionassociated with a unity density particle of the aero-dynamic diameter. The relationship between the realdiameters of spheres of various densities and theaerodynamic diameter, Daerl, is shown in Figure 3.

The slip correction C, is a semi-empirical factorthat corrects the Stokes Law of viscous resistance forthe effect of "slip" between the air molecules when theaerosol particles are almost as small or smaller thanthe free paths of the air molecules. 20 The slip correc-tion is approximated for spheres by:21

with

C = 1 + A(D- )Dreal

realX

A = aL + Pe 2with X the mean free path of the gas molecules, a A1. 26, 3 z 0. 45, and yn 1. 08. At sea level (760 mmHg) the mean free path, X, is equal to about 0. 0650 p.mfor air at 210 C.

The second common "aerodynamic diameter"Daer2, is one used by Mercer22 and other investiga-tors in working with impactors. Since it is not possi-ble to differentiate with an inertial sampler betweentwo spherical particles with D1 and D2 the particle di-ameters and p1 and P2 the densities, respectively, if:

D1 V/(P1CI) = D2 \/(p2C2) (6)(with C1 the slip correction for the first particle andC2 the slip correction for the second particle), theaerodynamic diameter, Daer2, has been defined forspherical particles as

Daer2 Dreal P C(D ) (7)

with Dreal the real diameter of the spherical particle,p its density, and C(Dreal) its slip correction for theparticle.

Since C depends upon the real diameter, the realdiameter of a spherical particle of known aerodynamicdiameter Daer2 cannot be explicitly stated. However,if it is assumed, for simplicity, that the slip correc-tion is satisfactorily described by equation 5 with A =

1. 26, then the real diameter is given by (with diam-eters and mean free path in micrometer units):

[(Da

)

D + (AX)J -AX(8

and Daerl and Daer2 in micrometer units, are ap-proximately relate as:

aerl \/ [(D )r2) + (AX)2] - AX (9)

If two investigators, one at sea level (e. g., SanFrancisco; New York) and the other at 5500 feet abovesea level (e. g., Albuquerque; Denver) both measurethe aerodynamic diameter, Daer2, of the same parti-cle, they will obtain slightly different values, sincethe mean free path of air molecules is about 20% dif-ferent with a concomitant difference in the slip correc-tion. If, however, these two observers measure dif-ferent particles and obtain the same value for Da r2they will be predicting correctly the behavior of tVeseparticles for their respective locations, but will ob-tain slightly different values for Daerl, the equivalentunit density spheres. These slight differences maynot be detectable within the limits of normal experi-mental error in this example but may be importantin other cases. The relationship between D 1 andDaer2 is shown in Figure 4. aer

Neither Daeri, nor Daer2 is a satisfactory "aero-dynamic diameter ' for processes that involve diffusion,such as the inhalation of small, submicron particlesbecause they both depend upon the particle density aswell as the physical size, but the diffusion coefficientdoes not. In inhalation studies a better definition foraerodynamic diameter might be "the equivalent unitdensity sphere in the process under consideration, "whether it be sedimentary, inertial, or diffusional.

If a respirable aerosol is perfectly log-normallydistributed with respect to aerodynamic diameter, itsdistribution of real sizes cannot be perfectly log-normal because of the effect of the variable slip cor-rection. However, as a useful approximation, the twodistributions may be satisfactorily described as log-normal, but their medians and geometric standard de-viations will differ. Shown in Figure 5 are some log-normal activity distributions of spherical particles ofplutonium oxide (p z 10. 5) plotted with respect toaerodynamic diameters, Daer2. The activity distribu-tion of these same aerosols are plotted with respect toreal diameter in Figure 6. An electron-micrograph ofa sample of such an aerosol with AMAD = 1.5 FLm isseen in Figure 7.

Clearly, caution is advisable when the termdiameter is used to describe aerosols. The various"sizes" which are used in the description of aerosolsize distributions or individual particles must be prop-erly interpreted in terms of the context, the techniques,and devices used for their determination, the manipu-lation involved in the applicable calculations, and theconventions involved.

FILTER SAMPLING

The collection of gross samples onto suitablefilter media is a useful technique for measuring massconcentration, chemical character, or radioactivecontent of aerosols. The mechanisms of filtrationhave been discussed and studied by many investiga-tors. 23-26 At least four processes are involved infiltration: Impingement, diffusion, electrostaticattraction, and sieving. Since impingement andsieving are more efficient collection processes forthe larger particles and electrostatic attraction andespecially diffusion are more efficient collection

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processes for the smaller particles, the samplingefficiency of filters with respect to aerosol particlesize tends to maximum values for very large and verysmall particles with a minimum collection efficiencyat some intermediate size. The particle size associ-ated with the minimum collection efficiency varieswith the type of filter and the sample flow rate per unitarea (linear velocity through the filter), but usually isin the range of 0. 1 to 0. 3 rim. It is for this reasonthat filter efficiencies are usually tested with particlesin this size range.

Samples can be collected with high efficiency withmembrane filters such as those available from Milli-pore Corporation (Bedford, Mass.) and from GelmanInstrument Co. (Ann Arbor, Mich.); however, airflow per unit area through membrane filters is limitedby their relatively high resistance. Microscopically,membrane filters appear spongelike with externalpores that are fairly uniform in size. They are ratedcommercially by their average pore diameter in mi-crometer units. However, these pore diameters can-not be used to directly estimate their minimum collec-tion efficiency nor the size at which the minimum oc-curs. The minimum for all sub-micrometer mem-brane filters usually exceeds 99%.27

Lower resistance, high efficiency filter mediainclude glass fiber filters such as available from Gel-man Instrument Co., (Ann Arbor, Mich.) and Micro-sorban filters (Delbag-Luftfilter, Berlin, West Ger-many). Both of these media are rated with minimumcollection efficiencies about 99. 9% at 0. 3 rim. Theirhigher flow rate per unit area properties make themparticularly useful for collecting samples of low con-centration radioactive aerosol. The Microsorban ischemically degradable for analytical purposes.

