development and evaluation of a prototype ultrafine particle ...

*Author to whom correspondence should be addressed.

J. Aerosol Sci. Vol. 30, No. 8, pp. 1001}1017, 1999( 1999 Elsevier Science Ltd. All rights reserved

Printed in Great Britain0021-8502/99/$ - see front matterPII: S0021-8502(98)00769-1

DEVELOPMENT AND EVALUATION OF A PROTOTYPEULTRAFINE PARTICLE CONCENTRATOR

Constantinos Sioutas*, Seongheon Kim and Mingchih Chang

Department of Civil and Environmental Engineering, University of Southern California,3620 South Vermont Avenue, Los Angeles, CA 90898, U.S.A.

(First received 5 June 1998; and in ,nal form 5 October 1998)

Abstract=This paper presents the development and experimental characterization of a prototypeultra"ne particle concentrator. In this system, ultra"ne particles pass over a pool of warm waterwhere they become saturated with vapor, and subsequently drawn through a condenser, kept ata lower temperature, that allows the ultra"ne particles to grow to super-micrometer size by vaporcondensation on their surface. In order to increase particle concentration, the grown particles aredrawn through a virtual impactor with an approximate 50% cutpoint at 1.5 km. The concentratedparticles from the minor #ow of the virtual impactor "nally pass through a di!usion dryer thatremoves the excess water on the ultra"ne particles and returns them back to their original size andrelative humidity. In its optimum con"guration, the ultra"ne concentrator operates at a sampling#ow rate of 106.5 or 110 l min~1 and concentrates the ultra"ne particles to 3.5 or 7 l min~1 by anenrichment factor of approximately 15 and 25.5, respectively. Our experimental results identi"edsaturation of the ultra"ne aerosols at 353C and cooling to 253C as the optimum temperatures foroperation of the ultra"ne particle concentrator. Lower temperatures either do not concentrate, orconcentrate less e$ciently the ultra"ne particles. Increasing the saturation temperature to 403C andcooling to 313C does not improve the concentration enrichment achieved by the optimum con"g-uration. Our results also indicated that the concentration enrichment does not depend on thechemical composition of the ultra"ne aerosol. Hygroscopic ammonium sulfate, volatile ammoniumnitrate, hydrophobic polystyrene latex and actual &&real-life'' indoor air ultra"ne particles were allconcentrated by practically the same factor. More importantly, the experimental results show thatparticle concentration occurs without any coagulation, which would have distorted the size distribu-tion of the original ultra"ne aerosols. ( 1999 Elsevier Science Ltd. All rights reserved

1 . INTRODUCTION

There is abundant epidemiological evidence associating increased air particulate pollutionand the incidence of adverse health e!ects on humans (Dockery et al., 1992; Schwartz andDockery, 1992). These associations have been primarily demonstrated for "ne particles (e.g.particles smaller than 2.5 km in size, also referred to as PM

2.5) and its components such as

sulfate (SO2~4

) and strong acidity (H`) (Bates and Sizto 1989; Thurston et al., 1993; Dockeryet al., 1994). Few investigators have recognized the importance of the complex nature ofambient particles (Amdur, 1989; Brain et al., 1976; Anderson et al., 1992). Previous studieshave demonstrated acute e!ects of inhaled components of "ne particles (0.1}2.5 km) usingarti"cial multicomponent mixtures (Kleinman et al., 1995 and 1997). Although these studieshave made an attempt to simulate ambient particle exposures, the approach did not fullyachieve a &&real-life'' situation. This discordance between the outcomes of laboratory andepidemiological studies indicated that arti"cial particles do not replicate the adverse e!ectsof the complex and heterogeneous mixtures that occur in ambient air (Lippmann, 1989).The di$culty in establishing causal relationships between ambient "ne and ultra"neparticle exposures and health e!ects suggested that new paradigms in controlled particleexposure studies are needed to elucidate possible toxicity mechanisms of ambient partic-ulate matter.

Recently developed ambient "ne particle concentrators (Sioutas et al., 1995a, b; Sioutaset al., 1997) have been used in order to expose animals and humans to &&real-life'' aerosols.

1001

These technologies focused on separating ambient particles in the "ne (or accumulation)mode (0.15}2.5 km) from the majority of the surrounding air volume in order to investigatehealth e!ects resulting from exposures to elevated (but realistic) levels of actual ambientparticles. Ambient "ne particle concentrations were enriched by as much as a factor of 35prior to being supplied to animal exposure chambers. Mortality has been observed alreadyin some of our studies exposing animals with pulmonary in#ammation and chronicbronchitis to concentrations of about 300 kgm~3 of ambient "ne particles in Boston(Godleski et al., 1998). Nevertheless, these systems cannot be used to increase the concentra-tion of particles below 0.15 km, known as ultra"ne particles.

Atmospheric ultra"ne particles, or Aitken nuclei, arise from gas-to-particle conversionand combustion processes, in which hot, supersaturated vapors undergo condensationupon being cooled to ambient temperatures (Whitby and Svendrup, 1980; Finlayson-Pittsand Pitts, 1986). Particles in the size range 0.01 to about 0.2 km are formed entirely duringthese processes. Although the mass fraction of the ultra"ne mode is negligible, this sizerange contains the highest number of ambient particles by counts as well as surface area.Because of their increased number and surface area, ultra"ne particles are particularlyimportant in atmospheric chemistry and environmental health.

