An Overview of Unipolar Charger Developments for Nanoparticle Charging

Charging of nanoparticles is an important process in aerosol sizing. A unipolar charger is one of the most important upstream components in aerosol particle sizing and measurement systems by electrical mobility analysis. The aim of particle charging for an electrical mobility analyzer is to impose a known net charge distribution on the aerosol particles for each size. Charger performance depends on the extrinsic charging efficiency and stable operation. A well-designed unipolar charger should provide high extrinsic charging efficiency and stability that can be accurately determined for any given operating conditions. Depending on the mechanisms used to generate the ionized gas, the chargers can be classified into: (i) a corona discharge chargers, (ii) a radioactive chargers, and (iii) a photoelectric chargers. In this article, a brief overview on the development of existing unipolar aerosol chargers for nanoparticles is presented. Descriptions of the operating principles as well as detailed physical characteristics of these chargers, including the corona discharge, ionizing radiation, and photoelectron emission, are given.

Related papers:

title: Particle Size Distribution Modification During and After Electrical Charging: Comparison between a Corona Ionizer and a Radioactive Neutralizer
abstract: This article presents the results of an experimental investigation carried out to examine the changes of the size distribution of ultrafine particles that take place in aerosol electrical chargers. Two types of commercially available chargers have been studied: an unipolar corona discharge ionizer, and a bipolar radioactive neutralizer. Particle diffusion losses to the walls modify the size distribution in both chargers to approximately the same extent. Additionally, particles can grow by Brownian coagulation in the radioactive neutralizer if the aerosol particle number concentration is above about 10 7 /cm 3 . In contrast, the extremely small volume of the corona ionizer used in the experiments, 2 cm 3 , prevents coagulation even when the aerosol concentration is very high. For particle number concentration above 10 7 /cm 3 aerosol coagulation takes place downstream of any of these two chargers. The experimental results have been compared with those obtained by numerical integration of the population balance equations including charging, coagulation and diffusion losses. In spite that the coagulation rate between charged particles of opposite polarity is one to two orders of magnitude larger than the corresponding rate for uncharged particles, the numerical calculations have shown that the expected electrostatic enhancement of coagulation is negligible, probably because the fraction of charged particles in the radioactive neutralizer is too low for the particle size range studied.

title: The statistical electrification of aerosols by ionic diffusion
abstract: A quantitative investigation of the diffusion of atmospheric ions onto spherical particles is reported. Special emphasis is given to random charging processes. The analysis shows that in typical cases about half of the particles are positively charged and the other half negatively charged. Nearly every particle carries at least one electronic charge and some twenty or more. Expressions are developed for the statistical distribution of the fractional number of droplets in relation to their charge. When particle collisions are infrequent, the average free charge of the positive and negative fractions depends only on the square root of the radius and the mean thermal energy of each droplet. It is shown that an equipartition is established between the electrical potential energy of each drop and its thermal kinetic energy. Typical particle electrification somewhat increases the coagulation rate normally attributable to diffusion alone. Systematic droplet electrification of a selected sign is superimposed on this random charging if the positive and negative light ion conductivities of the environment have appreciably different values. The calculated distribution and average charge magnitude agree with those observed on silica dust and new measurements on cloud particles.

title: Unipolar and bipolar diffusion charging of ultrafine aerosol particles
abstract: Unipolar and bipolar diffusion charging of monodisperse aerosol particles of 4–100 nm in diameter has been studied experimentally and theoretically. Particles are charged by unipolar or bipolar ions produced by α rays from Am-241. They are enlarged by condensation of DBP vapor on them using the particle size magnifier and the enlarged particles are admitted into an observation cell having parallel d.c. electrodes to distinguish charged particles from uncharged particles by TV camera. In the case of unipolar charging, the ratios of charged particles to total particles at various values of the ion concentration and charging time are found in quite good agreement with the solution of equations based on the Fuchs' formula for the combination probability of ion with particle and introducing ion properties reported in previous papers. From these results, the necessary conditions to bring uncharged particles to a desired charge distribution are clarified for various particle sizes. In the case of bipolar charging, the number ratios of charged particles to total particles in a steady-state are again found in quite good agreement with the solution of basic equations similar to those of unipolar charging using the Fuchs' theory for the combination probability and the same ion properties as those used for unipolar charging. The ratio of positively to negatively charged particles is found to be approximately 0.35:0.65, which is explained by the difference in physical properties between positive and negative ions.

title: Aerosol Measurement: Principles, Techniques, and Applications
abstract: In recent years, industry has become increasingly interested in modern aerosol measurement methods, not only to protect the health of their workers but also to augment productivity and thereby gain competitive advantage.  Principles, Techniques, and Applications, Second Edition offers scientists and practitioners the fundamental principles used in deciding which aerosol properties to measure and how to interpret the results. Divided into three parts, the material reviews the physical understanding of aerosols, covers specific instrumental techniques, and explains applications in fields ranging from health care to mining and upper-atmosphere research. Leading experts contribute to the review of such areas as direct-reading techniques, bioaerosol sampling, indoor air evaluations, industrial aerosol processing, and measurement in semiconductor clean rooms. Plus, all the chapters in this latest edition have been updated and some have been rewritten by new authors. Two new chapters have been added: one on historical aspects of aerosol measurements and the other on real-time single particle analysis.

