30LPM Cascade Paper

7/29/2019 30LPM Cascade Paper

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Aerosol Science 35 (2004) 281299

www.elsevier.com/locate/jaerosci

A compact multistage (cascade) impactor for thecharacterization of atmospheric aerosols

Philip Demokritou, Seung Joo Lee, Stephen T. Ferguson,Petros Koutrakis

Environmental Science and Engineering Program, Department of Environmental Health,School of Public Health, Harvard University, 401 Park Drive, P.O. BOX 15677,

Landmark Center-West, Boston, MA 02215, USA

Received 18 February 2003; accepted 22 September 2003

Abstract

This paper presents the development, laboratory characterization, and eld evaluation of a compact multi-

stage cascade impactor (CCI). The CCI operates at a ow rate of 30 LPM and consists of eight impaction

stages equipped with rectangular slit-shaped acceleration nozzles. A 47 mm backup Teon membrane lter

is used downstream of the eighth stage to collect particles smaller than 0:16 m. In each stage, particles

are retained by impaction onto the inert polyurethane foam (PUF) substrate. The major feature of this novelsampler is its ability to both fractionate by size and collect relatively large amounts of particles (mg quan-

tities) onto an inert polyurethane foam impaction substrates. Even though the impaction substrates are not

coated with adhesives such as grease or mineral oil, particle bounce and re-entrainment losses were found to

be insignicant. Impaction characteristics (cutpoint and sharpness of collection eciency curve) of the PUF

substrates are maintained for mass loadings of at least 25 mg, which is much higher than for other commonly

used rigid, at impaction substrates. The system was calibrated in laboratory experiments using polydisperse

aerosols. The 50% cutpoints of the eight stages were 9:9; 5:3; 3:3; 2:5; 1:7; 1:0; 0:47 and 0:16 m (aerody-

namic diameter), with pressure drops of 0:02; 0:02; 0:04; 0:06; 0:08; 0:27; 1:57 and 5:73 kPa. These pressure

drops are considerably lower than those obtained using at rigid impaction substrates with comparable cut-

points. Particle losses for each stage were less than 10% for particles smaller than 7 m and less than 20%

for particles larger than 7 m. The CCI was also compared with the collocated micro-orice impactor (MOI)in laboratory-controlled experiments using articially generated polydisperse aerosol. These laboratory tests

showed that the mass concentrations measured by the MOI are considerably lower than those measured by

the CCI (the average ratio of total mass concentration of MOI to CCI was 0.86), with the size distribution

measured by the CCI closer to that measured using the real time particle sizing instruments (SMPS, APS).

A eld comparison of CCI, the Harvard impactor (HI) and the federal reference method (FRM) for ne

Corresponding author. Tel.: +1-617-3848847.

E-mail address: [email protected] (P. Demokritou).

0021-8502/$ - see front matter? 2003 Elsevier Ltd. All rights reserved.doi:10.1016/j.jaerosci.2003.09.003
mailto:[email protected]:[email protected]


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282 P. Demokritou et al./ Aerosol Science 35 (2004) 281 299

particles showed a good agreement between the CCI and the reference samplers for particles smaller than

2:5 m.

? 2003 Elsevier Ltd. All rights reserved.

Keywords: Cascade impactor; Particle sampling; Collection eciency

1. Introduction

There is a great interest in investigating the toxicity of atmospheric particles through in vitro

cellular studies (Imrich, Ning, Koziel, Coull, & Kobzik, 1999; Soukup, Ghio, & Becker, 2000).

To adequately conduct these studies, it is necessary to collect relatively large amounts of ambient

particles (several mg). In addition, a major objective of particle toxicity studies is to investigate therelative toxicity of the dierent particle size fractions. Previously used particle samplers have two

major shortcomings: rst, the collection capacity of the impaction substrate is limited, and; second,

they have extensive particle bounce-o and re-entrainment. In order to minimize particle bounce-o

and re-entrainment, impaction substrates are usually coated with adhesives such as mineral oil or

grease, both of which provide a signicantly limited loading capacity ( John, Fritter, & Winklmayr,

1991; John & Sethi, 1993; Sehmel, 1980; Wall, John, Wang, & Coren, 1990). For reasonably small

amounts of collected particles, it was shown that the collection eciency of an impactor could

depend on the amount of particles collected on the impaction substrate (Pak, Liu, & Rubow, 1992).