Todate there has been limited epidemiological evidence linking respiratory health e!ectsand exposures to ultra"ne particles, primarily due to the lack of adequate methods forultra"ne particle measurement. Recent epidemiological studies (Heyder et al., 1996; Peterset al., 1997), however, demonstrated a stronger association between health e!ects andexposures to ultra"ne particles compared to "ne or coarse particles.

Inhaled ultra"ne particles deposit primarily in the lower respiratory tract and alveoli bydi!usional mechanisms (ICRP, 1994). The pulmonary toxicity of ultra"ne particles has beendemonstrated in several controlled laboratory exposure studies. Inhalation of fumes con-sisting mainly of ultra"ne particles lead to the well-known e!ects of metal or polymer fumefever (Drinker et al., 1927; Gordon et al., 1992). Oberdoster et al. (1992) have showed thatapparently &&inert'' dusts consisting of ultra"ne particles can be highly toxic to the lungbecause of the interstitial access across the alveolar epithelium where they interact withmacrophages.

Separation and concentration of any type of particles from the airstream typically isbased on particle inertia. Devices such as virtual impactors have been developed for thispurpose (Marple and Chien, 1980; Sioutas et al., 1994a). Nevertheless, the development ofvirtual impactors to concentrate particles as small as 0.01 km (e.g. &&inertia-less'' particles)has been a major technical challenge. Recently, hypersonic impactors operating at very lowpressures (typically on the order of 500 Pa or less) have been developed to sample ultra"neparticles (Fernandez de la Mora et al., 1990; Hering and Stolzenburgh, 1995; Olawoyinet al., 1995). Inertial separation of ultra"ne particles from the surrounding air becomespossible due to the combined e!ect of very high velocities achieved in the impactor due topressure reduction. Use of hypersonic virtual impactors to concentrate ultra"ne particles,however, can be problematic for the following reasons; (a) the separated (and concentrated)particles are at a very low pressure, which makes it impossible to conduct inhalationexposures. Any volatile constituents of the ultra"ne aerosol will be lost at low pressure.A mechanism that transports the concentrated ultra"ne particles from a low pressure toatmospheric pressure (i.e. a blower) may result in excessive particle losses, and; (b) highsampling #ow rates (500 l min~1, or higher) are practically impossible because they requireoperation in parallel of tens of thousands of such impactors.

An alternative method to inertially separate ultra"ne particles from the majority of thesurrounding air without subjecting them to low pressure condition by condensationalgrowth/virtual impaction was recently developed (Sioutas and Koutrakis, 1996). Ultra-"ne particles undergo condensational growth using supersaturation of water vapor and areseparated by the virtual impactor. At low #ow rate of 8 l min~1, the average particlecollecting e$ciency is 0.9 and the particle losses through the system were less than 5%.Neither the collection e$ciency nor losses were found to depend on the particle chemicalcomposition, hygroscopicity, or size distribution of the original aerosol. Other than

1002 C. Sioutas et al.

Fig. 1. Schematic of the prototype Ultra"ne Particle Concentrator and the experimental set-up forits characterization.

inertially separating ultra"ne particles grown to super-micrometer size and collecting themon a "lter, however, that system did not provide airborne concentrated ultra"ne aerosols,suitable for use in inhalation exposure studies. Furthermore, the delivery #ow rate of theconcentrated droplets was only 0.8 l min~1, clearly too small to be of any practical use ininhalation exposures.

This paper discuses the development and experimental characterization of a prototypeultra"ne particle concentrator. This system is an expansion of the work by Sioutas andKoutrakis (1996) to a larger scale, suitable for inhalation studies. Speci"cally, the ultra"neparticle concentrator samples particles at a #ow rate of 110 lmin~1 and concentrates themto a #ow of either 7 or 3.5 lmin~1. The prototype ultra"ne particle concentrator is operatedat high #ow rates and concentrates the ultra"ne particles with high collecting e$ciency andlow losses at atmospheric pressure. Therefore, it provides a powerful tool to conduct humanand/or animal exposures to real-time concentrated ultra"ne particles.

2. METHODS

2.1. Description of the ultra,ne particle concentrator

Figure 1 is the schematic of the Ultra"ne Particle Concentrator, as well as the experi-mental setup for its characterization. Air "rst passes over a pool of warm water to achievesaturation, and subsequently it is drawn through a condenser that allows the ultra-"ne particles to grow to super-micrometer size. In order to increase particle concentration,the grown particles are then drawn through a virtual impactor. The concentrated particlesfrom the minor #ow of the virtual impactor "nally pass through a di!usion dryer to removethe excess water on the ultra"ne particles and return them back to their original size andrelative humidity. A more detailed description of the components of the system is given inthe following paragraphs.