title: Unipolar Diffusion Charging of Small Aerosol Particles
abstract: A theory for unipolar ``diffusion charging'' of small aerosol particles is developed for the free‐molecule and transition regions. The free‐molecule theory is, of course, exact within the restrictions imposed on the physical system. The theory for the transition region is based on the Knudsen iteration solution of a relaxation model of the Boltzmann equation. Insufficient experimental data are available to permit detailed comparison with the developed theories.

title: On the theory of charging of aerosol particles by unipolar ions in the absence of an applied electric field
abstract: The charging of aerosol particles by unipolar ions in a gaseous medium and in the absence of an applied electric field has been analyzed from the standpoint of the kinetic theory and compared with theories based upon the macroscopic diffusion of ions and experimental data. It is shown that the quasi-steady charging rate can be expressed by the simple equation, d γ d τ = e −γ , where γ is a dimensionless particle charge and τ is a dimensionless particle charging time. The particle charging rate is shown to be unaffected by the mean free path of the ions, and hence by pressure. The theoretical equation shows good agreement with the experimental data when the mean thermal speed of the ions is taken as 1.18 × 104 cm./sec., which corresponds to a molecular weight of 460 for the corona ions used in the charging experiments. It is also shown that the theory based upon the solution of the continuous macroscopic diffusion equation for ions cannot be applied to the electrical charging of aerosol particles in the micron and submicron size range because of the low concentration of ions normally encountered and the essentially discontinuous nature of the charging process.

title: Experimental study of diffusion charging of aerosols
abstract: The electrical charging of aerosol particles by unipolar gaseous ions was studied theoretically and experimentally. The primary objective of the study was to make precise determinations of the aerosol particle charge under various conditions of charging and to compare the experimental results with those predicted by theory. Experiments were performed using monodisperse oleic acid aerosols generated by a vibrating orifice generator, monodisperse NaCl and DOP (di-octyl phthalate) aerosols generated by an electrostatic classification method, and sulfuric acid aerosols generated by the photo-chemical conversion of gaseous SO/sub 2/ in the smog chamber. The experiments covered a particle size range of 0.0075 ..mu..m to 5.04 ..mu..m diameter. The corresponding range in Knudsen number (Kn = lambda/sub i//a, lambda/sub i/ = mean free path of ions, a = particle radius) was from 0.0056 to 3.86. The charging parameter, n/sub 0/t, was varied between 2.56 x 10/sup 6/ to 5.1 x 10/sup 7/ ion-s/cc, where n/sub 0/ is the concentration of ions and t is the charging time. Comparisons of the results with available aerosol charging theories indicate that there is reasonable agreement between the theory and experiment in the continuum regime (Kn > 1) and the transition regimes (Kn approx. or equal to 1), where the ion mean free path is no longer small in comparison with particle size, there is considerable discrepancy between the experimental data and available charging theories. A semi-empirical equation was developed which agrees well with the experimental data over the entire range of particle size and charging parameters covered in the experiments. Theoretical results are presented showing the distribution of charges on the particles as a function of particle size and the charging parameter n/sub 0/t. 70 figures, 24 tables, 112 references. (auth)« less

title: Diffusion Charging of Aerosol Particles at Low Pressures
abstract: The diffusion charging of aerosol particles by unipolar ions has been studied experimentally over the pressure range, 0.0311 to 0.960 atm, using monodispersed aerosols of di‐octyl phthalate. The ratio, a/λ, of the particle radius to the mean free path of ions has been varied over the range from 1.0 to 66. Within the accuracy of the experiments, the particle charge has been found to be independent of the ratio, a/λ, and well represented by the equation, np=9.00Dp ln (1+1.02×10−5N0tDp), where np is the number of elementary units of charge acquired by the particle, Dp is the diameter of the particle in microns, N0 is the number concentration of the ions to which the particle has been exposed in ions/cc, and t is the charging time in seconds. This equation agrees with a theoretical equation derived by White when the molecular weights of the positive ions used in the charging experiments are assumed to be equal to 16 times that of air or 460.

title: On the performance of the electrical aerosol analyzer
abstract: The electrical aerosol analyzer has been studied using monodisperse aerosols. The aerosols were generated by the electrostatic classification principle described by Liu and Pui (1974a) and by the photo-chemical conversion of gaseous SO2 into sulfuric acid droplets. The measuring range of the instrument was found to be from 0·006 to 1·0 μm in particle dia. and the sensitivity was found to vary from 0·01 to 400 pA/(106 particles/cm3) over the range of the instrument. Data are presented showing the performance of the device as a function of particle size and as a function of the operating conditions of the diffusion charger.