Recently, we developed a new impaction substrate that does not require the use of adhesives

such as grease or mineral oil to minimize particle bounce-o and re-entrainment losses (US patent

#6, 435, 043 B1, 2002). This impaction substrate consists of polyurethane foam (PUF, density =20 kg=m3, Merryweather foam, OH) and can be used to collect large quantities of particles (mg

amounts) while maintaining its impaction characteristics (cutpoint and sharpness of collection e-

ciency curve). Polyurethane foam is a porous polymeric material with stable physical characteris-

tics, low chemical background (when cleaned properly) and high collection eciency characteristics

(Kavouras, Ferguson, Wolfson, & Koutrakis, 2000; Salonen et al., 2000). The use of polyurethane

foam substrates improves the performance of inertial impactors, as compared to both coated and

uncoated at plate substrates (Demokritou, Kavouras, Ferguson, & Koutrakis, 2002a,b; Kavouras

et al., 2000; Kavouras & Koutrakis, 2001; Salonen et al., 2000). Since for the same nozzle geom-

etry and owrate, lower cutpoints can be achieved as compared to those for at plate substrates,

the pressure drop is substantially reduced for the same size cutpoint ( Demokritou et al., 2002a,b;Kavouras et al., 2000; Kavouras & Koutrakis, 2001). Recently, we have used the PUF substrate in

the development of a high volume cascade impactor (Demokritou et al., 2002a,b) and the high vol-

ume low cutpoint impactor (Kavouras et al., 2000). These two methods have been used extensively

to characterize the physicochemical and toxicological properties of atmospheric aerosols.

In this paper, we present the development and evaluation of the compact cascade impactor (CCI)

that utilizes the PUF foam as the collection medium. This system consists of eight impaction stages,

equipped with rectangular nozzles. The major feature of the CCI is its ability to fractionate aerosols

by size and collect large amounts of particles onto small, inert impaction substrates without the use

of adhesives.


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P. Demokritou et al./ Aerosol Science 35 (2004) 281 299 283

2. Methods

2.1. Design considerations

According to impaction theory, the dimensionless Stokes number, Stk, is the governing parameter

for impaction and is dened as follows:

Stk=pd

2pUCc

9W; (1)

where is the dynamic viscosity of the air, dp is the diameter of the particle, p is the particle

density, W is the nozzle diameter, U is the jet velocity and Cc is the Cunningham slip correction

factor. The slip correction factor is given by (Hinds, 1999)

Cc = 1 + Kn2

(2:34 + 1:05e0:195Kn); (2)

where Kn is the dimensionless Knudsen number, 2=dp, dened by the ratio of two times the mean

air molecule free path () to the particle diameter (dp).

2.2. Description of the compact cascade impactor (CCI)

A fully assembled compact cascade impactor (CCI) and its internal parts are shown in Fig. 1. The

sampling device consists of eight impactor stages and an after lter. The CCI is compact (50 cm H

and 11 cm O.D.), and is relatively lightweight (3:5 kg). Each stage consists of a slit-shaped accel-eration nozzle and a rectangular polyurethane foam (PUF) impaction substrate (density = 20 kg=m3,Merryweather Foam, OH). The PUF substrates are easily inserted and removed from a substrate

base (Fig. 1), and securely transported to a laboratory for gravimetric or chemical analysis. The

acceleration nozzle width and length dimensions are shown in Table 1. The optimal thickness of the

polyurethane foam for all stages is 0:64 cm, which assures that particles do not penetrate through

the PUF to the substrate holder.

Ultrane particles (less than 0:16 m) are collected on a 47 mm diameter 2 m pore Teon mem-

brane lters (Cat. No. R2PJ047, Pall Life Sciences, Ann Arbor, MI). This lter material was previ-

ously characterized in our lab and has both a high collection eciency and a relatively low-pressure

drop (Demokritou et al., 2002a,b). Other lter materials such as polypropylene ber (Monadnock,

Grade 5300) can also be used to collect the ultrane particles.

2.3. Laboratory performance characterization of CCI

The experimental setup used for the laboratory performance characterization of the system is

shown schematically in Fig. 2. The apparatus consists of two major components: (1) the particle

generation system, and (2) the particle sampling and monitoring system. Due to the wide range of

CCI cutpoints, two dierent techniques were employed to generate and measure the polydisperse

particles for the calibration of the various stages.


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Fig. 1. The compact cascade impactor (CCI).

2.3.1. Aerosol generators

(1) For the 0.011.0 m size range: the polydisperse aerosol generator for the 0.011:0 m size

range is shown in Fig. 2. An aqueous solution of sodium chloride (NaCl) was atomized usinga collision-type atomizer (TSI model 3076). All the generated particles were drawn through the

dilution chamber (volume of 1:15 102 m3) with dry, particle-free room air. After dilution, themixed stream was drawn though the diusion dryer (TSI, model 3062) and then passed through a

Kr-85 neutralizer (TSI model 3012) to stabilize the particle electrostatic charge.

