A Study of Nanoparticle Aerosol Charging by Monte Carlo Simulations

A Direct Simulation Monte Carlo (DSMC) technique is applied for describing the dynamics of aerosol charging. The method is based on the transformation of known combination coefficients into charging probabilities. Changes in the particle charge distribution are computed as a stochastic game, calculating the time-step after each event. The simulations are validated by comparison with analytical solutions for unipolar aerosol diffusion charging and aerosol photocharging. The advantage of the DSMC method lies in the uncomplicated simulation of multi-dimensional systems that would result in very elaborate population balances. The DSMC method is used for simulation of the photocharging of moderately concentrated bicomponent polydisperse aerosols. By means of this method, the influence of the particle parameters (size, material) on the dynamics of the charge distribution in different size and material fractions has been studied. It is shown that charge separation between size or material fractions can be achieved for aerosol components with dissimilar work functions, while the total aerosol charge is zero.     

Calibration and numerical simulation of Nanoparticle Surface Area Monitor (TSI Model 3550 NSAM)

TSI Nanoparticle Surface Area Monitor (NSAM) Model 3550 has been developed to measure the nanoparticle surface area deposited in different regions of the human lung. It makes use of an adjustable ion trap voltage to match the total surface area of particles, which are below 100 nm, deposited in tracheobronchial (TB) or alveolar (A) regions of the human lung. In this paper, calibration factors of NSAM were experimentally determined for particles of different materials. Tests were performed using monodisperse (Ag agglomerates and NaCl, 7–100 nm) and polydisperse particles (Ag agglomerates, number count mean diameter below 50 nm). Experimental data show that the currents in NSAM have a linear relation with a function of the total deposited nanoparticle surface area for the different compartments of the lung. No significant dependency of the calibration factors on particle materials and morphology was observed. Monodisperse nanoparticles in the size range where the response function is in the desirable range can be used for calibration. Calibration factors of monodisperse and polydisperse Ag particle agglomerates are in good agreement with each other, which indicates that polydisperse nanoparticles can be used to determine calibration factors. Using a CFD computer code (Fluent) numerical simulations of fluid flow and particle trajectories inside NSAM were performed to estimate response function of NSAM for different ion trap voltages. The numerical simulation results agreed well with experimental results.     

Evaluation of an electrical aerosol detector (EAD) for the aerosol integral parameter measurement

The performance of an electrical aerosol detector (EAD; TSI Model 3070A) was experimentally evaluated for measuring the integral parameters of particles (i.e., total length concentration of particles, and the total surface area concentrations of particles deposited in a human lung). The EAD consists of a unipolar diffusion charger with an ion trap, and aerosol electrometer. We first evaluated the performance of the EAD charger. Both polydisperse and monodisperse particles of Ag, NaCl, and oleic acid (with the dielectric constants of infinite, 6.1 and 2.5) were then generated to evaluate the particle material effect on the EAD readout.     

Measuring surface area of airborne titanium dioxide powder agglomerates: relationships between gas adsorption,diffusion and mobility-based methods

Inhalation toxicology studies generally use the Brunauer, Emmett, and Teller (BET) gas adsorption method to measure total surface area of particles whereas occupational exposures are more readily measured by real-time mobility-based surface areas or active surface area measured with diffusion charger-based instruments. Three surface area measurement methods were studied: filter-based inert gas adsorption (BET method), diffusion charging, and mobility-based methods. The goal of the project was to investigate and develop a correlation between the measurement methods. The experimental design consisted of measuring surface area in a series of five trials for each of two powder types, fine and ultrafine titanium dioxide with primary particle sizes of 440 and 20 nm, respectively, and two aerosol concentrations. Diffusion charger instruments tended to underestimate the total particle surface area measured by the BET, but were well correlated with mobility-based surface areas obtained from a scanning mobility particle sizer. Filter-based gas adsorption methods and diffusion charging methods provide different but valuable information on total and active surface areas of particles, respectively. Results indicate they should not be used as predictors of one another.     

A numerical method for the analysis of polydisperse aerosol particles charging in a coaxial electrode system

In this paper, charging kinetics of polydisperse aerosol particles in a corona field of a coaxial electrode system is numerically analyzed for a logarithmic normal distribution of aerosol particle size. The particle charging and the particle current are calculated by using a charging model considering ion concentration and particle mobility. Particle charging profiles under varying ion density and electrical field intensity distributions of the charging chamber were revealed. A low charging profile in the transition region of bipolar corona field was demonstrated in the simulation results.     