title: Modeling particulate charging in ESPs
abstract: In electrostatic precipitators there is a strong interaction between the particulate space charge and the operating voltage and current of an electrical section. Calculating either the space charge or the operating point when the other is fixed is not difficult, but calculating both self-consistently is much more difficult. A method is proposed that makes the problem straightforward. An iterative solution is required, but the closure rate should be acceptable.

title: Measurements of charge imparted to fine particles by a corona discharge
abstract: IN electrostatic precipitation the diffusion mechanism of charging fine particles is extremely difficult to treat mathematically unless rather drastic simplifying assumptions are made. The size of these fine particles is comparable to the mean free path of the molecules of the gas. To treat this precisely one would have to include the effect of the field on the individual paths of the ions and consider the fact that the paths of the ions in the electric field are curves and not straight lines. If all of this is included in the differential equation it appears almost impossible to obtain any exact solution. White1 has given an excellent review of the solutions which have been obtained. In order to obtain reasonable mathematical equations all investigators have neglected the effect of the main electric field and have treated the problem by kinetic theory methods. Of course, since the field was neglected in deriving the equations, the calculated charge is independent of the field. In the past the Millikan oil-drop method has been the usual method for measuring the charge and with this method it was difficult to obtain sufficient experimental information to determine the agreement with calculations.

title: Electric aerosol particle counting and size distribution measuring system for the 0.015 to 1 ? size range
abstract: An electrical particle counter and size analyzer system having the following characteristics has been described. 1 Sizing range from 0.015 to 1.2 μ. 2 Classification range from 0.015 to 0.6 μ at 28 1/m aerosol sampling rate. 3 A unique unipolar diffusion charger that is stable, controllable and capable of near optimum charging performance. 4 A versatile mobility analyzer capable of discrete classification of particles with mobilities from 0.01 to 0.0002 (cm/sec)/(volt/cm) at aerosol flow rates up to 57 1/m. This analyzer is unique in using a filter located at the aft end of the current collector. This separation of the collecting electrode and the current collector permits the use of collector voltages up to 30 KV while maintaining background currents below 10 −14 amp. 5 Although the instrument described here is only semi-automatic, a completely automatic version of the EPC is entirely practical. An automatic EPC used together with an automatic CN and optical counter would permit continuous automatic size distribution measurement over the size range from 0.01 to 10 μ.

title: Performance of a unipolar ``square wave'' diffusion charger with variable nt-product
abstract: A recently developed unipolar diffusion charger is described in detail and is investigated. A square wave voltage across the charging zone guarantees minimized electrostatic particle losses. Subsequently, ultrafine particles can be efficiently charged employing high ion concentrations. An analytical description of the spatial distribution of ions in the charging zone illustrates the strong influence of the space charge field in the charging zone. The distribution of the charging parameter nt (product of the spatial ion concentration and the residence time of particles in the charging region) is shown. This calculated charging parameter is described by an averaged single nt . An increasing discrepancy between the experimental and theoretical charging efficiency size relation is observed by varying the voltage operation parameter in the charging area. The experimental efficiency size relation is fitted to theory, leading to an effective charging parameter nt ef . An extrinsic charging efficiency E ex , which accounts for particle losses, is defined. E ex is shown to exhibit a maximum with respect to nt ef . The maximum E ex is high in comparison with conventional charger designs, especially for particles in the ultrafine size range.

title: Aerosol Charger Using Sinusoidally Driven Ion Current from a Corona Discharge
abstract: Construction and testing of an aerosol charger capable of charging initially neutral 0.913–μm diameter plastic spheres to 200 elementary charge units in 20 ms is described. The charger utilizes a continuous corona discharge from fine wires located in cylindrical cavities in opposite walls of a rectangular aerosol flow channel of dimensions 1.00 × 5.56 × 2.54 cm. A 3000-Hz AC voltage with maximum amplitude of 10 kV is applied across the 1.00-cm channel gap to produce an ion current which reverses direction with the cycling of the electric field. Aerosol particles moving through the channel are impacted by unipolar ions from first one side and then the other. Lateral motion of charged particles is oscillatory at the frequency of the applied electric field. This serves to minimize loss of charged particles due to wall deposition. A method for calculating the acquired particle charge based on field charging theory is presented. Particle charge levels calculated are in satisfactory agreement with measured part...

title: Experimental design of high volume electrostatic charger
abstract: A corona charger was developed for charging aerosols having flow rates of 0.6–6.0 m3/s. To prevent wall loss transpiration flow from 0.6–4.8 m3/s was introduced at 90° from the principal direction of flow. The redesigned charging section could produce ion currents as high as 10−4 A for applied corona voltages below 10 kV. Calibration experiments with glass aerosols showed that particle penetration through the charging section could be substantially increased by the use of transpiration air, that the transpiration air did not alter the shape of the particle size distribution, and that for particles less than 0.5 μm, diffusion charging was the dominant mechanism.

