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Dichotomous Samplers Modified for

Use with Electron MicroscopyYaacov Mamane

a& Thomas G. Dzubay

b

a

Environmental Engineering, Technion, Haifa, 32000, IsraelbU.S. Environmental Protection Agency, Research Triangle

Park, NC, 27711

Published online: 08 Jun 2007.

To cite this article: Yaacov Mamane & Thomas G. Dzubay (1990): Dichotomous Samplers

Modified for Use with Electron Microscopy, Aerosol Science and Technology, 13:2, 241-248

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Dichotom ous Samplers Modified for Use withElectron MicroscopyYaacov MamaneEnvironmental Engineering, Technion, Haifa, Israel 32000Thomas G. DzubayU.S. Environmental Protection Agen cy, Research Triangle Park, NC 27711

e r g e sulfate artifacts up to 2 pm in diameter wereobserved by scanning electron microscopy for the fineparticle fraction collected in dichotom ous samplers. Theartifacts were attributed to small liquid particles thatpiled up o n the filter, coalesced, and later dried as largerparticles. Such a rtifacts were elim inated when particles

INTRODUCTIONTwo types of dichotomous samplers havebeen used for collecting size-fractionated at-mospheric particles on a pair of filters. Onetype uses a virtual impactor (Loo and Cork,19881, and the other uses Nuclepore filterswith different pore sizes in tandem (Cahill etal., 1977; Parker et al., 1977). Samplescollected in such devices are well suited foranalysis of total mass and bulk compositionbut are poorly suited for analysis of individ-ual particles by scanning electron mi-croscopy (SEM), a technique that requiresparticles to be spaced a few diameters aparton the filter. This is a consequence of thetypical size distribution of atmosphericaerosol for which the number of particlesper logarithmic unit of diameter D variesapproximately as D- (Junge, 1963). Sucha distribution will cause fine particles to becollected too densely while coarse particlesare collected too sparsely on filters with thesame area.The consequences of collecting fine parti-cles too densely were seen in a study con-

were collected in a modified dichotomous sampler inwhich 80% to 90% of the airflow was d iverted from thefine fraction filter. This airllow diver sion techniqu e wasused successfully with both virtual-impactor andtandem-filter types of dichotom ous samplers.

ducted in the Philadelphia-Camden area dur-ing summer 1982. Although both fine (


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242 Y. Mamane and T. G. Dzubay

GURE 1. Micrographs of particles collected inCamden, NJ, on 0.3-pm Nuclepore filter in a vir-tual impactor in (a) coarse fraction, ( b ) fine frac-tion, and ( c ) back side of fine fraction filter. Sam-pling period was 12-h beginning at 1800, July 31,1982.

their x-ray emissions, and only S was de-tected. Thus, these particles comprise theslightly acidic ammonium sulfate depositnoted above. If such a deposit were a uni-form film, its thickness would be 0.3 pm,which is comparable to many layers of theoriginal particles that we presume were pre-sent. During the sampling period, the rela-tive humidity was close to loo%, whichwould cause ammonium sulfate to deli-quesce. Thus, we assume that the originalS-rich fine particles coalesced and formed aiquid that later dried to form the largearticles seen in Figure 1b. Support for this

d by Figure I c , whichof the filter used to

collect fine particles. Particles there arelarger than the 0.3-pm filter pores and couldhave reached the back side only if theyflowed through the pores while in liquidstate. Estimates based on SEM indicated that15% of the sulfate reached the back side.

One way to collect adequately spaced fineand coarse particles would be to use a largerdeposit area for the fine fraction. For theJunge size distribution described above, itwould be necessary to increase the depositarea by a factor of 10 for each factor of 10decrease in particle size. An easier way toavoid pileup is to use identical filter diame-ters and divert a portion of the airflow fromthe fine filter. Below we describe modifiedvirtual impactor and tandem filter samplersthat use airflow diversion to collect fine andcoarse particles spaced adequately for SEWe also describe a hybrid dichotomous sam-pler that is suitable not only for SEM butalso for bulk composition analysis by X-rayfluorescence (XRF).