(2) For the 1.020.0 m size range: the aerosol generator for the 1.0 20:0 m size range is

also shown in Fig. 2. It uses an aqueous suspension of either hollow glass spheres (nominal size

220 m; density = 1:10 g=cm3 Polysciences, PA) or solid glass spheres (nominal size

310 m; density = 2:48 g=cm3 Polysciences, PA). A suspension of approximately 1:0 g of glass

beads per 200 ml of distilled water was continuously mixed with a magnetic stirrer, and aerosolized


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Table 1

Physical characteristics, ow parameters, and experimental results of CCI

Stage no. 1 2 3 4 5 6 7 8

Physical characteristics

Acceleration nozzle W (cm) 0.432 0.236 0.160 0.137 0.094 0.058 0.058 0.028

L (cm) 7.19 7.01 6.32 5.92 5.59 5.00 1.78 1.91

S=W 1.0 1.0 1.0 1.0 1.0 1.0 1.0 2.0

Substrate Width length 1:8 8:1 1:8 8:1 1:0 6:4 1:0 6:4 0:64 6:4 0:64 6:4 0:64 6:4 0:64 6:4(cm)

Material PUF PUF PUF PUF PUF PUF PUF PUF

Flow parameters

Re 926 950 1053 1125 1191 1330 3744 3495

U (m/s) 1.6 3.0 4.9 6.2 9.5 17.1 48.1 93.9

Experimental results

d50 (m) 9.9 5.3 3.3 2.5 1.7 1.0 0.47 0.16Stk 0.47 0.47 0.46 0.43 0.44 0.47 0.39 0.32

1.38 1.33 1.32 1.38 1.33 1.43 1.81 1.74

P (kPa) 0.02 0.02 0.04 0.06 0.08 0.27 1.57 5.73

Note. W, nozzle width; L, nozzle length; S, substrate-to-nozzle distance; Re, Reynolds number; U, nozzle air velocity;

d50, 50% cutpoint;

Stk, square root of Stokes number; , collection eciency curve sharpness; P, pressure drop.

using a nebulizer (Retec Model X-70/N, with air pressure at 7 psi) and then dried and diluted usingdry, particle-free room air.

2.3.2. Particle sampling and monitoring system

The output of the aerosol generator passed into the top of a cylindrical anodized aluminum dilution

tube (120 cm H5:08 cm O.D.). Supplemental air was drawn in through a HEPA lter at the top ofthe dilution tube. To assure aerosol mixing inside the tube, a round plate was placed inside the tube

downstream of the input ows. Each stage of the CCI was evaluated separately. Each impaction stage

was mounted inside the dilution tube, one at the time, as shown in Fig. 2. Particle concentrations were

measured for one minute durations using isokinetic sampling probes, alternating between upstream

and downstream of the sampler. The alternating measurements were repeated at least ve times toensure adequate precision. The upstream sampling port was placed approximately eight-duct diameters

downstream from the duct entrance. The downstream sampling port was similarly placed on a second

duct connected to the outlet of the impaction system. The reproducibility of the measurements was

5%. An aerodynamic particle sizer (APS, TSI Model 3310) and a scanning mobility particle sizer(SMPS, TSI Model 3080) were used to measure particles with diameter from 0.5 to 20 m and

from 0.01 to 1:0 m, respectively. Particle losses for each impaction stage were measured using the

tested impaction stage without the impaction substrate in place.

For the SMPS data, each particle size interval was converted from mobility equivalent diameter

to aerodynamic diameter. Assuming that the mobility equivalent diameter is equal to the equivalent


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Silica Gel

HEPAFilter

2 LPM

Mixing Chamber TSI 3012Kr-85 Neutralizer

TSI 3062Diffusion Dryer

NaCl Solution

TSI 3076Constant OutputAtomizer

SilicaGel

HEPAFilter

35 psi

7 psi

HEPA Filter

Mixing Chamber

HEPAFilter

SilicaGel

Excess

fluid

Nebulizer

BubblingAir

Glass beadsuspension

PUMP

PUMP

PUMP

PUMP

PUMP

PUMP

SupplementalFiltered Air

PARTICLE SAMPLING & MONITORING

TEST AEROSOL GENERATOR FOR SIZERANGE 1 - 20 m


PressureGauge

29.7 or 25.0 LPMHEPAFilter

Flowmeter

PUMP

Pressure

Gauge

SMPS / APS

Isokinetic Sampler

Isokinetic Sampler

Impactorstage

0.3 or 5.0 LPM

MagneticStirrer

HEPAFilter

Fig. 2. Experimental setup for laboratory characterization of CCI.