title: Nanoparticle Charging in a Twin Hewitt Charger
abstract: A new unipolar charger for aerosol nanoparticles has been developed. In this twin Hewitt charger two corona discharge zones are connected by a charging zone where the nanoparticle aerosol flows. Ions move into the charging zone alternating from each corona discharging zone by means of a square-wave voltage. The operation parameters of the device have been experimentally investigated at standard conditions with the goal to optimize the extrinsic charging efficiency in N2 carrier gas. It has been found that there exists an optimal length of the charging channel for each gas flow rate through the charger which minimizes losses of charged particles and at the same time having a sufficient large niont-product. Extrinsic charging efficiencies of some 30% for particles with a diameter of 10 nm are obtained.

title: Unipolar diffusion charging of aerosol particles in the transition regime
abstract: This paper presents experimental results of unipolar diffusion charging of monodisperse combustion aerosols in the size range of 10–300 nm. A Hewitt-type corona charger was designed for the purpose of the investigation and tested in a Tandem-DMA experimental configuration with particles produced by a combustion aerosol generator (CAG). Measurements of the mean charge and the actual charge distribution of the monodisperse samples have been performed, while the apparatus allowed for the investigation of the effect of pre-existing charge of the aerosols. Results are compared with Fuchs' limiting-sphere theory and good agreement is found in most cases. Calculations with the birth-and-death method, using combination coefficients determined by the limiting-sphere theory, show that a Gaussian-type distribution fits well the experimental results. It is shown that pre-existing charge on the particles, as long as it is lower than the mean charge dictated by the operating conditions ( N i t product) of the charger, does not affect the final charging state of the aerosol. Using the classification column of a differential mobility spectrometer (DMS) we performed similar measurements at sub-atmospheric pressures. Variation of the pressure in the range of 250–1000 mbar appears to have an effect on the average number of charges acquired by the particles, although the differences among the measurements lie within the uncertainty levels of the experimental arrangement.

title: Unipolar field charging of particles by electrical discharge: effect of particle shape
abstract: In order to control the properties of the pollutant particles (diameter, charge, concentration, aerosol flow rate) to be collected in a coagulation device, a corona charger (negative wire cylinder discharge) has been designed and tested. This paper presents results of di ethyl hexyl sebacat (DEHS) and NaCl particle charging in a corona discharge. The aerosol particles are charged in a high electric field (>10 6 V m −1 ) so as to check the validity of the field charging law. A special care is taken to characterise accurately the discharge, to calculate, for each experimental condition, the gaseous ion density and electric field in which particles get charged. The first point is that the field charging law fits well with experimental charging data for spherical particles in the intermediate size range (0.44– 1.13 μm ). Nevertheless, for cubic NaCl particles in the same charging conditions (i.e. ion density and transit time), the experimental charge level appears to be greater than predicted by classical field charging law. The only difference between DEHS and NaCl particles seems to be the particle shape, that could lead to an increase of saturation charge in the case of cubic particles, due to local electric field enhancement.

title: Approach to Characterization of a Diode Type Corona Charger for Aerosol Size Measurement
abstract: A semi-empirical method to determine the electrostatic characteristics of a diode type corona aerosol charger based on ion current measurement and electrostatic charging theory was presented. Results from mathematical model were in agreement with those from experimental investigation of the charger. Current-voltage characteristics, Nit product and charge distribution against aerosol size were obtained. It was shown that the space charge was significant and must be taken into account at high ion number concentration and low flow rate. Additionally, significant particle loss was evident for particles smaller than 20 nm in diameter where their electrical mobility was high. Increase in charging efficiency may be achieved by introducing surrounding sheath flow and applying AC high voltage. Overall, the approach was found to be useful in characterizing the aerosol charger.

title: GENERATOR FOR PRODUCING HIGH CONCENTRATIONS OF SMALL IONS. Technical Report No. 12
abstract: A free ion generator, capable of producing high concentrations of light ions with high efficiency, has been developed. lons generated in the corona about a needle point are forced through a sonic orifice and thence free of the accelerating field. Positive and negative ion concentrations as high as 10/sup 11/ ions/cm/sup 3/ in the sonic jet are produced. Ion outputs of 10/sup 14//sec using about 2.5 cfm of free air at 30 psig through a 1/16 inch orifice are achieved. The generator is capable of producing positive, negative or an equal mixture of positive and negative ions. The high ion concentrations decay very rapidly due to space charge and recombination effects. A theoretical and preliminary experimental study of ion decay is made. The ion concentration is found to drop off approximately as the (-2) power of the distance from the generator along the axis of the jet. Concentrations five feet from the ionizer along the jet axis are on the order of 10/sup 6/ ions/cm/sup 3/. (auth)

title: Airborne particle charging by unipolar ions in AC electric field
abstract: A method of particle charging by unipolar ion bombardment in an AC electric field was developed. This method allows particles to be charged up to the Pauthenier limit. The applied AC electric field allows many difficulties inherent to the charging in DC corona discharge to be overcome. Particles passing through the charger are not precipitated on one of the electrodes, but undergo only oscillations with a small amplitude. The charge acquired by any particle is uniquely related to its diameter so that this method of particle charging can be applied for particle size measurement.

