Multiple charging of ultrafine particles in a corona charger

The charge distribution on ultrafine aerosol particles in the size range below 35 nm has been measured using a corona-based unipolar charger, in which ion generation and aerosol charging take place simultaneously in the region around a sharp-point discharge electrode. The mean number of charges per particle predicted by Fuchs’ diffusion charging theory is in relatively good agreement with the experimental results, and this implies that diffusion charging is the predominant mechanism in spite that the electric field in the charging region is very high. Since a steady state is unattainable in unipolar charging, the charge distribution depends on the geometry and operating conditions of each particular charger. When the present device operates under the conditions (nt-product) which yield the maximum charging efficiency, double charge appears on particles with diameter as small as 15 nm. At larger values of nt, 32 nm particles can acquire up to five charges. The critical particle diameter above which multiple charging occurs is about four times smaller than for bipolar radioactive chargers. In order to use corona charging in aerosol particle size measurement by electrical methods, the required mobility data inversion is thus straightforward for particle diameter below about 15 nm, but becomes quite complex for larger sizes.     

Towards traceable particle number concentration standard: Single charged aerosol reference (SCAR)

A concept of realizing a standard for aerosol particle number concentration was tested, based on generating singly charged aerosol particles in the size range from 10 up to 500 nm. To this end, a device named single-charged aerosol reference (SCAR) was designed, built, and tested. The device is based on electrical charging of nanoparticles and subsequent growth of the particles. With an accurate measurement of volume flow and electrical current from the singly charged particles, the number concentration can be accurately, and in the end, traceably determined. Laboratory tests have shown that the device can be used to generate a narrow (GSD<1.3) particle size distribution of singly charged particles. The device can be used for traceable calibration of instruments measuring the number concentration of the particles.     

Aerosol synthesis of silicon nanoparticles with narrow size distribution—Part 1: Experimental investigations

A study on the feasibility of aerosol processing of nearly monodisperse silicon nanoparticles via pyrolysis of monosilane in a hot wall reactor is presented. For optimal conditions silicon nanoparticles with a geometric standard deviation of 1.06 were synthesized at a production rate of 0.7 g/h. The size of the particles could be precisely controlled in the range of 20–40 nm, whilst maintaining a geometric standard deviation in the range of 1.06–1.08, by proper choice of the governing parameters temperature, residence time and precursor concentration. The results show that narrow particle size distributions can only be obtained in the temperature range between 900 and 1100 °C, as long as both the initial silane concentration (1 mbar silane partial pressure) and the reactor total pressure are low (25 mbar). This regime for the production of narrow particle size distributions has not been identified in prior work on the thermal decomposition of silane. Narrowly distributed particles can be obtained under conditions where nucleation and particle growth are separated and the agglomeration rates are negligible.     

COUNTING EFFICIENCY OF THE API AEROSIZER

The Aerosizer (Amherst Process Instruments, Inc. Hadley MA) is a time-of-flight instrument frequently used to measure the size distribution of an aerosol. However, if the Aerosizer’s counting efficiency, defined as the number of particles counted divided by the total number entering the instrument, is not 100% or varies with particle size, the resulting size distribution will be inaccurate.Experiments were conducted to determine the effect of particle diameter, particle concentration, photomultiplier tube (PMT) voltage, and model type on the Aerosizer’s counting efficiency. To calculate counting efficiency, the number of particles between 0.3 and 10 μm recorded by the Aerosizer was divided by the number of particles of the same size collected on each stage of a cascade impactor.Particle diameter, aerosol concentration, Aerosizer model, PMT voltage, and the diameter interaction terms influenced counting efficiency. Counting efficiencies were less than 1% for particles smaller than 0.45 μm, and more than 100% for particles larger than 7 μm. Increasing the PMT voltage increased the counting efficiency for the smaller particles, but also created false, larger particles. Counting efficiency decreased as count rate increased for count rates greater than 20,000 particles per second. The Aerosizer LD counted particles more efficiently than the Aerosizer Mach 2 because of improved laser and optics systems. Four regression models that relate counting efficiency to the salient operating parameters were developed, one for each combination of Aerosizer model and photomultiplier tube voltage studied.     

