Determination of Particle Effective Density in Urban Environments with a Differential Mobility Analyzer and Aerosol Particle Mass Analyzer

Effective densities of atmospheric aerosols in various locations of the Los Angeles Basin were determined by a DMA-APM technique. Effective density was calculated by comparing voltage distributions of sampled atmospheric aerosols with PSL particles of known density. The five sites chosen for field experiments were: (1) Interstate-710 Freeway, impacted by heavy-duty diesel vehicles; (2) State Route CA-110, open only to gasoline vehicles; (3) Riverside, a receptor site known for secondary particle formation; (4) University of Southern California, a typical urban and industrial environment; and (5) Coast for marine aerosol. The size range selected for this study was from 50 nm to 414 nm. While 50 nm particles exhibited a single effective density multiple effective densities were measured for each of the other particle sizes as significant fractions of these particles are transported from background sources. Regardless of location, 322–414 nm particle effective densities were considerably lower than unity. The lowest effective densities (～ 0.1 g cm ^{? 3} ) were reported for I-710, confirming that diesel combustion aerosols are rich in chain agglomerates with large void spaces. Riverside exhibited high effective densities (～ 1.2–1.5 g cm ^{? 3} ) for 50–202 nm particles, which we hypothesize is due to transformations that occur during advection from Los Angeles. Measurements of diurnal variation of effective density at Riverside support this hypothesis. Overall, our results suggest that effective density declines as the particle mobility diameter increases irrespective of location. Fractal dimensions calculated from average effective densities were lowest for I-710 ( D _f = 2.41) and CA-110 (D _f = 2.54) aerosols, presumably due to the influence of vehicular combustion emission on these sites. By contrast, average fractal dimensions at USC, Riverside and Coast were found to be 2.79, 2.83, and 2.92, respectively. High fractal dimensions at these sites may be the effects of aging, moisture absorption and/or organic vapor condensation on the particles, which fills void space and makes particles more spherical.     

Comparison and Combination of Aerosol Size Distributions Measured with a Low Pressure Impactor,Differential Mobility Particle Sizer,Electrical Aerosol Analyzer,and Aerodynamic Particle Sizer

Data from a different mobility particle sizer (DMPS) or an electrical aerosol analyzer (EAA) has been combined with data from an aerodynamic particle sizer (APS) and converted to obtain aerosol mass distribution parameters on a near real-time basis. A low pressure impactor (LPI), a direct and independent measure of this mass distribution, provided information for comparison.

The number distribution of particles within the electrical measurement range was obtained with the DMPS and EAA. Data from the APS for particles greater than that size were used to complete the number distribution. Two methods of obtaining mass distribution parameters from this number data were attempted. The first was to convert the number data, channel by channel, to mass data and then fit a log-normal function to this new mass distribution. The second method was to fit a log-normal function to the combined number distribution and then use the Hatch-Choate equations to obtain mass parameters.

Both the DMPS / APS and the EAA / APS systems were shown to successfully measure aerosol mass distribution as a function of aerodynamic diameter. Careful operation of the measurement equipment and proper data manipulation are necessary to achieve reliable results. A channel-by-channel conversion from number to mass distribution provided the best comparison to the LPI measurement. The DMPS / APS combination furnishes higher-size resolution and accuracy than the EAA / APS system. A small gap was observed in the EAA / APS combined data; however, this did not seem to adversely affect the determination of mass distribution parameters.     

Determination of Arbitrary Moments of Aerosol Size Distributions from Measurements with a Differential Mobility Analyzer

A method to determine arbitrary moments of aerosol size distributions from differential mobility analyzer measurements has been proposed. The proposed method is based on a modification of the algorithm developed by Knutson and Whitby to calculate the moments of electrical mobility distributions. For this modification, the electrical mobility and the charge distribution have been approximately expressed by power functions of the particle diameter. To evaluate the validity of the approximation, we have carried out numerical simulations for typical size distributions. We have found that for typical narrowly distributed aerosols such as polystyrene latex particles and particles that arise in the tandem differential mobility analyzer configuration, the distribution parameters can be accurately determined by this method. For a log-normally distributed aerosol, the accuracy of the distribution parameters determined by this method has been evaluated as a function of the geometric standard deviation. We have also compared the accuracy of the proposed method with other existing methods in the case of the asymmetric Gaussian distribution.     