The saturator consists of a vertical glass cylindrical tube 40 cm long and 15 cm indiameter, partially "lled with distilled deionized water. The tube is submerged in a waterbath (Model, 1204, Sheldon, MFG, Inc., Cornellius, OR) that heats up the water duringthe experiments. The aerosol residence time is about 3 s to guarantee the saturation of thesampled air. The descending-ascending motion of particles as they enter and exit thesaturator further enhances mixing and saturation. The vapor-saturated aerosol then entersthe condenser, which consists of one horizontal aluminum tube, 80 cm long and 2.4 cm indiameter. These dimensions yield a residence time in the condenser of about 0.2 s. The

Prototype ultra"ne particle concentrator 1003

Table 1. Comparison of design and operating parameters of this work with a previously published condensationalgrowth system (Sioutas and Koutrakis, 1996) and the TSI 3022 CPC

Current work Sioutas and Koutrakis (1996) TSI CPC 3022

Flow Rate (l min~1) 106.5}110 8 0.3}1.5

SaturatorLiquid Deionized water Water 1-butanolTemperature (3C) 20}40 50 35Residence time (s) 3 10 6

CondenserTemperature (3C) 10}30 8 8Residence time (s) 0.2 0.5 0.2

condenser tube is submerged in a mixture of ice and water, plus a few grams of rock salt tokeep the solution at !93C. The condenser was designed to decrease the temperature ofsaturated aerosol by about 103C at a #ow rate in the range of 106.5}110 l min~1. The grownparticles are then drawn through the minor #ow of a virtual impactor connected immedi-ately downstream of the condenser. The concentrated droplets are "nally drawn througha di!usion dryer that removes excess moisture to become concentrated ultra"ne particles.

The di!usion dryer consists of a cylindrical screen, 1.8 cm in diameter, placed inthe center of a glass tube, 6 cm in diameter. Both glass tube and screen are 20 cm long. Theinner space between the two tubes is "lled with calcium sulfate (DrieriteTM), to remove theexcess water in the air stream. Relative humidity was measured immediately downstream ofthe dryer with a temperature/relative humidity probe (Cole-Parmer' Model 37960, Cole-Parmer' Instruments Co., Vernon Hills, IL). In all the experiments that will be described inthe following paragraphs, the measured relative humidity downstream of the di!usion dryerranged from 28% to 34%.

The e!ect of parameters including vapor temperature in the saturator and minor-to- total#ow ratio was investigated in order to determine an optimal con"guration that concen-trates ultra"ne particles with high collection e$ciency, low losses and high concentrationenrichment factor. Table 1 shows the comparison of the present design and operatingparameters to those by Sioutas and Koutrakis (1996) as well as those of a conventionalcondensation nucleus counter such as the TSI Condensation Particle Counter (CPC 3022,TSI Inc, St. Paul, MN).

2.2. Experimental characterization of the virtual impactor

The "rst series of experiments was done to characterize particle collection e$ciency andlosses of the virtual impactor used to concentrate the grown ultra"ne particles. The virtualimpactor has an acceleration nozzle diameter of 0.37 cm and collection nozzle diameter of0.55 cm. The impactor has been designed to have a theoretical 50% collection e$ciencycutpoint at about 1.5 km. The ratio of the collection-to-acceleration nozzles is chosen 1.5because it has been shown to minimize internal particle losses (Marple and Chien, 1980;Sioutas et al., 1994b).

The principal parameter determining particle inertial separation is the Stokes number ofa particle having a 50% probability of separation, St

50. St

50is de"ned as the ratio of the

product of the jet velocity, ;, and the particle relaxation time, q, vs the impactor's nozzlediameter (round nozzle) or width (rectangular nozzle), =, (Marple and Liu, 1974; Hinds,1982):

St50"

o1d250

C#;

9k=(1)

where d50

is the geometrical diameter of particle having a 50% probability of impaction,; is the average velocity of the jet (cm s~1), o

1is the particle density (g cm~3), k is the


Fig. 2. Experimental set-up for the characterization of the virtual impactor.

dynamic viscosity of the air (g cm~1 s), and C#is the Cunningham slip correction factor. The

slip correction factor is given by the equation (Hinds, 1982)

C#"1#

2

Pd1

[6.32#2.01 exp(!0.1095Pd1)] (2)

where P is the absolute pressure upstream of the impaction zone (in cmHg) and d1

is theparticle diameter in km. For the aforementioned values for ;, =, and P, the St

50value

corresponding to 1.5 km is approximately 0.6.Two pressure taps were placed in the major and minor #ows of the virtual impactor,

respectively. The pressure drop across the major #ow of the impactor at a #ow rate of 106.5and 110 lmin~1 was 25 and 28 kPa, respectively. The pressure drop between the airstreamin the minor #ow and the atmosphere, however, was only 1 kPa (e.g. the absolute pressureof the concentrated particles is about 0.99 atmospheres). This pressure recovery is veryimportant in using this system for inhalation studies, which cannot be conducted undera high vacuum.

