Measurement of Particle Density by Inertial Classification of Differential Mobility Analyzer Generated Monodisperse Aerosols

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1 Aerosol Science and Technology SSN: (Print) (Online) Journal homepage: Measurement of Particle Density by nertial Classification of Differential Mobility Analyzer Generated Monodisperse Aerosols W. P. Kelly & P. H. McMurry To cite this article: W. P. Kelly & P. H. McMurry (1992) Measurement of Particle Density by nertial Classification of Differential Mobility Analyzer Generated Monodisperse Aerosols, Aerosol Science and Technology, 17:3, , DO: / To link to this article: Published online: 11 Jun Submit your article to this journal Article views: 1023 View related articles Citing articles: 98 View citing articles Full Terms & Conditions of access and use can be found at

2 Measurement of Particle Density by nertial Classification of Differential Mobility Analyzer-Generated Monodisperse Aerosols W. P. Kelly and P. H. McMurry* Particle Technology Laboratory, Department of Mechanical Engineering, University of Minnesota, 111 Church St., S.E., Minneapolis, MN A density measurement technique based on the selection of a monodisperse aerosol with a differential mobility analyzer followed by classification according to aerodynamic diameter with an impactor has been designed and tested. Experimental results were obtained for several laboratory aerosols (dioctyl phthalate, (NH,),SO,, NaC, and H,S04 at a range of hnmidities) by using four different microorifice uniform deposit impactor stages with aerodynamic diameter cut- offs of pm. The average error in measured particle densities is 4% and a maximum error of 8% is observed for all of the materials tested except NaC, for which the measured effective density is 14% smaller than the true density. The discrepancy for NaCl is attributed to nonspherical particle shape. The system will be applied in the future to measure the densities of submicrometer atmospheric particles. NTRODUCTON Density plays an important role in determining aerosol transport properties. For example, settling velocities and inertial characteristics are dependent on density. Thus, density affects phenomena including dry deposition, cloud scavenging, and lung deposition. Densities must also be known to evaluate size-dependent collection efficiencies of inertial samplers such as impactors and cyclones. nertial samplers classify particles according to aerodynamic diameter, which is a function of particle density, shape, and size. Such devices are frequently used to obtain information on the size-resolved mass and chemical composition of aerosol particles. n order to convert such data from aerodynamic to geo- This is Particle Technology Laboratory Report No *To whom correspondence should be addressed. metric size distributions, it is necessary to account for the effect of density. One example of a problem that requires converting impactor data to geometric size distributions is the evaluation of aerosol optical properties (e.g., phase functions or scattering and absorption coefficients) with Mie theory (e.g., Ouimette, 1981; Sloane, 1983, 1984, 1986; Zhang, 1990). The optical properties of a particle depend on its size, shape, and refractive index, but not on its density. Evaluating aerosol optical properties with Mie theory involves integration over the aerosol size distribution; particle shape is typically assumed to be spherical. f inertial samplers are used to obtain size distributions, data must be converted to actual size before the integrals can be evaluated. n previous work of this type, densities were estimated from measured chemical composition in order to convert from aerodynamic to geometric sizes. Estimating densities from measure- Aerosol Sciencc and Technology 17: (1992) Elsevier Science Publishing Co., nc.