Although still in common usage, one of the poor-est filter media for aerosol sampling is the WhatmanNo. 41 (W. & R. Balston, Ltd., London, England)which has a minimum collection efficiency of only 15%for some sampling velocities. 28 The manufacturerstates that the Whatman No. 41 filter is 11.. . not rec-ommended for fine precipitates. "

In filter sampling of radioactive aerosols, the flowrate and total volume of air sampled are importantdata as is also true for other sampling systems.Estimates of radioactivity concentration require aknowledge of the total sample volume. There is nosubstitute for a careful measurement of sampling rateboth at the beginning and end of long samples sincesample collection usually causes increased resistanceacross the filter with concomitant reduction in sam-pling rate. Use of such systems as a critical orificebehind the filter will not prevent this reduction sinceflow through a critical orifice depends upon the up-stream pressure.

ELECTROSTATIC SAMPLERS

Radioactive aerosols can often be collected andstudied with electrostatic precipitators and electro-static mobility analyzers. The forces experienced bycharged particles in an electric field are used to causetheir collection in electrostatic samplers. Aerosolparticles are almost always charged and often highlycharged, so that electrostatic charge is an importantcharacteristic of particles which may have a major in-fluence on particle behavior.

Electrostatic precipitators provide both thecharging mechanism for the particles and the collectingelectric field. They usually employ unipolar chargingwith a corona discharge. A concentric configuration,as shown schematically in Figure 8, has been employedby Barnes, 29 Lauterbach et al. 30 and others. In theseconcentric electrostatic precipitators, a corona dis-charge is produced in the converging field lines near a

high-voltage electrode at the center of a conductive

grounded cylinder. Aerosols are drawn into the de-vice with a pump and the particles are charged uni-polarly and collected upon the inner surface of thetube by the forces in the electric field. A foil linerserves as the collection surface. Gross samples forradioactive counting and other analysis can be collect-ed in this way and since there is some separation inthe deposit based upon mobility, tentative estimatesmay be made concerning the size distribution of theradioactivity. Moderately high, constant flow ratesand collection of large samples are possible with thisconcentric design.

Electrostatic precipitation is also useful in col-lecting aerosol samples for electron-microscopic ex-amination. Morrow and Mercer31 describe a simplepoint-to-plane configuration for depositing aerosolsamples directly upon an electron-microscope sam-ple grid. A schematic drawing of the point-to-planeelectrostatic precipitator is shown in Figure 9. Asample is drawn into a 3/8" diameter cylindricalchannel at a chosen flow rate ranging from 50 cc/minto 1 liter/min. A sharp needle near one side of thechannel serves as a high-voltage electrode and pro-duces a corona discharge at about 7000 VDC. In di-rect opposition to this needle, on the other side of thechannel, a carbon-substrated electron-microscopegrid is mounted on a grounded post. Aerosols drawnthrough this device are unipolarly charged and col-lected randomly upon the grid by the action of theelectric forces. An electron-micrograph of a sampleof an aerosol collected with this device and shadowedwith chromium vapor is shown in Figure 7.

Mercer et al. 32 describe a similar instrument(Fig. 10) for collecting a very small sample upon thecenter of an electron-microscope grid. This electro-static precipitator uses a tritium source to provideions which are accelerated by a battery potentialthrough a biased aperture into the aerosol channel andtoward a grounded electron-microscope grid. Thision current effects unipolar charging of the aerosolparticles which are collected on the electron-micro-scope grid by the action of the electric forces. Sincesample flow rates may not exceed 10 cc/min forproper operation and since the total sample is depos-ited upon only about 0. 5 mm2 of the grid substrate,the instrument is particularly desirable for obtainingsmall samples of high concentrations of radioactiveaerosols. The distribution of particles on these sam-ples is related to particle size and must be carefullyanalyzed. Another electrostatic sampler for collect-ing samples for microscopic study is described byLiu et al. 33

Electrostatic aerosol analyzers are designed tocharacterize the state of electrostatic charge on aero-sol particles. Devices and methods for the analysisand study of the charge with respect to particle sizeand the electrostatic properties of aerosols have beendescribed by Daniel and Brackett, 34 35 Gillespie andLangstroth, 36 Yoshikawa et al, 37 Langer38 andothers. The mobility of charged aerosol particles invarious electric fields under a variety of conditions isthe primary characteristic upon which these electro-static analysis methods are based. The state ofelectrostatic charge associated with radioactive aero-sols may be an important property of the aerosolswhich can be measured with aerosol electrostaticmobility analyzers. A parallel plate electrostaticcharge and mobility analyzer has recently been de-scribed by Megaw and Wells. 39

An automatic electrical particle counter and sizeanalyzer has been developed by Whitby and Clark40(Whitby Aerosol Analyzer, Thermo-Systems, Inc.,St. Paul, Minn. ) for studying aerosols in the diameterrange of 0. 015 .tm to 1 rim. This instrument may alsobe used as an electrical mobility classifier. Aerosolsare charged in a reproducible manner, and the analysis

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of the mobility in a concentric precipitator is then re-latable to size with the known charging function.