The experimental setup for the characterization of the virtual impactor is shown in Fig. 2.Monodisperse aerosols were generated by atomizing dilute aqueous suspensions of #uor-escent polystyrene latex particles using a constant output nebulizer (HEART, VORTRANMedical Technology, Inc., Sacramento, CA). The range of monodisperse particles variedfrom 0.5 to 9 km. All the generated particles were drawn through a 1 l bottle to remove theexcess moisture and subsequently mixed with dry room air. The dry aerosol was drawnthrough a tube containing ten Po-210 neutralizers that reduces particle charges prior toentering the virtual impactor. A glass "ber "lter (2 km pore, Gelman Science, Ann Arbor,MI) was connected to each of the major and minor #ows, respectively, to collect the testparticles. Each "lter was connected to a pump with a calibrated #owmeter in line (Cole-Parmer' Instruments Co., Vernon Hills, IL). In addition, a similar glass "ber "lter wasconnected in parallel to the test system to measure the concentration of the generatedaerosol. At the end of each run, each glass "ber "lter was placed in 5 ml of ethyl acetate to


Table 2. Characteristics of the ultra"ne aerosols used in the experiments

MMD (GSD)* Number concentration rangeChemical composition (km) (particles cm~3)

Fluorescent polystyrene latex 0.05 (1.11) 1.5}1.95]105Fluorescent polystyrene latex 0.10 (1.05) 1.1}1.5]105Ammonium sulfate 0.09 (1.8) 1.45}2.0]105Ammonium nitrate 0.09 (1.9) 5.3}6.5]104Indoor air NM- 7.5}11]103Deionized water NM- 0.3}0.4]103

*MMD is the mass median diameter measured with the Microori"ce Uniform Deposit Impactor(MOUDI). GSD is the geometric standard deviation.-Not measured.

extract the #uorescent dye from the collected particles. The quantities of the #uorescent dyein the extraction solutions were measured by a Fluorescence Detector (FD-500, GTI,Concord, MA) to determine particle concentration. The collection e$ciency of the virtualimpactor was determined by dividing the amount of #uorescence on the minor #ow "lter tothe sum of the amounts collected on both major and minor #ow "lters. Furthermore,particle losses were determined by comparing the concentration determined with thereference "lter to the total concentration in the major and minor #ows of the virtualimpactor.

2.3. Experimental characterization of the ultra,ne particle concentrator

The experiment setup for the characterization of the Ultra"ne Particle Concentrator isshown in Fig. 1. Ultra"ne particles were generated by atomizing suspensions of ultra"neparticles with a constant output HEART nebulizer (VORTRAN Medical Technology, Inc.,Sacramento, CA). Di!erent types of suspensions were used for this purpose, includingmonodisperse 0.05 and 0.1 km PSL #uorescent latex particles (Polysciences Inc., Warrin-gton, PA) as well as polydisperse aerosols of ammonium sulfate and ammonium nitrate.Finally, ultra"ne indoor air particles were used as the test aerosol. The generated ultra"neaerosols were dried and neutralized in a process identical to that described previously in thetests to characterize the virtual impactor.

Table 2 summarizes the characteristics of the di!erent aerosols tested. Prior to testing theultra"ne particle concentrator, the size distribution based on mass of the ammonium sulfateand ammonium nitrate aerosols were determined using the Microori"ce Uniform DepositImpactor (MOUDI, MSP Corp, Minneapolis, MN). The performance of the MOUDI isdescribed by Marple et al. (1991). Measurement of the size-dependent mass concentration ofindoor air was not considered necessary for the following reason: the CPC measures totalparticle concentration by counts, which, for indoor or ambient air, is dominated by ultra"neparticles (Whitby and Svendrup, 1980; Seinfeld, 1986). The particle number concentrationfor pure deionized water was also measured to provide an estimate of background particlesrelated to impurities in the water. The dilution air in this series of tests was drawn througha HEPA "lter to ensure that only particles generated by atomizing deionized water arecounted by the CPC. As seen in Table 2, the concentration of these particles is very lowcompared to the concentrations of the test aerosols, thereby eliminating any ambiguities onthe nature of the aerosols used in each test.

The e!ect of the saturator temperature in activating and growing ultra"ne particles wasalso investigated. The temperature of the water bath was adjusted so that the ultra"neaerosol leaves the saturator a speci"c temperature. Five di!erent aerosol temperaturesettings were tried: 20, 25, 30, 35 and 403C, respectively. The temperature of the aerosol wasrecorded immediately upstream and downstream of the condenser as well as downstream ofthe di!usion dryer with a thermometer (VWR Brand, Cat61016-208). Total #ow rates wereset at 106.5 and 110 l min~1, respectively, depending on the minor #ow rate (e.g. 3.5 and


Table 3. Summary of the di!erent temperature settings tried for the evaluation of the ultra"ne particle concentra-tor. Data for vapor pressures and concentrations at di!erent temperatures are from McQuiston and Parker (1982)

Temperature of the Temperature of the Theoretical vaporsaturated aerosol, ¹

4cooled aerosol, ¹

#Theoretical concentration available

(3C) (3C) Supersaturation* for condensation (gm~3)-

20 11 1.82 7.825 15 1.80 10.330 21 1.67 12.435 25 1.70 16.540 31 1.59 20.2

*De"ned as the ratio of vapor pressures at ¹4and ¹

#, respectively.

-De"ned as the di!erence in the saturation vapor concentrations at ¹4and ¹

#, respectively.

