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A Novel Optical Instrument for Estimating Size Segregated Aerosol MassConcentration in Real TimeXiaoliang Wang a; George Chancellor a; James Evenstad a; James E. Farnsworth a; Anthony Hase a;Gregory M. Olson a; Avula Sreenath a; Jugal K. Agarwal a

a TSI Incorporated, Minnesota, USA

First published on: 01 September 2009

To cite this Article Wang, Xiaoliang, Chancellor, George, Evenstad, James, Farnsworth, James E., Hase, Anthony, Olson,Gregory M., Sreenath, Avula and Agarwal, Jugal K.(2009) 'A Novel Optical Instrument for Estimating Size SegregatedAerosol Mass Concentration in Real Time', Aerosol Science and Technology, 43: 9, 939 — 950, First published on: 01September 2009 (iFirst)To link to this Article: DOI: 10.1080/02786820903045141URL: http://dx.doi.org/10.1080/02786820903045141

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Aerosol Science and Technology, 43:939–950, 2009Copyright © American Association for Aerosol ResearchISSN: 0278-6826 print / 1521-7388 onlineDOI: 10.1080/02786820903045141

A Novel Optical Instrument for Estimating Size SegregatedAerosol Mass Concentration in Real Time

Xiaoliang Wang, George Chancellor, James Evenstad, James E. Farnsworth,Anthony Hase, Gregory M. Olson, Avula Sreenath, and Jugal K. AgarwalTSI Incorporated, St. Paul, Minnesota, USA

A novel optical instrument has been developed that estimatessize segregated aerosol mass concentration (i.e., PM10, PM4, PM2.5,and PM1) over a wide concentration range (0.001–150 mg/m3) inreal time. This instrument combines photometric measurement ofthe particle cloud and optical sizing of single particles in a singleoptical system. The photometric signal is calibrated to approximatethe PM2.5 fraction of the particulate mass, the size range over whichthe photometric signal is most sensitive. The electrical pulse heightsgenerated by light scattering from particles larger than 1 micronare calibrated to approximate the aerodynamic diameter of anaerosol of given physical properties, from which the aerosol massdistribution can be inferred. By combining the photometric andoptical pulse measurements, this instrument can estimate aerosolmass concentrations higher than typical single particle countinginstruments while providing size information and more accuratemass concentration information than traditional photometers. Ex-periments have shown that this instrument can be calibrated tomeasure aerosols with very different properties and yet achievereasonable accuracy.

INTRODUCTIONDetermining human exposure to aerosols is an important

component of health risk assessment in workplace and outdoorenvironments. When inhaled, aerosol particles can deposit indifferent regions of the respiratory track causing adverse healtheffects. The United States Environmental Protection Agency(US-EPA) uses PM10 and PM2.5 as the particulate matter (PM)pollution indices in its National Ambient Air Quality Standards(NAAQS). For workplace environments, the American Confer-ence of Governmental Hygienists (ACGIH) has established sam-pling conventions of respirable, thoracic, and inhalable aerosols(Vincent 1999). A comparison of the different sampling con-ventions is given in Figure 1 (U.S. EPA 1997a; Vincent 1999).

Both ambient air quality monitoring and human exposureassessment need an instrument that simultaneously measures

Received 12 September 2008; accepted 9 May 2009.Address correspondence to Xiaoliang Wang, Desert Research

Institute, 2215 Raggio Parkway, Reno, NV 89512, USA. E-mail:[email protected]

size dependent mass concentrations in real time over a wideconcentration range (PM10, PM2.5, PM1, or inhalable, thoracic,and respirable, etc.) (Chow 1995; Pui 1996; McMurry 2000).However, such an instrument has not yet been available.

The federal reference method (FRM) for determining com-pliance with mass based air quality standards is to use gravimet-ric filter samplers (U.S. EPA 1997b). Such a system typicallyincludes a size selective inlet to remove particles larger than aspecified size, a gravimetric filter that collects all particles be-low the cut-off size of the size selective inlet and a large pumpto draw a large volume of flow through the filter. The mass onthe filter is then used to determine the particulate mass concen-tration. The disadvantage of the gravimetric analysis is that it istime consuming and expensive.

There are several direct-reading instruments that measuremass concentrations in near-real time. They collect particles anduse a sensitive means to determine the particle mass. Examplesinclude the quartz crystal microbalance, the Tapered ElementOscillating Microbalance (TEOM) (Patashnick and Rupprecht1991), and the Beta gauge (Jaklevic et al. 1981). These in-struments have good mass sensitivity, but are not capable ofresolving particle size without a size selective inlet.

