Charging of Ultra-Fine Aerosol Particles by an Ozone-Free Indirect UV Photo-Charger

Particle charging by indirect photoemission may be an alternative to charging methods based on corona discharge or on bipolar charging by radioactive ion sources. An indirect charger using a low energy UV radiation is introduced and characterized in detail. Depending on the carrier gas, photoelectrons or ions formed by electron-attachment charge the particles by diffusion charging. The achieved charging efficiency is in the range of 20–70% for particle sizes of 20 to 100 nm. It can easily be modulated by a small voltage applied to the photoemitter. To achieve stable electron emission, glassy carbon with its very inert surface is used as photoemitter. Particle losses are very small. The charge distribution has been measured by a tandem DMA setup. The experimental results are compared with the theory of unipolar diffusion charging based on the Fuchs’ combination probability of ions with the particles. The charger characterization has been performed by carbon particles in a size range of 20–100 nm, produced by a spark discharge generator.

Copyright 2013 American Association for Aerosol Research     

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.     

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.     

Bipolar diffusion charging characteristics of single-wall carbon nanotube aerosol particles

Bipolar diffusion charging characteristics of airborne single-wall carbon nanotube (SWCNT) agglomerates were investigated in the mobility diameter range of 100–1000 nm. Neutral fractions of three types of SWCNT aerosols following bipolar charge equilibrium in a radioactive source were experimentally measured to infer their electrical charging characteristics. Significant deviation from Boltzmann and Fuchs stationary charge equilibrium was observed, with neutral fractions of SWCNT particles lower by 30–53% compared to that of spherical particles of the same mobility. Particles with mobility diameter larger than 400 nm showed high electrical charging efficiencies compared to that of mobility-equivalent spherical particles. Higher charging efficiencies of SWCNT particles were attributed to their higher electrical capacitance resulting from complex nonspherical morphologies. Numerical calculations using idealized fiber geometries confirmed the qualitative trend in the experimental data. The electrical capacitance of nanotubes particles deduced from experimentally measured neutral fractions were also found to be higher by a factor ranging from 1.6 to 4.6 compared to that of mobility-equivalent spherical particles, indicating high charge carrying capacity. The charging-equivalent diameters of nanotube particles were computed and were found to be higher than their mobility diameter by a factor of 2.85–4.34.     

A novel bipolar charger for submicron aerosol particles using carbon fiber ionizers

A simple and novel bipolar charging device using carbon fiber ionizers was developed to neutralize submicron aerosol particles without the generation of ozone. The ion currents of the positive and negative ions generated by carbon fiber ionizers were so chosen as to optimize particle neutralization. The particle penetration, charging probability and charge distribution resulting from the charger were investigated and compared to those from a Kr-85 radioactive neutralizer for the particles in the size range of 20–120 nm. Size distributions for various laboratory-generated aerosols (sodium chloride, ammonium nitrate, ammonium sulfate and glutaric acid) neutralized by the charger were also investigated and compared to those obtained without neutralization. Particle penetration in the charger was over 90% for particles larger than 20 nm. Charging probability and charge distribution for the charger were in good agreement with those from Kr-85 neutralizer and with theoretical estimations. Size distributions observed for the charger and Kr-85 neutralizer were also in good agreement for particles of different concentrations and various chemical compositions. The newly developed bipolar carbon fiber charger can neutralize submicron particles, at least as effectively as currently available radioactive neutralizers and with negligible ozone generation which is its major advantage.     

Monte Carlo simulation of pneumatic tribocharging in two-phase flow for high-inertia particles

Pneumatic transport, triboelectrostatic experimentation studying the removal of carbon from fly ash has shown the importance of particle charging on process performance. To further elucidate the influence of particle charging, a tribo-electrification model for charging spherical particles in vertical pipes is proposed. Unlike previous numerical approaches, the charging mechanism is studied in the framework of a charge relaxation process in which the equilibrium charge for uniform and localized charge distributions on the particle surfaces are determined. A recently proposed statistical model was also adopted to evaluate particle-wall collisions for wall-bounded flows, taking into account the influence of particle-particle collisions. Effects of particle concentrations, along with particle size and turbulence intensity, are also investigated. Model predictions are compared with experimental results, with good agreement found if the charge is assumed to be distributed on small surface sector of the particles. After defining suitable probability density functions for the variables considered, a Monte Carlo analysis is employed for simulating particle charge distributions and compared with published measurements to identify the important parameters during particle tribocharging.     