3 200 W. P. Kelly and P. H. McMurry ments of chemical composition can lead to errors because particles often consist of complex mixtures of species and phases. f such estimates are based on measurements of the average chemical composition in a given size range, then a priori assumptions must be made about mixing characteristics in order to infer densities. The purpose of the work described in this paper has been to develop a method for direct measurements of particle density. Such measurements will reduce the number of assumptions required to convert aerodynamic distributions to geometric size distributions. Previous Density and Shape Factor Measurement Techniques Owing to difficulties in independently measuring the effects of density, shape, and voids on the inertial properties of irregular particles, these parameters are often combined to give an effective particle density (also referred to in the literature as apparent density or aerodynamic density). Early measurements of apparent particle density were accomplished by using Millikan cells and are reviewed by Fuchs (1964). More recently, Allen and Raabe (1985) and Cheng et al. (1988) have used Millikan cells to measure the slip correction factor and dynamic shape factor of aggregated latex spheres. Though highly accurate, Millikan cell measurements yield information on only singleparticle characteristics. Therefore, it is difficult to measure densities of large numbers of particles so as to obtain representative averages. t is important to include large numbers of particles for systems such as atmospheric aerosols, where there are significant particle-to-particle variabilities. Millikan cells are also difficult to use in the submicrometer range owing to problems with electrostatically trapping extremely small particles. Hanel and Thudium (1977) present results for the mean bulk density of atmos- pheric particles. With this approach the mass of an aerosol deposit is obtained by a sensitive electronic balance, and its volume is measured with a specially designed pycnometer. The average density of the particles is the ratio of the measured mass and volume. Although this technique was refined to a point where bulk particle densities could be measured within 2%, it failed to resolve variations in density encountered with different particle sizes, and particle-to-particle variations could not be discerned. Also, this method measures the true particle material density, rather than an effective particle density which includes the effects of particle shape. These extremes of measuring densities of bulk samples or of individual particles can now be balanced by the use of steady-flow instrumentation. Recently, Wang and John (1987, 1989) quantified the effect of particle density on the response of an aerodynamic particle sizer (APS). With the effect of density on APS response known, the APS could be used to measure the densities of particles having a known size. The APS technique was extended with the addition of a cascade impactor to measure dynamic shape factors of nonspherical and porous particles by Brockmann and Rader (1990). Assumptions about particle density and either porosity factor or shape factor are necessary, leading to uncertainties near 25% for dynamic shape factor. One major limitation to the APS technique of particle density or dynamic shape factor measurement is the lack of resolution in the submicrometer range. A liquid displacement method and a density gradient centrifugation method have been used by Raabe et al. (1984) for measuring bulk particle density and density distributions within a sample, respectively. Coll and Oppenheimer (1987) have used a disk centrifuge for measuring the density of spherical and near-spherical particles. Although such techniques can be highly accurate ( %), they are

4 Particle Density Measurement by nertial Classification of Aerosols 201 not appropriate for submicrometer atmospheric particles which are likely to change properties when immersed in a liquid. DMA/ mpactor Density Measurement Technique The key to measuring the density of particles with a differential mobility analyzer (DMA) and an impactor lies in accurately measuring the size-dependent impactor collection efficiency. The system that was used in this research is similar to those reported by Hillamo and Kauppinen (1991) and Marple et al. (1991). These techniques use a DMA to select a monodisperse aerosol which is then Sampled with an impactor. The particle concentrations upstream and downstream of the impactor are measured in order to find the impactor collection efficiency. Particle size is varied by adjusting the DMA voltage. Hillamo and Kauppinen (1991) and Marple et al. (1991) used electrometers to measure concentrations, whereas condensation nucleus counters (CNCs) were used in the present work. The advantage of using CNCs is the high resolution of collection efficiency at low particle concentrations, such as those found in ambient sampling. This report explains the theory used in determining particle density for spherical particles or effective particle density for nonspherical particles based on the instrumentation used in this study. Experimental results for the density of wellcharacterized laboratory aerosols are then given. These data are used to test the accuracy of the system. n the future, the DMA/impactor technique will be used to find the average and range of atmospheric particle densities at several sizes in the submicrometer size range. THEORY When dealing with particles of unknown morphology and density, it is often neces- sary to express their behavior in aerosol samplers in terms of an equivalent diameter. Equivalent diameters for impactors and DMAs are based on the terminal velocities reached by particles under the influence of gravitational and electrostatic fields. rregular particles are then related to standard spheres by the concepts of particle density and dynamic shape factor. The following discussion is a brief review of the equivalent diameters encountered with DMAs and impactors. Hinds (1982), Kasper (1982), and Brockmann and Rader (1990) give definitions for several common equivalent diameters. From the notation of Brockmann and Rader, the mass equivalent diameter d, is the diameter of a solid sphere, which has the same mass as the irregular particle; the envelope equivalent diameter d, is the diameter a sphere of the same mass containing the same degree of voids as the original particle. The relationship between dm and d, is: where p, is inherent material density of the aerosol particle and p, is the effective density of the porous particle. The mobility equivalent diameter d, is the diameter of a sphere that has the same dynamic mobility as the particle in question and is the size parameter that is classified by a DMA. The aerodynamic equivalent diameter d, is the diameter of a unit density sphere which settles at the same speed as the particle. nertial classifiers such as impactors segregate particles according to aerodynamic diameter. The concept of dynamic shape factor x is introduced in order to relate the terminal velocity of an arbitrary particle to that experienced by a solid sphere of equal mass. The dynamic shape factor is defined as the drag force F experienced by the arbitrary particle to the drag force on the mass equivalent diameter sphere Fdm at the same velocity (Brockmann and Rader,