Real-time mass measurement can also be achieved with aphotometer if the aerosol is primarily fine aerosol (Armbrusteret al. 1984; Thomas and Gebhart 1994). Photometers measurethe scattered light flux from an ensemble of particles in the view-ing volume. They are sensitive to a wide dynamic range of par-ticle concentration. However, they do not resolve particle size.The photometric signal is dependent on particle properties suchas size, shape and refractive index. Therefore different cali-bration factors are needed for different aerosols. Furthermore,photometers are typically most sensitive to particle diametersclose to the wavelength (λ) of the light source. The photometricsignal per unit mass concentration drops sharply for particlesoutside of this size range (Gebhart 2001). Hence, photometersunderestimate mass concentration contributed by large particles(>5λ).

Optical particle counters (OPCs) are widely used to measureparticle size distributions in real time (Gebhart 2001). By as-suming aerosol density, shape, and refractive index, the “optical

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940 X. WANG ET AL.

FIG. 1. PM2.5, PM10, inhalable, thoracic, and respirable particulate mattersampling conventions promulgated by US-EPA and the ACGIH (U.S. EPA1997a; Vincent 1999).

equivalent size” distribution can be converted to mass distribu-tion (Binnig et al. 2007). The advantages of an OPC are that(1) it can count particle numbers very accurately when the con-centration is low; (2) it has very good signal to noise ratios forlarger particles (e.g., >1 µm); (3) it can be relatively inexpen-sive. The main disadvantages are that (1) the particle optical sizedoes not equal geometric size because it depends on the particleshape and refractive index; the error is further amplified whenconverting particle size distribution to mass concentration if theparticle density is not known; (2) it will underestimate particleconcentration due to coincidence errors caused by multiple par-ticles present in the viewing volume at the same time. ThereforeOPCs are typically only used in relatively clean environments.

Another widely used instrument that measures particle sizeand mass distributions in real time is the Aerodynamic Par-ticle Sizer (APS) (Agarwal 1981; Hairston et al. 1996) TheAPS measurement is less dependent on the particle refractiveindex and density than the OPC. Good agreement between theAPS and direct mass measurements has been reported (Sioutas1999). However, the APS cannot measure high concentrations.Moreover, the APS resolution decreases with the particle size.

In summary, filter sampling and direct mass measurementdevices presently available have a significant disadvantage ofnot being able to provide particle size information unless size-selective inlets are used. An OPC or APS suffers coincidencelosses at high concentrations.

In this article, we describe a novel light scattering instru-ment that estimates size segregated aerosol mass concentrationin real time over a wide concentration range. The novelty of thisinstrument is that it combines photometry and single particlesizing in one optical device. Photometers and OPCs are thoughtto be very different devices and their commonalities have beenignored (Cerni and Sehler 2008). In fact, both instruments in-herently detect photometric signal and single particle scattering

at the same time, although only one of these two signals isanalyzed. An OPC operates at low concentrations so that onlyone particle at a time is present in the viewing volume. Thephotometric signal from gas molecule scattering forms part ofthe background noise and limits the minimum particle size thatcan be measured by an OPC. On the other hand, a photome-ter can operate at high concentrations so that multiple particlescan be present in the viewing volume at the same time. Pulsesfrom single particle scattering are smeared in the averaging ofthe photometric signal. The instrument described in this papermeasures both the photometric signal and pulses generated byparticle larger than 1 µm. Such a combination allows this in-strument to measure higher particle concentrations than an OPCand provide size information that is not available in a photome-ter. In the following sections, we will first describe the theory ofoperation, and then present experimental data to compare thisinstrument with other devices for different aerosols. Its advan-tages and limitations will also be discussed.

THEORY OF OPERATION

Instrument DescriptionThe instrument being described has been commercialized as

the DustTrakTM DRX Aerosol Monitor by TSI Inc. (Model 8533and 8534). Hereafter it is referred to as DRX. A schematic dia-gram of the DRX is shown in Figure 2. Aerosol being measuredis drawn into the optical chamber in a continuous stream usinga pump at a total flow rate of 3 liter/min. One liter/min of theaerosol stream is passed through a HEPA filter to remove parti-cles. The clean air is then drawn back into the chamber aroundthe inlet nozzle as sheath flow to reduce particle recirculationand protect the optics from particle contamination. The remain-ing 2 liter/min of particle-laden flow continues through the inletand enters the optical chamber, where it is illuminated by a sheetof collimated laser beam with a wavelength of 655 nm. Scatteredlight in the scattering angle range of 90 ± 62◦ is captured by agold-coated spherical mirror and focused onto a photo detector.Particles exiting the optical chamber can be collected on an in-ternal filter for gravimetric or chemical analysis. The total flowrate of 3 liter/min is actively controlled by a feedback loop be-tween the pump and the flowmeter. The aerosol-to-sheath flowratio is maintained by selecting an orifice to balance the pressuredrops in the two flow paths. The HEPA filter for sheath flow isable to collect a significant amount of dust (∼50 mg) before achange is seen in the flow ratio.