7 l min~1, respectively). In each experiment, room air was drawn through the system forabout 30}45 min prior to generating particles to allow for temperature stabilization.Subsequently, the TSI Condensation Particle Counter (CPC 3022, TSI Inc., St. Paul, MN)was connected immediately upstream of the saturator and downstream of the di!usiondryer (as shown in Fig. 1) to measure the number concentrations of the original andconcentrated ultra"ne aerosols. For each saturator temperature, the "ve aerosol species,0.05 and 0.1 km #uorescent PSL particles, ammonium nitrate and ammonium sulfate, andambient indoor air, were tested for two di!erent minor-to-total #ow ratios of the virtualimpactor.

Table 3 presents a summary of the saturated and cooled aerosol temperatures (¹4and ¹

#,

respectively), the theoretical supersaturation achieved in each con"guration (equal to theratio of water vapor pressures at temperatures ¹

4and ¹

#, respectively), and the theoretical

vapor concentration available for condensation on ultra"ne particles. This concentration isequal to the di!erence in vapor concentrations at temperatures ¹

4and ¹

#, respectively. It

should be noted, however, that the available vapor does not only condense onto theultra"ne particles, but also it condenses on any other available solid surface (such as thewalls of the condenser). Therefore the data in Table 3 present only qualitative di!erencesbetween the various con"gurations tried.

3 . RESULTS AND DISCUSSION

3.1. Performance of the virtual impactor

Results from the characterization of the virtual impactor are shown in Figs 3}5. Figure 3shows the collection e$ciency and losses for a total #ow of 110 lmin~1 and a minor #ow of7 l min~1. Particle losses are generally 10% or less and do not seem to depend on particleaerodynamic diameter. Figure 4 shows the collection e$ciency and losses for a total #ow of106.5 l min~1 and a minor #ow of 3.5 lmin~1. Similarly, particle losses are 10% or less andgenerally do not depend strongly on particle size. In both con"gurations, the aerodynamicdiameter corresponding to a 50% collection e$ciency is about 1.5 km.

The sharpness of the collection e$ciency curve of an impactor can be de"ned in terms ofthe geometric standard deviation (p

'), which is the ratio of the aerodynamic particle

diameter corresponding to 84% collection e$ciency to the 50% cutpoint (Marple andWilleke, 1976). Based on this de"nition, the value of p

'is approximately 1.3 and 1.4 for

minor #ow rates of 3.5 and 7 l min~1, respectively (or equivalently, for minor-to-total #owratios of 0.033 and 0.065, respectively). In their theoretical analysis on the e!ect of theminor-to-total #ow ratio on the performance of virtual impactors, Marple and Chien (1980)showed that the steepness of the e$ciency curve increases as the minor-to-total #ow ratiodecreases. Our experimental results provide corroboration to the theoretical analysis ofMarple and Chien (1980).


Fig. 3. Particle collection e$ciency and losses of the virtual impactor. Total #ow rate: 110 lmin~1;minor #ow rate: 7 l min~1.

Fig. 4. Particle collection e$ciency and losses of the virtual impactor. Total #ow rate: 106.5 lmin~1;minor #ow rate: 3.5 l min~1.


Fig. 5. Concentration enrichment achieved by the virtual impactor at two di!erent minor #ow rates(3.5 and 7 l min~1).

Figure 5 shows the enrichment in the concentration of particles in the size range from0.75}9 km as a function of particle aerodynamic diameter. The concentration enrichment,CE, is given by the following equation:

CE"

Q505

q.*/

(1!WL)g7*, (3)

where Q505

and q.*/

are the intake and minor #ows of the impactor, respectively, and g7*

andWL are the collection e$ciency and fractional losses of the impactor. Figure 5 showsthat for a minor #ow of 3.5 lmin~1, the concentration enrichment increases sharply fromabout 5 to about 28 as particle aerodynamic diameter increases from 1 to 2.2 km. Theenrichment is practically the same for particle in the aerodynamic diameter range of2.2}9 km. Similarly, for a minor #ow of 7 l min~1, the concentration enrichment increasessharply from 3 to about 13 as particle aerodynamic diameter increases from 1 to 2.5 km. Forparticles having aerodynamic diameters in the range of 3 to 9 km, the enrichment value isabout 15 and practically independent of particle size.

The results of Figure 5 also suggest that the virtual impactor could be also used by itself(e.g. without the condensational growth component of the system) to concentrate particleshaving aerodynamic diameters in the range of 2.5 to 10 km. Ambient particles in this rangeare de"ned as coarse particles (Whitby and Svendrup, 1980; Finlayson-Pitts and Pitts, 1986)and technologies to concentrate these particles will be useful in assessing their toxicity,especially when conducted concurrently to inhalation studies using concentrated "ne orultra"ne particles.


Fig. 6. Ultra"ne particle concentration enrichment at saturation temperature of 353C (¹4) and

cooling temperature of 253C (¹#) for di!erent types of ultra"ne particles at two di!erent minor #ow

rates (3.5 and 7 lmin~1).