5 202 W. P. Kelly and P. H. McMurry Noting that F, is given by Stokes' law, the aerodynamtc drag force on an arbitrary particle can be expressed as: where v is the relative velocity between the particle and the gas, p is the gas viscosity, and C(d,) is the Cunningham slip correction factor (Allen and Raabe, 1982; Rader, 1989). rregularities in particle shape or internal voids within the particle will cause the shape factor to exceed unity. The separate contributions of voids and shape can be taken into account by expressing the shape factor as (Brockmann and Rader, 1990) where the particle porosity is defined as and where the envelope shape factor K accounts for the effect of shape on drag. The mobility equivalent diameter d, of particles classified by a DMA is (Liu and Pui, 1974; Knutson and Whitby, 1975): where e is the fundamental unit of charge, n is the number of elementary charges on the particle, V is the DMA center rod voltage, L is the length between the aerosol inlet and outlet slits, q, is the clean sheath air inlet flow rate, q, is the excess air outlet flow rate, C is the slip correction factor, p is the viscosity of the gas, and r, and r, are the concentric inner and outer electrode radii. Notice that particle density does not effect the size selected by the DMA. From the def- inition of electrical mobility and with Eq. 3, it is straightforward to show that for porous or nonspherical particles this mobility equivalent diameter is related to the mass equivalent diameter dm by Similarly, the relationship between aerodynamic diameter d, and mass equivalent diameter is where p, is unit density (1 g/cm3). Therefore, the mass equivalent, mobility equivalent, and aerodynamic diameters are related by the following equation: f the particles are spherical (K = 1; d, = d,) then it follows from Eqs. 1, 4, and 5 that the relationship between aerodynamic and mobility equivalent diameters reduces to: f, in addition, the spherical particles are also nonporous, then the mobility equivalent diameter is equal to the envelope equivalent diameter and, using Eq. 1, this relationship simplifies to For the case of particles having arbitrary shape and porosity it is seen by comparing Eqs. 9 and 10 that the grouping of terms [C(drn)/C(d,)x]& can be interpreted as an effective particle density. Many of the measurements reported in this article were done with particles known to be spherical, either because they are liquid or because they were observed to be spherical in an electron microscope. For these particles the particle density

