Counting efficiency determination from quantitative intercomparison between expansion and laminar flow type condensation particle counter

Heterogeneous nucleation of supersaturated n-butanol vapor on neutral silver particles of different size has been investigated at variable nucleation temperatures using a fast expansion chamber and a commercial continuous flow type condensation particle counter (CPC). In addition, the theoretical supersaturation profile was calculated for the commercial CPC in order to conflate the results with the measured onset saturation ratio of the expansion type size analyzing nuclei counter (SANC). A comparison of the experimental results of the SANC measurements with the Kelvin equation shows that the heterogeneous nucleation starts below the Kelvin curve. By introducing a correction factor for the Kelvin equation based on the SANC measurements, the theoretical detection efficiency for a laminar flow type CPC could be derived. It was the first time that a validation between theoretical saturation ratio profiles and measurements conducted with different methods to generate supersaturation was achieved. We made the observation that reducing the nucleation temperature generally leads to enhanced counting efficiencies. Consequently, the cutoff diameter of regular butanol CPCs can be significantly reduced by simply lowering the condenser temperature.

Copyright © 2019 The Authors. Published with license by Taylor & Francis Group, LLC     

Experimental Study of Particle Deposition on Semiconductor Wafers

A sensitive method for detecting particle deposition on semiconductor wafers has been developed. The method consisted of generating a monodisperse fluorescent aerosol, depositing the known-size monodisperse aerosol on a wafer in a laminar flow chamber, and analyzing the deposited particles using a fluorometric technique. For aerosol particles in the size range of 0.1–1.0 μm, the mobility classification-inertial impaction technique developed by Romay-Novas and Pui (1988) was used to generate the monodisperse test aerosols. Above a particle diameter of 1.0 μm, monodisperse uranine-tagged oleic acid aerosols were generated by a vibrating-orifice generator. The test wafer was a 3.8-cm diameter silicon wafer placed horizontally in a vertical laminar flow chamber which was maintained at a free stream velocity of 20 cm/s. A condensation nucleus counter and an optical particle counter were used to obtain the particle concentration profile in the test cross section and to monitor the stability of aerosol concentration during the experiment. The results show that the measured particle deposition velocities on the wafers agree well with the theory of Liu and Ahn (1987) in the particle size range between 0.15 and 8.0 μm. The deposition velocity shows a minimum around 0.25 μm in particle diameter and increases with both smaller and larger particle size owing to diffusional deposition and gravitational settling, respectively.     

A multi-channel electrical mobility spectrometer with wedge geometry—Design and first evaluation

This paper presents a new design for a multi-channel electrical mobility spectrometer which measures the lognormal size distribution and number concentration of aerosol particles in the size range 5–300 nm with a short response time. The spectrometer charges particles in the test sample by unipolar corona discharge, they are then classified into 16 channels by electrical mobility. Charged particles are detected in the channels by individual aerosol electrometers, giving an electrical mobility spectrum for the sample.The main aspect of the spectrometer design is a wedge-shaped classifier with flat electrodes. This allows a flow to be drawn from the classifier at 16 different levels/channels with minimal disturbance to the remaining flow, hence filter based aerosol electrometers can be used for detection. The varying field within the classifier caused by the wedge shape is advantageous to the classification and optimised through the selection of the wedge angle.Also presented is an alternative technique for inferring the lognormal size distribution of an aerosol from a measured electrical mobility spectrum. This involves using a theoretical model of the instrument to simulate the output mobility spectra for a large number of aerosol samples with lognormal size distributions. The resulting data library can be searched against a measured electrical mobility spectrum to find the corresponding size distribution.The experimental work presented in this paper is a first evaluation of this spectrometer and includes measurement of the classifier transfer functions, basic calibration of the charger, and finally testing the spectrometer's performance on some simple unimodal lognormal aerosol samples.     