The Effect of Nanoparticle Convection-Diffusion Loss on the Transfer Function of an Aerosol Particle Mass Analyzer

The existing theoretical response spectra of APM-3600 agree well with the experimental data for submicron particles larger than 100 nm in the electrical mobility diameter but not for nanoparticles. In this study, a 2-D numerical model was developed to predict the transfer function and response spectra of APM-3600 based on the detailed simulation of flow and particle concentration fields. It was found that recirculation flows existed in the annular classifying region and APM's inlet and outlet regions, which led to enhanced convection-diffusion loss of nanoparticles compared to that without considering flow recirculation. As a result, the APM underestimates the mass of naonoparticles due to the shift of the peak position of the transfer function to a larger diameter than the targeted diameter. The response spectra calculated with the simulated transfer function agree well both in shapes and peak values with the experimental data present in a previous study for both nanoparticles and submicron particles larger than 100 nm. The predicted particle masses also agree well with the PSL's experimental data of the article.

Copyright 2014 American Association for Aerosol Research     

Mass Range and Optimized Operation of the Aerosol Particle Mass Analyzer

We investigated, theoretically, the mass range in which an aerosol particle mass analyzer (APM) can be used for classification, and how the APM classification performance can be optimized. We listed factors that set limits to the APM, which were constraints of the rotation speed and the voltage, as well as requirements on the APM classification performance parameter, λ, that guarantee at least minimal performance in both resolution and penetration. We introduced the APM operation diagram, which is a tool to visualize the limits and mass range. We proposed to operate the APM that was considered in this study with the λ value set within the range from 0.25 to 0.5 for optimum classification performance by balancing both resolution and penetration. The mass range for the APM, with the λ value maintained between 0.25 and 0.5, was calculated to be from 0.003 to 2000 fg, which corresponds to the diameter range from 20 to 1600 nm for the density of 1 g/cm³. To verify the validity of the mass range and the idea of the optimized operation, we carried out experiments on an APM with polystyrene and sodium-chloride particles that were classified by electrical mobility. We found that the APM was able to provide bell-shaped spectra down to 12 nm, and was able to perform mass classification with an accuracy better than 5% down to 50 nm. Underestimation of mass and reduction of resolution and penetration were observed at sizes smaller than about 30 nm.     

Design Considerations and Performance Evaluation of a Compact Aerosol Particle Mass Analyzer

A compact aerosol particle mass analyzer (APM) of which the size of the classifier was significantly reduced than that of the first commercial model (Kanomax Model 3600) was developed. Firstly, requirements for desired performance in classifying particle mass were set forth. Secondly, a theoretical framework for the design parameters of an APM that satisfies the requirements was formulated. Thirdly, the design parameters were determined that satisfies the requirements while reducing the instrument size. The requirements include the condition that the classification range covers from 0.001 to 1000 fg (approximately 12 to 1200 nm in size for spherical particles having the density of 1 g/cm³), and the condition that both the classification resolution and particle penetration in this mass range are higher than certain specified values. A prototype having the design parameters determined according to this theoretical framework was constructed, and its performance was evaluated experimentally. The external dimensions of the electrodes of the compact APM are approximately 140 mm in length and 60 mm in diameter. It was confirmed that the performance of the compact APM operated at the aerosol flow rate of 0.3 L/min was comparable to that of the Model 3600 APM operated at 1 L/min. Because of the reduced size and of the resultant improved portability, it is expected that the compact APM is readily applicable to field measurements.