The signal from the photo detector is digitized and processedby the photometric direct current (DC) voltage offset analyzerand the single particle pulse height analyzer. The photometricsignal is used to estimate PM2.5 mass concentration, and singleparticle counting is used to estimate mass distributions for par-ticles larger than 1 µm. These two results are then combinedto calculate the size segregated mass concentration. Figure 3schematically depicts signals from the DRX. The backgroundvoltage of the photo detector is due to light scattering from air


SIZE SEGREGATED AEROSOL MASS CONCENTRATION MEASUREMENT 941

FIG. 2. Schematic diagram of the DRX. The particle beam, laser beam, and the axis of the mirror are orthogonal to each other.

molecules, stray light inside the optical chamber and electricalnoise. At low number concentrations, the photometric voltagesignal is nearly zero. Particles larger than the detection limit willbe sized and counted. At high number concentrations, reliablephotometric signals can be measured. Smaller particles can nolonger be counted accurately due to high coincidence errors,while larger particles can still be sized. For this reason, the DRXcounts only particles larger than 1 µm (estimated aerodynamicdiameter) to calculate single particle masses. The conversionfrom the optical equivalent diameter to aerodynamic diameteris discussed in a later section. Consistent with the generally ac-cepted practice of calibrating photometers with Arizona RoadDust (ARD), the DRX is calibrated to represent ultrafine ARD(ISO 12103-1, A1 dust) equivalent concentration as default. Acustom calibration is needed if the aerosol of interest is differentfrom A1 dust.

PM2.5 Mass Concentration by PhotometryThe specific photometric response, which is defined as the

flux of scattered light per unit mass concentration of aerosol, is afunction of particle properties, including refractive index, shape,density and size distribution (Gebhart 2001). Figure 4 showsthe theoretically calculated specific photometric response of theDRX to idealized ARD (assuming spheres with refractive indexm = 1.54, density ρp = 2.65 g/cm3) of various lognormal sizedistributions defined by the mass median diameter and geomet-ric standard deviation (GSD). As expected, the specific photo-metric signal is strongest when the particle diameter is close tothe laser wavelength (655 nm). Therefore, a photometer is mostsensitive to fine particles. It will underestimate masses of verysmall and very large particles. For this reason, the DRX uses thephotometric signal to determine the PM2.5 concentration, anduses single particle measurement to obtain improved accuracy

FIG. 3. Schematic drawing of signals detected by the DRX.


942 X. WANG ET AL.

FIG. 4. Specific photometric response as a function of Arizona Road Dustsize distribution.

for larger sizes, such as PM2.5–10. This technique allows PM2.5

and PM10 to be calibrated separately to obtain best accuracy.Since the DRX typically operates without a PM2.5 impactor onits inlet, light scattering from particles larger than 2.5 µm willalso contribute to the photometric signal. This contribution isaccounted for by photometric signal calibration.

Photometric signals of a given aerosol are linearly propor-tional to the mass concentration when multiple scattering can beneglected. Figure 5 shows such linear relationships for the PM2.5

fraction of Emery oil (EO, refractive index m = 1.4645, densityρ p = 0.87 g/cm3) and A1 particles. Results from measurementand Mie scattering calculation show very good agreement. Thislinear relationship allows a simple gravimetric calibration to

FIG. 5. Linear correlations between the PM2.5 Emery oil (EO) and A1 dustconcentration and the DRX photometric response.

yield a photometric calibration factor (PCF) for a given aerosol:

PCF = Gravimetric PM2.5 mass concentration

DRX PM2.5 mass concentration for A1 dust. [1]

The DRX PM2.5 mass concentration for A1 dust is calculatedby (Photometric voltage × RARD), where RARD is a calibrationfactor that converts the photometric voltage to the PM2.5 massconcentration of A1 dust. The PM2.5 concentration of the aerosolunder test is then

PM2.5 mass concentration

= DRX PM2.5 mass concentration for A1 dust × PCF. [2]

The default PCF is 1. When aerosol properties (refractive in-dex, shape, density and size distribution) are different from thecalibration aerosol, a new PCF needs to be determined. To finda new PCF using Equation (1), one must sample the aerosolwith an external gravimetric filter with a PM2.5 impactor andthe DRX simultaneously, collecting enough mass on the gravi-metric filter. If the aerosol is stable, one can first measure thePM2.5 gravimetric mass concentration using the internal filter ofthe DRX with a PM2.5 impactor installed on the DRX inlet, thenremove the impactor and run the DRX for a couple of minutesto obtain an average PM2.5 reading.