Modification of Laminar Flow Ultrafine Condensation Particle Counters for the Enhanced Detection of 1 nm Condensation Nuclei

This paper describes simple modifications to thermally diffusive laminar flow ultrafine condensation particle counters (UCPCs) that allow detection of ～1 nm condensation nuclei with much higher efficiencies than have been previously reported. These non-destructive modifications were applied to a commercial butanol-based UCPC (TSI 3025A) and to a diethylene glycol-based UCPC (UMN DEG-UCPC). Size and charge dependent detection efficiencies using the modified UCPCs (BNL 3025A and BNL DEG-UCPC) were measured with high resolution mobility classified aerosols composed of NaCl, W, molecular ion standards of tetra-alkyl ammonium bromide, and neutralizer-generated ions. With negatively charged NaCl aerosol, the BNL 3025A and BNL DEG-UCPC achieved detection efficiencies of 37% (90× increase over TSI 3025A) at 1.68 nm mobility diameter (1.39 nm geometric diameter) and 23% (8× increase over UMN DEG-UCPC) at 1.19 nm mobility diameter (0.89 nm geometric diameter), respectively. Operating conditions for both UCPCs were identified that allowed negatively charged NaCl and W particles, but not negative ions of exactly the same mobility size, to be efficiently detected. This serendipitous material dependence, which is not fundamentally understood, suggests that vapor condensation might sometimes allow for the discrimination between air “ions” and charged “particles.” As a detector in a scanning mobility particle spectrometer (SMPS), a UCPC with this strong material dependence would allow for more accurate measurements of sub-2 nm aerosol size distributions due to the reduced interference from neutralizer-generated ions and atmospheric ions, and provide increased sensitivity for the determination of nucleation rates and initial particle growth rates.

Copyright 2012 American Association for Aerosol Research     

Filtration of Multi-Walled Carbon Nanotube Aerosol by Fibrous Filters

Filtration efficiency of multi-walled carbon nanotube (MWCNT) aerosol by fibrous filter was evaluated experimentally. Mono-mobility test aerosols with electrical mobility diameter of 100, 200, and 300 nm were generated by the atomization of MWCNT aqueous suspension followed by mobility classification with a differential mobility analyzer (DMA). By analyzing the shape of classified aerosol particles under a scanning electron microscope, it was found that the DMA-classified 300 nm particles were fibrous in shape and had uniform diameter of about 60 nm and length of 2.1 micrometer. On the other hand, 100 nm and 200 nm particles contained a fairly large amount of multiply charged fibrous particles with a larger diameter. These test aerosols were challenged to a medium performance fibrous filter at various filtration velocities. As a result, fibrous particles were captured by fibrous filter at a higher collection efficiency than the spherical particles with the same mobility. By analyzing the single fiber capturing efficiency, interception incorporating the rotation of fibrous particles is found to be the dominant capturing mechanism for the fibrous particles in the studied size range.     

Inertial Separation of Ultrafine Particles Using a Condensational Growth/Virtual Impaction System

ABSTRACT

A system for the separation of ultrafine particles (i.e., particles smaller than 0.1 μm) has been developed and evaluated. Ultrafine particles are first grown by means of supersaturation to a size that can be easily separated in a virtual impactor. Thus, inertial separation of ultrafine particles occurs without subjecting them to a high vacuum. The condensational growth/virtual impaction system has been evaluated using monodisperse 0.05 and 0.1 μm fluorescent PSL particles, as well as polydisperse ultrafine ammonium sulfate and potassium nitrate aerosols. The generated aerosols were first drawn over a pool of warm water (50°C) where they became saturated. Subsequently, the saturated aerosol was drawn through a cooling tube (8°C) where particles grew due to supersaturation to sizes in the range 1.0–4.0 μm. By placing a virtual impactor with a theoretical 50% cutpoint of 1.4 μm downstream of the condenser, ultrafine particles were separated from the majority (i.e., 90%) of the surrounding gas. The sampling flow rate of the virtual impactor was 8 L/min and its minor-to-total flow ratio was 0.1. For these operating conditions, the particle collection efficiency of the virtual impactor averaged to about 0.9 for particle concentrations in the range 7 × 10⁴-5 × 10⁵ particles/cm³. Particle losses through the system were found less than 5%. Increasing the particle concentration to levels in the range 106–107 particles/cm³ resulted in a decrease in the collection efficiency of the virtual impactor to about 50–70%, presumably due to the smaller final droplet size to which the ultrafine particles grew for the available supersaturation.     