title: Characterization of unipolar electrical aerosol chargers—Part II:: Application of comparison criteria to various types of nanoaerosol charging devices
abstract: The framework presented in Part I for comparing the performance of electrical aerosol chargers is applied to published data as well as to new measurements with unipolar corona chargers of very different designs in the size range between 10 and 60 nm. Variables include the internal flow conditions, the ion production arrangement, and the type of electrical field in the mixing zone between ions and particles. Comparisons are made in terms of losses of charged particles vs. charging efficiency and/or vs. average charge per particle. It was found that each charger has a characteristic curve of losses vs. charging effectiveness, comprising data points for a broad range of electrical and ion generation conditions. The results further suggest that the high charge levels and low losses can be achieved by a charger design combining short residence times (associated with turbulence) and high ion concentrations. A carefully adjusted sheath flow is somewhat helpful (at the cost of diluting the aerosol), whereas an AC electric field in the mixing zone of ions and particles does not contribute to improving the charge–loss relationship, even under laminar flow conditions. The uniformity of charging conditions for a weakly charged nanoaerosol in a given charger can be characterized without the need to determine n·t-products, from a plot of measured intrinsic average charge vs. intrinsic charging efficiency and comparing with predictions from charging theory.

title: Unipolar charging of nanometer aerosol particles in a corona ionizer
abstract: A corona ionizer for unipolar charging of nanometer-sized aerosol particles has been constructed and evaluated. Charging efficiency and particle deposition loss (electrostatic and diffusional) have been measured for particle mobility-equivalent diameters below 10 nm , at different aerosol flow rates, different corona polarity and voltages, and also at two different positions of the corona electrode tip. Even though electrostatic deposition of charged particles within the ionizer was relatively large at high corona voltages, charging efficiencies as high as about 30% for 10 nm particles could be achieved for positive and negative corona. This figure is almost one order of magnitude higher than that attainable with radioactive bipolar chargers, and similar to that recently reported for other types of unipolar chargers developed by several researchers.

title: The measurement of charging efficiencies and losses of aerosol nanoparticles in a corona charger
abstract: A methodology is proposed for the measurement of a number of parameters relevant to the performance evaluation of aerosol corona chargers. These parameters are intrinsic and extrinsic charging efficiencies, and diffusion and electrostatic particle losses. The methodology is essentially the same as that used in earlier works, except that free ions are removed just after the charger outlet in order to minimize the extent of possible after-charging effects which might lead to measurement errors. However, the experimental results show that after-charging is negligible and that, consequently, practically all the effective ion–particle collisions take place before the aerosol leaves the charger. Formation of new particles during corona discharge, which could in principle be an additional cause of measurement error, has not been observed in the working voltage range of the charger. Particle diffusion and electrostatic losses have been measured at varying values of the voltage applied to the charger: for a given particle size, diffusion loss decreases and electrostatic loss increases as the charger voltage is increased. The intrinsic charging efficiency increases with particle size and charger voltage. In contrast, the extrinsic charging efficiency, which is the parameter of importance in practice, attains a maximum value for a given charger voltage in such a manner that this optimum voltage depends on particle size.

title: Development and performance test of a unipolar diffusion charger for real-time measurements of submicron aerosol particles having a log-normal size distribution
abstract: We designed and evaluated a unipolar diffusion charger. This charger consisted of a corona discharge, mixing, and ion trap zones. For the performance evaluation of the diffusion charger, sodium chloride (NaCl) particles smaller than 0.1 μ m in diameter, and dioctyl sebacate (DOS) particles of 0.1– 0.7 μ m , were used. The diffusion charger was evaluated with two parameters. One parameter was particle loss and the other parameter was charging characteristics which were the product of penetration and average number of charges ( P · n ). The parameters were measured by varying values of the voltage applied to the diffusion charger. The total particle losses were below 15%. The number of charges was almost linear with respect to the particle diameter. In order to reduce the time required to obtain the desired particle size distribution in the laboratory experiments, we estimated the geometric mean diameter and the log-normal size distribution of the generated submicron particles by using a diffusion charger as well as a condensation particle counter (CPC, TSI model 3025A) and an aerosol electrometer (TSI model 3068A). The estimated size distribution can be used for real-time measurements after calibration.

title: 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
abstract: 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.  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.

title: Unipolar charging of nanoparticles by the Surface-discharge Microplasma Aerosol Charger (SMAC)
abstract: In this paper, we report the development of a novel unipolar charger for nanoparticles, a system that achieves low particle loss and high charging efficiency without the use of sheath air. The efficient unipolar charging of the system is realized mainly by the surface-discharge microplasma unit, a device previously applied with good success to the neutralization or charging of submicron particles [Kwon et al., 2005, ; 2006, ]. The unipolar charger generates unipolar ions using the surface discharge of a single electrode with a DC pulse supply. This marks an advance from our previous method of generating bipolar ions with the use of dual electrodes in earlier studies. We evaluated the efficiency of the penetration (or loss) and charging of nanoparticles in the size range of 3–15 nm, then compared the charging efficiencies measured with those predicted by diffusion charging theory. More than 90% of inlet nanoparticles penetrated the charger (less than 10% of the particle were lost) without the use of sheath air. Other chargers have only realized this high penetration efficiency by relying on sheath air flow. Moreover, the measured charging efficiencies agreed well with those predicted by diffusion charging theory and were somewhat higher and more size-dependent than the charging efficiencies of other nanoparticle chargers.