When the aerosol being measured has a size distribution andrefractive index similar to that of the calibration aerosol, den-sity becomes the dominant factor (O’Shaughnessy and Slagley2002). The PCF is approximately proportional to the particledensity. Table 1 shows the measured PCF for various non-light-absorbing aerosol materials. Note that the PCF is in reasonableagreement with the ratio of the density of the aerosol under mea-surement to that of ARD. Therefore, it is possible to estimatethe PCF for a non-light-absorbing aerosol if the particle density(ρp) is known:

PCF ∼= ρp/ρARD. [3]

TABLE 1Properties and measured photometric calibration factor (PCF)

of several aerosol materials

Arizonaroad dust

Ammoniumsulfate

Sodiumchloride

Emeryoil

Refractive 1.54 1.534 1.544 1.4645index

ρp (g/cm3) 2.65 1.77 2.17 0.87ρp /ρARD 1.00 0.67 0.82 0.33PCF 0.99 ± 0.17 0.61 ± 0.04 0.74 ± 0.10 0.32

Tests were repeated three times for those data presented with uncer-tainties (one standard deviation).



FIG. 6. DRX pulse height as a function of PSL diameter. Data were obtainedby both Mie scattering calculation and pulse height distribution measurement.

Mass Distribution by Single Particle MeasurementOPCs have been widely used for particle size distribution

measurement. An OPC measures the optical equivalent diame-ter (dopt ), which is defined as the diameter of the test aerosol thatscatters the same amount of light as the calibration aerosol. TheDRX optical equivalent diameter is calibrated with polystyrenelatex (PSL) spheres. The theoretical and measured relationshipsbetween pulse height and PSL size are shown in Figure 6. Labo-ratory tests show that the DRX has ∼50% counting efficiency for0.5 µm PSL and it can accurately size PSL up to approximately15 µm.

To estimate mass distribution from optical particle sizing andcounting, the following assumptions are made.

First, it is assumed that concentrations of particles larger than1 µm are low enough for typical measurements so that coinci-dence errors are negligible. A dead-time coincidence correctionalgorithm (Hering et al. 2005) is implemented in the DRX, whichreduces coincidence errors to approximately 10% at about 7000particle/cm3. Errors will increase if the concentration of parti-cles larger than 1 µm is greater than this concentration.

Second, it is assumed that the presence of smaller particlesthat cause an increased photometric voltage will not affect thesizing accuracy of larger particles (≥1 µm). Earlier studies haveshown that the presence of high concentrations of smaller par-ticles will skew the OPC size distribution due to the statisticalfluctuations in the number of particles in the viewing volumeduring successive time intervals (Whitby and Liu 1967; Whitbyand Willeke 1979; Lekhtmakher and Shapiro 2004). Accord-ing to Whitby and Willeke (1979), if the average number ofsmaller particles (those with diameters 1/4 decade smaller thanthe lowest OPC diameter) in the viewing volume is less than1, the spurious pulses generated by these particles will not sig-nificantly affect the measured size distribution. The DRX has aviewing volume of 0.021 mm3. Therefore, as long as the con-centration of particles in the size range of ∼0.56–1 µm is less

than ∼50,000 particles/cm3, the DRX measurement of 1–2.5 µmparticles (which is used to calculate PM1) will not be affected.PM1 concentration will be less accurate if the concentration of0.56–1 µm is greater than 50,000 particles/cm3.

Third, it is assumed that particles are spherical. The parti-cle mass concentration (cm,i) of each OPC channel i can becalculated by

cm,i = cn,i × π

6ρp d3

ve,i , [4]

where cn,i is the concentration of channel i, ρp is particle density,and dve is the volume equivalent diameter of channel i. The DRXhas 2048 pulse height channels.

Since the sampling conventions (see Figure 1) are defined interm of aerodynamic diameter, we need to convert the dve to theaerodynamic diameter (da) as follows (Hinds 1998):

dve = √ρoχ/ρp da, [5]

where ρ0 is unit density (1 g/cm3) and χ is the dynamic shapefactor. Since we are only interested in relatively large particles,the slip correction term has been neglected. The dynamic shapefactor is assumed to be 1. Then Equation (4) can be recast interms of aerodynamic diameter as:

cm,i = cn,i × π

6ρ0 d3

a,i

√ρ0/ρp. [6]

By default, the DRX reports the aerodynamic diameter of ARD,and ρp = ρARD (2.65 g/cm3). If a photometric calibration wasperformed, Equation (3) is used to estimate ρp.