A High Volume Apparatus for the Condensational Growth of Ultrafine Particles for Inhalation Toxicological Studies

A high volume (2500 LPM) system for the condensational growth of ultrafine particles was developed and evaluated using indoor air as a test aerosol. The main features of this system are the following: (a) ultrafine particles grow condensationally to supermicron sizes using high purity deionized water as a condensing medium; (b) the supersaturation ratio is adjustable and can be precisely controlled; (c) the system can operate for a wide range of ambient air temperature and relative humidity conditions; and (d) a thermal dryer is used to return the condensationally grown particles back to their original size. Restoring the original ambient size distribution and preserving the composition of the ambient ultrafine particles is very important for inhalation studies. The system is fully automated and has computerized feedback controls. In addition, saturation of the aerosol with water vapor occurs at close to ambient temperatures to minimize particle losses of volatile components. Saturation of sample air is obtained using a direct steam-injecting, fully modulating electric humidifier. The sample air after saturation is drawn through the supersaturator, which is a refrigerant-to-air heat exchanger and is cooled down to obtain the desirable supersaturation ratio. Supersaturation ratios can be precisely adjusted, with the optimum operational level found to be in the range of 2 to 3. The performance of the system was evaluated as a function of critical operation parameters, including the supersaturation ratio as well as the saturation and supersaturation temperatures. A series of virtual and conventional impactors was used to characterize the condensational growth of ultrafine particles. This new high volume apparatus was shown to grow ambient ultrafine particles to supermicron sizes with a particle size growth of approximately 1.8 w m. Particle losses in the system were found to be minimal (about 10%). The thermal dryer was used successfully to restore the grown particles back to their original size distribution. Particle concentration, aerosol temperature, and residence time (aerosol flow) are key parameters shown to affect the performance of the thermal dryer was used successfully to restore the grown particles back to their original size distribution. Particle concentration, aerosol temperature, and residence time (aerosol flow) are key parameters shown to affect the performance of the thermal dryer.     

Measuring aerosol active surface area by direct ultraviolet photoionization and charge capture in continuous flow

Abstract

Direct ultraviolet photoionization electrically charges particles using a mechanism distinct from diffusion charging. The purpose of this study is to evaluate aerosol photoemission theory as a function of aerosol particle size, concentration, material, and morphology. Particles are classified using an aerodynamic aerosol classifier (AAC) and subsequently measured with a scanning mobility particle sizer (SMPS) and photoionization measurement system in parallel. This configuration allows direct comparison of photo-emission from high concentrations of initially neutral, monodisperse aerosols with different morphologies or materials. Under all examined conditions, the overall photoelectric yields of particles of self-similar material (silver and unconditioned soot) and morphology (sintered spheres and agglomerates) are each linearly proportional to the second moment of the mobility-equivalent diameter distribution, even in the transition regime (mobility diameter 30–200?nm), with agglomerate silver particles resulting in 5× higher photoelectric yield than unconditioned soot from a propane flame. It is shown for the first time that the photoelectric yield is significantly higher (2.6×) for fractal-like agglomerate silver particles than sintered, close-packed spherical particles of the same material and mobility-equivalent diameter, which is inferred to be due to the larger material surface area exposed externally to the particle surroundings. It is demonstrated that photoelectric measurements of aerosols reflect the photoelectrically active surface area which depends on the particle morphology and therefore the state of sintering.

Copyright © 2019 American Association for Aerosol Research     

Development of portable aerosol mobility spectrometer for personal and mobile aerosol measurement

We describe development of a portable aerosol mobility spectrometer (PAMS) for size distribution measurement of submicrometer aerosol. The spectrometer is designed for use in personal or mobile aerosol characterization studies and measures approximately 22.5×22.5×15 cm and weighs about 4.5 kg including the battery. PAMS uses electrical mobility technique to measure number-weighted particle size distribution of aerosol in the 10–855 nm range. Aerosol particles are electrically charged using a dual-corona bipolar corona charger, followed by classification in a cylindrical miniature differential mobility analyzer. A condensation particle counter is used to detect and count particles. The mobility classifier was operated at an aerosol flow rate of 0.05 L/min, and at two different user-selectable sheath flows of 0.2 L/min (for wider size range 15–855 nm) and 0.4 L/min (for higher size resolution over the size range of 10.6–436 nm). The instrument was operated in voltage stepping mode to retrieve the size distribution in approximately 1–2 min. Sizing accuracy and resolution were probed and found to be within the 25% limit of NIOSH criterion for direct-reading instruments. Comparison of size distribution measurements from PAMS and other commercial mobility spectrometers showed good agreement. The instrument offers unique measurement capability for on-person or mobile size distribution measurement of ultrafine and nanoparticle aerosol.     