6 Particle Density Measurement by nertial Classification of Aerosols 203 was obtained with Eq. 11 and compared with accepted literature values. Some experiments were done with particles known to be nonspherical. n this case both the shape and porosity can affect measured effective densities. Because the effective density is equal to [C(d,)/~(d,)~l~,, independent information on shape and porosity effects on dynamic shape factor and mass equivalent diameter is necessary to infer density, p,. n evaluating densities for nonspherical particles it should be noted that x is an orientation-dependent particle property, and preferential alignment of nonspherical particles can occur in both the DMA and the impactor. These effects are likely to be especially significant for particles with large aspect ratios (e.g., fibers). The nonspherical particles used in this study are nearly cubic and no explicit attempt is made to account for the effects of such alignment. n summary, equations for evaluating the effective densities of homogeneous spherical particles or of nonspherical or porous particles based on the measurement of equivalent aerodynamic and equivalent mobility diameters have been presented. The particle density calculated for irregular or porous particles should be considered an effective density, which may be equal to or less than the bulk material density, depending on the particle morphology. ndependent information on particle shape factors is required to infer true particle densities for irregular particles. EXPERMENTAL Figure 1 is a schematic diagram of the apparatus used to measure particle density. This system consists of apparatus for generating monodisperse particles in the pm diameter range, and a single-stage inertial impactor for measuring the aerodynamic diameters of these particles. The aerodynamic cutoff diameter of the impactor is determined using a pair of single-particle counting CNCs (Model 3760, TS, nc., St. Paul, MN) to measure the concentrations of particles upstream and downstream of the impactor. Monodisperse aerosols are generated by using a DMA to select particles of a known electrical mobility from a polydisperse aerosol produced by atomizing 0.1 volume percent solutions of dioctyl phtha- late (DOP), sulfuric acid (H2S04), ammonium sulfate [(NH4),S0,], or sodium chloride (NaCl). The DOP was diluted in high-pressure liquid chromatography grade 2-propanol (isopropyl alcohol) and dried by using activated charcoal. The remaining chemicals were diluted in deionized, filtered water. The low ion concentration charger described by Gupta and McMurry (1989) is used to improve the monodispersity of the DMA-generated particles by reducing multiple charging. Particles flowing out of the DMA are passed through a 210~o neutralizer in order to bring them to a charge distribution near Boltzmann equilibrium. Relative humidity is an important variable when sampling hygroscopic particles whose water content, size, and density are sensitive to relative humidity. n the present work, sulfuric acid is used as a model hygroscopic compound. Sulfuric acid particles were sampled at several different relative humidities in order to test the system's ability to measure changes in relative humidity-dependent densities. The subsystem used to adjust the aerosol humidities is similar to that described by McMurry and Stolzenburg (1989). Humidity levels are measured in the DMA excess air outlet line using a thermocouple and a dew point hygrometer (Model 1100, General Eastern nstruments, Woburn, MA). These data are not adjusted for flow-induced changes in relative humidity associated with pressure drop across the impactor or local cooling within the high speed jets. While an approach similar to that of Fang et al. (1991) could be used to

7 Compressed Air Monodisperse Aerosol Generator Relative Humidity WAir A polydisperse aerosol Electrostatic Classifier excess air L Critical Orifice! monodisperse aerosol Data Acquisition and Conwl m. m. :m.b.~.g.m.g.m.m.9.m.9.g.m.s.m.;! Mom1! mpactor 1 Condensation Nucleus Counter Condensation Nucleus Counter p nertial Classifier Vacuum Pump 3.. FGURE 1. Schematic diagram of thc DMA/impactor density measurement,system.

