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Atmospheric Environment 42 (2008) 828–832

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Technical note

Collection efficiencies of bioaerosol impingers forvirus-containing aerosols

Andrew Dart�, Jonathan Thornburg

Center for Aerosol Technology, RTI International, 3040 Cornwallis Road, Research Triangle Park, NC 27709, USA

Received 29 June 2007; received in revised form 5 November 2007; accepted 5 November 2007

Abstract

Impingers are common bioaerosol sampling instruments. In the last 20 years, impinger performance for collection of

viable and non-viable bioaerosols in the 0.1–10mm size range has been well documented. An ideal impinger has high

collection efficiency for particles 40.1mm. This research explored how well the 500ml Greenburg–Smith (G–S) impingers

collected particles between 0.1 and 2mm, simulating virus aerosols, with minimal fluid evaporation during sample

collection intervals near 60min. Various impingers have differing dimensions that come into play when collecting viable

samples of bioaerosols. The sampling environment can be modified to create optimum impingement conditions. Moderate

differences in flow rate and impingement fluid volume may show distinct differences in the capture rate of viable samples

within the bioaerosol impinger.

r 2007 Elsevier Ltd. All rights reserved.

Keywords: Greenburg–Smith; Bioaerosol; Impinger; Virus-containing aerosol; Impingement; Viability

1. Introduction

Impingers are common aerosol and bioaerosolsample collection instruments. Liquid impingers foraerosol sampling were introduced in 1914 by theUnited States Public Health Service and UnitedStates Bureau of Mines’ in response to industrialhealth concerns to total suspended particulatematter (Greenburg and Bloomfield, 1932). In thelast 20 years, impinger performance for collection ofviable and non-viable bioaerosols in the 0.1–10 mmsize range has been well documented (Henningsonet al., 1988; Grinshpun et al., 1997; Willeke et al.,1998).

e front matter r 2007 Elsevier Ltd. All rights reserved

mosenv.2007.11.003

ing author.

ess: adart@rti.org (A. Dart).

An ideal impinger has high collection efficiencyfor particles 40.1 mm, allows sample collection forextended periods, and does not decrease micro-organism viability. However, a successful balance ofthese objectives is difficult. Collection fluid evapora-tion, and resulting impacts on microorganismviability and collection efficiency, becomes a pro-blem if an extended sample collection is desired(Henningson et al., 1988). Maximum efficiency forsurface and subsurface impingement is found duringthe first 60min of sample collection. Lin et al. (1997)found that particle re-aerosolization, particlebounce, and particle penetrations exponentiallyincrease after 60min of sampling mainly due to eva-poration of the collection fluid. Willeke et al. (1998)and Lin et al. (1999) experimented with alter-nate collection fluids to allow increased sampling

.

ARTICLE IN PRESSA. Dart, J. Thornburg / Atmospheric Environment 42 (2008) 828–832 829

collection time with limited success at maintainingmicroorganism viability. Other experimental de-signs were incorporated such as centrifugal motion(Willeke et al., 1998) or submersed impingement(Grinshpun et al., 1997) to reduce evaporation whilemaintaining high collection efficiency.

One possible approach to extend sample collec-tion time is to increase the collection fluid volume ofthe impinger. Other bioaerosol sample collectiontechniques that allow extended sample time exist,such as centrifugal samplers with continuous flow,but were not investigated as part of this research.A review of commercially available impingersidentified a model with a maximum volume of500ml, the Greenburg–Smith (G–S) impinger. Thisimpinger was invented in 1922 for industrial totalsuspended particulate matter sampling (Greenburgand Bloomfield, 1932). The research here withinexplored whether the 500ml G–S impinger providesacceptably high collection efficiency for particlesbetween 0.1 and 2 mm, simulating part of the sizerange for virus aerosols, with minimal fluid eva-poration over extended sample collection periods.The actual size distribution of virus bioaerosol isnot known and is dependent upon the compositionof the suspension matrix. These tests investigated areasonable range of particle size range in whichvirus bioaerosols are found and at which currently

Dehumidified

nitrogen source

Regulator

P

TSI-3076 particle atomizer

Charge neutra

Flow measu

rotame

Fig. 1. G–S impinger

available impingers function. The tests were notmeant to be inclusive of the entire potential sizerange of the virus bioaerosol. If acceptable collec-tion efficiencies were observed, subsequent micro-organism viability testing would be conducted.