Characterizing the performance of two optical particle counters (Grimm OPC1.108 and OPC1.109) under urban aerosol conditions

The performance of Grimm optical particle counters (OPC, models 1.108 and 1.109) was characterized under urban aerosol conditions. Number concentrations were well correlated. The different lower cut-off diameters (0.25 and 0.3 μm) give an average difference of 23.5%. Both detect less than 10% of the total particle concentration (0.01–1 μm; Differential Mobility Analyzer), but in the respective size ranges, differences are <10%. OPC number size distributions were converted to mass concentrations using instrument-specific factors given by the manufacturer. Mass concentrations for OPC1.108 were 60% higher than for OPC1.109 and (in case of OPC1.109) much lower than those measured with an impactor in the relevant size range or a TSP filter. Using the C-factor correction suggested by the manufacturer, OPC1.109 underestimated mass concentrations by 21% (impactor) and by about 36% (TSP filter), which is in the range of comparability of co-located different mass concentration methods (Hitzenberger, Berner, Maenhaut, Cafmeyer, Schwarz, &; Mueller et al., 2004).     

COAGULATION OF CIGARETTE SMOKE PARTICLES

Experimental measurements on the deposition of cigarette smoke particles (CSP) in the human airways have produced results that are inconsistent with typical deposition data based on particle size. Previous work relating to hygroscopic growth indicates that hygroscopicity alone can not account for this discrepancy. The present study investigates coagulation of CSP modeled as a polydisperse-charged aerosol as a possible explanation. The results of the model more accurately predict the experimental coagulation data for mainstream CSP than models that treat CSP as a monodisperse or polydisperse-uncharged aerosol. An aerosol with an initial charge distribution based on Boltzmann equilibrium yields slightly larger coagulation rates than the mainstream CSP polydisperse-charged model. The numerical results indicate that the size and charge distribution of sidestream CSP, with a concentration of 10⁶ particles cm^-3, remain stable. In 2 s, the size distribution of mainstream CSP, with a concentration of 10⁹ particles cm^-3, shifts to a larger size while becoming flatter and wider. The diameter of average mass increases from 0.29 to 0.5 μm. Numerical results confirm experimental reports for mainstream CSP, which indicate that the total number of charged particles increases with time and, in the early stages of coagulation, the amount of charge per particle cannot be estimated based on the particle size. This study shows that polydisperse-charged CSP, allowed to coagulate for 2 s in the mouth, will not produce size distributions that yield the observed deposition of CSP. However, additional coagulation will take place as the CSP travels through the respiratory tract, which will be investigated in future work.     

New bio-aerosol collector using a micromachined virtual impactor

For collection and concentration of bioaerosols, we designed and evaluated a single stage virtual impactor, which was fabricated by micro-electro-mechanical systems (MEMS) process. The cut-off diameter of 1 μm was selected, since 1 μm is the lowest size as used in the US Government Joint Biological Point Detection System [Haglund, J. S., & McFarland, A. R. (2004). A circumferential slot virtual impactor. Aerosol Science and Technology, 38, 664–674; Moshier, T., & Buonaugurio, T. (2000). Joint Biological Point Detection System (JBPDS) requirements and design interplay. Proceedings of the First Joint Conference on Point Detection for Chemical and Biological Defense, October 23–27, 2000, Williamsburg, VA.] The design value of a 1 μm cut-off diameter required a nozzle width and thickness of 880 and 200 μm, respectively. The virtual impactor was evaluated for physical and biological collection efficiencies. For the performance evaluation of physical collection efficiency and wall loss, polystyrene latex (PSL) particles were generated from an atomizer and their size distribution was measured using an aerodynamic particle sizer (APS, TSI model 3321) and a scanning mobility particle sizer (SMPS, TSI model 3936). The measured cut-off diameter was 0.95 μm, which agreed with the calculated results (=0.94 μm) determined with a commercial computational fluid dynamics (CFD) package, FLUENT, and the measured wall loss was below 33.5%. For the performance evaluation of biological collection efficiency, Staphylococcus epidermidis bioaerosols were dispersed into air by a nebulizer. The bioaerosols were measured using APS and sampled with a bioaerosol sampler. The overall physical collection efficiency based on the number concentration was 73.8±3%, which was similar to the one based on the number of colonies (=76.7±7%). We found that most of the bioaerosols collected and concentrated by our virtual impactor were viable.     

Estimating aerosol particle charging parameters using a Bayesian inversion technique

A Bayesian inversion routine is described in which data from tandem differential mobility analysis (TDMA) can be used to determine fundamental parameters for charging models as a function of particle size. The measurement and inversion techniques were verified using simulated data for particles in the 50–500 nm range undergoing unipolar diffusion charging as described by Fuchs’ limiting sphere theory, for which the fundamental charging parameter is the product of the unipolar ion concentration and residence time within the charger, the so-called nt product. Under conditions where the average particle charge is greater than one unit charge, the inversion routine is precisely and accurately able to determine the nt value. The inversion routine breaks down, however, when the average number of charges per particle is well below unity, i.e. for low nt values or for small particle sizes. Incorporation of charging efficiency data in addition to TDMA data can allow for use of the inversion routine when the average number of charges per particle is low. With its limitations known, the inversion routine was applied to determine the nt value for the unipolar charger used in the TSI electrical aerosol detector (EAD), model 3070A, for particles in the 50–200 nm size range. The effective nt value within the EAD charger decreased with decreasing particle size, implying that charged particle losses occur within the EAD charger. The described inversion routine is unique in its ability to determine size dependent charge model parameters, and can be utilized with any given charging model. It is expected that this technique will be of use in advancing the understanding of aerosol particle charging models and in future design of unipolar chargers.     