8 Particle Density Measurement by nertial Classification of Aerosols 205 account for the effects of these phenomena on aerodynamic classification of sulfuric acid particles by the microorifice uniform deposit impactor, such corrections are compound specific. Our objective is to use this approach for measuring the densities of particles having unknown composition. The experimental errors for sulfuric acid data should give an indication of the uncertainties that might be expected for hygroscopic atmospheric particles. A laboratory prototype DMA is used to select particles of a known size from a polydisperse aerosol. Kinney et al. (1991) have shown that 3% accuracy in particle diameter can be achieved with the DMA. Because particle densities calculated by Eqs are based on the mobility diameter squared, the corresponding uncertainty in density is - 6%. A total DMA flow rate of 5.0 L/min in this experiment allows for a maximum particle size near 0.6 pm, whereas the aerosol in- and outflows of 0.4 L/min create a narrow distribution of sizes. At the exit from the DMA the aerosol flows through a single-stage MOUD. Four different orifice plates having aerodynamic cut points ranging from 0.56 to 0.12 pm are used. We refer to these four orifice plates as stages a, b, c, and d, in order of decreasing cut size. For any given particle material, the electrical mobility equivalent cut point for the impactor is found by adjusting the DMA voltage. Because the MOUD orifice plates are designed to operate at 30 L/min, a fraction of each plate is masked in order to maintain the standard jet velocity at the operating flow rate of 1.5 L/min. n order to prevent bounce of the solid (NH,),SO, and NaCl particles, grease (Dow Corning Corp, Midland, M) diluted in toluene is applied to aluminum substrates with a cotton swab and then heated to give a smooth coating. Liquid particles (DOP,H,SO,) are collected on uncoated aluminum substrates. Particle concentrations upstream and downstream of the impactor are detected by using TS Model 3760 CNCs. Flow through these instruments is controlled by 1.5 L/min critical orifices, which maintain a constant volumetric flow rate. CNC concentrations are adjusted to account for the difference in mass flow rate between the upstream and downstream CNCs that is caused by the pressure drop through the impactor. This concentration correction ranges from 3% to 10% depending on the impactor stage and is applied to all of the data taken. Although the concentrations given by the upstream CNC are typically found to be between 1% and 3% higher than the downstream CNC when sampling the same aerosol without the impactor in place, no systematic correction factor is applied to account for this concentration variation because the error is small and variable. Errors associated with variations in concentration of the sampled aerosol are avoided by sampling simultaneously upstream and downstream1 of the impactor. Once the flow rates are properly adjusted and temperature and pressure levels are recorded, the DMA voltage is set and the CNC digital pulses are counted by using PC1 data acquisition and control cards (Burr Brown/ntelligent nstrumentation, Tucson, AZ) installed in a Zenith 2000 personal computer. Particles are sampled at a single size until a 0.5% statistical uncertainty level for collection efficiency is reached. Data is collected at a range of sizes to determine the size-dependent impactor collection efficiency. This procedure usually requires < 1 min per data point, making it possible to obtain many points along the collection efficiency curve within a short time interval. RESULTS AND DSCUSSON Collection efficiency curves are plotted in Figures 2-5 for impactor stages a-d. ndividual graphs for each stage show a family

9 W. P. Kelly and P. H. McMurry Equivalent Mobility Diameter (pm) FGURE 2. Collection efficiency versus equivalent mobility diameter for stage a of a MOUD. The two sets of data for (NH,),SO, were obtained on different days under similar experimental conditions, and provide an indication of repeatability. of parallel curves with materials of different density being spread laterally, as would be expected. Data for particles of materials having lower densities are found to the right, while materials having higher densities are shifted to the left. t follows, for example, that because the density of sulfuric acid increases with decreasing relative humidity, the sulfuric acid collection efficiency curves in Figures 3 and 4 shift towards smaller particle sizes as relative humidity decreases. Note also the smoothness of the curves and their approach to the ideal conditions of complete capture at large sizes and complete penetration at small sizes. The repeatability for testing the same material on different runs is illustrated with the (NH,),SO, data in Figure 2 and the DOP data in Figure 4. Equivalent mobility diameters were converted to aerodynamic diameter with Eq. 11 by using the bulk material densities from the literature. This transformation implicitly assumes that the particles are spherical and nonporous. Relationships between collection efficiencies and these aerodynamic diameters are shown for stage c in Figure 6. Similar results were obtained for the other stages. Note that, except for NaCl, the data very nearly collapse to a single curve, as would be expected if the particles were spherical and nonporous. For all materials excepting NaC1, the relative standard deviation in the aerodynamic diameter at the 50% cut point is 2.3% for stage b and 2.0% for stage c. These standard deviations in aerodynamic cut points are on the order