2. Methods

The two G–S impinger models (Ace Glass Inc.,Vineland, NJ) tested were designed to operate at28 lpm with 300ml of fluid. The two impingersdiffered in the impingement nozzle type and the ballinlet size. The G–S 18/9 (Model 7536-13) has a ballinlet with an outer diameter of 18mm and an innerdiameter of 9mm leading to the impingement nozzlethat ends 10mm above the vial surface. The G–S 28/15 (Model 7536-32) has an inlet inner diameter of15mm that tapers first to 9mm and then to 2mm toform a nozzle located 6mm above an impactionplate. The challenge aerosol was a mixture of0.2–2.0 mm polystyrene latex spheres (Duke Scien-tific, Palo Alto, CA) in de-ionized water nebulizedwith a constant output atomizer (Model 3076, TSIInc., Minneapolis, MN) operating at 241 kPawith HEPA-filtered compressed nitrogen (Fig. 1).The impinger fluid was Difco Nutrient broth(Becton Dickinson and Company, Franklin Lakes,NJ) supplemented with 0.5% NaCl (Mallinckrodt

lizer: Crypton-85 source

rement

ter

DownstreamUpstream

4-port crossover

LAS-X

Particle counter

GS-Impinger

test apparatus.

ARTICLE IN PRESSA. Dart, J. Thornburg / Atmospheric Environment 42 (2008) 828–832830

Baker, Phillipsburg, NJ) and 0.5% antifoam(Sigma-Aldrich, St. Louis, MO). A manometermonitored the pressure drop across the impinger.A rotameter downstream of the impinger regulatedthe flow and kept the system at positive pressure.

Impinger collection efficiency was measured bycomparing real-time particulate concentrationsupstream and downstream from the impinger. ALAS-X (Particle Measurement Systems, Boulder,CO) isokinetically sampling at 250 cm3min�1, alter-nating between upstream measurement and down-stream measurements at 60-s intervals over 20min,provides 10 efficiency measurements per experi-ment. Experimental variables were impinger flow(14.2 and 28.3 lpm) and fluid volume (50, 100 and300ml). These variables were selected in an attemptto maximize collection efficiency. Each experimentalcondition was replicated once.

3. Results

Fig. 2a and b shows the influence of flow and fluidvolume on the average collection efficiency of eachimpinger type. Error bars represent 1 standarddeviation in the measured collection efficiency. Linet al. (1997) documented the collection efficiency of

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28/15, 14 Lpm, 300 ml28/15, 14 Lpm, 100 ml28/15, 14 Lpm, 50 mlAGI-4

Particle Diameter (µm)

Fig. 2. Influence of flow rate and fluid volume on average collection ef

collection efficiency data is shown for comparison. (a) G–S 18/9 efficie

was 28 or 14 lpm and volume was o100ml. (b) G–S 28/15 efficiency cur

because collection fluid was aerosolized under these conditions. Error

the AGI-4, shown for comparison against the G–Simpingers.

The G–S 28/15 impinger operating at 14 lpm and50ml of fluid showed the highest collection effi-ciency across all particle sizes. The plate below thenarrow nozzle created sufficient turbulence to breakthe air stream into small bubbles that kept thelargest particles entrained in the fluid, minimizedfluid separation, and produced a pressure drop ofup to 8.5 kPa. However, a large percentage ofparticles o0.5 mm were not collected by theimpinger. This was thought to be a consequenceof particles being caught by bubbles in the liquidand re-aerosolized when the bubbles burst at thesurface. This effect was magnified at higher fluidvolumes, thus decreasing the collection efficiency.At 28 lpm, the G–S 28/15 impinger generatedspurious aerosol (efficiency curves not shown)regardless of fluid volume. The additional kineticenergy at the higher flow was sufficient to transportparticles out of the impinger and aerosolize theimpinger fluid.

Re-aerosolization of the PSL spheres, and hencelower collection efficiency, was more pronouncedfor the G–S 18/9 impinger. When operated with300ml of impinger fluid, the straight, larger

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18/9, 14 Lpm, 300 ml18/9, 28 Lpm, 300 ml18/9, 28 or 14 Lpm, 100 or 50 mlAGI-4

Particle Diameter (µm)

ficiency of polystyrene latex spheres of varying diameters. AGI-4

ncy curves. Note that the collection efficiency was 0% when flow

ves. Data for G–S 28/15 impinger at 28 lpm is not shown in part b

bars represent 1 standard deviation of the average.