Figures 2 and 3 show the virtual impactorand tandem filter samplers that were used.For the virtual impactor, the inlet flow rateF is

where F, and F,, are the flow rates throughthe coarse and fine filters, respectively, andF;, is the rate of diverted fine particle air-flow. For tandem filters

Table 1 summarizes the flow rates andfilter media. Nuclepore filters are well suitedfor electron microscopy analysis, and 2-pmTeflon filters are efficient for particles assmall as 0.03 pm (Liu et al., 1983) and aresuitable for bulk sample analysis. Filter di-ameters were 37 rnm, and deposit areas

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Modified Dichotomous Samplers

FIGURE 2. Modified virtual impactor sampler. Flow rates aregiven in Table 1.

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Y. Mamane and T. G. Dzubay

COARSEPARTICLE

FILTER

FINEPARTICLE

FILTER '

FIGURE 3. Modified tandemFlow rates are given in Table 1.

filter sampler.

were 6.4 cm2. Samplers 1, 2, and 3 shownin Table 1 were used in the present experi-ment. Sampler 4 in Table 1 is suggested as acost-effective design for future use.

The virtual impactor was designed byLoo and Cork (1988) and made by SierraInstruments, Inc. It separated fine and coarseparticles at D,, = 2.5 pm (aerodynamic di-ameter for 50% efficiency). We modified it,as shown in Figure 2, by boring a hole in itsside to divert some of the fine particles to a1.2-cm diameter pipe with flow rate Fa. Itwas assumed that fine particle concentra-tions were equal in the two flow streams. APM-10 inlet (Sierra Andersen, model SA246) removed particles larger than 10 pm.

The tandem filter sampler, shown in Fig-

ure 3, collected coarse particles on a 5-pmpore Nuclepore filter at 1.8 cm s- ' facevelocity. For those conditions, D,, = 2 pmfor liquid particles, and bias due to bouncefor solid particles was small (John et al.,1983). The decision to minimize bounce byusing low face velocity necessitated verylong sampling periods. A 0.3-pm Nucleporefilter was used to collect fine particles effi-ciently (Liu et al., 1983). An impactor onthe inlet removed particles larger than D,,= 10 pm.

To test the effectiveness of the modifiedair samplers, we collected several pairedsamples on the roof of our laboratory, 10 mabove ground in Research Triangle Park,NC, during summer 1987. For several 6- to

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TABLE 1. Operational Characteristics of Dichotomous Samplers Used in Present S tudy andHybrid V irtual Impactor Sug gested for Future StudiesFine fractiona

Sampler Path f 1 Path f2 Coarse fraction1. Tandem filters

Flow rate (Llmin)FilterTypical analyses

2. Standard virtual impactorFlow rate (Llmin)FilterTypical analysis

3. Modified virtual impactorFlow rate (Llmin)FilterTypical analysis

4. Hybrid virtual impactorFlow rate (Llmin)FilterTypical analysis

0.10.3-pm Nuclepore

SEM15

2-pm TeflonXRF

1.7-3.40.1-p Nuclepore

SEM13.33

2-pm TeflonXRF

0.8NoneNone

0NoneNone

12-14NoneNone1.67

0.2-pm NucleporeSEM

0.95-pm Nuclepore

SEM1.67

2-pm TeflonXRF

1.70.3-pm Nuclepore

SEM1.67

0.2-pm NucleporeXRF and SEM

'?Paths l and f2 pertain to flow rates Ffl and FfLhown in Figures 2 and 3

44-h periods, a modified virtual impactorwas operated beside a standard virtual im-pactor. For several 2- to 7-day periods, amodified tandem filter sampler was operatedbeside a standard virtual impactor. The lat-ter had to be operated intermittently (7 minevery hour) to avoid clogging its filters.Flow rates were recorded daily and at theend of each period.

Teflon filters from the standard virtualimpactor were analyzed by gravimetry andXRF. Portions of the Nuclepore filters werecoated with carbon and analyzed in an Am-ray (model 1000) SEM equipped with anenergy dispersive X-ray spectrometer(EDX). Electron beam conditions were 30-keV energy, 100-pA current, 26" sampletilt, and 12-mm working distance.