title: Performance study of a unipolar aerosol mini-charger for a personal nanoparticle sizer
abstract: We designed and constructed a corona-discharge-based unipolar mini-charger and experimentally evaluated its performance. The simple design and compact size make the prototype mini-charger well suited for use with portable aerosol sizing instruments based on particle electrical mobility techniques. In this work, we optimized the extrinsic charging efficiency of the prototype mini-charger for two different aerosol flow rates (i.e., 0.3 and 1.5 lpm). The optimal settings for the prototype operation are: (i) a corona current of 1 μ A and an ion driving voltage of 40 V for the 0.3 lpm flow rate, and (ii) 2 μ A and 120 V for the 1.5 lpm flowrate. Both intrinsic and extrinsic charging efficiencies of the prototype mini-charger at the optimal operational conditions were then evaluated for particles in diameters ranging from 10 to 200 nm. The intrinsic charging efficiency of the prototype reaches 100% at 20 nm for the 0.3 lpm flow rate, and 45 nm for the 1.5 lpm flow rate. The higher intrinsic charging efficiency at the low flowrate is due to the longer residence time of particles in the device. The extrinsic charging efficiency, however, is higher for the 1.5 lpm flow rate than for the 0.3 lpm flow rate due to charged particle loss in the prototype. Charge distributions of test monodisperse particles of different sizes were also measured by the Tandem-DMA technique.

title: Development and performance test of a micromachined unipolar charger for measurements of submicron aerosol particles having a log-normal size distribution
abstract: We designed and evaluated a chip-type unipolar charger based on a micro-electro-mechanical systems (MEMS) process for a compact, portable, inexpensive, and easy-to-deploy instrument to measure real-time aerosol particle size distribution with reasonable accuracy. The charger was evaluated using two parameters: particle loss and charging characteristics which were the product of penetration (p) and average number of charges (np). Particle losses were below 16.6%, and p·np was approximately proportional to d p 2 . After evaluation, the number concentrations and the geometric mean diameters of NaCl aerosol particles were estimated using a method presented in Park, An, and Hwang (2007a) and were compared to scanning mobility particle sizer (SMPS) data. To simulate the charger, a commercial computational fluid dynamics (CFD) software, FLUENT (6.3 version), with external user defined function (UDF) code, was used to solve the electric field, flow field, average particle charge, and particle trajectories. The measured data of particle loss, average particle charge, number concentration, and geometric mean diameter were in good agreement with the results calculated by FLUENT.

title: Effect of needle cone angle and air flow rate on electrostatic discharge characteristics of a corona-needle ionizer
abstract: In this study, the corona-needle ionizer was designed, constructed, and characterized. Experimental characterizations of the electrostatic discharge in terms of current–voltage relationships of the corona ionizer, including the effects of discharge electrode cone angle and air flow rate were presented. It was found that the charging current and ion concentration in the charging zone increased monotonically with corona voltage. Conversely, discharge currents decreased with increasing angle of the needle cone. The negative corona was found to have higher current than the positive corona. At higher air flow rates, the ion current and concentration were found to be relatively high for the same corona voltage. The effect of air flow rate was more pronounced than the corona voltage. It was also shown that the ion penetration through the ionizer decreased with increasing corona voltage, and increased with increasing air flow rate. The highest ion penetration through the ionizer of the 10° needle cone angle was found to be about 93.7 and 7.7% for positive and negative coronas, respectively. The highest ion penetration for the needle cone angle of 20° was found to be 96.6 and 6.1% for positive and negative coronas, respectively.

title: Study of a Nanoparticle Charger Containing Multiple Discharging Wires in a Tube
abstract: A unipolar charger containing multiple discharging wires in a tube (inner diameter: 50 mm) was developed and tested in order to increase the aerosol flow rate and the charging efficiency of nanoparticles. Four gold wires of 25 µm in diameter and 15 mm in length were used as the discharging electrodes to generate positive ions (Ni) from 2.72 × 108 ions/cc to 3.87 × 109 ions/cc in concentration at the discharging voltage of + 4.0 ∼ + 10 KV. Monodisperse NaCl particles of 10 ∼ 50 nm in diameter were used to test the charging efficiency and the particle loss of charged particles with different aerosol flow rates, corona voltages and sheath flow rates. The sheath air near the tube wall was found to increase the extrinsic charging efficiency, and the highest efficiency was obtained at + 6.0 KV discharging voltage, 10 L/min aerosol flow rate and 9 L/min sheath flow rate. The extrinsic charging efficiency increased from 10.6% to 74.2% when the particle diameter was increased from 10 to 50 nm. The TDMA (tandem differential mobility analyzer) method was used to determine the charge distribution and the mean charge per particle and it was found that the Fuchs charging theory corrected for the extrinsic charging efficiency matched with the experimental data very well.