volume diameter (Kasper, 1982), the conversion was made using (Peters, Chein, & Lundgren, 1993)

Cada =

pCm

odm; (3)

where da is aerodynamic diameter, dm is the mobility equivalent diameter, Ca is the slip correction

factor for the aerodynamic diameter, Cm is the slip correction factor for the mobility equivalent

diameter, is the dynamic shape factor, p is the density of the particle, and 0 is the unit density

(1 g=cm3). For sodium chloride particles, the dynamic shape factor and particle density were assumed

to be = 1:08 (that of a cube) and 2:2 g=cm3, respectively.


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The collection eciency for a given particle size, E(da), for each impaction stage, was calculated

using

E(da) = C(da)T(da)

= T(da)P(da)T(da)

; (4)

where T(da); C(da) and P(da) are the total, collected and penetrated particles of aerodynamic

diameter of da, respectively. The collection eciency data were tted using the Boltzmann sigmoidal

algorithm (Origin, MicroCal Software Inc.) using

E(da) =A1 A2

1 + e(dax0)=dx

+ A2; (5)

where x0 is the median aerodynamic diameter; dx is the width of the tting; and A1 and A2, are the

coecients determined by the algorithm. The sharpness () of the collection eciency curve was

calculated using

=

d84:1

d15:9; (6)

where d84:1 and d15:9 are the sizes of particles having collection eciencies of 84.1% and 15.9%,

respectively (Hinds, 1999).

2.4. Particle loading capacity experiment

In conventional impactors, a known limitation is the collection eciency change as a function of

the deposited on the impaction substrate particles. This eect may introduce a signicant sampling

artifact and thereby cause an unacceptable reduction in the useful duration of sampling. Previous

investigations of the eect of particle loading on the commonly used at surface impaction substrates

showed that signicant changes occur for both particle bounce and cuto diameter, even for particle

loading of less than 1:0 mg (Kenny, Gussmann, & Meyer, 2000; Misra, Singh, Shen, Sioutas, &

Hall, 2002; Tsai & Cheng, 1995; Turner & Hering, 1987).

Experiments were performed to investigate the eect of substrate particle loading on the collection

eciency curve. Stage no. 4 (50% cutpoint: 2:5 m) was selected to investigate the loading eect on

PUF. As the loading was built up over time on the substrate, consecutive particle number concentra-

tions were measured up- and down-stream of the sampler (see Fig. 2). Particle collection eciencywas calculated based on Eq. (4) as a function of loaded mass on the PUF substrate. Loaded mass,

M, on the substrate was calculated using

M =

mj

ni

Ni; j ei; j pd3pi

6; (7)

where, i is the ith number of n channels in APS, j is the jth minute, Nij is the upstream par-

ticle number, eij is the collection eciency, p is the particle density, and dp is the particle

diameter.


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TSI 3400A Fludized Bed

Aerosol GeneratorSMPS / APS


PUMP Silica

HEPA

35 psi

30.0 LPM

140.0 LPM

30.0 LPM

HEPA

CCI

PUMP

TSI 3062

Diffusion Dryer

TEST AEROSOL GENERATOR FOR SIZERANGE 0-1m

NaCl Solution

TSI 3076Constant OutputAtomizerDry & filtered air

35 psi

Mixing ChamberDry& filtered airTSI 3012Kr-85 Neutralizer

60.0 LPM

Mixing Duct

SupplementalFiltered Air

SAMPLING SYSTEM

MOI

PUMP

PUMP

Fig. 3. Laboratory intercomparison experimental setup.

2.5. Laboratory intercomparison experiments

Laboratory experiments were conducted to compare the size distribution measured by both the

microorice impactor (MOI, MSP Corporation, Minneapolis, MN) and by the CCI, with that obtained

using real time instrument measurements (SMPS, APS). The experimental setup for these experiments

is shown in Fig. 3. In order to simulate the bi-modal size distribution of atmospheric aerosols,

two separate aerosol generators were used in parallel to generate polydisperse aerosols in the 020 m size range. Fine Test Dust (FTD, Powder Technology Inc., Burnsville, MN) and NaCl were

used as the test aerosols. FTD was dried over 24 h prior its use in a temperature of 50C and

aerosolized using a Fluidized Bed Aerosol Generator (TSI Model 3400A). An aqueous solution

of sodium chloride (NaCl) was atomized using a Collision type atomizer (TSI model 3076) as

outlined above, in the impactor characterization experiments. The mixture of aerosolized FTD and

NaCl was then passed through the Kr-85 Neutralizer to reduce the particle charge to close to the

Boltzmann equilibrium distribution. The aerosol was then introduced into a circular aluminum tube

(ID=5:08 cm). Isokinetic probes were placed at a distance of more than 10 times of tube diameter

to sample aerosol for both the cascade impactors and the real time instruments (SMPS/APS).