Copyright 2013 American Association for Aerosol Research     

Sizing of Aerosol in Gases Other Than Air Using a Differential Mobility Analyzer

The differential mobility analyzer (DMA) is a device that sizes aerosol particles based on their electrical mobility. The relationship between particle size and mobility depends, among other factors, on three gas specific parameters, namely, dynamic viscosity, mean free path, and Cunningham slip correction factor C c . Provided these parameters are known, DMA theory is expected to be valid independent of gas type. The present study demonstrates the sizing accuracy of DMAs for gases other than air using monodisperse polystyrene latex (PSL) spheres with nominal diameters of 60 nm, 149 nm, and 343 nm in He, Ar, H 2 , CO 2 , and N 2 O. Eliminating possible systematic errors due to uncertainties in DMA geometry and nominal PSL diameter by normalizing the measured PSL diameters to their respective diameters measured in air, the sheath flow rate Q sh and C c are expected to be the main sources for measurement errors. Since C c data are lacking for PSL spheres in gases other than air, an expression given by Allen and Raabe (1985b) was used to approximate C c . The experimental results of the present study are consistent with a 2% accuracy of this expression for C c , which is considerably better than the 5% accuracy estimated by Rader (1990) for a similar expression for oil drops. Finally, we discuss other aspects of operating a DMA with gases other than air, namely, flow meter calibration and dependence of electrical breakdown voltage on gas type. In the present study a thermal mass flow meter (MFM) was used to measure Q sh . Calibration of this MFM revealed that the gas specific MFM correction factors ( K factors) provided by the technical literature can be highly inaccurate (here between -12% and +31%). More accurate K factors are presented.     

Fast Mixing Condensation Nucleus Counter: Application to Rapid Scanning Differential Mobility Analyzer Measurements

Condensation nucleus counters (CNCs) exhibit slower time response than expected due to mixing effects within the detector.This mixing produces an exponential distribution of delay times with a characteristic mixing time m that ranges from 0.1 s to 0.9 s for commonly used instruments and limits their usefulness for measuring rapidly changing aerosols. Moreover, when used as detectors in the scanning electrical mobility spectrometer (SEMS; also known as scanning mobility particle sizer, SMPS), CNCs limit the speed with which size distribution measurements can be made. In order to overcome this limitation, a new, fast-response mixing CNC (MCNC) has been developed and characterized. The time response of this new detector and TSI Models 3025 and 3010 CNCs has been measured using a spark source to generate an aerosol pulse. The mixing induced response smearing of this new detector, m , of this instrument is 0.058 s, which is significantly shorter than either of the other instruments tested. Its lower detection limit is about 5 nm diameter. The high aerosol flow rate of the MCNC (0.65 l min -1 ), fast time response, and low detection limit make it an ideal detector for SEMS/SMPS measurements. Using this MCNC as a detector for the SEMS, size distribution measurements over the 5 nm to 140 nm range have been made in 3 s with minimal distortion. The size distribution of a coagulation aerosol was effectively recovered by deconvolution with scans as short as 1 s. Uncertainties in the 1 s scans result, in part, from electronics problems in the scanning DMA.     

The Influence of Relative Humidity on Nanoparticle Concentration and Particle Mass Distribution Measurements by the MOUDI

A humidity control system was operated upstream of two collocated MOUDIs (micro-orifice uniform deposit impactors) for sampling ambient aerosol particles. One MOUDI used silicone-grease-coated aluminum foils (ALs) as the impaction substrates and was considered as the reference impactor, while the other used uncoated ALs or uncoated Teflon filters (TFs) as the impaction substrates for quantifying the effect of different relative humidities (RHs) and impaction substrates on the PM_0.1 concentrations and mass distributions of ambient PMs. Test results showed that decreasing RH in general increased particle bounce from uncoated substrates with the bounce from uncoated ALs being more severe than that from uncoated TFs. Particle bounce did not influence the overall mass distribution of ambient fine particles when RH ranged between 40% and 80%, whereas it led to undersampling of particles greater than 2.5 μm in aerodynamic diameter severely. Oversampling of PM_0.1 occurred by as much as 95%–180% or 25%–55% when the MOUDI used uncoated ALs or TFs, respectively, as RH was reduced from 50% to 25%. Particle bounce was found to be negligible, and PM_0.1 and PM_2.5 could be sampled accurately with less than 5% error at the RH of 75%–80% or 65%–80% when uncoated ALs or TFs were used, respectively.     