Table 4. Concentration enrichment achieved by the ultra"ne particle concentrator as a function of particle typeand minor #ow rate. The temperatures of the aerosol upstream and downstream of the condenser are 35 and 253C,

respectively

Concentration Concentrationupstream of the downstream of thesaturator di!usion dryer Concentration

Aerosol type (particles cm~3) (particles cm~3) enrichment

Minor #ow: 7 lmin~1Indoor air 8.1 ($0.9)]103 1.3 ($0.2)]105 15.1 ($0.8)0.05 km PSL 1.7 ($0.2)]105 2.5 ($0.2)]106 14.8 ($0.9)0.1 km PSL 1.3 ($0.2)]105 2.0 ($0.1)]106 15.3 ($0.7)Ammonium nitrate 5.9 ($0.6)]104 8.9 ($0.3)]105 15.1 ($0.6)Ammonium sulfate 1.7 ($0.2)]105 2.6 ($0.1)]106 15.2 ($0.8)

Minor #ow: 3.5 l min~1Indoor air 8.1 ($0.9)]103 2.1 ($0.2)]105 26.0 ($1.1)0.05 km PSL 1.7 ($0.2)]105 4.2 ($0.2)]106 24.7 ($1.8)0.1 km PSL 1.3 ($0.2)]105 3.5 ($0.2)]105 27.1 ($1.4)Ammonium nitrate 5.9 ($1)]104 1.4 ($0.2)]106 23.8 ($0.8)Ammonium sulfate 1.7 ($1)]105 4.3 ($0.2)]106 25.1 ($1.3)

The particle concentration data correspond to averages of at least three tests.

3.2. Performance evaluation of the ultra,ne particle concentrator

To initiate the parametric evaluation of the ultra"ne particle concentrator, we set thetemperature of the water bath so that the aerosol temperatures upstream and downstreamof the condenser (¹

4and ¹

#, respectively) are 35 and 253C. For these values of ¹

4and ¹

#we evaluated the concentration enrichment achieved by the ultra"ne concentrator for "vedi!erent ultra"ne aerosol species and for two di!erent minor-to-total #ow ratios of thevirtual impactor. Results from this "rst series of tests are shown in Table 4 and summarizedin Fig. 6.


Figure 6 shows clearly that the concentration enrichment at a minor #ow rate ofeither 3.5 or 7 l min~1 does not depend on particle species. The average concen-tration enrichment for indoor air, 0.05 km PSL, 0.1 km PSL, ammonium nitrate andammonium sulfate particles is by 15.1, 14.8, 15.3, 15.1 and 15.2, respectively, when thevirtual impactor operates with a minor #ow of 7 l min~1. When the virtual impactoroperates with a minor #ow of 3.5 l min~1, the enrichment for indoor air, 0.05 km PSL,0.1 km PSL, ammonium nitrate and ammonium sulfate particles becomes 26.0, 24.7, 27.1,23.8 and 25.1, respectively.

These values for the concentration enrichment are identical to the maximum obtainableconcentration factors when dry, coarse particles (e.g. larger than about 3 km in aerodynam-ic diameter) are sampled by the virtual impactor. [Figure 5 shows that the maximumconcentration enrichment obtained for particles in the aerodynamic diameter range of 3 to9 km is by a factor of about 15 and 27 at minor #ow rates of 7 and 3.5 lmin~1, respectively].A very important implication of this result is that it shows conclusively that no particlecoagulation occurs during the concentration enrichment process. If any coagulation hadoccurred, the measured number concentrations downstream of the di!usion dryer (thus theenrichment factors) would have been substantially smaller than the maximum obtainablevalues, shown in Figure 5.

An important consideration regarding the ultra"ne particle concentrator's performancewas its ability to increase the concentration of particles without substantial losses of volatileor semi-volatile materials during sampling. Ammonium nitrate is one of the most predomi-nant volatile constituents of ambient particles, thus comparisons in the concentrationenrichment based on ammonium nitrate would illustrate the magnitude (if any) of theselosses during particle sampling and concentration. As ultra"ne particles are heated duringsaturation, losses due to volatilization of nitrate could be possible. It should be noted thattests with pure ultra"ne ammonium nitrate particles would tend to exaggerate such losses,since the vapor pressure of any substance over the surface of a particle increases withdecreasing particle size (Finlayson-Pitts and Pitts, 1986).

Figure 6 shows the average concentration enrichment of ultra"ne ammonium nitrateparticles is practically identical to that observed for non-volatile ammonium sulfate orpolystyrene latex particles. This suggests that no signi"cant volatilization losses occurduring the concentration enrichment process. Ammonium nitrate dissociates to ammoniaand nitric acid, with its dissociation constant increasing exponentially with temperature.However, the dissociation constant decreases sharply as the relative humidity (RH) exceeds90}95% (Stelson and Seinfeld, 1982). For example, even at 503C and at RH"95%, thedissociation constant of ammonium nitrate is approximately 7 ppb2, which is the value ofthe dissociation constant at 183C. Therefore, despite the increase in the aerosol temperature(which would have increased exponentially the value of the dissociation constant), satura-tion of the aerosol seems to prevent nitrate losses due to volatilization.