10 Particle Density Measurement by nertial Classification of Aerosols & DOP Y 77% RH - A 52% RH 26% RH 11% RH Equivalent Mobility Diameter (pm) FGURE 3. Collection efficiency versus equivalent mobility diameter for stage c of a MOUD c-- DOP - DOP 75% RH % RH - --+E- 33% RH 13% RH % RH -- m4)2s04 NaCl Equivalent Mobility Diameter (pm) FGURE 4. Collection efficiency versus equivalent mobility diameter for stage d of a MOUD.

11 W. P. Kelly and P. H. McMurry Equivalent Mobility Diameter (pm) FGURE 5. Collection efficiency vcrsus equivalent mobility diameter for stagc b of a MOUD. of the 3% sizing uncertainty for DMAs (Kinney et al., 1990). A scanning electron microscopic examination of the DMAclassified NaCl particles revealed that they were nonspherical, which probably explains the anomalous behavior of these particles. n order to find the equivalent mobility diameter at 50% collection efficiency (dbso), linear regression was used to determine the best-fit relationship between measured collection efficiencies in the 20-80% range and mobility equivalent diameter. Correlation coefficients for the linear fitted equations all exceeded 0.99; the average correlation coefficient was 0.996, indicating very linear data in the region chosen for analysis. More complicated curve fits were not justified based on the fact that day-to-day variations in the measurements exceeded the enhanced accuracy of a more complex curve fit. n order test the system's ability to measure density, it was necessary to choose one or more reference materials as calibration standards to determine the aerodynamic cut size (da5,,) of each impactor stage. These aerodynamic cut sizes are obtained with Eq. 11 by using literature values for particle density and the measured values for dbso. The average of ddi0 values for DOP and (NH4),S04 was chosen to characterize the aerodynamic out size for the impactors because these materials cover the range of densities that might be expected for atmospheric particles ( p,,, = g/cm3 and P[NH,),SO, = g/cm3). Based on this approach the average aerodynamic diameters for stages a, b, c, and d were found to be 0.563, 0.266, 0.175, and pm, respectively. Errors in these aerodynamic cut points could be introduced by the evaporation of

12 Particle Density Measurement by nertial Classification of Aerosols u- DOP DOP % RH 60 62% RH -----~ % RH - A 13% RH 40 9% RH 30 PJH4)2S A NaCl Equivalent Aerodynamic Diameter (pm) FGURE 6. Collection cfficiency versus equivalent aerodynamic diamcter for stage c of a MOUD showing that thc aerodynamic cut point is pm. DOP particles between the DMA outlet and the impactor inlet, or by nonspherical shape effects for the (NH,),SO, particles. Estimates based on the work of Rader et al. (1987) suggest that errors associated with DOP evaporation should be small in comparison with other uncertainties for stages a-c, although could have been significant at stage d. The (NH,),SO, particles were examined in a scanning electron microscope and appeared to be spherical, thereby supporting the spherical shape assumption of our analysis. Our measurements showed that aerodynamic cut sizes for both of these materials were within 2.6% for all stages, with an average discrepancy of 1.8%. Based on the consistency of these results for all four stages, we believe that the measured aerodynamic cutoff diameters were not significantly affected by either evaporation or particle shape. By using DOP and (NH,),SO, to determine the aerodynamic 50% cut point of the impactor, the experimental effective densities of the remaining aerosols have been determined from their mobility diameter cut point values using Eqs By comparing these measured values with known densities, it is possible to determine the accuracy of the density measurements made by this system. The results of these comparisons are shown in Tables 1 and 2. The tabulated densities for impactor stages b and c show that the maximum error for H,SO, particles at various relative humidities is 7.7% while the average error 3.6%. Similar accuracy is expected for stages a and d, where only the calibration aerosols (DOP and