Measurements of Particle Loss Functions in a Differential Mobility Analyzer (TSI,Model 3071) for Different Flow Rates

Particle losses in a differential mobility analyzer (TSI, Model 3071) caused by diffusive deposition and Brownian diffusion are measured for particles in the diameter size range between 3 and 100 nm. For small sampling and aerosol flow rates (0.3 liters/min) at 20 nm, nearly 50% of the primary particles are lost; and for 2 liters/min, the particle losses have to be considered in the diameter size range below 30 nm (50% at 7 nm). From the measured penetration values, an effective tube length is derived which is useful to calculate particle losses for other flow rates through the analyzer.     

Determination of Particle Size Distribution of Ultra-Fine Aerosols Using a Differential Mobility Analyzer

The size analysis of ultrafine aerosol particles using a differential mobility analyzer combined with a CNC is discussed from two standpoints: (1) particle loss caused by Brownian diffusion in the analyzer, and (2) data reduction procedure where Fuchs' charging theory is applied. As a result, it has been found that (1) particle loss becomes significant when particle size is smaller than about 15 nm, and (2) a simple and practical data reduction procedure may be used, where the stationary bipolar charge distribution given by Boltzmann's law is modified by using Fuchs' charge distribution in the smaller size range.     

Evaluation of a New Particle Trap in a Laser Desorption Mass Spectrometer for Online Measurement of Aerosol Composition

We have developed a new analyzer for the online measurement of aerosol composition: a particle trap laser desorption mass spectrometer (PT-LDMS). The main components of the instrument include an aerodynamic lens, a particle trap enclosed by a quartz cell, a quadrupole mass spectrometer (QMS), a vacuum chamber incorporating the above components, and a carbon dioxide (CO₂) laser (wavelength 10.6 μm). The aerodynamic lens generates a beam of submicron particles, which is focused on a small area on the particle trap. The particle trap consists of custom-made mesh layers, the structure of which was newly designed using engineering techniques for micro electro mechanical systems (MEMS). A large number of mesh frames are well arranged in the trap, and particles can be efficiently captured after multiple impactions on the frames. The CO₂ laser is used to vaporize aerosol compounds captured on the particle trap. The evolved gas confined within the quartz cell is analyzed using an electron impact ionization (EI) QMS to quantify the chemical composition of the particles. The concept of the PT-LDMS and first evaluation of its performance are presented, specifically focusing on the structure and performance of the particle trap.     

Extending the Capabilities of Single Particle Mass Spectrometry: II. Measurements of Aerosol Particle Density without DMA

Particle density is an important and useful property that is difficult to measure because it usually requires two separate instruments to measure two particle attributes. As density measurements are often performed on size-classified particles, they are hampered by low particle numbers, and hence poor temporal resolution. We present here a new method for measuring particle densities using our single particle mass spectrometer, SPLAT. This method takes advantage of the fact that the detection efficiency in our single particle mass spectrometer drops off very rapidly as the particle size decreases below 100 nm creating a distinct sharp feature on the small particle side of the vacuum aerodynamic size distribution. Thus, the two quantities needed to determine particle density, the particle diameter and vacuum aerodynamic diameter, are known. We first test this method on particles of known compositions and densities to find that the densities it yields are accurate. We then apply the method to obtain the densities of particles that were characterized during instrument field deployments. We illustrate how the method can also be used to measure the density of chemically resolved particles. In addition, we present a new method to characterize the instrument detection efficiency as a function of particle size that relies on measuring the mobility and vacuum aerodynamic size distributions of polydisperse spherical particles of known density. We show that a new aerodynamic lens used in SPLAT II improves instrument performance, making it possible to detect 83 nm particles with 50% efficiency.     