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MOI operates at 30 LPM and was used to classify particles in the following size intervals:

60:175; 0:1750:56; 0:561:0; 1:01:8; 1:83:2; 3:25:6; 5:610 and 10 m, using 37 mm Teon

membrane lters (Cat. No. R2PJ037, Pall Life Sciences, Ann Arbor, MI) as impaction substrates.

Sample duration for both cascade impactors was 90 min. Tests were conducted using both a rela-tively high particle concentration of about 5:0 mg=m3 and a relatively low concentration of about

0:4 mg=m3. At the end of each run, PUF substrates, the 47 mm Teon after-lter of CCI, and

37 mm Teon lers of MOI were equilibrated over a 48-h period in a temperature and relative

humidity-controlled room (40 5% RH and 21 2:8C). Just prior to gravimetric analysis boththe lters and the PUF substrates were exposed to 210Po ionizing units (Staticmaster Model 2U500,

NRD Inc., Grand Island, NY) for more than 10 s, to minimize eects of electrostatic charge. For all

samples, 100% replicate weighing was performed both before and after sampling, with a third weigh-

ing done when the rst two was not within 5 g. This procedure was implemented to improvethe precision of the gravimetric analysis and to exclude outliers (Allen, Oh, Koutrakis, & Sioutas,

1999). Average particle concentrations were also calculated based on the real time data obtained bythe collocated APS and SMPS.

2.6. Field study

For the intercomparison study to measure the particulate ne mass (PM2:5), two conventional

Harvard PM2:5 impactors (HI; Marple, Rubow, Turner, & Spengler, 1987), and a PM2:5 Federal

Reference Method impactor (FRM, Rupprecht and Patashnick, Albany, NY) were collocated with

the CCI system on the roof of Harvard Countway Library in Boston, MA. Samples were collected

during the month of September 2002, with durations of 48 h. Initial and nal sampler ows for the

samplers were measured with calibrated rotameters (Matheson, East Rutherford, NJ).

The PM2:5 HI collects particles on 37 mm Teon membrane lters (Pall Life Sciences, Ann Arbor,

MI) at 10 LPM, while the FRM uses a 47 mm lter operating at 16:7 LPM. Final weighing was

done within 10 days after sampling, following EPA guidelines, as described above.

3. Results and discussion

3.1. Impactor characterization experiments

The collection eciency curves for each of the eight cascade impactor stages are shown in

Fig. 4. The experimentally calculated cutpoints (d50; m), the collection eciency sharpness ()and pressure drop (P, kPa) are also presented in Table 1.

As expected, the experimentally determined cutpoints and

Stk values of the porous impaction

substrates are smaller than the theoretically calculated value for a at rigid surface impaction sub-

strate, assuming a

Stk value at 0.7. This cutpoint decrease was experimentally conrmed for both

rectangular and round nozzles and was presumed to be due to the penetration of some air stream-

lines into the porous polyurethane foam surface (Kavouras et al., 2000; Kavouras & Koutrakis,

2001). Huang and Tsai (2003) also showed that this is due to the penetration of air into PUF

causing a higher inertial force for particles near the surface of the porous PUF than that of the at

substrate. The same results were reported in theoretical/experimental investigations (Huang & Tsai,


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Aerodynamic particle diameter (m)

0.1 1 10

Collec

tionefficiency,

%

0

10

20

30

40

50

60

70

80

90

100

5 268 7 4 3 1

Fig. 4. Collection eciencies as a function of aerodynamic diameter for each stage of CCI.

2003; Huang, Tsai, & Shih, 2001). This shows that for the same nozzle geometry and owrate,

lower cutpoints can be achieved as compared to at rigid substrates, and therefore, the pressure drop

is substantially reduced for the same cutpoint (Demokritou et al., 2002a,b; Kavouras et al., 2000;

Kavouras & Koutrakis, 2001).