RESULTS AND DISCUSSIONModified vs. Standard Virtual ImpactorsFigure 4 shows micrographs of particles col-lected with modified and standard virtualimpactors for a 24-h period during summerat Research Triangle Park. Coarse particles

were adequately spaced for SEM, as Figure4 a illustrates. Figure 4b and c show strik-ing differences in the size and spacing offine particles from each sampler. (As notedin Figure 4, the filter pore sizes were dif-ferent, but this is of little consequence.)Figure 4b shows that most of the fine parti-cles collected in the modified virtual im-pactor were spheres smaller than 0.4 pm;only S was detected in their EDX spectra.Figure 4c shows that particles collected inthe standard virtual impactor appear as 0.5-to 2-pm artifacts. Such artifacts result fromhigh particle loading and high relative hu-midity (> 80%) at night, which cause sul-fate particles to deliquesce and coalesce asthey did in the Philadelphia-Camden study.Artifact formation is avoided by using amodified virtual impactor to collect a lightlyloaded fine fraction sample as in Figure 4 b.

Modified Tandem Filter SamplerFine and coarse particles were collected ontandem filters during a 7-day period andanalyzed by SEM. Use of airflow diversion

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Y. Mamane and T. G . Dzubay

. Micrographs of particles collected si-multaneously at Research Triangle Park for the24-h period beginning at 1700 on July 28 , 1987; (a )coarse fraction on 0.3-pm Nuclepore in modifiedvirtual impactor, ( b ) ine particles on 0.1-pm Nu-clepore filter collected with 1.7-Llmin flow rate inmodified virtual impactor, and ( c ) fine particles on0.3-pm Nuclepore filter collected with 14-Llminflow rate in standard virtual impactor.

enabled particle spacing to be adequate forboth size fractions. Micrographs indicatedthat coarse fraction and fine fraction parti-cles were qualitatively similar to those shownin Figures 4 a and b, respectively.

Modified and standard virtual impactorswere run side by side at Research TrianglePark under conditions specified in Table 1.Nuclepore filters from theimpactor were analyzed by Sfilters from the standard virtual impactorwere analyzed by gravimetry and XRF. Ta-bles 2 and 3 summarize results for a 44-hperiod that began on August 7, 1

The regions analyzed by SE70.5 x 104-pm fields for the coarse fractionand nine 35.5 x 52.1-pm fields for the finefraction. Size and morphological character-istics were recorded, and X-ray spectra wereobtained for all particles with diameter1 .4 and 1 .5 pm in the fine and coarsefractions, respectively. Particles were classi-fied into categories based on spectral andmorphological criteria described by Dzubayand Mamane (1989). The total volume ofsampled particles was calculated from thenumber of particles counted, the effectivediameter for each size range and the fraction

TABLE 2 . Coarse Particle Results for S a m ~ l e srom Research Triangle Park. NCCounts in diameter range (pm) Total Mass

Category 5 1.5 1 2 . 1 1 3 .1 5 . 1 1 7 . 1 5 10 counts (ng m-3)Minerals

SilicatesQuartzGypsumOthers

BotanicalOrganicFly ash (coal)Fe-richS-richTotal (SEM)Total (grav.)

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TABLE 3. Fine Particle Results for Sample from Research Triangle Park, NCCounts in diameteru range (ym) Total Mass

Category 0 . 5 5 0 . 7 5 0 .9 5 1 . 1 1 . 3 1 . 5 5 1 . 7 2 . counts (ngm-3)-Mineral (SEM) 5 7 8 10 11 6 4 5 1 450 k 170

Fly ash (SEM) 2 1 3 1 1 1 9 60 + 40Botanical (SEM) 1 5 1 2 7 16 250 * 110organic (SEM) 11Sulfate (XRF) 6710 * lo00Mass (grav.) 18,340+ 900

OOver 200 particles larger than 0.4 pm were analyzed by SEM-EDX; 87 are shown above, and the rest were sulfate particles.The flow rate through the filter analyzed by SEM-EDX was 3.3 Llmin.b ~ o s trganic particles were chain aggregates.

of filter area scanned. Mass concentrationswere calculated from the densities character-istic of each particle type and the air volumesampled. Accuracy estimates were based oncompounding errors from counting statis-tics, 15% uncertainty in density and an as-sumed 30% error in estimating particle vol-ume.