Challenges of measuring nascent soot in flames as evidenced by high-resolution differential mobility analysis

Nascent soot particles with mobility diameters ≤10 nm were measured in an ethylene/air premixed flame to shed light on the challenges and potential artifacts affecting studies on soot inception by differential mobility analysis (DMA) techniques. The size distribution functions (SDFs) of particles with charge acquired either naturally or diffusively upon ion seeding were measured at several positions in the flame using rapid-dilution probing and a high-resolution DMA for different values of the ratio of dilution ratio to residence time (DR/Δt). The SDFs are roughly bimodal with a sub-3 nm mode and a larger one that appears either downstream in the flame or for low DR/Δts. Soot nuclei smaller than 3 nm preferentially acquire positive charge, which brings into question the assumption of steady-state charging probability of flame sampled soot nuclei in the bipolar diffusion neutralizer. The approximately polarity-symmetric lognormal SDF of larger particles is attributed to nuclei coagulation. Naturally charged particles increase in number when lowering DR/Δt, suggesting either their collisional charging by flame chemi-ions or particle nucleation by condensation of neutral molecules on ions or both. The critical conditions for suppressing particle coagulation and charge redistribution in the sampling system were not achieved under most conditions, despite the fact that values of DR/Δts were more favorable to such a suppression in the present experiment as compared to other studies in the literature. As a result, the identification of this “asymptotic” regime, which is critical to determine the parent SDFs and the charge state of nascent soot in the flame, is still elusive.

© 2016 American Association for Aerosol Research     

Unipolar Charging of Nanosized Aerosol Particles Using Soft X-ray Photoionization

A unipolar charging device based on a soft X-ray (<9.5 keV) photoionization was developed to investigate the charging efficiency of aerosol nanoparticles. Unipolar charging using a 241 Am charger was also evaluated as a comparison with the characteristics obtained by X-ray charging. The production rate and the concentration of ions generated by the X-ray and 241 Am unipolar chargers were estimated from ion current measurements. Theoretical calculations by the unipolar diffusion charging theory were also carried out and the calculated data were compared with the experimental results. For acquiring a high number of standard nanoparticles, the classification of monodisperse nanoparticles from polydisperse aerosol particles using the X-ray unipolar charger and a differential mobility analyzer (DMA) was also evaluated. The ion production rate of the X-ray unipolar charger was at least 5.5 times higher than that of the 241 Am unipolar charger and the ion concentration was about three times higher. Therefore, the X-ray unipolar charger showed a higher capability for charging aerosol particles of 10-40 nm size in diameter than the 241 Am charger. The charging state of particles produced by the X-ray unipolar charger was in good agreement with theoretical calculations. The X-ray unipolar charger developed herein has potential for use in charging a high number concentration of nanoparticles for use in nanotechnology investigations.     

Ozone-free post-DBD aerosol bipolar diffusion charger: Evaluation as neutralizer for SMPS size distribution measurements

A postplasma neutralizer for submicron particles size measurements by mobility analysis has been evaluated. Bipolar ion currents have been measured downstream a dielectric barrier discharge (DBD) to estimate the ion fluxes at the inlet of charging volume and the n_i·τ product that define the theoretical maximal concentration that can be neutralized. Charge distributions were measured versus DBD voltage, aerosol diameter and concentration for monodisperse aerosols. It is confirmed that the charge distribution of particles depends on the ratio of initial positive and negative ion currents controlled by the DBD voltage leading to a tuneable mean charge of aerosol in this post-DBD bipolar charger. As expected from Gunn's law, the mean charge and the variance are proportional to particle diameter above 50 nm and independent of the aerosol concentration. The size distributions measured with ⁸⁵Kr and post-DBD neutralizer present the same modal diameters and a maximal overestimation of the total concentration of 10%, for aerosol from 15 to 730 nm with concentrations up to 6 × 10¹² m^?3. This post-DBD bipolar charger can be used for submicron aerosol neutralization and thus for scanning mobility particle sizer size distribution measurements in air as well as in nitrogen to suppress ozone downstream DBD.