Table 1 summarizes the characterization of critical parameters for each stage of the CCI. These

include the cutpoint, the

Stk, the collection eciency curve sharpness (), and the pressure drop

across the stage. For all stages, the values are adequate to assure good separation of particles larger

than the cutpoint from the airstream (Fig. 4). Even for the lowest cutpoint (stage 8), the pressuredrop is quite low (5:73 kPa), and is signicantly lower than that of impactors that use at rigid

impaction substrates (for the same cutpoint), and thus minimizes losses of semivolatile components

from collected atmospheric particles (Kelly et al., 1994).

Fig. 5 shows the combined particle losses for all eight stages (based on measurements for each

individual stage) as a function of the particle aerodynamic diameter. Inertial particle losses within the

CCI may be caused by the turbulent ow in the acceleration nozzle region, at ow turns, and at the

ow exit/entrance between stages. Particle losses inside each stage were determined by measuring

particles up- and down-stream each stage, with the substrate removed. It is important to note that the

collection eciencies for each stage (see Fig. 4) include particles collected on both the impaction


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Aerodynamic particle diameter (m)

0.1 1 10

Part

icleLoss,

%

0

10

20

30

40

50

60

70

80

90

100

345678 1

Stage Number (cutpoint)

2

Fig. 5. Interstage particle losses as a function of aerodynamic diameter.

substrate and particles lost on stage walls and acceleration nozzle. Coarse particle (2.510 m) losses

increased from 5% for 4 m particles, up to 20% for particles with diameter larger than 8 m. Fine

particle (2:5 m) losses are minimal (less than 5%). Particle losses in the last stage (0 :16 m)

were higher (about 10%), due to the increased diusional deposition of these small particles. Similarresults were previously reported for other cascade impactors (Demokritou et al., 2002a,b; Kavouras

et al., 2000; Marple, Rubow, & Behm, 1991).

3.2. Particle loading experiments

Fig. 6 summarizes the experimental results of particle loading eect on the impaction characteristics

of PUF. Fig. 6(a) shows the change of cut-o diameter (d50), as a function of loaded mass on PUF

(for stage 4, with a cutpoint of 2:5 m). The cutpoint increased from 2.5 to 2.6 and 2 :7 m, for

10.9 and 24 mg of loaded mass, respectively. These mass loadings correspond to 121 and 264 h,

respectively, of sampler operation at an average ambient PM 2:5 concentration of 50 g=m3

(or 48 hat 126 and 276 g=m3, respectively). This should be considered a negligible eect on the cutpoint,

compared to the eect when using an oiled at substrate, for which the d50 decreased from 2.45

to 2:25 m (about 8.2% decrease from the initial cutpoint) for a particle loading of only 1:5 mg(Kenny et al., 2000).

Fig. 6(b) presents the collection eciency as a function of particle mass loading for particle

sizes of 9.7 and 1:7 m, which represents particles signicantly larger and signicantly smaller than

the cutpoint (d50 = 2:5 m), respectively. The collection eciencies for the 9:7 m particle remain

unchanged and close to 100% for loading up to about 25 mg. This is an indication that there is

negligible particle bounce and re-entrainment, which is usually more pronounced for large particle


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Loaded mass, M (mg)

0 5 10 15 20 25

d50

(m)

1.50

1.75

2.00

2.25

2.50

2.75

3.00

3.25

3.50

Change of d50

Loaded mass, M (mg)

0 5 10 15 20 25

Collectioneffic

iency(%)

0

10

20

30

40

50

60

70

80

90

100

da = 9.7 m

da = 1.7 m

(a)

(b)

Fig. 6. Particle loading test for an impaction stage of 30 LPM CCI ( d50 =2:5 m). (a) Change of 50% cut-o aerodynamic

diameter vs. loaded mass. (b) Collection eciency vs. loaded mass for da = 1:7 and 9:7 m, respectively.

sizes. This conrms that the PUF substrate maintains its ability to minimize particle bounce even

with relatively high loading conditions. It is worthwhile to point out that also for a greased coated

impaction substrate, particle bounce became signicant for a particle mass loading of only about

1 mg, due to a change in the impaction surface characteristics (Tsai & Cheng, 1995). Similarly,

for the particle size smaller than the cutpoint (1:7 m), the collection eciency was relatively low


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Table 2

Laboratory comparison of the particulate mass collected with the MOI, the CCI and SMPS/APS

Test MOI stage size range (m) and collected mass (mg) Total

collected

0:175 0.175 0.56 0.56 1.0 1.0 1.8 1.83.2 3.25.6 5.6 10 10 mass (mg)