Investigation of Aerosol Surface Area Estimation from Number and Mass Concentration Measurements: Particle Density Effect

For nanoparticles with nonspherical morphologies, e.g., open agglomerates or fibrous particles, it is expected that the actual density of agglomerates may be significantly different from the bulk material density. It is further expected that using the material density may upset the relationship between surface area and mass when a method for estimating aerosol surface area from number and mass concentrations (referred to as “Maynard's estimation method”) is used. Therefore, it is necessary to quantitatively investigate how much the Maynard's estimation method depends on particle morphology and density. In this study, aerosol surface area estimated from number and mass concentration measurements was evaluated and compared with values from two reference methods: a method proposed by Lall and Friedlander for agglomerates and a mobility based method for compact nonspherical particles using well-defined polydisperse aerosols with known particle densities. Polydisperse silver aerosol particles were generated by an aerosol generation facility. Generated aerosols had a range of morphologies, count median diameters (CMD) between 25 and 50 nm, and geometric standard deviations (GSD) between 1.5 and 1.8. The surface area estimates from number and mass concentration measurements correlated well with the two reference values when gravimetric mass was used. The aerosol surface area estimates from the Maynard's estimation method were comparable to the reference method for all particle morphologies within the surface area ratios of 3.31 and 0.19 for assumed GSDs 1.5 and 1.8, respectively, when the bulk material density of silver was used. The difference between the Maynard's estimation method and surface area measured by the reference method for fractal-like agglomerates decreased from 79% to 23% when the measured effective particle density was used, while the difference for nearly spherical particles decreased from 30% to 24%. The results indicate that the use of particle density of agglomerates improves the accuracy of the Maynard's estimation method and that an effective density should be taken into account, when known, when estimating aerosol surface area of nonspherical aerosol such as open agglomerates and fibrous particles.     

Determination of the Mean Mobility of Aerosol Nanoparticles Classified by Differential Mobility Analyzers

MonteCarlo simulations of diffusive particle trajectories, as well as Stolzenburg's model calculations, have shown that the mean mobility of the particles classified by a differential mobility analyzer (DMA) at a given applied voltage may differ from the theoretical one inferred from the Knutson–Whitby equation if the particles are withdrawn from the tails of the particle mobility distribution. In this case, the true mean mobility, defined as the mean mobility of the particles classified at the specified voltage, can be precisely measured by a second DMA operating in series with the first one (tandem DMA). However, if particles are extracted from the central part of the distribution, their mobility can be correctly measured with a single DMA. Besides showing the importance of the usage of the tandem DMA technique for accurate measurements of mobility, this article provides an analytical expression which, if the mobility distribution of the polydisperse aerosol fed to the DMA is known, allows an accurate estimation of the true (mean) mobility of the classified particles.

Copyright 2014 American Association for Aerosol Research     

Diffusion Distortions in a Differential Mobility Analyzer: The Shape of Apparent Mobility Spectrum

The effect of molecular and or small-scale turbulent diffusion in a differential mobility analyzer (DMA) is described in terms of apparent mobility spectrum. Without restricting generality, the normalized apparent spectrum, i.e., the apparent spectrum of unimobile particles with unity charge density is introduced. An approach based on the calculation of the probabilities of the random displacements of particles around their regular trajectories enables us to derive analytical expressions for the normalized apparent spectrum. A particular derivation is carried out for the case of a second-order DMA with one collecting electrode and variable electric field strength. Explicit analytical equations of various approximation degrees have been derived. The normalized apparent spectrum of the particular DMA shows a remarkable asymmetry; its mobility mode is shifted toward lower mobilities. The derived equations serve as a basis for the estimation of the spectral resolution of the DMA. The equations can also be used for a proper design of the DMA, reducing the effect of diffusion. Once the normalized apparent spectrum is known, a possibility appears to improve the resolution of the DMA by solving a relevant equation and eliminating the effect of diffusion in such a way.     

