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.     

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.     

Experimental study of a new corona-based unipolar aerosol charger

A corona-based unipolar aerosol charger has been constructed, and its performance has been systematically evaluated. The prototype consists of completely separated corona ionization and charging chambers. With this configuration the electrostatic loss of charged particles is eliminated, and particle loss by diffusion and the space charge effect is minimized by the angular injection of the ionizer flow and the rapid exit of charged particles. The charger performance was optimized by varying different operational parameters, i.e., total and ionizer flowrates, and ion concentration. It was found that operation with one corona ionizer gave higher extrinsic charging efficiency than operation with two ionizers. The corona-discharge current has negligible effect on the charging performance. Operating the charger at a total flowrate of 5 lpm, with 1.0 lpm flow in each of the two ionizers, gave the highest extrinsic charging efficiency. Further, the performance of prototype charger was not compromised even at a total flowrate of 10 lpm. The charger provides higher extrinsic charging efficiency than other corona-based unipolar chargers. Extrinsic charge distributions for particles of different sizes were at last measured by the tandem-DMA technique.     

The effect of particle morphology on unipolar diffusion charging of nanoparticle agglomerates in the transition regime

We investigated the effect of particle morphology on unipolar diffusion charging of nanoparticle agglomerates consisting of multiple primary spheres. In the unipolar diffusion charging of non-spherical agglomerates, geometric surface area and electrical capacitance of particles, which are related to particle morphology, are known as important parameters to determine mean charge per particle. From mobility analysis we found that the geometric surface area of chain-like agglomerates is only larger than that of spherical particles with the same mobility diameter for mobility size range below d_m=80 nm. We estimated the electrical capacitance of agglomerates with a newly developed model based on electrostatics and mobility theories. The results show that the electrical capacitance of chain-like agglomerates becomes significantly larger compared to that of spheres with the same mobility diameter as particles become larger. Our analysis results indicate that loose agglomerates have larger mean charge per particle compared to compact particles with the same mobility diameter because the electrical capacitance of agglomerates becomes larger as particle morphology becomes looser. Our experimental data show that mean charge per particle for silver agglomerates is larger than that for fully coalesced silver spheres with the same mobility diameter as agglomerates by about 24%. The experimental data is in good agreement with estimates of mean charge per particle for silver agglomerates.     

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.     

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.     

The effect of dielectric constant of materials on unipolar diffusion charging of nanoparticles

We investigated the dependence of unipolar diffusion charging of nanoparticles on the dielectric constant of the particle material experimentally. The examined nanoparticles (10–200 nm) cover a wide range of dielectric constant but have almost the same spherical or compact morphology. Measurements of both intrinsic charged fraction and mean charge per particle show very small differences among different materials. The level of the small difference is consistent with the estimation by Fuchs’ [(1963). On the stationary charge distribution on aerosol particles in bipolar ionic atmosphere. Geofisica Pura e Applicata, 56, 185–193] theory.     

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.     

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.     

The efficiency of diffusional particle collection onto wire grids in the mobility equivalent size range of 1.2–8 nm

The diffusional deposition efficiency of monomobile, singly charged nanoaerosols onto electrically grounded metal wire mesh was measured on the basis of electrometer current data. The overall experimental uncertainty was of the order of 1% of measured penetration and 1% of measured particle mobility. The aerosol was either WO_x produced by evaporation/condensation and classified with a special high flow DMA into mobility classes between 1.2 and 8 nm—or electrosprayed THAB ions with a mobility equivalent diameter of 1.44 or 1.76 nm for the monomer and dimer, respectively.The experimental data were compared first to a model by Cheng and Yeh for diffusion deposition onto wire screens. Very good agreement was found down to approximately 3–4 nm, however, with a progressive deviation toward lower than predicted penetration values (predicted by pure diffusion theory (over ?25% at 1.2 nm)). The WO_x data agree very well with penetration data obtained for THAB monomer and dimer. For the larger size range above 4 nm, our data are also in excellent agreement with recent data by Thomas et al. It can be concluded that, no thermal rebound effect exists for charged particles in the electrical mobility diameter size range down to 1.2 nm. Lower than predicted penetrations measured below about 3 nm are most likely due to a small contribution by the image charge effect coupled with diffusion. Comparisons between the electrical and diffusional mobility of THAB ions show that the monomer is split into at least two different electrical mobility peaks, however, with the same diffusion coefficient, thus indicating the possible existence of structural isomers for the THAB monomer.     

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.     

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.     

Experimental and modelled efficiencies during the filtration of a liquid aerosol with a fibrous medium

The aim of this study was to determine and model efficiency during the filtration of a liquid aerosol through a fibrous filter. A series of experiments demonstrated that liquid particle filtration is different from solid particle filtration in that a drainage state appears, characterized by a constant pressure drop at the end of filter clogging. Moreover, during filter clogging, the number efficiency presents a minimum level for particles close to 100 nm in diameter (the most penetrating particle size). The results also reveal that during filter clogging there is a decrease in the medium's performance for particles smaller than 100 nm and an increase in efficiency for particles with a diameter >200 nm. Both effects are induced by the amount of liquid collected in the medium. Finally, a model is proposed to describe filter efficiency during clogging with a liquid aerosol.     

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.     

The effect of the generation and handling in the acquired electrostatic charge in airborne particles

The measurement of the charge distribution in laboratory generated aerosols particles was carried out. Four cases of electrostatic charge acquisition by aerosol particles were evaluated. In two of these cases, the charges acquired by the particles were naturally derived from the aerosol generation procedure itself, without using any additional charging method. In the other two cases, a corona charger and an impact charger were utilized as supplementary methods for charge generation. Two types of aerosol generators were used in the dispersion of particles in the gas stream: the vibrating orifice generator TSI model 3450 and the rotating plate generator TSI model 3433. In the vibrating orifice generator, a solution of methylene blue was used and the generated particles were mono-dispersed. Different mono-aerosols were generated with particle diameters varying from 6.0 × 10^− 6 m to 1.4 × 10^− 5 m. In the rotating plate generator, a poly-dispersed phosphate rock concentrate with Stokes mean diameter of 1.30 × 10^− 6 m and size range between 1.5 × 10^− 7 m and 8.0 × 10^− 6 m was utilized as powder material in all tests. In the tests performed with the mono-dispersed particles, the median charges of the particles varied between − 3.0 × 10^− 16 C and − 5.0 × 10^− 18 °C and a weak dependence between particle size and charge was observed. The particles were predominantly negatively charged. In the tests with the poly-dispersed particles the median charges varied fairly linearly with the particle diameter and were negative. The order of magnitude of the results obtained is in accordance with data reported in the literature. The charge distribution, in this case, was wider, so that an appreciable amount of particles were positively charged. The relative spread of the distribution varied with the charging method. It was also noticed that the corona charger acted very effectively in charging the particles.     

Enhancement of Extrinsic Charging Efficiency of a Nanoparticle Charger with Multiple Discharging Wires

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.     

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.