AERODYNAMIC SAMPLERS

Particularly valuable in relating the size distribu-tion of radioactive aerosols to their expected behaviorin such situations as inhalation are the sampling de-vices which separate particles of different sizes ac-cording to their aerodynamic properties. These de-vices usually use one of the following basic aerodynam-ic characteristics: (a) sedimentation under gravity orunder centrifugal forces, (b) inertial momentum duringa change of direction, or (c) natural Brownian motionor diffusion of the particles. The aerodynamic analy-ses based upon the first two of these characteristicsare directly comparable; the third, diffusional char-acteristics, are different.

One of the most popular of the inertial separators,the cascade impactor, was invented by May. 41 Meth-ods of calibration, manner of use, and theory ofoperation of this instrument have been developed anddescribed by a number of authors including May, 41Davies et al, 42, 43 Ranz and Wong, 44 and Mercer. 45-

An aerosol sample is drawn at a constant ratethrough a series of successively smaller jets, eitherround holes or rectangular slits, with a collectionsurface very close to the exit of each jet and perpen-dicular to the direction of flow. At each stage, theaerosol particles must make a right angle change indirection to follow the air streams; larger particlesare unable to negotiate the turn and impact upon thecollector. A cross sectional view of a basic fourstage impactor is shown in Figure 11. The averagesize that will be collected at each stage is successive-ly smaller. An efficient filter usually serves as thefinal stage to collect all of the smaller particles whichsuccessfully pass through the impactor. The gradedsamples of the aerosol obtained are analyzed with re-spect to the total amount of material or radioactivityon each collection surface. Usually a suitable mathe-matical function is fitted with impactor data to de-scribe the aerodynamic size distribution of the radio-activity.

The collection efficiency of each impactor stagedepends upon a number of factors: (a) the linear veloc-ities of the air streams (usually described by a hypo-thetical average, UO, equal to the volumetric flow ratedivided by the area of the jet; (b) the jet opening shapeand size (conveniently described by the jet width forslits or the jet diameter for holes, either of which maybe called W); (c) the aerosol particle shape; (d) theparticle size (diameter, D, if the particle is spheri-cal); (e) the particle density, P; (f) the slip correction,C (important for smaller particles); (g) the air vis-cosity, rl; (h) the jet-to-collector separation distance,S; and (i) the jet depth, J. Ranz and Wong44 used thedimensionless parameter, \, defined for sphericalparticles as:

p D U C

18 n W (10)and plotted collection efficiency versus V'i. Theparameter, ,Il', is proportional to the particle diam-eter. Based upon the principles of physical similarityand dimensional analysis, an efficiency curve of thistype will be applicable to another impactor stage of thesame type but of a different size which has the sameratios, S/W and J/W, and conforms in other ways asto physical similarity. The size separation that occursis not perfect in that the collection efficiency of eachstage gradually goes from zero to unity over a smallsize range. One useful technique for analyzing datainvolves use of the "effective cutoff" particle size, de-fined as the particle size for which the collection effi-ciency of a particular impactor stage is equal to 0. 5.

For simplicity the assumption is made that only parti-cles larger than the effective cutoff size are collectedby the impactor stage. This method was used by Ranzand Wong44 and Mitchell and Pilcher. 50 It also hasbeen compared with other methods and recommendedby Mercer.47 A least squares method for fitting amathematical distribution function, such as a log-normal function, to cascade impactor data has beendescribed by Raabe and Tillery. 51 A simple graphi-cal procedure for fitting a log-normal function to im-pactor data involves plotting the cumulative distribu-tion (fraction of activity below a given effective cutoffdiameter) on log-probability coordinate paper versusthe aerodynamic diameter to estimate the AMAD andagg Low flow rate impactors, such as the Mercer im-

pactor52 shown in Figure 12, are useful for studyingthe aerodynamic size distribution of radioactive aero-sols of high specific activity or high activity concen-tration. The Mercer impactor has a series of sevenimpactor stages, each consisting of a single roundhole jet, with round glass cover slips used as the col-lection surfaces. Each stage is fitted with an 0-ringseal into a 1-5/8" diameter by 2" long cylindricalhousing with inlet probe and outlet filter. Typically,the Mercer impactor has jets designed for sampleflow rate between 50 cc/min and 1 1/min; flow rate isusually chosen so that the last stage has an effectivecutoff diameter of about 0. 3 Lm and the first stageabout 4 1Lm. Operating times are limited by samplebuildup to a few minutes.

High flow rate impactors, such as the Andersenimpactor (2000 Inc., Salt Lake City, Utah) and theLundgren impactor (Environmental Research Corp.,St. Paul, Minnesota) are useful for characterizingradioactive aerosols of low specific activity or lowactivity concentrations. The Andersen Impactor53 hasround jets of about the same sizes as the Mercer im-pactor but each stage is a 3-3/4" circular disk inwhich 400 holes serve as the impactor jets and circu-lar metal disks serve as the collection surfaces. Thestages are gasketed and held together as a unit withthree large springs. Sample flow rates are normallyabout 1 cfm (28. 8 1/min) and sample times may bequite long if sample buildup is not prohibitive. Asuitable filter holder can be placed at the exit to col-lect the small particles that negotiate the impactor.The Lundgren impactor54 uses large rectangular slitsfor the jets. It consists of four rectangular jet im-pactor stages with rotating cylindrical drums as thecollection surfaces. Since the drums can be rotatedat speeds from 1 rpm to 1 revolution/24 hours, thesample can be analyzed with respect to time of collec-tion and sample buildup is less prohibitive. The nor-mal sampling rate is in the range of 3 to 5 cfm (- 90to 150 1/min). The first stage has a particle effectivecutoff diameter of about 10 p.m and the last about 0. 3p.m. A suitable filter holder can be placed at the exitto collect the small particles which negotiate the im-pactor.