title: Enhancement of Extrinsic Charging Efficiency of a Nanoparticle Charger with Multiple Discharging Wires
abstract: A unipolar charger with multiple discharging wires has been developed and investigated to enhance the extrinsic charging efficiency of nanoparticles by using sheath air near the wall of the charger. The applied voltage of the charger ranged from +4.0 to +10 kV, corresponding to corona current from 0.02 to 119.63 μA. Monodisperse NaCl particles of 10 ∼ 50 nm and Ag particles of 2.5 ∼ 10 nm in diameter were produced to test the performance of the charger with multiple discharging wires and to investigate the particle loss at different sheath flow rates, corona voltages and sheath air velocities. Results showed that the optimal efficiency in the charger was obtained at +9 kV applied voltage, 10 L/min aerosol flow rate and 20 L/min sheath air flow rate. The extrinsic charging efficiency increased from 2.86% to 86.3% in the charger as the particle diameter increasing from 2.5 to 50 nm. The TDMA (tandem-differential mobility analyzer) technique was used to investigate the charge distribution, and the charge distributions in the exit were obtained at the optimal operating condition.

title: A novel unipolar charger for ultrafine aerosol particles with minimal particle losses
abstract: A unipolar diffusion charger with minimal particle losses in the ultrafine size range has been developed. This charger uses two radioactive α-sources to produce ions which are drawn into the charging region by an alternating electric field. The aerosol flow in the centre of the charging region is surrounded by a particle-free sheath air flow to prevent particle losses. Charged aerosol particles move in a “zigzag” manner in the charging channel under the influence of the alternating electric field. Positive gas ions, with higher electrical mobilities than ultrafine aerosol particles, homogeneously fill the entire charging region. Here, the aerosol becomes unipolarly charged. The charging efficiency is strongly dependent on the residence time of the particles in the ion cloud. The ion current was measured in the charging region to determine the ion concentration. The residence time of the particles in the ion cloud was calculated for each stream line of the aerosol flow. The losses of ultrafine particles in the charger were experimentally determined. The unipolar charge distribution was measured and calculated by integrating the theoretical charge distribution over all stream lines of the aerosol flow.

title: A High Efficiency, High Throughput Unipolar Aerosol Charger for Nanoparticles
abstract: A novel aerosol charger has been developed, which has high efficiency and high throughput especially for nanometer particles in the size range of 3–50 nm. Unipolar charging with high ion concentration and long charging time is used to obtain the high charging efficiency. High throughput is achieved by reducing particle loss within the charger. This is accomplished by directing ion flow and aerosol flow in the same direction and by the use of sheath air flow. The charger configuration is of a longitudinal design – the direction of aerosol stream and ion stream are flowing parallel along the longitudinal axis of the charger. The charger consists of four sections: the inlet zone, the ion production zone, the unipolar charging zone, and the exit zone. In the inlet and ion production zones, unipolar ions are generated using Po210 radioactive sources with an electric field designed to separate the positive and negative ions, and to focus the selected unipolar ions into the core region of the charger. The ions with the selected polarity is then attracted to the charging zone by an uniform electric field created by a series of ring electrodes applied with a linear ramped voltage. Aerosol entering the charger is sheathed with clean gas flow in order to keep the aerosol in the core region. A novel exit design with a reversed electric field is incorporated in order to minimize the charged particles loss. The performance of the charger is first evaluated using computer simulation and then constructed for experimental validation. Experiment data have demonstrated that the charger achieves 90% and 95% charged-particles penetration efficiency and with 22% and 48% extrinsic charging efficiency at 3 and 5 nm particle sizes, respectively. These performance data represent significant improvement, over a factor of 10, compared with the existing chargers.

title: Charging of particles in unipolar coronas irradiated by in-situ soft X-rays: enhancement of capture efficiency of ultrafine particles
abstract: The charging of*ne particles in unipolar coronas irradiated by sof t X-rays (3 :5–9:5 keV ;� =0 :13–0:41 nm) was studied. Voltage–current characteristics were used to examine the corona inception voltage in presence and absence ofX-ray irradiation and to estimate the ion concentrations. The capture characteristics ofsynthesized particles: iron oxide, sodium chloride, silica and titanium dioxide, were established in coronas, with and without X-ray irradiation. Enhanced charging ofultra*ne particles in coronas was observed in conjunction with soft X-ray irradiation. A positive corona with X-ray irradiation resulted in the highest charging e)ciencies, followed by a negative corona with X-ray, X-ray only, negative corona only and *nally positive corona only. Particle charging theories available in the literature were used to qualitatively explain the observed trends. ?