Air sampling filtration media: Collection efficiency for respirable size-selective sampling

The collection efficiencies of commonly used membrane air sampling filters in the ultrafine particle size range were investigated. Mixed cellulose ester (MCE; 0.45, 0.8, 1.2, and 5 μm pore sizes), polycarbonate (0.4, 0.8, 2, and 5 μm pore sizes), polytetrafluoroethylene (PTFE; 0.45, 1, 2, and 5 μm pore sizes), polyvinyl chloride (PVC; 0.8 and 5 μm pore sizes), and silver membrane (0.45, 0.8, 1.2, and 5 μm pore sizes) filters were exposed to polydisperse sodium chloride (NaCl) particles in the size range of 10–400 nm. Test aerosols were nebulized and introduced into a calm air chamber through a diffusion dryer and aerosol neutralizer. The testing filters (37 mm diameter) were mounted in a conductive polypropylene filter-holder (cassette) within a metal testing tube. The experiments were conducted at flow rates between 1.7 and 11.2 l min^?1. The particle size distributions of NaCl challenge aerosol were measured upstream and downstream of the test filters by a scanning mobility particle sizer (SMPS). Three different filters of each type with at least three repetitions for each pore size were tested. In general, the collection efficiency varied with airflow, pore size, and sampling duration. In addition, both collection efficiency and pressure drop increased with decreased pore size and increased sampling flow rate, but they differed among filter types and manufacturer. The present study confirmed that the MCE, PTFE, and PVC filters have a relatively high collection efficiency for challenge particles much smaller than their nominal pore size and are considerably more efficient than polycarbonate and silver membrane filters, especially at larger nominal pore sizes.     

The formation and removal characteristics of aerosols in ammonia-based wet flue gas desulfurization

The characteristics of aerosol generation were studied experimentally in an ammonia-based wet flue gas desulfurization process. Particle size distributions and concentrations, morphologies and compositions before and after desulfurization were measured using an electrical low pressure impactor and scanning electron microscopy, respectively. The results show that aerosols can be generated between ammonia and sulfur dioxide resulting in gas-phase reaction and the aerosol concentration at the outlet of scrubber is significantly higher than at the inlet. Before desulfurization the particles are primarily silica-alumina minerals including O, Al, Si and C, while after ammonia-based desulfurization aerosol particles have smooth surfaces with regular structures, such as cubic and prismatic crystals, and contain principally O, S and N. Particle sizes up to 10 μm were measured, but the majority of the aerosol particles are in the submicron range. Separation of submicron particles from flue gas is difficult by conventional desulfurization scrubber; however high removal efficiency can be achieved based on the enlargement of the particles by heterogeneous condensation. The influence of the supersaturation degree and liquid–gas ratio on the removal efficiencies of the particles are demonstrated additionally. It indicates that aerosols generated in ammonia-based desulfurization process can be deliquesced and gradually enlarged in a high humidity flue gas, but the supersaturation degree higher than 1.2 is required for heterogeneous nucleation of water vapor with ash particles from coal combustion.     

Response of Particle Measuring Systems Airborne ASASP and PCASP to NaCl and Latex Particles

The response of a Particle Measuring Systems, Inc. (PMS) airborne Passive Cavity Aerosol Spectrometer Probe (PCASP)-100X and a PMS airborne Active Scattering Aerosol Spectrometer Probe (ASASP)-100X to nearly monodisperse aerosols of NaCl and polystyrene latex spheres in the size range 0.074–1.07 μm diameter (± 5%) are examined. Particles < 0.34 μm are size classified by electrical mobility using a Thermo System Inc. Electrostatic Classifier. The particles are also sized with the aid of a scanning electron microscope. Three equivalent diameters for the near cubic NaCl particles are considered: average of length and breadth, and cross section equivalent and volume equivalent diameters. For the linear diameter, the probes' response to latex and NaCl particles is quite similar. However, the use of cross section and volume equivalent diameter leads NaCl to be sized significantly less than the latex spheres consistent with reported differences between spherical (i.e., latex) and cubical geometry (i.e., NaCl). Using the linear diameter the lower detection limit of the ASASP-100X is found to be 0.175 μm ±5%, not 0.120 μm as given by the manufacturer. The lower detection limit of the PCASP-100X is found to be 0.125 μm ±5%, compared with the value of 0.10 μm suggested by PMS. In spite of this discrepancy the PCASP still represents a significant advancement for the airborne measurement of aerosol size distributions. Size distributions measured simultaneously with the two probes from an aircraft agreed more favorably with the new calibrations than with those of the manufacturer.     