The cascade centripeter55 is a newer inertialseparator which has been popular in health physicswork. It is essentially a cascade impactor in whichthe impaction stages are replaced by small holes infiltered collection chambers so that the undesirablebuildup of material that may occur on the collectionplates of an impactor is avoided. These separatorscan operate for long periods without disturbing theparticle separation characteristics of the stages, butthey suffer from high (- 50%) interstage losses.

Another group of aerodynamic samplers that areuseful in characterizing radioactive aerosols arecentrifugal devices which make use of centrifugalforces on an aerosol flow to collect particles. Instru-ments using centrifugal separation vary from verysimple one-stage cyclones, which create a fast circu-

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lar motion of air to remove larger particles56 57 tovery sophisticated aerosol centrifuges. Simple cy-clones are useful in estimating the respirable dustfractions of aerosols. 58

Among the best of the centrifugal separators foraccurate aerodynamic size distribution analyses ofsmall samples of radioactive aerosols are the "spin-ning spiral" aerosol spectrometers.59-62 Thesedevices use the principle of centrifugal separation ofparticles in a spinning spiral duct cut in a specialcentrifuge rotor. An aerosol sample (usually nomore than 500 cc/min) is drawn by negative pressureinto the duct through an inlet slit near the axis ofrotation of the rotor and near the inner wall of theduct where it meets a laminar stream of clean air.The aerosol bearing air entering the duct through theslit forms a flowing sheath along the inner wall of theduct with the clean air layer filling the rest of theduct. The rotor is spun rapidly by a suitable motorat the desired constant speed. The aerosol particlesmove across the clean air stream because of thecentrifugal force provided by the spinning of the rotorand collect on a foil lining the outer wall of the chan-nel. While moving across the stream, the particlesexperience different drag resistance depending upontheir aerodynamic size characteristics. Particleswith larger terminal settling speeds (or aerodynamicdiameters) collect on the foil at shorter distancesdown the channel than the smaller particles. Theparticles are nicely separated in the process andcollected according to their aerodynamic equivalentsizes, so that at a given position on the collectionfoil only particles within a small range of aerodynam-ic diameters are collected. Since the spiral channelmoves farther from the center of rotation with length,the aerosol particles encounter a continuously in-creasing centrifugal force. This characteristic tendsto enhance the collection of particles of relativelysmall sizes. Aerodynamic size resolution is usuallyabout 3%. After sampling, the foil can be removedfrom the spiral housing and cut up or scanned forradioactivity analysis, or previously placed electron-microscope grids can be removed for size measure-ments. Particles having aerodynamic diameters toosmall to completely cross the stream will pass outof the end of the channel unless provision is made tocollect them by filtration.

Among the spinning spiral aerosol spectrometersthere are three types in common usage: The StoberSpiral Centrifuge, 59 b (Ivan Sorvall Co., Newtown,Conn.) the Los Alamos Spiral Centrifuge AerosolSpectrometer (Los Alamos Scientific Laboratory,Los Alamos, New Mexico)61 and the Lovelace Aero-sol Particle Separator62 (Lovelace Foundation forMedical Education and Research, Albuquerque, NewMexico). The Stober spectrometer and Los AlamosSpectrometer both consist of a lengthy spiral about180 cm long. The rotor diameter in both of thesemachines is about 25 cm. The St8ber duct is 3. 3 cmin depth and the Los Alamos duct is about 5 cm deep.The St8ber channel consists of a series of arcs ofcircles while the Los Alamos channel is a partialArchimedean spiral. The Los Alamos Spiral Cen-trifuge Aerosol Spectrometer, shown in explodedview in Figure 13, has a number of design advan-tages over the Stober spiral centrifuge includingsealed bearing aerosol sampling port, improvedaerosol inlet design, and absence of thermal regu-lator jacket. Both use a sophisticated centrifugemotor with accurate speed control. Although theSt8ber Spectrometer and the Los Alamos Spectro-meter separate particles with high resolution overthe aerodynamic diameter range from 10 p.m to 0. 1p.m with rotational speed about 3000 rpm, the sizeand weight of these devices and the fact that thedeposit is spread over a long foil render the use ofthese instruments difficult for field applications.

No provision is made in either of these devices forcollecting the small particles which are not collectedon the foil, but an exhaust filter can be used.

The Lovelace Aerosol Particle Separator, shownin exploded view in Figure 14, employs a 18 cm diam-eter rotor and a novel expanding spiral channel but 46cm long. The duct is 3. 18 cm deep. It has a sealedbearing aerosol sampling port and requires no ther-mal jacket. Either an elaborate centrifuge motor forlaboratory studies or a simple single speed inductionmotor for field studies can be used. Particles can beseparated and collected on the foil over the aerody-namic diameter range of 5 to 0. 5 [Jm with rotationalspeed about 3600 rpm and all particles below 0. 5 pLmare collected with a special filter arrangement insideand at the end of the duct so that these smaller parti-cles are also separated with respect to aerodynamicproperties. Because of its unique design character-istics and small size, the Lovelace Aerosol ParticleSeparator is handier to use than the other spiral aero-sol spectrometers. The distributions of particles atdifferent positions usually have standard deviationsaround 3%.