The ARD aerodynamic diameter da,ARD is converted fromthe PSL equivalent diameter dopt,i using a size calibration factorfor ARD (SCFARD) by

da,ARD,i =√

ρARD/ρ0 SCFARD dopt,i . [7]

It is puzzling to find that the value of SCFARD, which is deter-mined by calibration described later, is approximately 0.6. Thissuggests that da,ARD,i

∼= dopt,i from Equation (7). However,ARD has a refractive index (1.54) only slightly lower than thatof PSL (1.59). If the light scattering intensity for ARD were cal-culated as spheres using Mie theory, SCFARD would be approxi-mately 1.01, and da,ARD would be about 1.65dopt . An SCFARD of∼0.6 suggests that ARD scatters a lot more light than that pre-dicted by Mie theory assuming spherical shape. This is probablybecause ARD is very irregular in shape (Hindman et al. 1982).Many studies have shown that the light scattering intensity ofa non-spherical particle, when measured at off-axis scatteringangles, is much higher than that predicted by Mie theory (Geb-hart 1991; Friehmelt and Heidenreich 1999). It is still not clearto us, however, why the measured photometric signal is in goodagreement with Mie theory prediction as shown in Figure 5.


944 X. WANG ET AL.

When the aerosol being measured is different from ARD,a custom size calibration factor (SCF) is needed to convertthe aerodynamic diameter of ARD to that of the aerosol beingmeasured for the same optical equivalent diameter:

SCFi = da,i

da,ARD,i

= da,i√ρARD/ρ0 SCFARD dopt,i

. [8]

Ideally, the SCF should be a size dependent function. For sim-plicity, we assume SCFi is relatively constant over the size rangeof 1–15 µm. Therefore only one SCF value is applied. Equation(6) can then be rewritten as

cm,i = cn,i × π

6ρ0 (da,ARD × SCF)3

√ρ0/ρp. [9]

The particle mass concentration from Equation (9) is inte-grated and grouped into one of the four size bins: PM1–2.5,PM2.5–4, PM4–10, and PM>10.

There are two approaches to determine SCF. The first methodinvolves measuring the penetration efficiency of a PM2.5 im-pactor with the DRX. The SCF can be calculated from the ratioof 2.5 µm (the impactor cut-off size) to the DRX measuredsize corresponding to 50% penetration efficiency. This methodis similar to those of calibrating an OPC to measure aerody-namic particle sizes (Marple and Rubow 1976; Friehmelt andHeidenreich 1999; Binnig et al. 2007). The second method in-volves two gravimetric calibrations, one for PM2.5 and the otherfor PM10. The PCF and SCF are then calculated using Equations(1) and (9), which force an agreement between the PM2.5 andPM10 readings of the DRX and the two gravimetric measure-ments.

Size Segregated Mass Concentration by CombiningPhotometry and Single Particle Measurement

Once PM2.5 mass concentration is obtained from photometry,and PM1–2.5, PM2.5–4, PM4–10, and PM>10 mass fractions areobtained from single particle sizing, these two results may becombined to obtain the size segregated mass concentration asfollows:

PM1 = PM2.5 − PM1−2.5

PM2.5 = Photometric Voltage × RARD × PCF

PM4(Respirable) = PM2.5 + PM2.5−4 [10]

PM10(Thoracic) = PM4 + PM4−10

TPM = PM10 + PM>10.

Ideally, each size channel needs to be multiplied by the res-pirable, PM10 or thoracic penetration efficiencies in Figure 1to obtain respective mass fractions. For simplicity, this step isnot incorporated. Therefore, the DRX will overestimate particlessmaller than the cut-off sizes but underestimate those larger thanthe cut-off sizes. These two fractions offset each other to some

FIG. 7. Experimental setup for the DRX mass accuracy test.

extent and the total error is deemed insignificant compared toother uncertainties.

PERFORMANCE TESTSTo test the performance of the DRX, we carried out a series

of experiments to compare DRX with a TEOM (Series 1400a,Thermo Scientific), a photometer (TSI Model 8520 DustTrak),and an OPC (TSI Model 8220) for several aerosols.

DRX vs. DustTrak 8520 and TEOM for DifferentLaboratory Generated Aerosols

In this experiment, we compared the DRX size segregatedmass concentrations with TEOM for the default calibrationaerosol (A1 dust) and three other aerosols (coarse ARD A4 dust,hematite and petroleum coke dust, all purchased from PowderTechnology Inc., Burnsville, MN). The objectives are (1) toverify that the DRX can accurately measure A1 dust at variousconcentrations, (2) to characterize the DRX error for differentaerosols, and (3) to find out whether the DRX can be calibrated toaccurately measure mass concentrations of aerosols very differ-ent from A1 dust. The DRX was also compared to the DustTrak8520 for A1 and A4 dusts to show their different sensitivities tothe aerosol size distribution.