High loading 0.564 0.757 1.023 2.571 2.871 1.366 0.168 0.031 9.35

Low loading 0.067 0.119 0.134 0.245 0.311 0.118 0.017 0.001 1.013

Test CCI stage size range (m) and collected mass (mg) Total

collected

0:16 0.16 0.47 0.471.0 1.01.7 1.72.5 2.5 3.3 3.35.3 5.39.9 9:9 mass (mg)

High loading 0.452 0.491 0.795 1.228 2.132 2.03 2.533 1.859 0.352 11.871

Low loading 0.061 0.068 0.090 0.117 0.178 0.149 0.325 0.098 0.000 1.085

Test SMPS/APS size range (m) and collected mass (mg) Total

collected

0:16 0.160.47 0.471.0 1.0 1.7 1.7 2.5 2.53.3 3.35.3 5.39.9 9:9 mass (mg)

Low loading 0.014 0.073 0.076 0.097 0.174 0.206 0.242 0.080 0.001 0.964

(approximately 20%) and also remained unchanged with loading, an indication that there is no

signicant distortion of the collection eciency curve in general.

This superior performance of this porous substrate (PUF) in terms of loading capacity is mostlikely due to the lack of formation of a hummock-like deposit during particle loading on the

surface, which is a distinctive feature for rigid, at impaction substrates. In this case, the particles

are deposited inside the pores of the foam, resulting in a much larger holding capacity.

3.3. Comparison of CCI and MOI and real time methods

Table 2 summarizes the results from the laboratory comparison of the MOI, the CCI and the

real time instruments (SMPS/APS) for both relatively high (about 10 mg) and relatively low (about

1 mg) total collected mass. Fig. 7 shows the mass size distributions measured by all three methods,

for the low total concentration conditions. It is obvious that the size distribution obtained fromthe CCI measurements agrees well with that of the real time instrumentation (SMPS/APS), while

the MOI measurements do not show as a good agreement. The MOI measurements indicate lower

coarse particle concentrations, and higher ne particle concentrations than the other two methods. A

previous study showed that particle bounce of coarse particles occurs in the MOI stages when using

Teon membrane lters as impaction substrates (Marple et al., 1991). The results observed here are

consistent with bounce-o the coarse particles from the upper MOI stages, which are then collected

downstream on the ne particle stages. Similar ndings were found for the high concentration test,

an indication that particle bounce of coarse particles can occur with the MOI, for both high and low

ambient total particle concentrations.


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Aerodynamic Diameter (Dp), m

0.1 1 10

Mass

Concen

tra

tion

/log

(Dp

),

g/m

3

0.0

5.0

10.0

15.0

20.0

25.0

Compact Cascade Impactor

MOI

SMPS/APS data

Fig. 7. Laboratory intercomparison of the particulate mass concentration measured with the MOI, the CCI, and the

SMPS/APS at low loading particle concentration.

The ratio of MOI to CCI mean total mass collected was 0.79 and 0.93 for the high and low-

concentration tests, respectively. One possible reason for these results is that multiple nozzles of

the MOI impaction stages are expected to have higher wall losses by diusion than the single slit

nozzles of the CCI stages. It is likely that the presumably higher amount of particle bounce forthe higher concentration test was one reason why there was a smaller ratio of MOI to CCI to-

tal mass for this experiment. Visual observation also indicated that there was signicant particle

deposition on the walls and the multiple nozzles of the MOI. It is also possible that use of an un-

coated substrate (Teon membrane lter) for the MOI contributed to the dierences between the two

samplers. This would be in agreement with the ndings of higher wall and nozzle losses, when un-

coated impaction substrates were used, as reported by Sioutas, Chang, Kim, Koutrakis, and Ferguson

(1999).

3.4. Field study results

PUF is a porous material, and is known to absorb varying amounts of water vapor from the air,

with increasing absorption for increasing relative humidity. In order to investigate the weight change

of the PUF as a function of the relative humidity in the RH- and temperature-controlled balance

room, ve laboratory blank substrates (for each of the three substrate sizes used by the CCI) were

weighed daily for 1 week. It was shown that a mean of 0.17% mass dierence occurred for the

largest (5%) dierence in relative humidity (RH). For a 48-h sampling period, this corresponds to

4.1, 1.8 and 1:0 g=m3, for nominal PUF masses of 104, 46, and 25 mg, respectively. To correct

eld test substrates for the eects of relative humidity, laboratory blanks were used. The dierence

in mass for each eld test substrate, before and after sampling, was adjusted to take into account