The spiral centrifuges are useful for studies ofradioactivity aerodynamic size distribution, specificactivity size distribution, physical density distribution,and shape factor distribution for a radioactive aerosol.All three of the instruments described may suffer incurrent designs from inlet losses for the larger parti-cles which may affect evaluation of the activity aero-dynamic size distribution for particles larger than afew micrometers in diameter.

Dynamical measurements of the diffusion proper-ties of aerosols yield a different aerodynamic char-acter because the diffusion coefficients are independentof particle density or mass and depend only upon par-ticle size. 63 Instruments and methods for studyingdiffusion extend from single diffusion tubes as firstdescribed by Townsend64 to elaborate parallel platediffusion batteries.65-7 Aerosols of very smallparticles drawn through a diffusion tube or battery areforced to travel a relatively long distance in a con-fined channel or channels. Since the particles are inconstant random Brownian motion, many hit the wallof the channel and are firmly held by adhesive andother forces. More of the smaller particles are re-moved from the air stream since they have largerdiffusion coefficients, however, separation betweensizes is not sharp.

CHEMICAL PROPERTIES

Evaluation of the chemical properties of radio-active aerosols is an important but difficult area ofcharacterization. Small samples may have too littlemass of the radioactive material to allow conventionalchemical techniques to be employed. When possible,infrared spectroscopy or other inztrumental techniquescan be useful for studying chemical composition.Clearly, valence state and chemical compound identi-fication is important to predicting the fate of inhaledradioactive aerosols.

A useful test of a sample of a radioactive aerosolis the measurement of its apparent solubility, particu-larly in simulated or real physiological fluids, undernon-equilibrium conditions. Mercer68 has discussedthe importance of particle size distribution in deter-mining the solubility of inhaled aerosols of sparinglysoluble chemical forms. Not only is the chemicalform important in determining the basic solubilitycharacteristics of an aerosol but also the surfacearea-to-mass ratio is important in that the solubilityof particles expressed as the mass fraction dissolvedper unit time is inversely proportional to the particlediameter under non-equilibrium conditions. Materialswhich are normally referred to as insoluble may be

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cleared from the lungs after being inhaled as submi-crometer particles by the sparingly small dissolutionthat occurs with the enhancement of solubility whichis caused by the particle size effect. Kanapilly et al. 69have described an in vitro system for measuring thesolubility rates for sparingly soluble materials undernon-equilibrium conditions. This system uses a con-tinually flowing stream of serum simulant passing anaerosol sample sealed in membrane filters. Dis-solved material is carried off in the stream and mea-sured. Kanapilly et al. have found the dissolutionhalf-time for radio-labeled fused clay (normally con-sidered an insoluble material) to be about 780 daysfor 1 p.m particles at 370 C.

Unfortunately, the dissolution rate constants forvarious chemical forms of familiar materials are notreadily available at this time because chemists havecustomarily described solubility in terms of theequilibrium solubility product which does not apply tonon-equilibrium dissolution as occurs with particlesin thy lung or in the flowing system of Kanapilly etal. According to Mercer, film diffusion kineticsdoes not control the dissolution of sparingly solublematerials, but rather the dissolution rate is given by

dMdt= -kS (11)

where M is the particle mass, S is the particle sur-face area, t is time and k is the dissolution rate con-stant of specific solubility which has units of mass orradioactivity dissolved per unit time per unit surfacearea of the particles. Equation (11) is equivalent toEquation (12):

dF k (12)dt -D_

with F the mass fraction, D the particle diameter andk a constant equal to ka /p am with k the rate con-stant of specific solubility, as the particle surfaceshape factor, am the particle mass shape factor, andp the particle density. Evaluation of the functionaldissolution rates for unknown radioactive aerosolsprovides valuable presumptive information about thechemical forms of the aerosols as well as their sizedistributions.

REFERENCES

1. Symposium on New Methods for Particle SizeDetermination in the Subsieve Range, Washington,D.C., March 4, 1941, Am. Soc. Testing Mater.Spec. Tech. Publ. No. 51 (1941).

2. Symposium on Particle Size Measurement,Boston, Mass., June 26-27, Am. Soc. TestingMater. Spec. Tech. Publ. No. 234 (1958).

3. The Physics of Particle Size Analysis, Brit. J.Appl. Phys. Suppl. 3, London, The Institute ofPhysics (1954).

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5. G. Herdan, Small ParticleStatistics, ElsevierPublishing Co., Amsterdam, Netherlands (1953).

6. Riyad R. Irani and Clayton F. Callis, ParticleSize: Measurement, Interpretation, and Applica-tion, John Wiley and Sons, Inc., New York, N.Y.(1963).

7. F. Kottler, The distribution of particle sizes,Part I., J. Franklin Inst. 250 339-356 (1950).

8. F. Kottler, The distribution of particle sizes,Part II., J. Franklin Inst. 250 419-441 (1950).

9. F. Kottler, The goodness of fit and the distribu-tion of particle sizes, Part I., J. Franklin Inst.251 499-514 (1951).

10. F. Kottler, The goodness of fit and the distribu-tion of particle sizes, Part II., J. Franklin Inst.251 617-641 (1951).

11. T. Hatch and S. P. Choate, Statistical description

of the size properties of non-uniform particulatesubstances, J. Franklin Inst. 207 369-387 (1929).

12. Otto G. Raabe, Particle size analysis utilizinggrouped data and the log-normal distribution, J.Aerosol Sci. 2, 289-303 (1971).

13. J. Cartwright, Particle shape factors, Ann.Occup. Hyg. 5, 163-171 (1962).

14. J. Raymond Hodkinson, The effect of particleshape on measures for the size and concentrationof suspended and settled particles, Am. Id. Hyg.Assoc. J. 26, 64-71 (1965).