The experimental setup is shown in Figure 7. The test dustwas aerosolized with a TSI 3400A Fluidized Bed Aerosol Gen-erator. Aerosol concentration was varied during the test by ad-justing dust feeding speed and the air flow through the bed. ADRX, a TEOM, a TSI 8520 DustTrak with a PM10 impactor,and a TSI 3321 APS sampled the aerosol concurrently from amixing chamber. A fan inside the mixing chamber ensured uni-form concentration inside the chamber. Before this experiment,the TEOM was compared with gravimetric filter measurementsand found to agree within ±15%. The TEOM was used as themass reference in this experiment. The TSI 8520 DustTrak isa photometer. It was used in this experiment to compare the



FIG. 8. APS measured mass distributions of several aerosols generated bythe fluidized bed.

differences between a photometer and the combined photome-ter and OPC measurements implemented in the DRX. The APSwas used to monitor aerosol size distributions. The aerosol to theAPS was diluted by a factor of 5 to avoid coincidence errors. Thedata acquisition rate was once a second for the DRX and Dust-Trak 8520, and about once every 4 s for the TEOM. The TEOMinternal averaging time was set to 5 s to obtain fast response.

The experimental procedure is as follows: (1) A bed flowand a dust feeding speed of the fluidized bed were chosen togenerate A1 dust PM2.5 mass concentration around 2 mg/m3.The APS was used to monitor the aerosol size distribution untilit stabilized. The concentration of ∼2 mg/m3 was chosen be-cause it was high enough to yield a stable TEOM reading whilelow enough to avoid overloading the TEOM in one run. (2) TheDRX was calibrated with the stable A1 dust. This step set thebaseline calibration factors for later comparisons. (3) The DRXwas compared to the TEOM at various A1 dust concentrationsgenerated by varying the fluidized bed settings. (4) The test dustin the fluidized bed was changed and a PM2.5 concentration∼2 mg/m3 was generated and allowed to stabilize. (5) The DRX

calibrated with A1 dust was compared to the TEOM to char-acterize errors due to the difference between A1 dust and thetest aerosol. (6) The DRX was calibrated with the aerosol undertest. (7) The fluidized bed settings were varied, and the DRXand TEOM were compared at various concentrations of the testaerosol. (8) Steps 4–7 were repeated for different test dusts.

Figure 8 shows the mass concentration distribution of thefour aerosols under typical stable fluidized bed operating con-ditions. The geometric mass mean aerodynamic diameter (dg)and standard deviation (σg) of these size distributions are listedin Table 2.

Figure 9 shows the A1 dust mass concentration measuredby the TEOM and the DRX during step (3) of the experimentalprocedure described above. A PM10 impactor was installed onthe inlet of the TEOM. The DRX did not have an impactor onits inlet. Note that the PM10 concentration of the DRX is ingood agreement with the TEOM over a wide range. The DRXsimultaneously measured PM1, PM2.5, PM4, PM10, and TPM.The TEOM response time is about 6 s slower than the DRX,hence the TEOM showed a delayed response. Figure 10 showsthe PM2.5 and PM4 mass concentrations measured by the TEOMand the DRX. A PM2.5 or PM4 impactor was installed on the inletof the TEOM, and the DRX remained without an impactor onits inlet. The agreement between these two instruments is alsogood for these two mass fractions. Figure 11 plots the PM2.5,PM4 and PM10 mass concentration linear correlation betweenthe TEOM and DRX. The raw data were numerically averagedevery 5 s, and the difference in response time was adjusted.Because the TEOM and DRX were zeroed with filtered air, theregression lines were forced through the origin. As can be seen,the DRX agrees with the TEOM within approximately ±10%for all three mass fractions.

Table 2 lists the DRX/TEOM mass concentration ratios forvarious aerosols measured before and after the DRX was cali-brated with the aerosol under test. Also listed are the calibrationfactors obtained in step (6) of the experimental procedure. Notethat as expected, the DRX would have significant errors whenthe test aerosol had different size distribution, refractive index,or density from A1 dust. However, after calibrated with the testaerosol, the DRX agreed with TEOM within ±10% for both

TABLE 2Geometric mass mean aerodynamic diameter (dg) and standard deviation (σg) of aerosol size distributions, DRX/TEOM ratiowhen the DRX was calibrated with A1 dust, DRX calibration factors for different aerosols, and the DRX/TEOM ratio after the

DRX was calibrated with the corresponding aerosol

Size distribution DRX/TEOM, A1 dust cal Cal factors DRX/TEOM, after cal

Aerosol dg (µm) σg PM2.5 PM10 SCF PCF PM2.5 PM10

ARD (A1 dust) 2.12 1.57 1.00 1.00 1.00 1.00 — —ARD (A4 dust) 2.93 1.68 1.31 0.93 1.08 0.76 1.01 1.06Hematite 4.10 1.49 1.83 0.46 1.57 0.55 0.97 1.02Petroleum coke 3.54 1.74 1.46 0.63 1.35 0.69 1.00 1.01


946 X. WANG ET AL.

FIG. 9. A1 dust concentration measured by the TEOM and the DRX. TheTEOM had a PM10 impactor on its inlet, while the DRX did not have animpactor on its inlet.