In addition to the tests described above, the mass concentrations of ammonium nitrateupstream and downstream the ultra"ne concentrator were measured in few additionalexperiments to con"rm that no nitrate losses occur through the system. The Harvard/EPAAnnular Denuder System (HEADS, Koutrakis et al., 1988) was used as the reference. TheHEADS consisted of an annular denuder, coated with sodium carbonate to remove nitricacid from the air sample, followed by a sodium carbonate-coated glass "ber "lter to collectammonium nitrate particles. The HEADS sampled immediately upstream of the saturator.An identical HEADS was connected to the minor #ow of the virtual impactor, downstreamof the di!usion dryer. Both HEADS samplers operated at 7 l min~1. At the end of eachexperiment, the glass "ber "lters were extracted with 0.15 ml ethanol in 5 ml ultrapurewater. After sonication for about 5 min, the extracts were analyzed for nitrate by ionchromatography.

Results from these tests are shown in Table 5. The concentration enrichment values forultra"ne ammonium nitrate particles based on mass are virtually identical to those basedon particle counts (Figure 6), which con"rms that concentration enrichment occurs withoutany substantial loss of nitrate.


Table 5. Mass concentrations of ultra"ne ammonium nitrate particles measured upstream and down-stream of the ultra"ne particle concentrator. Total #ow: 110 lmin~1; minor #ow rate: 7 l min~1. Thetemperatures of the aerosol upstream and downstream of the condenser are 35 and 253C, respectively

Concentration upstream Concentration downstreamof the saturator of the di!usion dryer Concentration

Experiment no. (kgm~3) (kg m~3) enrichment

I 22.9 325.2 14.2II 14.7 225.1 15.3III 28.4 445.9 15.7IV 11.3 163.9 14.5V 19.6 298.0 15.2




In subsequent experiments, the temperature in the saturator was varied, and the perfor-mance of the ultra"ne concentrator for each ultra"ne particle species and each minor-to-total #ow ratio was evaluated. Results from these tests are summarized in Figs 7}10.Figure 7 shows the concentration enrichment for indoor air, ammonium sulfate, am-monium nitrate, and 0.05 and 0.1 km PSL particles for two di!erent virtual impactorcon"gurations at aerosol temperatures upstream and downstream of the condenser (¹

4and

¹#, respectively) of 20 and 113C. The results of Fig. 7 show that none of the ultra"ne

aerosols becomes concentrated at these temperature settings. The supersaturation achievedin this con"guration is 1.82, which should be su$cient to activate particles as small as0.003 km. The smallest particle size (d*) that can be activated by supersaturation is given bythe Kelvin equation (Hinds, 1982)

d*"4pM ln(S)

oR¹

, (4)





where S is the supersaturation, M, o, and p are the molecular weight, density, and surfacetension of the condensing liquid, and R is the ideal gas constant. It is more likely thatultra"ne particles become activated by the supersaturation, but the vapor concentrationthat is actually available for condensation does not grow the ultra"ne particles to a size thatcan be concentrated by the virtual impactor (e.g. particles probable grow to sizes smallerthan 1 km).

Figure 8 shows the concentration enrichment for indoor air, ammonium sulfate, am-monium nitrate, and 0.05 and 0.1 km PSL particles for two di!erent virtual impactorcon"gurations at aerosol temperatures upstream and downstream of the condenser (¹

4and

¹#, respectively) of 25 and 153C, respectively. Indoor air ultra"ne particles seem to be

marginally concentrated, whereas the concentration of ammonium nitrate, ammoniumsulfate and 0.05 km particles increases by a factor of about 3 ($0.4). The concentration of0.1 km particles increases by factor of about 5. The higher concentration enrichmentobtained for 0.1 km PSL particles could be due to their somewhat larger initial size,compared to the rest of the ultra"ne particles. This may also explain the reason for whichthe lowest enrichment is obtained for indoor air particles. The size distribution of indooraerosols, measured by counts, is dominated by particles smaller than 0.05 km (Thatcher andLayton, 1995), which apparently do not grow to a droplet size that can be concentrated bythe virtual impactor. Although studies on technologies using condensational growth (suchas the TSI Condensation Particle Counters) have shown that the ultra"ne particles grow tothe same "nal size (Ahn and Liu, 1990), it is possible that the combination of parameters inour system such as the temperatures of the saturated and cooled aerosol, as well as therelatively short residence times in the saturator and condenser do not allow all particles toreach their "nal droplet size.

Figure 9 shows the concentration enrichment for indoor air, ammonium sulfate, am-monium nitrate, and 0.05 and 0.1 km PSL particles for two di!erent virtual impactor


con"gurations at aerosol temperatures upstream and downstream of the condenser (¹4and

¹#, respectively) of 30 and 213C, respectively. The growth of ultra"ne particles in this

con"guration seems to be more uniform across the di!erent particle species than those atlower temperatures. The average concentration enrichment values for minor #ow rates of7 and 3.5 l min~1 range from approximately 8 to 13 for a minor #ow rate of 7 l min~1, andfrom about 15 to 19 at a minor #ow rate of 7 lmin~1, respectively. By examining theconcentration enrichment curves of the virtual impactor (Fig. 5), we could hypothesize that,for these temperature settings, ultra"ne particles grow to about 1.4}1.7 km on the average.This higher enrichment is evidently due to the higher vapor concentration available forcondensation in this con"guration (12.4 gm~3, compared to 7.8 and 10 gm~3, correspond-ing to ¹