Copyright © 2017 American Association for Aerosol Research     

The Bipolar Diffusion Charging of Nanoparticles: A Review and Development of Approaches for Non-Spherical Particles

Theoretical and experimental analyses of the steady state, bipolar diffusion charge distribution on nanoparticles are reviewed. This charge distribution plays a critical role in electrical mobility measurements of nanoparticle size distribution functions, where it is approximated via empirical regression equations. While the regression approach has been broadly successful, there remain several unresolved issues related to charge distribution calculations. Specifically, research to date has not revealed a method to reliably calculate nanoparticle-ion collision rates in the presence of strong attractive potentials, charge distribution predictions do not routinely consider the mass and (electrical) mobility distributions of the charging ions, and calculation approaches applicable to both spherical and nonspherical particles have not been compared to experimental data. In light of these issues, we examine the steady-state bipolar charge distribution on gold nanospheres and gold nanorods via tandem differential mobility analysis (TDMA). We compare measurements to regression equations as well as to Brownian Dynamics (BD) simulations, which take ion mobility and mass distributions as inputs. These distributions were measured using a DMA coupled to a mass spectrometer. Both regression equations and BD simulations are found to agree reasonably well with measurements in air, and we find that particle mobility diameter has a much greater influence on charging than particle morphology. Results support the use of BD calculations to predict bipolar charge distributions when ion properties are known. Nevertheless, our work supports continued use of regression equations when such information is not available.

Copyright © 2015 American Association for Aerosol Research     

The Effect of Particle Pre-Existing Charge on Unipolar Charging and Its Implication on Electrical Aerosol Measurements

We investigated the effect of particle pre-existing charges on unipolar charging. Particles carrying a defined number and polarity of pre-existing charges were used to study the unipolar charging process in a unipolar diffusion charger with positive ions. It was found that the particles initially carrying negative charges have almost the same amount of positive charges as the initially uncharged particles after passing the test charger; and the particles initially carrying more positive charges have more final charges. An analytical solution of a model for particle charge distribution of initially charged particles was provided for unipolar charging based on Fuchs' theory and the birth-and-death theory. The N _ion t value used in this model was obtained by fitting the experimental data of average charge on particles for initially uncharged particles. The results from the analytical solution show very good agreements with experimental data regarding the relationship between the pre-existing charge and the final charge on particles (50–200 nm in this study). Experimental tests of the response of Nanoparticle Surface Area Monitor (NSAM) against initially charged particles demonstrated that NSAM could have a large response deviation (more than 20% in the tested charge level) depending on the particle size and the amount of pre-existing positive charges on particles. Modeling of NSAM response showed similar deviation and predicted that when pre-existing charge is high enough, the NSAM response can be as large as 5 and 9 times of the uncharged particle response for alveolar and tracheobronchial surface area concentration, respectively.     

Electrical mobility and size distribution of aged Pb nanometer carriers in nitrogen gas

Radon and thoron progeny are positively charged clusters of radioactive atoms and other molecules and are neutralized through several mechanisms when they are aged. The charge status of radon clusters has been shown to be a major factor influencing their size distribution. In the present work, we attempted to determine simultaneously the activity size and charge distributions of thoron progeny using an electrical mobility spectrometer and a graded diffusion battery. These measurements allow us to study the dynamics of the charge neutralization process. Our data showed that ²¹²Pb generated in dry N₂ atmospheres (RH < 3.4%) had a bimodal distribution. The small cluster mode with a size range of 0.5–2.5 nm was comprised of mainly neutral progeny, whereas a significant portion of the large nucleation mode (2.5–10 nm) consisted of charged progeny (36–72% based on activity) depending on the aging time. The relative humidity (RH) had a major effect on the charge neutralization process. As the RH increased from 3.4 to 17%, the proportion of charged progeny decreased steadily indicating charge neutralization. At the same time, the fraction of the nucleation mode decreased. At RH 13%, the progeny consisted of essentially neutral molecular clusters with a single size mode about 1 nm. Therefore, the mean particle size decreased in the neutralization process, consistent with previous observations. As the concentration of the nucleation fraction decreased, the charge fraction of these nanometer particles increased from 72% to essentially 100%. These results show the importance of charged progeny in the formation and disappearance of nucleation mode.     