Development and experimental evaluation of aerodynamic lens as an aerosol inlet of single particle mass spectrometry

This study described a development and an experimental evaluation of an efficient aerodynamic lens inlet of the single particle mass spectrometry. Several key designing parameters and systematic factors were investigated for the whole lens system through a full numerical simulation. From many tests for various designs of the system, we showed that Mach number was not an independent parameter but interrelated well with flow Reynolds numbers and pressures upstream of the orifices. By manipulating the parameters, we showed for the first time a possibility that there exist a universal correlation between optimal Stokes number and a new factor incorporating the other dimensionless variables and a design parameter. The universality was confirmed by the full simulation results. We demonstrated that the new design of the system was capable of focusing ultrafine aerosols in the size range of 30–700 nm. At two different operating conditions, the formations of sub-millimeter beams of 30–300 nm NaCl aerosols are verified by light scattering imaging as well as microprobe observation of deposited aerosol beams. Finally, the measured sizes of aerosol beams agree reasonably well with those from the simulations as a function of particle size.     

A miniature disk electrostatic aerosol classifier (mini-disk EAC) for personal nanoparticle sizers

We have developed a miniature disk electrostatic aerosol classifier (mini-disk EAC) for use in electrical mobility-based personal nanoparticle instrumentation for measurement of personal exposures to nanoaerosols. The prototype consists of two parallel disk electrodes separated by an electrically insulating spacer, to create the particle classification zone. The aerosol enters and exits the classification zone along the bottom disk electrode. An additional, particle-free sheath flow is used to improve the measurement resolution. The transmission measurement of the mini-disk EAC for DMA-classified particles shows that particle losses due to diffusion and electrical image forces were low. The particle penetration at 10 nm diameter (the designed lower size limit for the classifier) was 67% when the prototype was operated at the aerosol and sheath flow rates of 0.5 and 1.0 l min^?1, respectively. The performance of the mini-disk EAC was experimentally characterized using the particle cutoff curves that describe their penetration through the classifier as a function of applied voltage across the two disk electrodes. Based on the measurement of particle penetration at different aerosol and sheath flows, it was found that the aerosol and sheath flow rates of 0.5 and 1.5 l min^?1 were optimal for classifier operation. Finally, a semi-empirical model was also developed to describe the transfer function of the mini-disk EAC for non-diffusive particles.     

An annular diffusion channel equipped with a track detector film for long-term measurements of activity concentration and size distribution of nanometer 218Po particles

A new method is presented to estimate mean activity concentration and mean size distribution of nanometer ²¹⁸Po over long time, i.e. several weeks in indoor atmospheres. It uses an annular channel, equipped with an LR 115 solid-state alpha-track detector, as diffusion sampler. Design, experimental characterisation and analytical methods are described. The calibration was carried out in a 0.35 m³ test radon chamber without aerosol particles by comparing the number of tracks registered on the film detector with the nanometer ²¹⁸Po particle concentrations measured in parallel with a classical single screen-filter method. A calibration factor of 0.107 tracks cm^-2 h^-1 was established. The size distribution of these nanometer radioactive particles in the range of 0.5–5 nm was reconstructed from the track density recorded on the detector film with non-linear inversion techniques.     

Duration of tangent-linear regime in sectional multi-component aerosol dynamics

In this article, a tangent-linear multi-component aerosol dynamical box model was studied. In particular, the length of time over which the tangent-linear dynamics dominate the total evolution of a particle number concentration perturbation is determined. In the experiments, three non-linear model initial number concentration distributions, perturbation distributions, and ambient vapour concentrations are used. Results indicate, based on analysis of the evolution of number distribution perturbation, that in pure nucleation events the tangent-linear regime persists for about 1.5 h, and can last for several hours in cases of weak nucleation. Analysis of the perturbation evolution with emphasis on particle surface area or volume indicates that the dynamics is tangent-linear over a much longer period. In conclusion, high ambient sulphuric acid vapour concentration maintains a large number concentration of small particles (below about 10 nm) through nucleation. These particles are the prime source of non-linearity limiting the length of the tangent-linear regime in a box model context. The results have relevance, but are, however, not directly applicable, for instance, in three-dimensional chemical transport and air-quality modelling aiming to incorporate data assimilation techniques, such as four-dimensional variational data assimilation.     