4"20 and 253C, respectively). The values of the concentration enrichment corre-

sponding to either minor #ow rate are lower than those obtained at ¹4"353C and

¹#"253C (main con"guration), for the same reason; there is more vapor available when

the aerosol becomes saturated at 353C and is cooled down to 253C.Finally, Fig. 10 shows the concentration enrichment for indoor air, ammonium sulfate,

ammonium nitrate, and 0.05 and 0.1 km PSL particles for the two di!erent virtual impactorcon"gurations at aerosol temperatures upstream and downstream of the condenser (¹

4and

¹#, respectively) of 40 and 313C, respectively. The average concentration enrichment is by

a factor of 14.4 ($0.3) and 25.5 ($0.7) at a minor #ow rate of 7 and 3.5 l min~1, respectively.The enrichment values are practically identical to those obtained at ¹

4"353C and

¹#"253C (main con"guration) and similarly do not depend on the type of ultra"ne

particles used as the test aerosol. The main con"guration (¹4"353C and ¹

#"253C) was

therefore considered our optimum con"guration, as it results in the highest concentrationenrichment obtainable at the lowest possible saturation temperature. This feature mini-mizes the energy requirement for particle growth as well as the risk of losses of volatilecompounds from the ultra"ne aerosols.

In order to use this technology for inhalation exposure studies to ambient ultra"neaerosols, a device separating the "ne and coarse ambient particles (e.g. those havingaerodynamic diameter larger than 0.1}0.2 km) must be used upstream of this system.Technologies such as conventional impactors with small 50% cutpoints, already availablein the literature, can be either readily used (Sioutas et al., 1997), or scaled-up (Marple et al.,1991; Hering et al., 1997) to separate particles larger than 0.15}0.2 km from the air stream atsu$ciently high #ow rates, and with relatively low pressure drop (e.g. less than 10 kPa).One additional feature of an ultra"ne particle concentrator for inhalation exposures shouldbe the inclusion of devices that remove soluble gases (such as ammonia, sulfur dioxide,ozone and nitric acid) from the air sample, so that these gases do not interfere with theultra"ne particles in the presence of water. Technologies such as di!usion denuders(Coutant et al., 1989; Eatough et al., 1993; and Koutrakis et al., 1993) can be used toe$ciently remove these gases even at #ow rates as high as 110 l min~1. Finally, an issue thatshould be addressed in future toxicology studies is potential redistribution of water solublecompounds from deep in the particle to the surface, which may enhance particle toxicity.

4 . SUMMARY AND CONCLUSIONS

We have developed and evaluated experimentally a prototype ultra"ne particle concen-trator. Ultra"ne particles are "rst grown by means of supersaturation to a size that can beeasily concentrated by a virtual impactor. The concentrated droplets are dried in a di!usiondryer to obtain a concentrated ultra"ne aerosol.

The condensational growth/virtual impaction system has been evaluated using monodis-perse 0.05 and 0.1 km #uorescent PSL particles, as well as polydisperse ultra"ne ammoniumsulfate, ammonium nitrate and indoor air particles. The concentrator operated at a #owrate of either 106.5 or 110 l min~1, of which 3.5 or 7 min~1 were drawn through the minor#ow of the virtual impactor. The performance of the concentrator was evaluated at di!erentaerosol saturation and cooling temperatures. Our experimental results identi"ed saturationof the ultra"ne aerosols at 353C and cooling to 253C as the optimum temperatures for


operation of the ultra"ne particle concentrator. Lower temperatures either do not concen-trate, or concentrate less e$ciently the ultra"ne particles. Increasing the saturation temper-ature to 403C and cooling to 313C does not improve the concentration enrichment achievedby the system.

Our results indicated that the concentration enrichment does not depend on the chemicalcomposition of the ultra"ne aerosol. Hygroscopic ammonium sulfate, volatile ammoniumnitrate, hydrophobic polystyrene latex and actual &&real-life'' indoor air ultra"ne particleswere all concentrated by practically the same factor. The average concentration enrichmentwas by a factor of 15.1 ($0.4) and 25.5 ($1.9) for minor #ow rates of 7 and 3.5 l min~1 (or,equivalently, for minor-to-total #ow ratios of 0.033 and 0.064, respectively). These enrich-ment factors are very similar to the maximum concentration factors that could be obtainedby either con"guration of the virtual impactor, thereby suggesting that ultra"ne particleconcentration is achieved with a very high e$ciency and low losses. More importantly, theexperimental results show that particle concentration occurs without any coagulation,which would have distorted the size distribution of the original ultra"ne aerosols.

It should be emphasized that this investigation is a pilot study whose main goal was todemonstrate the feasibility of concentrating e$ciently ultra"ne particles using a largecutpoint virtual impactor, with minimum losses and at a reasonably high output #ow rate.If higher output #ow rates are desired, more than one of these systems could be placed inparallel. The simplicity of the design of this technology (e.g. low-energy input, small volumeand use of water instead of organic #uids as a working liquid) makes it ideal for use ina modular con"guration.

Acknowledgements*The development and evaluation of the prototype ultra"ne particle concentrator was sup-ported by the U.S. Environmental Protection Agency under the subcontract d5345-078160 from the HarvardSchool of Public Health to the University of Southern California. Finally, a patent application has been "led to theUnited States Patent O$ce (USC File No. 2819).

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