Comparison of four mobility particle sizers with different time resolution for stationary exposure measurements

Exposure to airborne ultrafine and nanoparticles has raised increased interest over the recent years as they may cause adverse health effects. A common way to quantify exposure to airborne particles is to measure particle number size distributions through electrical mobility analysis. Four mobility particle sizers have been subject to a detailed intercomparison study, a TSI Fast Mobility Particle Sizer (FMPS), a Grimm Sequential Mobility Particle Sizer (SMPS+C), and two TSI Scanning Mobility Particle Sizers (SMPSs), equipped with two different condensation particle counters (CPC). The instruments were challenged with either NaCl or diesel soot particles. The results indicate that the sizing of all tested instrument was similar with only the FMPS size distributions consistently shifted toward smaller particle sizes. The Grimm SMPS generally measured higher concentrations and broader distributions than the TSI instruments. The two Grimm DMAs agreed well with each other; however, the TSI SMPS results showed a reproducible dependence on the flow rates. While TSI and Grimm SMPS delivered consistent results for sodium chloride (NaCl) and diesel soot, the FMPS seemed to react differently to the changing particle source than the SMPSs, which may be caused by either the different morphology or particle size dependent effects. For NaCl particles, the FMPS delivered the narrowest distributions and concentrations comparable with TSI SMPSs, whereas for diesel soot, it delivered the broadest distributions and higher concentrations than TSI SMPSs.     

气溶胶生长过程的微观动力学研究

气溶胶作为大气颗粒物的重要成分对大气环境以及人体健康均有重要影响.其中气溶胶的吸湿性作为影响其在大气中存活时间以及物理化学性质的重要因素受到广泛关注.目前对于气溶胶颗粒的吸湿性研究较为简单,对于气溶胶颗粒物微观结构对吸湿性的影响较少.本文根据Langmuir吸附模型、Fick扩散定律建立了单颗粒的气-粒作用模型,分析了气溶胶颗粒物的均匀分布结构(H2O/C2H2O4)以及非均匀分布结构(H2O/C2H2O4/H2SO4)对其吸湿性的不同影响。研究结果发现,气溶胶颗粒物的非均匀分布结构会影响其吸湿过程的快慢;对于理想流体构成的气溶胶分子,由于分子之间不存在分子间相互作用,因此气-粒间的水分子交换更快,颗粒物外侧浓度首先升高,接着在浓度差的作用下,内侧颗粒逐渐向外扩散,直至达到平衡;相较与大尺寸的颗粒,在相同体积比的条件下,小颗粒由于气溶胶颗粒物的接触角大,因此表面张力大,水分更难流失;此外,由于大尺寸的气溶胶颗粒物表面积体积比较小,因此颗粒失水速度低,需要更多的时间达到相平衡。     

Influences of different powders on the characteristics of particle charging and deposition in powder coating processes

This experimental study investigated the influences of two different powder systems (coarse and ultrafine) on particle charging and deposition characteristics during electrostatic powder coating processes. Results disclosed that, despite their differences in particle sizes, the two powders behave similarly in deposition process, commonly resulting in a cone-shaped deposited pattern in the inner portion of the substrate and an increase of deposited particles in the fringe region. However, their different properties lead to the discrepancies in their deposition efficiencies, which account for a higher efficiency with the coarse powder. The study further revealed that the coarse powder is superior to the ultrafine powder in charging in-flight particles, which directly contributes to its higher deposition efficiencies. Furthermore, it was disclosed that the two powders exhibit distinct characteristics in charging deposited particles. Compared to the coarse powder, the ultrafine powder is more uniform in charging deposited particles, mainly owing to its greater particle number and higher specific surface area but less mass. In particular, the charging efficiency of overall deposited particles decreases for the ultrafine powder but increases for the coarse powder with increased charging voltage, closely related to their particle properties. However, both powders decrease in charging efficiency of deposited particles with extended spraying duration due to back corona intensifying with spraying.     