15. W. bt5ber, A. Berner, and R. Blaschke, Theaerodynamic diameter of aggregates of uniformspheres, J. Colloid Interface Sci. 29, 710-719(1969).

16. P. Kotrappa, Shape factors for quartz aerosol inrespirable size range, J. Aerosol Sci. 2, 353-359 (1971).

17. W. Stober, Dynamic shape factors of non-spher-ical aerosol particles, Third Int. RochesterConf. on Environmental Toxicity: Assessment ofAirborne Particles, University of Rochester,New York, June 17-20 (1970).

18. C. N. Davies, Size, area, volume and weight ofdust particles, Ann. Occup. Hyg. 3, 219-225(1961).

19. P. E. Morrow (Chairman, Task Group on LungDynamics), Deposition and retention models forinternal dosimetry of the human respiratory tract,Health Phys. 12, 173-208 (1966).

20. E. Cunningham, On the velocity of steady fall ofspherical particles through fluid medium, Proc.Roy. Soc. (London) Ser. A. 39, 357-365 (1910).

21. Arnold E. Reif, Aerosols: Physical properties,instrumentation, and techniques, pp. 168-244, inAviation Medicine Selected Reviews, C. S. White,W. R. Lovelace II, and F. G. Hirsch (Eds.),Pergamon Press (1958).

22. T. T. Mercer, M. I. Tillery, and H. Y. Chow,Operating characteristics of some compressedair nebulizers, Am. Ind. Hyg. Assoc. J. 29, 66-78 (1968).

23. C. Y. Chen, Filtration of aerosols by fibrousmedia, Chem. Rev. 55, 595-623 (1955).

24. J. Pich, Theory of aerosol filtration by fibrousand membrane filters, in Aerosol Science, pp.223-285, C. N. Davies (ed.) Academic Press,Inc., New York (1966).

25. I. B. Stechkina and N. A. Fuchs, Studies onfibrous aerosol filters, I. Calculation of diffusion-al deposition of aerosols in fibrous filters, Ann.Occup. Hyg. 9, 59-64 (1966).

26. A. A. Kirsch and N. A. Fuchs, Studies on fibrousaerosol filters, III. Diffusional deposition of aero-sols in fibrous filters, Ann. Occup. Hyg. 11, 299-304 (1968).

27. W. J. Megaw and R. D. Wiffen, The efficiency ofmembrane filters, AERE-R4211, Atomic EnergyResearch Establishment, Harwell, England (1963).

28. Ronald G. Stafford and Harry J. Ettinger, FilterEfficiency vs Particle Size and Velocity, LA-4650,Los Alamos Scientific Laboratory, Los Alamos,New Mexico (1971).

29. E. C. Barnes, Atmospheric sampling by electro-static precipitation, in Air Pollution, Proceedingsof the United States Technical Conference on AirPollution, McGraw-Hill Book Company, Inc., NewYork, (1953).

30. Kenneth E. Lauterbach, Thomas T. Mercer,Alastair D. Hayes, and Paul E. Morrow, Effi-ciency studies of the electrostatic precipitator,A.M.A. Arch. Ind. Hyg. Occup. Med. 9, 69-75(1954).

31. Paul E. Morrow and Thomas T. Mercer, A point-to-plane electrostatic precipitator for particle sizesampling, Am. Ind. Hyg. Assoc. J. 25, 8-14(1964).

69

32. T. T. Mercer, M. I. Tillery, and M. A. Flores,An Electrostatic Precipitator for the Collectionof Aerosol Samples for Particle Size AnalysisAEC Research and Development Report, LF-7,Lovelace Foundation for Medical Education andResearch, Albuquerque, N.M. (1963).

33. Benjamin Y. H. Liu, Kenneth T. Whitby andHenry H. S. Yu, Electrostatic aerosol samplerfor light and electron microscopy, Rev. Sci.Instrum. 38, 100-102 (1967).

34. J. H. Daniel and F. S. Brackett, An electricalmethod for investigating the nature and behaviorof small, airborne, charged particles, J. Appl.Phys. 22, 542-554 (1951).

35. J. H. Daniel and F. S. Brackett, Investigation ofsmall, airborne, charged particles by an electri-cal method, A.M.A. Arch. Ind. Hyg. Occup.Med. 3, 505-519 (1951).

36. T. Gillespie and G. 0. Lanstroth, An instrumentfor determining the electrical charge distributionin aerosols, Can. J. Chem, 30, 1056-1068 (1952).

37. Herbert H. Yoshikawa, George A. Swartz, JohnT. MacWaters, and Wade L. Fite, Electrostaticparticle size analyzer, Rev. Sci. Instrum. 27,359-362 (1956).

38. G. Langer, Electrostatic Classification of Sub-micron Airborne Particles, AEC Progress Re-port, ARF 3187-8, Armour Research Foundationof Illinois Institute of Technology, Germantown,Md. (1963).

39. W. J. Megaw and A. C. Wells, A high resolutioncharge and mobility spectrometer for radioactivesubmicronmetre aerosols, J. Sci. Instrum. (J.Physics E) 2, 1013-1016 (1969).

40. Kenneth T. Whitby and William E. Clark, Elec-tric aerosol particle counting and size distribu-tion measuring system for the 0. 015 to 1 1i sizerange, Tellus 17, 573-586 (1966).