Ultrafine Particle Sampling with the UNC Passive Aerosol Sampler

A model is presented to describe the collection of ultrafine particles by the UNC passive aerosol sampler. In this model, particle deposition velocity is calculated as a function of particle size, shape and other properties, as well as a function of sampler geometry. To validate the model, deposition velocities were measured for ultrafine particles between 15 and 90 nm in diameter. Passive aerosol samplers were placed in a 1 m ³ test chamber and exposed to an ultrafine aerosol of ammonium fluorescein. SEM images of particles collected by the samplers were taken at 125 kX magnification. Experimental values of deposition velocity were then determined using data from these images and from concurrent measurements of particle concentration and size distribution taken with an SMPS. Deposition velocities from the model and from the experiments were compared and found to agree well. These results suggest that the deposition velocity model presented here can be used to extend the use of the UNC passive aerosol sampler into the ultrafine particle size region.     

Nucleation of Ethylene Glycol Vapor and Growth of Sub-10-nm Particles in Nanoparticle Size Magnifier

We investigated the rates of heterogeneous and homogeneous nucleation of ethylene glycol vapor onto sub-10-nm particles in a newly developed condensation device called nanoparticle size magnifier (NanoPSM). The saturation ratio in the NanoPSM is precisely controlled by vapor-feeding system and mixing section, which are designed based on an earlier particle size magnifier (PSM) developed by Okuyama et al. (1984). Size-classified NaCl nanoparticles smaller than 10 nm in mobility diameter are used as heterogeneous nuclei for the condensation of ethylene glycol vapor. The activation efficiency and growth rate of the activated nuclei are determined by a pulse height analysis using an optical particle counter (OPC). A computer fluid dynamics (CFD) simulation is employed to calculate the profiles of the gas velocity, temperature, vapor concentration, and resulting supersaturation in the NanoPSM. Annular high-supersaturation region is generated around the mixing boundary between cold aerosol and hot vapor. The experimental activation efficiency is 50% for 4.5-nm and 0.8% for 2 nm NaCl particles, through the subsequent growth of droplets to 2 μm in diameter. The experimental data are in fairly good agreement with the predicted activation efficiencies based on the classical Kelvin-Thomson theory when the local profiles of supersaturation are taken into account.     

Physical and Chemical Characteristics and Volatility of PM in the Proximity of a Light-Duty Vehicle Freeway

Abstract

Volatility properties of ultrafine particles were analyzed next to State Route 110 (Pasadena freeway CA), a light-duty vehicle freeway where heavy-duty traffic is prohibited. In addition, mass concentration and chemical composition of particulate matter (PM) were measured in coarse, accumulation, and ultrafine modes. On weekdays from 17 May to 4 June 2004, measurements were performed in two locations, one very close to the freeway (within 2.5 m from the curb) and one at a distance of about 50 m from the freeway. For measurement of mass and chemical composition, the study employed in each location a micro-orifice uniform deposit impactor (MOUDI) and a modified high-volume sampler. Both instruments sampled with the same size cutpoints: a coarse mode from 2.5 to 10 μm, an accumulation mode from 0.18 to 2.5 μm, and an ultrafine mode of particles less than 0.18 μm in aerodynamic diameter. Alternately, a tandem differential mobility analyzer (TDMA) was used at the two sites. A heater between the two DMAs evaporated volatile material from the monodisperse aerosol, size selected by the first DMA. The second DMA analyzed the losses of volatile components. The ultrafine number concentrations next to the freeway were 46,000 cm^?3 on average during the sampling period. The MOUDI ultrafine mass concentration, nitrate, and EC were higher next to the freeway than at the background site farther from the freeway. The other components analyzed in the ultrafine mode had similar concentrations next to the freeway and at the background site. Volatility ranged from about 65% volume losses of 120 nm particles heated to 110°C to 95% of 20 nm particles. The 20 nm aerosol was only internally mixed, whereas increasing nonvolatile fractions were found for 40 nm (6% next to the freeway), 80 nm (20%), and 120 nm (28%) aerosols.     