Bipolar diffusion charging of high-aspect ratio aerosols

Recent studies have raised concerns over applicability of the conventional charging theories to non-spherical particles such as soot aggregates and single-walled carbon nanotube aerosols of complex shape and morphology. It is expected that the role of particle structure and shape on particle diffusion charging characteristics may be significant in the submicron size range for carbon nanotubes (CNTs) and nanofibers (CNFs). In this study, we report experimental data on equilibrium charging characteristics of high-aspect ratio aerosol particles such as CNFs and multi-walled CNTs (MWCNTs) when exposed to a bipolar ion atmosphere. A neutral fraction was measured, i.e., the fraction of particles carrying no electrical charge. A differential mobility analyzer (DMA) was used to classify aerosols, leaving a bipolar radioactive charger to infer the bipolar charging characteristics at different mobility diameters in the submicron size range. The measured neutral fractions for CNF aerosol particles were lower than the corresponding Boltzmann values by 24.4%, 42.0%, and 45.8% for mobility diameters of 400 nm, 600 nm, and 700 nm, respectively, while the neutral fractions for measured aerodynamic diameters of 221 nm, 242 nm, and 254 nm were much lower than those expected by Boltzmann charge distribution, by 43.8%, 63.1%, and 67.3%, respectively. Neutral fractions of spherical particles of polystyrene latex (PSL) and diethylhexyl sebacate (DEHS) particles, measured under identical experimental conditions and procedure, agreed well with the Boltzmann charge distribution. The measured neutral fractions for MWCNT aerosol particles were lower than the corresponding Boltzmann values by 22.3%–25.0% for mobility diameters in the size range from 279 nm to 594 nm. Charging-equivalent diameters of CNF particles correlated well with either mobility diameter or equal-area diameter, which were found to be larger than their mobility or equal-area diameters by up to a factor of 5 in the size range of 400 nm–700 nm, while those of MWCNT particles were larger than the corresponding diameters by a factor of 2 in the size range of 279 nm–594 nm.     

Results of potential exposure assessments during the maintenance and cleanout of deposition equipment

This study is a compilation of results obtained during the cleanout of deposition equipment such as chemical vapor deposition or physical vapor deposition The measurement campaigns aimed to evaluate the potential exposure to nanoaerosols in the occupational environment and were conducted in the workspace. The characterization of aerosols includes measurements of the concentration using condensation particle counters and measurements of the size distribution using fast mobility particle sizer, scanning mobility particle sizer, and electrical low pressure impactor (ELPI). Particles were sampled using collection membranes placed on the ELPIs stages. The samples were analyzed with an SEM?CEDS to provide information including size, shape, agglomeration state, and the chemical composition of the particles. The majority of the time, no emission of nanoparticles (NPs) was measured during the use of the molecular deposition equipment or when opening the chambers, mainly due to the enclosed processes. On the other hand, the maintenance of the equipment, and especially the cleanout step, could induce high concentrations of NPs in the workplace following certain processes. Values of around 1 million particles/cm³ were detected with a size distribution including a high concentration of particles around 10?nm.     

Conceptual limitations and extensions of lung-deposited Nanoparticle Surface Area Monitor (NSAM)

Nanoparticle Surface Area Monitor (NSAM, TSI model 3550 and Aerotrak 9000) is an instrument designed to measure airborne surface area concentrations that would deposit in the alveolar or tracheobronchial region of the lung. It was found that the instrument can only be reliably used for the size range of nanoparticles between 20 and 100 nm. The upper size range can be extended to 400 nm, where the minimum in the deposition curves occurs. While the fraction below 20 nm usually contributes only negligibly to the total surface area and is therefore not critical, a preseparator is needed to remove all particles above 400 nm in cases where the size distribution extends into the larger size range. Besides limitations in the particle size range, potential implications of extreme concentrations up to the coagulation limit, particle material (density and composition) and particle morphology are discussed. While concentration does not seem to pose any major constraints, the effect of different agglomerate shapes still has to be further investigated. Particle material has a noticeable impact neither on particle charging in NSAM nor on the deposition curves within the aforementioned size range, but particle hygroscopicity can cause the lung deposition curves to change significantly which currently cannot be mimicked with the instrument. Besides limitations, possible extensions are also discussed. It was found that the tendencies of the particle deposition curves of a reference worker for alveolar, tracheobronchial, total and nasal depositions share the same tendencies in the 20–400 nm size range and that their ratios are almost constant. This also seems to be the case for different individuals and under different breathing conditions. By means of appropriate calibration factors NSAM can be used to deliver the lung deposited surface area concentrations in all these regions, based on a single measurement.     