PM2.5 and PM10. Figure 12 shows an example comparison be-tween the DRX and TEOM for various petroleum coke concen-trations of PM2.5 and PM10 mass fractions. The DRX measuredfive mass fractions simultaneously, but only the one correspond-ing to the TEOM impactor cut-off size was plotted. Note thatalthough petroleum coke has very different properties than theA1 dust, the DRX and TEOM have very good agreement. Thisexperiment clearly demonstrates that after calibration with thetest aerosol the DRX can measure size segregated mass con-centrations quite accurately. The calibration factors reflect theproperty differences between A1 dust and the test aerosols. Adetailed explanation of the mechanisms that lead to these cali-bration factor values is out of the scope of this article.

When the DRX and DustTrak 8520, both calibrated with A1dust, were used to measure A4 dust, the PM10 channel of theDRX was 93% of the reference TEOM concentration, while theDustTrak 8520 was only 69% of the reference concentration.The reason for this underestimation by the DustTrak 8520 canbe seen in Figure 8, where A4 dust has more coarse particlesthan A1 dust. Since the DustTrak 8520 is a photometer, it wouldunderestimate mass concentration of coarse particles due totheir lower specific photometric responses (Figure 4). On theother hand, the DRX uses optical counting to measure coarseparticles greater than 1 µm and uses photometric signal for thePM2.5 fraction. Its PM10 accuracy is less affected by the sizedistribution shift. Therefore, comparing to a simple photometer,the DRX not only provides size segregated information, but alsosignificantly reduces PM10 measurement error due to particlesize distribution shifts.

DRX vs. TEOM for Ambient AerosolsIn this experiment, we used a DRX and a TEOM to measure

aerosol concentrations from a receiving dock at TSI Inc. To re-duce uncertainties due to relative humidity, aerosols were driedby a diffusion dryer before measurement. Both instruments first

FIG. 10. PM2.5 and PM4 mass concentrations of A1 dust measured by TEOMand DRX. The TEOM had a PM2.5 or PM4 impactor on its inlet, while the DRXdid not have an impactor. The other mass fractions measured by the DRX werenot plotted for the sake of clarity.

sampled filtered and dried air for 24 h to measure zero drift. Itwas found that the 30 min averaged zero drift was 2.0 ± 0.9µg/m3 for the TEOM and 0.5 ± 0.1 µg/m3 for the DRX over24 h. The TEOM drift had a temporal pattern while the DRX driftwas random. The mean zero drift values were subtracted fromraw concentrations in later data processing. Next we calibratedthe DRX for aerosols in the receiving dock against the TEOMand obtained the DRX calibration factors of 0.8 and 0.418 forSCF and PCF, respectively. Then we applied a PM10 impactorto the inlet of the TEOM and ran it in parallel with the DRX for2 days. After that we replaced the PM10 impactor with a PM2.5

impactor on the inlet of the TEOM, and ran the two instrumentsfor another 4 days.

Figure 13a–b presents the 30 min averaged PM10 (a) andPM2.5 (b) levels measured by the DRX and TEOM over the sam-pling period. Figure 14a–b shows the respective linear regressionbetween the DRX and the TEOM. It can be seen that the TEOM



FIG. 11. Linear correlation between the DRX and the TEOM for A1 dustPM2.5, PM4, and PM10 mass fractions.

and DRX tracked each other closely. Figure 13 clearly revealsseveral temporal PM concentration patterns on the receivingdock. On weekdays, aerosol concentration started to increasearound 8:00 when the first delivery truck arrived. The concen-

FIG. 12. PM2.5 and PM10 mass concentrations of petroleum coke dustmeasured by TEOM and DRX. The TEOM had a PM2.5 or PM10 impactor onits inlet, while the DRX did not have an impactor. The other mass fractionsmeasured by the DRX were not plotted for the sake of clarity.

tration remained high during the day due to activities such asmoving and opening shipping boxes. The concentration reacheda peak around 19:00 when a janitor swept and vacuumed thefloor. After that, the concentration kept decreasing until nextmorning when work started again. The concentration remainedat low levels during the weekend. The difference between thePM2.5 and PM10 concentrations measured by the DRX alsoshowed a temporal pattern. This difference was large duringa work shift, indicating that shipping goods handling or floorcleaning generated a large amount of coarse particles. On theother hand, the difference was much smaller during nights orweekends, indicating ambient background aerosols were typi-cally fine particles. Note that the correlation between the DRXand the TEOM was reasonably good, although it was not asgood as those for laboratory generated aerosols as shown inFigure 11. This was presumably due to two reasons: (1) the


948 X. WANG ET AL.

FIG. 13. Aerosol concentration measured by TEOM and DRX from a TSIreceiving dock: (a) a PM10 impactor was installed on the TEOM inlet; (b) aPM2.5 impactor was installed on the TEOM inlet. The DRX did not have anyimpactor on its inlet. The DRX simultaneously measured 5 mass fractions.Only PM2.5 and PM10 were plotted.

heterogeneous properties of aerosols from the receiving dock,which caused measurement uncertainties with the DRX; (2) thetemporal variations of the TEOM zero drift, which caused rel-atively large uncertainties with the TEOM, especially at lowconcentrations.