Measurement of Metal Nanoparticle Agglomerates Generated by Spark Discharge Using the Universal Nanoparticle Analyzer (UNPA)

Nanoparticle agglomerates play an essential role in the manufacturing of many nanomaterials and are commonly found in combustion products. Conventional aerosol instruments based on equivalent spheres are not directly applicable to the measurement of nanoparticle agglomerates. The increasing interest in real-time assessment of the structure of engineered nanoparticle agglomerates and the mass concentration of potentially hazardous agglomerates (e.g., diesel soot, welding fume) makes an instrument devoted to online structure and mass measurements for nanoparticle agglomerates highly desirable. A recently developed instrument, universal nanoparticle analyzer (UNPA), utilizes the close relation between agglomerate structure and unipolar charging properties and infers agglomerate structure from measurement of the average charge per agglomerate. It was used in this study to characterize in situ the structure of metal nanoparticle agglomerates generated by spark discharge, to study the effects of sintering on the structure of these agglomerates, and to make real-time assessment of their airborne mass concentration. The primary particles sizes measured by UNPA for the gold (Au), silver (Ag), and nickel (Ni) agglomerates are in reasonable agreement with the TEM (transmission electron microscopy) sizing results, d _p = 7.9 ± 1.5, 11.8 ± 3.2, and 6.6 ± 1.0 nm, respectively. In addition, findings from the study of agglomerate structural change during sintering using the UNPA sensitivity coincide with results from TEM and mobility analyses. With regard to the mass concentration of silver agglomerates at room temperature, good agreement was found under our experimental conditions between results given by UNPA, the effective density, and the gravimetric measurement.

Copyright 2012 American Association for Aerosol Research     

Comparison Between the Theoretical and Experimental Performance of a Differential Mobility Analyzer with Three Monodisperse-Particle Outlets

Differential mobility analyzers (DMAs) with more than one monodisperse-particle outlet can offer a number of advantages compared to conventional single monodisperse-particle outlet designs. A generalized theoretical model and experimental measurements describing the performance of a DMA with 3 monodisperse-particle outlets have been independently reported in the literature. The objective of this article is to compare the theoretical predictions with the measurements. Resolutions determined by the theoretically predicted transfer functions for the three monodisperse-particle outlets are compared with measurements when the DMA was operated under different operating conditions. Predictions and measurements show good agreement when the DMA is operated at low sheath flow rates and for aerosol outlets relatively far from the aerosol inlet. For aerosol outlets relatively near the inlet there is evidence that the discrepancy between theoretical predictions and measurements may disappear at higher sheath flow rates, but the chances of flow disturbances in the classifier increase as well. The theory for multiple monodisperse-outlet DMAs is thus seen as successful in predicting the performance of this instrument, provided that disturbances in the flow field are avoided.

Copyright 2013 American Association for Aerosol Research     

Size Measurement of Polystyrene Latex Particles Larger than 1 Micrometer using a Long Differential Mobility Analyzer

We describe a newly constructed annular-type differential mobility analyzer (DMA) with an effective electrode length of 60 cm, which is longer than that of our original DMA (40 cm length). This long DMA was developed to extend the classification size of particles measuring up to 1.5 μm. As an application of this DMA, the mean diameters and standard deviations were determined for six samples of monodisperse polystyrene latex (PSL) particles ranging from 0.94 to 1.27 μm in nominal diameter. These PSL particles suspended in double-distilled water were aerosolized by a glass nebulizer and then introduced into the DMA. The mean diameters and standard deviations of these PSL particles were calculated by Ehara's method and compared with the nominal diameters and uncertainties. There was good correlation between the nominal diameters of these samples—particularly for recently certified samples—and the measured diameters. Classification of aerosol particles more than 1 micrometer in diameter using this DMA will be useful for many purposes.