41. K. R. May, The cascade impactor: An instru-ment for sampling coarse aerosols, J. Sci.Instrum. 22, 187-195 (1945).

42. C. N. Davies and Mary Aylward, The trajectoriesof heavy, solid particles in a two-dimensional jetof ideal fluid impinging normally upon a plate,Proc. Phys. Soc. (London) 64 B, 889-911 (1951).

43. C. N. Davies, Mary Aylward, and DorothyLeacey, Impingement of dust from air jets,A.M.A. Arch. Ind. Hyg. Occup. Med. 4, 354-397 (1951).

44. W. E. Ranz and J. B. Wong, Jet impactors fordetermining the particle-size distributions ofaerosols, A.M.A. Arch. Ind. Health Occup. Med.4, 464-477 (1952).

45. T. T. Mercer, On the calibration of cascade im-pactors, Ann. Occup. Hyg. 6, 1-14 (1963).

46. T. T. Mercer, The stage constants of cascadeimpactors, Ann. Occup. Hyg. 7, 115-125 (1964).

47. T. T. Mercer, The interpretation of cascade im-pactor data, Am. Ind, Hyg. Assoc. J. 26, 236-241 (1965).

48. T. T. Mercer and H. Y. Chow, Impaction fromrectangular jets, J. Colloid Interface Sci. 27,75-83 (1968).

49. T. T. Mercer and R. G. Stafford, Impactionfrom round jets, Ann. Occup. Hyg. 12, 41-48(1969).

50. R. I. Mitchell and J. M. Pilcher, Design andcalibration of an improved cascade impactor forsize analysis of aerosols, in Proc. Fifth AEC AirCleaning Conference, TID-7551, pp. 67-84 DTS.Dept.of Commerce, Washington, D.C. (1950).

51. 0. G. Raabe and M. I. Tillery, A method for

A multi-stage, low flow rate cascade impactor,Aerosol Sci. 1, 9-15 (1970).

53. Ariel A. Andersen, New sampler for the collec-tion, sizing and enumeration of viable airborneparticles, J. Bacteriol. 76, 471-484 (1958).

54. Dale A. Lundgren, A aerosol sampler for deter-mination of particle concentration as a function ofsize and time, J. Air Pollution Control Assoc.17, 225-229 (1967).

55. R. F. Hounam and R. J. Sherwood, The cascadecentripeter: A device for determining the concen-tration and size distribution of aerosols, Am.Ind. Hyg. Assoc. J. 26, 122-131 (1965).

56. M. Kippmann and W. B. Harris, Size-selectivesamplers for estimating "respirable' dust con-centrations, Health Phys. 8, 155-163 (1962).

57. Hans Breuer, Investigations on the suitability ofcyclones for the selective sampling of respirablefine dusts, pp. 523-530, in Inhaled Particles andVapours II, C. N. Davies (ed.) Pergamon Press,New York, N.Y. (1967).

58. Paul E. Morrow, Evaluation of inhalation hazardsbased upon the respirable dust concept and thephilosophy and application of selective sampling,Am. Ind. Hyg. Assoc. J. 25, 213-236 (1964).

59. W. Stober and H. Flachsbart, Size-separatingprecipitation of aerosols in a spinning spiralduct, Environ. Sci. Technol. 3, 1280-1296 (1969).

60. W. St6ber and H. Flachsbart, High resolutionaer odynamic size spectrometry of quasi-mono-disperse latex spheres with a spiral centrifuge,J. Aerosol Sci. 2, 103-116 (1971).

61. Owen R. Moss, Harry J. Ettinger and James R.Coulter, A Modified Spiral Centrifuge AerosolSpectrometer, LA-DC-11871, Los AlamosScientific Laboratory, Los Alamos, New Mexico(1971).

62. P. Kotrappa and M. E. Light, Design and per-formance of the Lovelace Aerosol Particle Sepa-rator (submitted for publication), LovelaceFoundation for Medical Education and Research,Albuquerque, New Mexico (1971).

63. N. A. Fuks, I. B. Stechkina, and V. A.Staroselsky, Determination of distribution ofaerosol particles according to size by diffusionmethod, translated for Atomics International,Canoga Park, Calif. from Inzy-Fiz. Zh. , AkadNauk Belorussk, SSR, 5 100-103 (1962).

64. J. S. Townsend, The diffusion of ions into gases,Phil. Trans. Roy. Soc. London Ser.A. 193, 129-158 (1900).

65. W. DeMarcus and J. W. Thomas, Theory of aDiffus ion Battery, Protective Equipment Evalu-ation Program, ORNL-1413, Oak Ridge NationalLaboratory, Oak Ridge, Tenn. (1952).

66. J. W. Thomas, The diffusion battery method foraerosol particle size determination, J. ColloidSci. 10, 246-255 (1955).

67. L. W. Pollak and A. L. Metnieks, On the deter-mination of the diffusion coefficient of hetero-geneous aerosols by the dynamic method, Geofis.Pura Appl. 37, 183-190 (1957).

68. T. T. Mercer, On the role of particle size in thedissolution of lung burdens, Health Phys. 13,1211-1221 (1967).

69. G. M. Kanapilly, 0. G. Raabe, C. H. T. Gohand R. A. Chimenti, Dissolution of small radio-active particles, pp. 40-51, in Fission ProductInhalation Program Annual Report 1969-1970,LF-43, Lovelace Foundation for Medical Educa-tion and Research, Albuquerque, New Mexico(1970).

fitting a size distribution function by least-squaresemploying cascade impactor data (Abstract), Am.Ind. Hyg. Assoc. J. 29, 102 (1968).