13 210 W. P. Kelly and P. H. McMurry TABLE 1. Density Results for MOUD Stage b Equivalent Accepted Relative mobility bulk humidity cutoff density Aerosol (96) ( pm) (g/cm3y Sulfuric acid Sulfuric acid Sulfuric acid Sulfuric acid Data are based on DOP and (NH,),S04 calibration showing that das0 = pm For stagc b a Bray (1970) Experimental particle Experimental density error (g/cm3) (%) (NH,),SO,) were tested. The 14% discrepancy between the measured effective density and the bulk density that is observed for NaCl exceeds expected experimental uncertainties. We believe the large difference between the effective density and the bulk material density of NaCl is due to irregularity in the shape of these particles. The measured NaCl density becomes the correct value if a dynamic shape factor of 1.08 is assumed. The dynamic shape factor for cubes in the continuum regime is also (Hinds, 1982). Thus, although the particles produced in our system were not all perfect cubes, and although our particles were in the transition regime, the value of the shape factor from our data is in the range that might be expected for compact crystalline particles. Figure 7 shows the experimentally measured density of sulfuric acid versus that given by Bray (1970) at a range of relative humidities. The k 6% vertical error bars are associated with DMA sizing uncertainties, and do not include other effects such as uncertainties in relative humidity. Our measured densities are about 8% lower than the accepted values at low (20-50%) humidities, but are very close to the true values at high ( > 50%) humidities. The 4% H,SO, sizing errors observed in our study are similar in magnitude to the 5% errors reported by Fang et al. (1991), who studied the effect of flowinduced relative humidity changes on sulfuric acid sizing by MOUD. Although our errors are consistent with these previous observations, we do not understand the reason for the systematic bias at low TABLE 2. Density Results for MOUD Stage c Relative humidity Aerosol (%) Sulfuric acid 75.0 Sulfuric acid 61.5 Sulfuric acid 33.4 Sulfuric acid 13.1 Sulfuric acid 9.0 NaCl - Equivalent mobility cutoff ( p.m) Accepted Experimental bulk particle Experimental density (g/cm3) density (g/cm3) error (%) 1.217" 1.278" 1.400" 1.521" 1.554" 2.165~ rt Data are based on DOP and (NH,)*S04 calibration showing that DaSU = pm for stage c. a Bray (1970). CRC Handbook of Chemistry and Physics (1988). 'This error may have been due to particle shape rather than thc measurement technique '

14 Particle Density Measurement by nertial Classification of Aerosols Relative Humidity (%) FGURE 7. Sulfuric acid density as a function of relative humidity according to Bray (1970) and the present experiment. relative humidity; it does not appear to be associated with flow-induced relative humidity changes. CONCLUSONS A new technique using a DMA and an impactor has been developed which is capable of measuring submicrometer particle density with errors of < 8% for spherical, hygroscopic particles in the pm diameter range. Uncertainties in measured densities of spherical, nonhygroscopic particle densities are - 4%. Laboratory aerosols that were spherical included DOP, H,S04, and (NH,),S04. The effective density of NaCl is 14% less than the accepted bulk material density. We believe this discrepancy for NaCl was due to the nonspherical shape of these particles. Results for the density of sulfuric acid reaffirm the work done by Fang et al. (1991) that sizing errors in MOUD do not exceed 5% for relative humidities < 80%, despite the effects of flow-induced humidity changes. The DMA/impactor system can be used alongside cascade impactors to provide direct measurement of ambient particle density in the submicron size range. Such information reduces assumptions that must be made to convert aerodynamic size to geometric size when working with impactor data. The experimental apparatus can also be used for impactor calibration, studies of particle bounce in impactors, and the measurement of particle shape factors. Support for this research was provided by Electric Power Research nstitute Contract No. RP REFERENCES Allen, M. D., and Raabe, 0. G. (1985). J. Aerosol Sci. 16:57-67.

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