Charge Distribution of Incipient Flame-Generated Particles

We report the size and electrical charge distributions of incipient nanoparticles generated in atmospheric pressure hydrocarbon/air premixed flames in conditions prior to the onset of soot particles. The particle size and charge distributions are measured by Differential Mobility Analysis (DMA) and compared to theoretical charge distributions predicted for flame conditions. The results show that the charge distribution attained in flames is well predicted by Boltzmann theory for all particles, including even the smallest incipient particles with diameters in the 1–3 nm size range. In flame conditions that produce only particles smaller than 3 nm, the charge fraction of particles agrees with that predicted by Boltzmann theory near the flame temperature (1700 K). In flame conditions with ‘bimodal’ particle size distributions, the charge fraction of the smallest particles agrees with the Boltzmann prediction at maximum flame temperature, while the charge fractions of larger particles agree with Boltzmann theory at temperatures that coincide with the local temperature near the probe surface (1000–1200 K). The results of this paper show that the temperature of the Boltzmann charge fraction that best agrees with the measured charge fraction for each particle size gives the local temperature of their last coagulation event. The smaller particles, which retain their charge fraction predicted by Boltzmann at the maximum flame temperature, do not thermalize by coagulation in the cool region near the probe evidencing low probability for charge transfer as well as for coagulation.     

Bipolar Charging of Ultrafine Aerosol Particles

The stationary bipolar charging characteristics of aerosol particles in the size range between 4.5 and 40 nm have been studied using a new technique whereby the particles neutralized by a ²⁴¹Am radioactive source are enlarged and directly observed in an electric field. The number ratio of charged particles to total particles obtained in this study was found to deviate from the charge distribution obtained from Boltzmann's law and to agree well with that calculated with the bipolar charging theory of Fuchs using his values for the ion properties. The ratio of positively charged to negatively charged particles was found to be approximately 0.35:0.65.

     

Collision-Based Ionization: Bridging the Gap between Chemical Ionization and Aerosol Particle Diffusion Charging

In diffusion charging theory, it is assumed that each ion–particle collision leads to the transfer of charge from ion to particle, and that charge transfer will not occur upon collision between a vapor molecule and a charged particle. However, in chemical ionization, charge transfer can occur in two directions—from charge-donating ion to vapor molecule and back from charged vapor molecule to the original charge-donating species. Both aerosol diffusion charging and chemical ionization are collision-based charge transfer processes, and for particles only slightly larger than vapor molecules (aerosol clusters), the line between diffusion charging and chemical ionization becomes blurred. We examined the charge transfer from aerosol clusters (positively charged amino acid clusters) in the ～1.0 nm size range to neutral vapor molecules (trimethylamine) at atmospheric pressure by using a combined experimental and theoretical approach. It was found that for singly charged amino acid cluster ions composed of 1, 2, and 3 amino acid molecules, the rate of charge transfer to trimethylamine vapor molecules was clearly observable, particularly for clusters composed of 1 and 2 molecules. The charge transfer rate for singly charged clusters with 4 or more amino acid molecules was consistently close to 0, indicating that the rate of charge transfer from clusters to vapor molecules is size dependent. The charge transfer rates also varied with cluster's chemical composition. Overall, this study demonstrates that small aerosol clusters (～0.5 nm) can lose charge through collisions with vapor molecules, which is typically not considered in diffusion charging theories.     

An Improved Method for Charging Submicron and Nano Particles with Uniform Charging Performance

An improved method for charging submicron and nano silver particles with uniform charging performance was developed. Monodisperse silver particles were grown into microdroplets through condensation. The aerodynamic diameter and GSD of the condensed droplets were the same regardless of their original diameter. The diameter of the droplets increased from 1.7μm to 2.5 μm as the temperature of the saturator increased from 45°C to 55°C. They were charged by an indirect corona-based charger, in which the ion-generation zone is followed by a particle-charging zone through which the condensed droplets pass. The charges of the droplets were controlled by varying the droplet size, ion concentration, and strength of electric field in the charger. The solvent of the charged droplets was evaporated in an evaporator. The size distribution of the evaporated particles was measured by SMPS spectrometer and compared with their original size distribution. The particles after evaporation were slightly larger than their original particles, due to recondensation. The total charge and number concentration of the particles were measured by aerosol electrometer and CPC, to calculate the average charge. Their electrical mobility distribution was measured by SMPS spectrometer without a neutralizer, to calculate the charge distribution and average charge of the evaporated particles. The results showed the average charges of the particles were similar, regardless of initial diameter and manner of calculation. The charge distributions of the evaporated particles were identical, except for 16.9 nm particles. Ion evaporation phenomenon of particles smaller than 40 nm in diameter was not detected.     