Proton µ‐PIXE mapping,AFM imaging and size statistics of mineral granules in a dental composite

We applied proton microbeam particle‐induced X‐ray emission (µ‐PIXE) for mapping Ca, Zr, Ba and Yb, and atomic force microscopy (AFM) for imaging the surface landscape of a dental composite which releases Ca²⁺ and F^? for the protection of hard dental tissues. Three areas ～250 × 250 µm² located ～0.5–2 mm apart on a smooth surface specimen were mapped with 3.1 MeV protons focused to a ～3.0 µm spot and at ～3.9 µm pixel size sampling. The maps evidenced particles with diameters of 3.2–32 µm (Ca), 20–60 µm (Zr), ≤ 4 µm (Ba) and 10–50 µm (Yb). Cross‐section area histograms of Ca‐rich particles fitted with 2–6 Poisson functions revealed a polydisperse size distribution and substantial differences from an area to another, possibly implying large local variations of Ca²⁺ released in the hard tissue near a dental filling of a few millimeters in diameter. Such imbalances may lead to low local Ca²⁺ protection of the dental tissue, favoring the onset of secondary caries. Similarly, AFM images showed high zone‐dependent differences in the distributions of grains with apparent diameters of 1–4 µm, plausibly recognized as Ca‐ and Ba‐containing particles. In a simple model based on demineralization data, lateral diffusion of Ca²⁺ between adjacent domains containing high‐ and low‐area Ca‐rich grains is described by exponential concentration gradients. These gradients may generate appreciable electromotive forces, which may enhance electrochemically the local tissue demineralization. Similar effects are to be expected in the protective action of F^? ions released from microgranules of YbF₃ and of Ba fluoroaluminosilicate glass. Copyright © 2010 John Wiley & Sons, Ltd.     

Size of the Detection Area of a Phase-Doppler Anemometer for reflecting and refracting particles

Measurements of particle size distributions in multi-phase flows with a phase-Doppler anemometer yield incorrect results if polydisperse particles are investigated. For weighting biased size distributions, different in situ methods, requiring the size of the detection area, are known, but all of these weighting procedures are restricted to very small measuring volumes if off-axis instrument configurations are considered. Moreover, the weighting functions have some disadvantages in the case of poor statistics in single size classes or the results are not suitable for determining the size of the detection area for particles which are larger than the beam waist. Therefore, the intention in this work was to measure the size of the detection area for different kinds of monodisperse particles, different instrument configurations and varied instrument sensitivities experimentally and to develop an improved weighting procedure that copes with the above difficulties. The application of the results obtained from the investigations with monodisperse particles to measured particle size distributions and volume flux densities of polydisperse water droplets in a spray cone of an atomizer confirms the applicability of this weighting procedure. It is still restricted to directed flows, perpendicular to the fringes.     

Coagulation of charged particles in a dusty plasma

An investigation is made of characteristic features in the behavior of small particles in a dusty plasma attributable partly to the suppression of coagulation as a result of monopolar charging for particle sizes smaller than the Debye shielding length and partly to the reduction in the effect of charging for larger particles. Similarity relations linking the plasma composition and particle charge with the parameters of the dust component are used to determine the range of parameters for which the linear approximation of the particle charge as a function of their sizes holds. A modified classical theory of coagulation in the diffusion approximation is used to study some anomalies in the behavior of the particle size distribution. It is established that unlike an ordinary aerosol, in a dusty plasma the dispersion of the distribution and the average particle size may decrease with time. It is shown for the first time that a long-lived “quasi-liquid” state of a dusty plasma may be established as a result of the anomalous behavior of the size distribution function of coagulating charged particles.     

In‐Situ Determination of the Charging of Nanometer and Submicron Particles at High Temperatures

Thermal charging of submicron and nanometer particles has been studied for model aerosols of TiO₂ and SiO₂ as well as Al‐Si (aluminosilicate) at 1 000 °C with a new quasi in‐situ technique. The size dependence of the particle separation efficiency for electrostatic precipitation was determined. The charging state of the particles was obtained from evaluating the global Deutsch number for precipitation in an electric field applied to a laminar flow based on particle trajectory considerations.     

Digestive Ripening,Nanophase Segregation and Superlattice Formation in Gold Nanocrystal Colloids

A novel digestive ripening process is shown to narrow the particle size distribution from a highly polydisperse dodecanethiol ligated gold colloid. Unlike the Ostwald ripening process, the digestion occurs through transferring materials from large particles to small particles. Temperature-induced size segregation can further select the particle sizes. By using these two methods, highly ordered superlattices using nanocrystals as building blocks can be synthesized directly from a polydisperse colloid.     