DRX vs. OPCIn this experiment, we compared the DRX with an OPC

for measuring monodisperse particles at various concentrations.The experimental setup is shown in Figure 15. Emery oil par-ticles were generated by atomizing 0.6% Emery oil solution inisopropanol. A differential mobility analyzer (TSI Model 3081)selected 0.5 µm monodisperse aerosols (Knutson and Whitby1975), which was mixed with makeup flow and sampled by a

FIG. 14. Scattered plot of the DRX vs. TEOM 30 min average concentrations:(a) PM10, (b) PM2.5.

FIG. 15. Experimental setup to compare DRX with an OPC.



FIG. 16. A comparison of DRX and OPC measuring mass concentration of0.5 µm monodisperse Emery oil particle at various concentrations.

DRX, a TSI 8220 OPC, and a TSI 3775 Condensation ParticleCounter (CPC) simultaneously. The OPC has 100% countingefficiency for 0.5 µm Emery oil particles. However, it has verycoarse size channels (0.3–0.5, 0.5–1, 1–3, 3–10, and >10 µm),which would introduce significant errors when converting sizedistributions to masses. Therefore, it was only used as a counterin this experiment. The mass concentrations by the OPC andthe reference CPC were calculated by multiplying number con-centration by the mass of a 0.5 µm Emery oil particle. TheDRX photometric signal was calibrated to 0.5 µm Emery oil,and the optical sizing was adjusted for Emery oil. The aerosolconcentration was varied by adjusting the dilution bridge.

Figure 16 shows a comparison of the DRX and OPC massconcentration against the reference CPC concentration. Notethat the DRX agreed with the reference concentration within15% in the mass concentration range of ∼1–1000 µg/m3 (cor-responding to a number concentration range of ∼18–18,000particle/cm3). The deviation became greater at lower concen-trations due to low signal-to-noise level. On the other hand, theOPC agreed with the reference concentration within 15% fromvery low concentration up to ∼10 µg/m3 (180 particle/cm3). Itstarted to underestimate concentration at higher concentrationsdue to coincidence errors. In many measurement applications,the number concentration of particles smaller than 1 µm is muchhigher than those larger than 1 µm. This experiment demon-strates that the DRX can measure aerosol concentrations moreaccurately than an OPC in a dusty environment.

DISCUSSIONS AND CONCLUSIONSWe have developed a unique instrument, the DustTrakTM

DRX, that estimates size segregated particulate mass concen-tration in real time. This optical instrument uses photometryto measure PM2.5 mass concentration and uses single particlesizing and counting to produce size segregated mass fractions.

The DRX is more accurate than an OPC when measuringhigh concentrations. It measures the mass contribution from theindistinguishable smaller particles with the photometric signal,and only sizes and counts individual particles larger than 1 µm.It further employs a dead-time correction to reduce coincidenceerrors.

The DRX is also superior to simple photometers. It offersadditional information about particle sizes. Furthermore, it isalso more accurate in estimating mass contribution from largerparticles (≥1 µm), which is significantly underestimated bysimple photometers.

The DRX is calibrated using A1 dust. Measurements withA1 dust demonstrate that the DRX agrees with a TEOM massconcentration within approximately ±10%. It offers a fasterresponse time and data acquisition rate. While the TEOM onlymeasures one size fraction determined by the impactor, the DRXprovides simultaneous size segregated aerosol mass concentra-tion information. When the aerosol of interest is quite differentfrom A1 dust, a custom calibration is needed to obtain a pho-tometric calibration factor (PCF) and a size calibration factor(SCF). Experiments performed with different aerosols show thatthe DRX can measure mass concentration with reasonable ac-curacy after it is calibrated with the aerosol under test.

Since the DRX measures aerosol mass concentrations us-ing the light scattering principle, its accuracy is affected bythe shape, size distribution, refractive index and density ofthe aerosol being sampled. Therefore, although the instrumentis suitable for measuring the relative concentration change ofaerosols having constant properties such as those typically en-countered in the workplace, it might have considerable uncer-tainty when it is used for atmospheric aerosol monitoring due tothe temporal and geographical variations of aerosol properties.Previous studies have shown, however, that when operated withconsistent methodology, light scattering measurement deviceswill yield results highly correlated to atmospheric aerosol massconcentrations, especially over short time periods (Waggoneret al. 1981; Chow et al. 2002). A preliminary measurementof ambient aerosols from a receiving dock showed reasonableagreement between the DRX and the TEOM. The DRX is there-fore a useful real-time indicator of short-term variations of am-bient aerosol.

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