title: Probing aerosols by photoelectric charging
abstract: Particles suspended in a gas may emit photoelectrons upon irradiation with ultraviolet light. It is shown that photoemission from small particles involves material and surface dependent physical processes that yield new information, especially on ultrafine particles not accessible to the well‐known diffusion charging. An apparatus is presented for the measurement of the electric mobility and the charge generated on particles in the atmosphere using a monochromatic low‐pressure Hg arc. Particles generated by combustion of organic material, e.g., auto exhaust, are charged with high efficiency. This offers a new way to detect and/or remove such particles from the air. In outside air the concentration of photoelectrically active particles has a diurnal cycle which also depends on the meteorological conditions. Surprisingly, highly mobile particles are detectable even during rainfall.

title: Multiple wavelength aerosol photoemission by excimer lamps
abstract: Submicron aerosol particles of carbon, different metals and exhaust particles from a diesel engine are characterized via the charge p they obtain by photoelectron emission when irradiated by specially designed excimer lamps. The high intensity of these monochromatic lamps makes it possible to charge all particles to the limit defined by Coulomb recapture of the electron. This maximum charge pmax(R, (Φ) depends only on the particle radius R and photothreshold Φ∞ and can therefore be used for identification and separation of different materials as well as detection of small changes in the state of the surface in terms of the outermost atomic layer. The latter effect is demonstrated on C, Cu and diesel particle coated with Zn or surface oxidized with O3. The function pmax(R, Φ) for spherical particles holds well for agglomerated structures, if particle size is defined by the electric mobility of the particles. As long as less than 10% of pmax are reached at short radiation time or small intensity, the photoemission probability μ linearly depends on a photoemission parameter c related to the absorption behaviour of the particles. The charge distribution under these conditions is explained by Poisson statistics. The parameters Φ∞, c and R can all be determined using excimer lamp photoelectric sensors containing lamps of different wavelengths. Excimer lamps represent an interesting alternative to conventional charging principles in electrostatic precipitators for gas cleaning.

title: Indirect photoelectric diffusion charging of submicron aerosols
abstract: A new method of charging submicron particles in clean inert gases has been developed. Monodisperse sodium chloride aerosols in the diameter range of 10–50 nm were generated as test aerosols. They were charged by the attachment of photoelectrons emitted from different metal surfaces inside the aerosol flow duct. The metal surfaces were irradiated by u.v.-light of 185 or 254 nm wavelength. The influences of particle diameter, particle number concentration, photoemitting material, electrical potential and trace compounds in the carrier gas on the charging efficiency have been investigated.

title: REMOVAL OF AIRBORNE PARTICLES BY A DEVICE USING UV/PHOTOELECTRON METHOD UNDER REDUCED PRESSURE CONDITIONS
abstract: A modified version of the previously developed tubular particle-removal device based on the UV/photoelectron method (Shimada et al. (1997) ) is manufactured to investigate removal efficiency of aerosol particles suspended in air at a reduced pressure down to 20 torr. The present measurements with monodisperse particles of 0.458 μm in diameter show the dependence of the removal efficiency on pressure, flow rate and electric field in the electrostatic precipitation device. To predict the removal efficiency, a theoretical analysis for the particle collection processes in the precipitator is made, in which particle charging by negative ions, and particle transport by air flow, diffusion and electrical migration are numerically calculated taking into account the effects of reduced pressure. The calculated results reproduce the measured removal efficiencies well. It is found in the calculation that the Fuchs' approach to ionic particle charging needs modification to predict charging of highly charged particles by ions of the same polarity. It is also demonstrated that the removal efficiency is increased to 100% when the precipitator is equipped with an additional set of electrodes that collects charged particles.

title: Single Charging of Nanoparticles by UV Photoionization at High Flow Rates
abstract: The feasibility of UV photoionization for single unipolar charging of nanoparticles at flow rates up to 100 l· min −1 is demonstrated. The charging level of the aerosol particles can be varied by adjusting the intensity of the UV radiation. The suitability of a UV photocharger followed by a DMA to deliver monodisperse nanoparticles at high aerosol flow rates has been assessed experimentally in comparison to a radioactive bipolar charger ( 85 Kr, 10 mCi). Monodisperse aerosols with particle sizes in the range of 5 to 25 nm and number concentrations between 10 4 and 10 5 cm −3 have been obtained at flow rates up to 100 l· min −1 with the two aerosol chargers. In terms of output particle concentration, the UV photoionizer performs better than the radioactive ionizer with increasing aerosol flow rate. Aerosol charging in the UV photoionizer is described by means of a photoelectric charging model that relies on an empirical parameter and of a diffusion charging model based on the Fuchs theory. The UV photocharger behaved as a quasi-unipolar charger for polydisperse aerosols with particles sizes less than 30 nm and number concentrations ∼10 7 cm −3 . Much reduced diffusion charging was observed in the experiments, with respect to the calculations, likely due to ion losses onto the walls caused by unsteady electric fields in the irradiation region.