Morphology of single-wall carbon nanotube aggregates generated by electrospray of aqueous suspensions

Airborne single-wall carbon nanotubes (SWCNTs) have a high tendency to agglomerate due to strong interparticle attractive forces. The SWCNT agglomerates generally have complex morphologies with an intricate network of bundles of nanotubes and nanoropes, which limits their usefulness in many applications. It is thus desirable to produce SWCNT aerosol particles that have well-defined, unagglomerated fibrous morphologies. We present a method to generate unagglomerated, fibrous particles of SWCNT aerosols using capillary electrospray of aqueous suspensions. The effects of the operating parameters of capillary electrospray such as strength of buffer solution, capillary diameter, flow rate, and colloidal particle concentration on the size distributions of SWCNT aerosols were investigated. Results showed that electrospray from a suspension of higher nanotube concentration produced a bimodal distribution of SWCNT aerosols. Monodisperse SWCNT aerosols below 100 nm were mostly non-agglomerated single fibers, while polydisperse aerosols larger than 100 nm had two distinct morphologies: a ribbon shape and the long, straight fiber. Possible mechanisms are suggested to explain the formation of the different shapes, which could be used to produce SWCNT aerosols with different morphologies.     

Surface science with aerosols

Experimental surface science with aerosol particles under atmospheric conditions is becoming a realistic possibility. The first part of this critical review focuses on nano-scopic aerosols generated in combustion of organic fuels at ambient pressures. The bizarre shape of soot agglomerates resists a simple definition of size and surface area. Yet a measure of the size known as the mobility diameter can be extracted from the mobility of the particles in their carrier gas. The total surface area must be divided into an active and a passive part. At the active surface, mass, energy, and momentum is exchanged with the molecules of the carrier gas. The active surface thus determines the dynamical properties of the particles. The passive surface is the surface enclosed in the interior as well as the surface in bays or cracks or, with larger particles, in the dead point of the laminar flow; it determines particle properties on a longer time scale. Simple automatic portable sensors measure the number density of airborne particles, their “size” and a characteristic fingerprint of the surface chemistry, making it possible to determine the source from which the particle was emitted. The response time of the sensors is ～1 s, hence one can monitor dynamical changes of the particles such as adsorption of water in the atmosphere. In the second part we examine a number of surface science techniques that have been used to characterize surfaces important to atmospheric chemistry in more detail, in particular the uptake of water and the influence of surfactants. We illustrate the application of these techniques to the investigation of alkali halide surfaces as a function of relative humidity. Finally we give first examples on how infrared spectroscopy and synchrotron-based ambient pressure X-ray photoelectron spectroscopy have been used to study more realistic aerosol particles, under conditions of ambient humidity. These examples show that in situ chemical analysis of the particles is possible with third generation synchrotron X-ray sources. In the near future, X-ray lasers might reveal the fast dynamics of chemical processes as well. Thus it is within reach to study aerosols under the conditions of the stratosphere. Stratospheric aerosols can reduce the insolation of the earth and may become one of the last resorts of humanity to counteract the effects of global warming.     

Use of an electrical aerosol detector (EAD) for nanoparticle size distribution measurement

Recently, Nanoparticle Surface Area Monitor (NSAM, TSI model 3550) and EAD (EAD, TSI Model 3070A) have been commercially available to measure the integral parameters (i.e., total particle surface area and total particle length) of nanoparticles. By comparison, the configuration of the EAD or NSAM is similar to that of electrical mobility analyzer of the early generation for particle size distribution measurement. It is therefore possible to use the EAD or NSAM as a particle sizer. To realize the objective of using the EAD as a sizer, we characterized the average electrical charges of monodisperse particles passing through the EAD particle charger and ion trap set at voltages ranging from 20 to 2500 V. The average charge data collected at different ion-trap voltages were then summarized by the empirical correlation using the parameter of Z _p *V, where Z _p is the particle electrical mobility and V is the ion-trap voltage. A data-reduction scheme was further proposed to retrieve the size distribution of sampled particles from the EAD readout at different ion-trap voltages. In the scheme, the functional format of each mode in a number size distribution of particles was assumed as log-normal, but the number of modes in an entire size distribution is not limited. A criterion was used to best fit the simulated EAD readouts with experimental ones by varying the count median diameter (CMD), geometric standard deviation (σ _g), and total particle number (N _t ) of each mode in a particle size distribution. Experiments were performed to verify the proposed scheme.