52. T. T. Mercer, M. I. Tillery and G. J. Newton,

70

EXAMPLE LOG-NORMAL DISTRIBUTIONCMD= 1.0 g = 2.0

-DIAMETER OF AVERAGE MASS (2 056Om)

-AREA MEDIAN (2.61411m)AREA MEAN (3 324 ym)

MASS MEDIAN (4 226Lm)

,7 MASS MEAN (5.374/km)

2 3 4 5 6 7

PARTICLE DIAM. (microns)

Figure 1. An example of a normalized histo-gram constructed from the data ofthe sizes of particles in an aerosolsample.

10

E =1

U)

70

6

W-

0

PARTICLE DIAMETER (MICRONS)

Figure 2. An example of the log-normal distri-bution function used for aerosol par-

ticle distributions in normalizedlinear form for count median diam-eter, CMD = 1. 0 ILm and geometricstandard deviation, ag = 2. 0, show-ing the mode, median, mean of thesize distribution, the surface area

distribution median and mean diam-eters, the mass distribution medianand mean, and the diameter of aver-

age mass.

REAL DIAMETER Fm

Figure 3. The relationship between the real di-ameters of spherical particles ofvarious densities (P) and the aerody-namic diameters, Daerl, which are

diameters of the aerodynamicallyequivalent unit density spheres.

71

CZ

C3

KI~

FK K~

K),

K-

MM- -

v. DIAETE D m

AERODYNAMIC DIAMETER (D /=p)FLmFigure 4. The relationship between the two

aerodynamic diameters, Daeri,which is the diameter of the aerody-namically equivalent unit densitysphere, and Daer2, which is equal toDreal V(pC), shown in micrometerunits for two altitudes.

AMAD

AERODYNAMIC DIAMETER (gm)

Figure 5. Activity distributions of aerosols ofspherical particles of 239PuO2 (p

10. 5) which are log-normal for aero-

dynamic sizes (Daer2) plotted for dis-tributions with various activitymedian aerodynamic diameters(AMAD) and ag = 1. 8.

72

0

z0I-

CD

I-

U)

4

REAL DIAMETER (pzm)

Figure 6. Activity distributions of aerosols ofspherical particles of 239PuO2 (p q

10. 5) which are log-normal for aero-

dynamic sizes (Daer2) plotted versusreal size for distributions with variousactivity median aerodynamic diam-eters (AMAD) and with ag = 1. 8.

Figure 7. Electron-micrograph of an aerosolsample shadowed with chromiumvapor of 239PuO2 particles with p b10. 5, activity median aerodynamicdiameter, AMAD F 1. 5 Lm, andag 1.8.

FILTER a HOLDER

FLOWMETER- f £ ~~~aPUMP

HIGH VOLTAGE ELECTRODE

Figure 8. Schematic drawing of a concentricelectrostatic precipitator which col-lects aerosols upon a groundedcylindrical tube under the unipolarcharging influence of a corona dis-charge and electrical forces in thefield around a high-voltage centralelectrode.

HIGH VOLTAGE ELECTRODE

FILTER a HOLDER

F/LOWME TER8i PUMP

GRID MOUNT a CONTACT ROD

Figure 9. Schematic drawing of the basic designof a point-to-plane electrostatic pre-cipitator for collecting samples ofaerosols onto an electron microscopegrid for size-distribution analysis. Ahigh-voltage needle supplies unipolarions from a corona discharge; thegrid mounting is grounded. Thisdrawing illustrates the point-to-planeprecipitator designed by Morrow andMercer. 31

73

I0

0

0

4

AMAD

.5p.m AMAD

+2100V

0

0

+2040V

3 Filter

Microscope Grid

Ground

Figure 10. An electrostatic precipitator using atritium source to produce ions forcollecting small samples of aerosolsfor particle size analysis. Aerosolis drawn through the flow channel, astream of unipolar ions produced bythe tritium source interacts with theparticles under the influence of bat-tery supplied potential, and the par-ticles are attracted to the groundedelectron microscope grid. (Illustra-tion from Mercer et al. 32

Aerosol Entry

0 ~~~~RingsJet Collector Spacers

0*ig

Cover Plate

Cover Slips..,

0

0 Rii

ipport Springs

Figure 11. Schematic cross-sectional drawingof a simple four stage, cascade im-pactor followed by a backup collec-tion filter; a pump after the filterdraws the sample. An aerosol sam-ple must change direction of flow ateach stage causing particle collectionby impaction on the cover slips.

Figure 12. Schematic cross-sectional view (notnecessarily to scale) of a Mercer-type seven-stage, round-jet cascadeimpactor designed to collect smallaerosol samples at flow rates be-tween 50 cc/min and 1 1/min (seeMercer et al., reference 52).

ibrone Filter

Vacuum Applied

74

X AEROSOL

AEROSOL INLET,,' < _ABEARING ASSEMBLY

_//

Figure 13. Schematic drawing of an explodedview of the Los Alamos ScientificLaboratory Spiral Centrifuge Aero-sol Spectrometer from Moss et al.,reference 61. (Kindly provided byMr. Harry J. Ettinger and Mr. OwenR. Moss, H-5, Los Alamos Scien-tific Laboratory.)

)L ENTRY

BEARINGHOUSING

TOPDISK

) _ SPIRALCHANNEL Figure 14. Schematic drawing of an exploded

view of the Lovelace Aerosol Particle Separator from Kotrappa andLight, reference 62. (Kindly pro-

CLEAN vided by Dr. P. Kotrappa and Mr.LMINATOR Max E. Light, Aerosol Physics

Dept., Lovelace Foundation.)

75