Unipolar Charging of Fine and Ultra-Fine Particles Using Carbon Fiber Ionizers

A simple and novel unipolar charger using carbon fiber ionizers was developed to effectively charge fine and ultra-fine aerosol particles without the generation of ozone. The particle penetration in the charger was investigated for non-charged, neutralized, and singly charged particles in the size range of 20–200 nm. Particle loss and the intrinsic, exit and extrinsic charging efficiencies of fine and ultra-fine particles were also investigated for non-charged particles at different applied voltages to the charger. Particle penetrations in the charger were nearly 100% for particles larger than 20 nm, irrespective of the initial particle charging state. Particle losses in the charger could be decreased by decreasing the applied voltage to the charger from 4.0 kV to 2.3 kV. The intrinsic charging efficiencies were proportionally increased with the applied voltage, whereas the exit charging efficiencies were almost independent of the applied voltage. Therefore, the extrinsic charging efficiency of the charger becomes higher for the lower applied voltage (2.3 kV), at which about 60% of 20 nm particles were charged. Little (less than 4 ppb) to no ozone was generated under all operation conditions. It can be concluded that the newly developed unipolar charger using carbon fiber ionizers can charge fine and ultra-fine particles at least as effectively as currently available unipolar chargers, but with the major advantage of negligible ozone generation, a highly desirable feature if the charged particles are to be used for chemical or biological analysis.     

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.     

Performance of bipolar diffusion chargers: Experiments with particles in the size range of 100 to 900 nm

Accurate measurement of particle size distribution using electrical-mobility techniques requires knowledge of the charging state of the sampled particles. A consistent particle charge distribution is possible with bipolar diffusion chargers operated under steady-state condition. Theoretical steady-state charge distributions for bipolar charging are well established but recent studies have shown that the performance of particle chargers is a strong function of particle size, particle concentration, ion source, and charger operating conditions. Most of these studies have focused on particles smaller than 100 nm and the applicability of these results for particles larger than 100 nm must be investigated. In this study, experimentally obtained singly-charged and doubly-charged fractions are compared against theoretical predictions for particles in the size range of 100 to 900 nm. The experimental results show that the commercial soft X-ray charger performs as theoretically-predicted over the range of conditions studied while the performance of other commonly used radioactive chargers (⁸⁵Kr and ²¹⁰Po) are dependent on source strengths, flowrates, particle charge polarities, and particle sizes. From measurements of particle residence times and ion concentrations in different test bipolar chargers, prior observations of flowrate-dependent charging fractions can be explained. Additionally, the results from this study are used to determine an acceptable time period for usage of the commercial TSI 3077A ⁸⁵Kr chargers for steady-state charging as a function of flowrate.

Copyright © 2018 American Association for Aerosol Research     

Aerosol Charge Neutralization by a Mixing-Type Bipolar Charger using Corona Discharge at High Pressure

An aerosol neutralizer called the Mixing-type Bipolar Charger using Corona-Discharge at High Pressure (MBCCHP) was developed. In the MBCCHP, a corona discharge (High-Pressure Corona Ionizer; HPC Ionizer) induced by high frequency voltage (>100 Hz) at high pressure (>0.2 MPa) is used to generate bipolar ions at high concentration (1–3 × 10⁹ ions/cm³) that are then mixed with aerosol particles flowing in a charging chamber where no external electric field is present. The charging performance of the MBCCHP was evaluated by comparing the measured and theoretical number ratios of positively and negatively charged particles to the total number of particles, and by comparing those of negatively charged to positively charged particles for an equilibrium charge distribution. The theoretical and measured results agreed well in the particle size range of 5–80 nm. Particle loss in the MBCCHP for the size range of 5–100 nm was less than 15%, and particle generation from the electrode due to spattering or from the carrier gas containing SO_x due to chemical reaction was either negligible or not observed. The MBCCHP can effectively provide aerosol particles in the equilibrium charge state. Advantages include (1) no selective deposition of charged particles by an electric field, (2) no generation of new particles by reactive molecules, such as atmospheric pollution gases contained in a sample aerosol by chemical reactions with active species, such as OH radicals, produced by discharge, and (3) no effect of carrier gases of the sample aerosol on the ion properties.