Retrieving the ion mobility ratio and aerosol charge fractions for a neutralizer in real-world applications

Abstract

Electrical mobility size spectrometers (with a neutralizer, an electrical mobility classifier, and a detector as key components) are widely used to measure aerosol size distributions. The performance of a neutralizer is often evaluated separately from the spectrometer. In real-world applications of a neutralizer, i.e., typically with uncontrolled composition of the neutralizer carrier gas including trace constituents that can lead to variabilities in properties of positive and negative ions, charge fractions may differ from those predicted by widely used aerosol charging models with fixed ion properties and subsequently cause significant uncertainties in reported aerosol size distributions. In this study, we proposed an empirical method to retrieve the variations in neutralizer ion properties and aerosol charge fractions when measuring aerosol size distributions. Our approach requires measuring both positively and negatively charged particles using the electrical mobility size spectrometer to provide information on the performance of the neutralizer. Bipolar diffusion charging theories were applied to illustrate that aerosol charge fractions are governed by the mobility ratio of positive and negative ions. Positively and negatively charged particles measured by the spectrometer can be used to estimate the mobility ratio of positive and negative ions for the neutralizer. A modified Gunn and Woessner’s formula can then be used to calculate aerosol charge fractions from the retrieved ion mobility ratio. These charge fractions can be used for size distribution data inversion. Both simulated aerosols and experiments were used to evaluate the proposed method. We found that this new method can capture the variations in neutralizer ion properties and aerosol charge fractions under various conditions and help to achieve more accurate measurement of aerosol size distributions.

Copyright © 2018 American Association for Aerosol Research     

Influence of Sampling Artefacts on Measured PM,OC, and EC Levels in Carbonaceous Aerosols in an Urban Area

The registration efficiency of the TSI model 3025 ultra-fine condensation particle counter for Ag and NaCl particles of between 2 and 20 nm in diameter was determined. Taking into account the different shapes of the input aerosol size distributions entering the differential mobility analyzer (DMA) and the transfer function of the DMA, the counting efficiencies of condensation nucleus counters (CNC) for monodisperse Ag and NaCl particles were estimated. In addition, the dependence of the CNC registration efficiency on the particle concentration was investigated.     

Investigation of Surface Properties of Ultrafine Particles by Application of a Multistep Condensation Nucleus Counter

The fate of atmospheric ultrafine particles is determined by their size, chemical composition, and especially by their physical and chemical surface properties. To characterize the surface of ultrafine particles, their behavior as condensation nuclei can be used. Monodisperse ultra-fine particles with different surface structures were investigated by observing the onset of droplet formation at a fixed electrical mobility diameter. Droplet growth was detected by application of a multistep condensation nucleus counter (CNC). The particles were generated under well-controlled conditions and monodisperse fractions were obtained using an electrostatic classifier. For studying the influence of changes in the surface structure, ultrafine sulfuric acid droplets were coated with different organic materials. Different surface films required different supersaturations for droplet growth depending on the molecular structure and layer thickness of the material used for coating. Therefore it was concluded that certain compounds, enriched on the particle surface, affect condensation of water vapor in such a way that higher supersaturations are required in comparison to the particle core material. Additionally, it was observed that remarkably high supersaturations of water vapor were required for condensation on particles consisting of the following materials: metals, carbon, and Aerosil (spherical silica particles).     

Total Deposition of Ultrafine Hydrophobic and Hygroscopic Aerosols in the Human Respiratory System

The deposition of ultrafine particles in the human respiratory system was studied under a variety of steady breathing conditions. The monodisperse aerosols tested were hydrophobic kerosene heater, aluminosilicate, and hygroscopic NaCl in the size range of 0.03 to 0.4 μm in diameter. The results for all aerosols show that particle deposition increases with an increase in tidal volume, but with a decrease in breathing frequency. Also, deposition during breath holding increases nearly exponentially with an increase in time. However, particle deposition as a function of particle size is different between hydrophobic and hygroscopic aerosols in the size range tested. The hydrophobic aerosols increase with a decrease in particle size, while hygroscopic aerosols show minimum value in the size range of 0.06 to 0.09 μm. The hydrophobic kerosene heater and aluminosilicate particle deposition confirms the recent theoretical calculations of Yeh and Schum (1980), Yu and Diu (1982), and Yu and Hu (1983). The NaCl particles show a 3.5 to 4.5 fold growth under conditions approximating the respiratory tract.     

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.