ARTICLE IN PRESSA. Dart, J. Thornburg / Atmospheric Environment 42 (2008) 828–832 831

diameter impingement nozzle released large pocketsof air that rose through the fluid and burst at thesurface. These air pockets created a path for theparticles to follow and allow for easy re-aerosoliza-tion, thereby lowering the collection efficiency asinitially described by Lin et al. (1997). Lin foundthat collection efficiencies drastically decreasedwhen a critical minimum collection volume wasreached, he attributed this to particle bounce off thebottom of the glass impinger. The flow rate partedthe fluid and exposed bare glass at the bottom of theimpinger. This was attributed to the diameter of theimpinger nozzle and since the i.d. of the impinge-ment nozzle of both the AGI-4 and the AGI-30 is�1.5mm (orders of magnitude smaller than the i.d.of each instrument used here), the effects of thisparticle bounce would inevitably be much morepronounced. This phenomenon was more pro-nounced at higher sample flow rates. When theG–S 18/9 impinger fluid volume was 100ml or less,fluid separation caused significant particle bounceoff the bottom of impinger which reduced thecollection efficiency to zero. Henningson et al.(1988) found particle bounce to be the mostdetrimental to bacteria viability when testing theAGI-4. The velocity of the effluent leaving theimpingement nozzle in the AGI-4 is 340m s�1; muchhigher than the 75.5m s�1 exit velocity from the28/15 impingement nozzle.

Multivariate linear regression analysis determinedwhether the G–S model, flow, and fluid volumeinfluenced the collection fluid evaporation rate andpressure drop across the impinger (Table 1). Theairflow rate through the impinger was the only

Table 1

Multivariate analysis of determinants of evaporation rate and

pressure drop

Variable Coefficient 95% CI P-value

Evaporation rate

Intercept �0.017 �0.121 to 0.087 0.748

Flow 0.127 0.056 to 0.197 o0.001

Volume 0.007 �0.004 to 0.019 0.161

G–S model �0.004 �0.039 to 0.030 0.801

Pressure drop

Intercept �29 �46 to �13 0.004

Flow 0.26 0.01 to 0.52 0.041

Volume 0.02 �0.01 to 0.05 0.179

G–S model 18.2 11 to 25 o0.001

Significant P-values (o0.05) are italicized.

parameter correlated with evaporation rate. G–Simpinger model and initial fluid volume were notstatistically significant parameters. The evaporationrate from either impinger was o0.001mlmin�1 perlpm of air flow. Therefore, at flow rates for maxi-mum collection efficiency, both G–S impingerscould sample for 250min before collection efficiencydecreased due to re-aerosolization of the concen-trated PSL spheres from the reduced volume ofimpinger fluid. As expected, pressure drop acrossthe impinger varied between G–S impinger modelsand airflow rate. The pressure drop across the G–S28/15 impinger was 18 times greater than thatimposed by the G–S 18/9 impinger due to thepresence of the impaction plate. The limited rangeof airflow rates investigated, 14.2 and 28.3 lpm, hada minor effect on pressure drop. The fluid volumewas statistically insignificant, despite the six-foldrange in volumes tested. The significant y-interceptprobably resulted from the minor pressure dropimposed by portions of the impingers other than thefluid, such as the inlet nozzle.

4. Conclusions

This research was motivated by the need for abioaerosol impinger with high collection efficiencyover sample collection periods 460min that main-tained virus bioaerosol viability. The collectionefficiency of the G–S 18/9 and 28/15 impingerswas investigated because of their large fluidvolumes. However, the data showed that neithermodel is suitable for high-efficiency collection ofbioaerosols o0.7 mm. The G–S 28/15 impingeroperating at 14 lpm with 50ml of fluid collectioncollects particles 40.7 mm with an efficiency equiva-lent to the AGI-4, and may be a suitable option ifsample collection periods 460min are desired.However, the fraction of virus bioaerosol o0.7 mmwill not be collected by either impinger.

References

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collection efficiency of impingers. Aerosol Science and

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