The SEM and gravimetric results forcoarse particle mass concentration agreewithin the estimated uncertainties. Majorcomponents of the coarse fraction were min-erals (49%) ,botanical matter (44%1, and flyash (7%). Components of the fine fractionmass deduced by SEM were minerals(2.3%), botanical matter (1.6%), and fly ash(0.3%). Most of the sulfate particles weresmaller than 0.4 pm and were close to thedetection limit of our SEM-EDX instru-ment. The ratio of fly ash to the sum ofminerals and fly ash is 12+ 6% for bothsize fractions. Many of the mineral andbotanical particles were enriched in S. Al-though similar S enrichment has been re-ported for much larger particles (Mamaneand Noll, 19851, the effect is more prorni-nent for the fine fraction examined in thepresent study.

CONCLUSIONSWhen aerosol was collected in a standardvirtual impactor, sulfate-rich artifacts ap-peared in the 0.5- to 2-pm size range on thefine fraction filter. Such artifacts resulted

from multiple layers of smaller particles thatdeliquesced during high relative humidity,coalesced and later dried. This phenomenonwas observed for samples from both thePhiladelphia-Camden area and Research Tri-angle Park, NC. The artifacts are eliminatedby diverting 80 to 90% of the airflow fromthe fine filter in both virtual impactor andtandem filter samplers. In the present studywe operated two dichotomous samplers sideby side to provide samples for analysis ofboth bulk composition and individual parti-cles. H~wever, such samples could havebeen obtained more simply by operating thehybrid virtual impactor described in Table1.

We thank Tom Lemmons, John Miller, and Rachel Wardfor assistance during this project. Although research de-scribed in this article has been conducted at the U.S.Environmental Protection Agency, it has not been sub-jected to Agency review and, therefore, does not necessar-ily reflect the views of the Agency, and no official en-dorsement should be inferred. Mention of commercialproducts and company names does not constitute endorse-ment by the Agency.

REFERENCESCahill, T. A., Ashbaugh, L. L., Barone, J. B., Eldred, R.

A., Feeney, P. J . , Flocchini, R . G., Goodart, C .,Shadon, D. J. , and Wolfe, G. W. (1977). J. AirPollut. Control Assoc. 27:675-678.

Dzubay, T. G., and Mamane, Y. (1989). Atmos. Envi-ron. 23:467-476.

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Dzubay, T. G., Stevens, R. K ., Gordon, G. E., Olmez,I., Sheffield, A. E., and Courtney, W. J . (1988).Environ. Sci. Technol. 22:46-52.John,W., Hering, S., Reischl, G., Sasaki, G., and Goren,S. (1983). Atmos. Environ. 17:373-382.

Junge, C . E. (1963). Air Chemistry and Radioactivity.Academic Press, New York.Liu, B. Y. H., Pui, D. Y. H., and Rubow, K. L. (1983).In Aerosols in the Mining and Industrial WorkEnvironments. (V . A. Marple and B. Y. H. Liu,eds.). Ann Arbor Science, Ann Arbor, MI, Vol. 3,pp. 989-1038.

Loo, B. W., and Cork, C. P. (1988). Aerosol Sci.Technol. 9:167-176.Mamane, Y., and Dzubay, T. G. (1988). J . Water AirSoil Pollut. 37:389-405.Mamane, Y., and Noll, K. E. (1985). Atmos. Environ.19:611-622.Parker, R. D., Buzzard, G. H., Dzubay, T. G., and Bell,3. P. (1977). Atmos. Environ. 11:617-621.Whitby, K. T. (1978). Atmos. Environ. 12:135-159.Received 2 September 1988; accepted 6 November 1989

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dichotomous samplers modified for

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