Performance evaluation of long differential mobility analyzer (LDMA) in measurements of nanoparticles

Performance of a long differential mobility analyzer (LDMA) in measurements of nanoparticles was evaluated experimentally and numerically. In the evaluation of the LDMA measurements, silver particles in a size range of 5–30 nm were used under an increased flow rate. The numerical calculation method was used for calculating the particle trajectory in the LDMA, and the results were used for a comparison with Stolzenburg's transfer function. Using the CFD method, the flow around the aerosol inlet slit was analyzed, and the resulting particle mobility distribution was compared with that for an ideal flow. The resulting flow effect on the penetration efficiency caused by the inlet and exit slits were negligible when a well-designed system was used. The experimental measurements of mobility distributions were in good agreement with the theoretical prediction of particle size ranges over 10 nm, but some discrepancies were observed when particle size ranges were below 10 nm in size. The numerical calculation estimated the discrepancy found below the 10 nm particle size ranges.     

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.     

Charge distributions of aerosol dioctyl sebacate particles charged in a dielectric barrier discharger

The collection efficiencies of submicron aerosol particles using a two-stage, dielectric barrier discharge (DBD) type electrostatic precipitator have been reported previously [Byeon et al. (2006). Collection of submicron particles by an electrostatic precipitator using a dielectric barrier discharge. Journal of Aerosol Science, 37, 1618–1628]. In this paper, the charge distributions of aerosol dioctyl sebacate (DOS) particles, which had a mobility equivalent diameter of 118, 175, and 241 nm and were charged in a DBD charger, were examined using a tandem differential mobility analyzer (TDMA) system at applied voltages of 9–11 kV and frequencies of 60–120 Hz. The mean number of elementary charges for positively or negatively charged particles increased slightly with increasing applied voltage or frequency. However, the number of elementary charges increased significantly with increasing particle size. At any applied voltage and frequency, the charge distributions of these particles of these sizes indicated asymmetric bipolar charging. The positive-to-negative charge ratios were 10.4, 4.7, and 3.0 for particle sizes of 118, 175, and 241 nm, respectively, at a DBD voltage and frequency was 9 kV and 60 Hz, respectively. Fluorometric analysis showed that average positive-to-negative charge ratios were 11.5, 4.9, and 3.7 for particle sizes of 118, 175, and 241 nm, which agrees well with the TDMA results. Further fluorometric analyses with larger particles (514 and 710 nm) and higher frequencies (1 and 2 kHz) showed that the positive-to-negative charge ratio reached almost unity with increasing particle size or frequency.     

Design and performance test of a multi-channel diffusion charger for real-time measurements of submicron aerosol particles having a unimodal log-normal size distribution

It is important to develop a simple and fast method for measuring the sizes of submicron particles in both laboratories and fields. In our previous studies, Park, An, and Hwang [(2007). Development and performance test of a unipolar diffusion charger for real-time measurements of submicron aerosol particles having a log-normal size distribution. Journal of Aerosol Science, 38, 420–430] and Park, Kim, An, and Hwang [(2007). Real-time measurement of submicron aerosol particles having a log-normal size distribution by simultaneously using unipolar diffusion charger and unipolar field charger. Journal of Aerosol Science, 38, 1240–245], we introduced methodologies that our lab made unipolar charger could lead to detection times of under 5 s in conjunction with an electrometer and a condensation particle counter (CPC), and under 3 s with two electrometers.However, both methodologies require an appropriate assumption of the geometric standard deviation of particle sizes. In this paper, we introduce a methodology for determining the geometric standard deviation of particle sizes as well as the geometric mean diameter and the total number concentration of particles. For this purpose, a diffusion charger that consisted of discharge zone, mixing and charging zone, and three flow channels for obtaining three different residence times and average charges of particles in the channels, was designed and tested. For determining the average particle charge, various methods including theoretical calculations and the tandem differential mobility analyzer (TDMA) method were used. The results obtained from the different methods agreed well with each other. To compare the size distribution with the data that were measured through a scanning mobility particle sizer (SMPS), sodium chloride (NaCl) particles were used. The estimated results by using a data inversion algorithm were less than those measured by SMPS by around 22% for the total number concentration and 10% for both the geometric mean diameter and the geometric standard deviation. Furthermore, the detection time was under 3 s.     