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Table 3

Mean concentration, precision, limit of detection (LOD), and coecient of variation (CV) for CCI

Stage No. of sample (n) Mean conc. (g=m3) Precision (g=m3) LOD (g=m3) CV

1 18 5.72 0.59 1.62 0.10

2 18 5.67 0.60 1.62 0.11

3 18 3.41 0.29 1.10 0.09

4 18 1.92 0.21 1.10 0.11

5 18 1.16 0.18 0.49 0.15

6 18 1.26 0.22 0.49 0.18

7 18 2.54 0.33 0.49 0.13

8 18 2.96 0.71 0.49 0.24

changes in the corresponding blanks. No humidity eects were observed for the Teon lters usedfor the impaction stages of the MOI and the back-up lter of the CCI.

The limit of detection (LOD), precision, and coecient of variation (CV) were also calculated for

the PUF substrates of each stage. LOD was calculated based on three times the standard deviation

of mass value for the eld blanks. Precision was obtained as the root mean square (RMS) dierence

for the paired values of CCI, divided by

2. CV was calculated as the precision divided by the

mean concentration. The LOD, precision, CV values of every stage are shown in Table 3. The mean

ambient concentrations of each stage in Boston were all higher than the LOD values.

The particle mass concentration, as a function of the aerodynamic diameter for the CCI, for two

representative sampling periods, were plotted in Fig. 8. The size distributions are both bimodal which

is typical for the Boston area (Long, 2000). Figs. 9(a) and (b) show good agreement between the

PM2:5 mass measurements for the CCI vs. the HI and the FRM, respectively. PM2:5 concentrations

for the CCI were calculated using the sum of the mass concentrations for the 5th, 6th, 7th, and 8th

stages, and the backup lter.

4. Summary and conclusion

A 30 LPM compact eight stage-cascade impactor (CCI) was developed and evaluated. This sys-

tem measures particle sizes between 10 and 0:16 m, with a backup lter to collect the remaining

ultrane particles, and has a total pressure drop of about 8:0 kPa. This relatively low-pressure drop

signicantly reduces the potential for volatilization losses of semi-volatile particle components. Thepolyurethane foam used for the impaction substrates results in insignicant particle bounce-o and

re-entrainment, without using adhesives such as grease or mineral oil as coatings. Interstage losses

of ultrane and ne particles were less than 10% and were less than 20 for coarse particles. The

system adequately maintained its impaction characteristics for a loading of up to 25 mg, which is

signicantly better than the performance of other cascade impactors that use at, rigid substrates.

The compact cascade impactor was also evaluated in a laboratory and eld study and compared

with the microorice impactor (MOI), Harvard impactor (HI) and the Federal reference method

(FRM) for ne particles. The average CCI collected mass was found to be higher than that of

the MOI. The dierence is most likely due to the particle bounce-o from the uncoated impaction


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0.1 1 10

MassConcentration/log(Dp),g

/m3

0

5

10

15

20

25

Sep. 23 - 25, 2002


0.1 1 10

MassConcentration/lo

g(Dp),g/m

3

0

5

10

15

20

25

Sep. 25 - 27, 2002

Fig. 8. Mass size distribution measured by CCI in Boston.

substrates and the increased wall and nozzle particle losses for the MOI. The size distribution

obtained by CCI agrees well with that of the real time instrumentation (SMPS/APS). The eld study

results show good agreement between the CCI and the two reference methods (HI and FRM) for

PM2:5 mass concentration.


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HI PM 2.5 Concentration (g/m3)

0 2 4 6 8 10 12 14 16 18 20 22

CCIPM

2.5

Concen

tra

tion

(g

/m3)

0

2

4

6

8

10

12

14

16

18

20

22

Data

1 : 1

Regression

CCI=1.05*HI-1.48

r 2=0.92

FRM PM2.5 Concentration (g/m

3)

0 2 4 6 8 10 12 14 16 18 20 22

CCIPM

2.5

Concen

tra

tion

(g

/m3)

0

2

4

6

8

10

12

14

16

18

20

22

Data

1 : 1

Regression

CCI=1.17*FRM-0.08

r2=0.97

(a)

(b)

Harvard Impactor vs. CCI

FRM vs. CCI

Fig. 9. Intercomparison between CCI, HI, and FRM for PM 2:5 mass concentrations during eld study in Boston.

Acknowledgements

The laboratory performance evaluation of the sampler was supported by the EPA/Harvard Particle

Health Eects Center (EPA Grant No. R827 353-01-0) and the NIEHS Harvard Center (NIEHS

Grant No. ES00002).


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