A fast integrated mobility spectrometer with wide dynamic size range: Theoretical analysis and numerical simulation

A fast integrated mobility spectrometer with wide size range (WSR-FIMS) is described. The WSR-FIMS greatly enhances the dynamic size range of the original FIMS [Kulkarni, P., & Wang, J. (2006a). New fast integrated mobility spectrometer for real-time measurement of aerosol size distribution—I: Concept and theory. Journal of Aerosol Science, 37, 1303—1325; Kulkarni, P., & Wang, J. (2006b). New fast integrated mobility spectrometer for real-time measurement of aerosol size distribution—II: Design, calibration, and performance characterization. Journal of Aerosol Science, 37, 1326—1339] by employing a non-uniform electric field. The strength of this electric field varies over three orders of magnitude along the width of the separator, allowing particles of a much wider size range to be classified and measured simultaneously. A theoretical framework is developed to derive the transfer function, resolution, and transmission efficiency of the WSR-FIMS. Two representative operation configurations are simulated, and the results show the WSR-FIMS can simultaneously measure particles ranging from 10 to 1470 nm, therefore greatly reducing the measurement time from minutes required by scanning mobility particle sizer (SMPS) to 1 s or less. The WSR-FIMS also has a higher size resolution than typical SMPS over most of its measurement size range. For typical ambient aerosols, the simulations show that 1 s measurements using the WSR-FIMS provide good counting statistics.     

Filtration and inactivation of aerosolized bacteriophage MS2 by a CNT air filter fabricated using electro-aerodynamic deposition

Carbon nanotubes (CNTs) were coated on a sample of glass fiber air filter medium at atmospheric pressure and room temperature using electro-aerodynamic deposition (EAD). In the EAD method, CNTs (diameter: 50 nm, length: 2–3 μm) were aerosolized, electrically charged, and injected through a nozzle. A voltage was applied externally between the ground nozzle and a planar electrode on which the sample was located. The charged CNTs were deposited on the sample in a vertically standing posture even at a low flow velocity. Before the deposition experiment, a calculation was performed to determine the applied voltage by simulating the electric field, flow field, and particle trajectory. Using CNT-coated filter samples, virus aerosol filtration and anti-viral tests were carried out using the aerosol number counting method and the plaque counting method, respectively. For this purpose, bacteriophage MS2 was aerosolized with an atomizer. The particle filtration efficiency was increased to 33.3% in the most penetration particle size zone (100 nm) and the antiviral efficiency of the CNT filter was 92% when the coating areal density was 1.5 × 10⁹ #/cm². The susceptibility constant of virus to CNTs was 0.2 cm²/μg.     

Charge neutralization of submicron aerosols using surface-discharge microplasma

In this study we investigated the charging characteristics of a novel aerosol neutralizer (Surface-discharge Microplasma Aerosol Charger; SMAC) based on the dielectric barrier discharging. The surface discharge was induced by supplying positive and negative DC pulses with a pair of micro-structured electrodes. We confirmed the occurrence of the surface discharge by measuring the microdischarge current, and evaluated the charging performance of the SMAC as a particle neutralizer by measuring the penetration efficiency, neutralizing probability, and charge distribution for particles in the size range of 10–200 nm. The SMAC was found to obtain a particle penetration exceeding 90% for the whole particle size range. The neutral fraction obtained by the SMAC showed good agreement with a bipolar diffusion charging theory and the fraction obtained by an ²⁴¹Am radioactive source when the SMAC was optimized for aerosol neutralization with the offset voltage control. The charge distributions of negatively and positively charged particles by the SMAC and the ²⁴¹Am neutralizer were in good agreement also. The charge balance of positive and negative particles obtained by the SMAC was effectively controlled by adjusting the offset voltage on each electrode. This is the first study to demonstrate the successful use of dielectric barrier surface discharge to bring particles of 10–200 nm to an equilibrium charging state in a controllable manner.     

Influence of particle size distributions on yield stress and viscosity of cement–fly ash pastes

The rheological properties of blended cement-based materials depend strongly on mixture proportions and the characteristics of the components. In this study, design of experiments is used to investigate the influence of three variables (cement particle size distribution (PSD), fly ash PSD, and ratio of fly ash to cement) at each of four levels on the yield stress and viscosity of blended pastes. Both rheological parameters are seen to vary over several orders of magnitude for the evaluated design space. Physical characteristics of the powders, such as cement and total particle densities and total particle surface area, are computed for each mixture. A percolation-type relationship is observed between yield stress and cement particle (number) density. While neither apparent nor plastic viscosities were particularly well described by the commonly employed Kreiger–Dougherty equation, plastic viscosities were found to be linear functions of either total (cement + fly ash) particle surface area or total particle density.