Development of a fast-response, high-resolution electrical mobility spectrometer

A short electrical mobility spectrometer (EMS) for measuring aerosol size distribution has been developed and presented [Intra and Tippayawong, Korean J. Chem. Eng., 26, 1770, 2009]. In this work, further improvement of the short EMS into a fast-response, and high resolution instrument is presented. This was done by (i) improvement in particle charging, (ii) utilization of faster flow rate, and (iii) adoption of higher number of electrode rings. The so-called ??long?? EMS consists of three main parts: a particle charger, a long multi-channel size classifier column, and a multichannel electrometer. Performance of the long EMS was preliminarily tested using polydisperse, carbonaceous aerosol particles generated by a diffusion flame. Preliminary test results showed that the long EMS performed comparatively well, and gave faster response and higher resolution than the short EMS. It was a valuable aerosol instrument available for measuring size distribution of aerosol particles.     

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.     

Extending the Faraday cup aerosol electrometer based calibration method up to 5 µm

A Faraday cup aerosol electrometer based electrical aerosol instrument calibration setup from nanometers up to micrometers has been designed, constructed, and characterized. The set-up utilizes singly charged seed particles, which are grown to the desired size by condensation of diethylhexyl sebacate. The calibration particle size is further selected with a Differential Mobility Analyzer (DMA). For micrometer sizes, a large DMA was designed, constructed, and characterized. The DMA electrical mobility resolution was found to be 7.95 for 20?L/min sheath and 2?L/min sample flows. The calibration is based on comparing the instrument’s response against the concentration measured with a reference Faraday cup aerosol electrometer. The set-up produces relatively high concentrations in the micrometer size range (more than 2500 1/cm³ at 5.3?µm). A low bias flow mixing and splitting between the reference and the instrument was constructed from a modified, large-sized mixer and a four-port flow splitter. It was characterized at different flow rates and as a function of the particle size. Using two of the four outlet ports at equal 1.5?L/min flow rates, the particle concentration bias of the flow splitting was found to be less than ±1% in the size range of 3.6?nm–5.3?µm. The developed calibration set-up was used to define the detection efficiency of a condensation particle counter from 3.6?nm to 5.3?µm with an expanded measurement uncertainty (k?=?2) of less than 4% over the entire size range and less than 2% for most of the measurement points.

Copyright © 2018 American Association for Aerosol Research     

Flow and Pressure Balancing in a Fluid Interface Impactor

Fluid interface impactors, such as virtual and opposing jet impactors, require particular attention to flow and pressure balance in the incoming and outgoing airstreams. The relationship of flow rate to pressure drop has been studied in an opposing jet aerosol classifier of rectangular design. In tests of the sensitivity of classifier operation, changes in pressure differences across the inlets and the outlets were associated with changes in the flow balances of 0.5 and -1.0%/mm w.g., respectively. These flow sensitivities may be used in conjunction with cut size sensitivities to conclude that the appropriate measurements to use for classifier control are the static pressure differences between the two inlets and between the two outlets, rather than the mass flow rates of the four jets. For aerosol size classification tests, a nebulizer-virtual impactor polydisperse aerosol generator was designed to reduce the concentration of small particles and make the generator compatible with dynamic optical particle counting. The aerodynamic cut diameter is Stk ^1/2 ₅₀ = 0.49 ± 0.08 for the standard pressure balance condition. Classification sharpness, using 84 and 16% efficiency sizes, is 1.43 ± 0.03 at the standard condition and can be reduced to 1.11 ± 0.02 at other conditions.     

In-Situ Characterization of Metal Nanopowders Manufactured by the Wire Electrical Explosion Process

In order to understand the size distributions of metal nanopowders inside manufacturing equipment operated at elevated pressures, a scanning mobility particle sizer is used to carry out in-situ measurements of metal nanopowders manufactured by the wire electrical explosion process. A pressure reducer and rotating disk diluter are used for conditioning metal nanopowder samples appropriate for real-time aerosol instruments operated at atmospheric pressure. Based on measurement data collected downstream of the evaporation chamber, the production of metal nanopowders shows good stability and uniformity for a total number concentration of approximately 5 × 10⁷ particles/cm³, and a unimodal size distribution with a mean diameter of approximately 170 nm. Using an aerosol electrometer and two sets of electrostatic classifiers, positively charged particles slightly outnumber negatively charged particles. The performance of the rotating disk diluter is confirmed by comparing the size distributions of metal nanopowders diluted with five different dilution factors, ranging from 235 to 2500. SEM and TEM image analysis indicates that most metal nanopowders manufactured by this process consist of aggregated particles, and their size distributions obtained from SEM images are similar to those measured by the SMPS. The changes in particle size distribution at each stage of the manufacturing process, including the evaporation chamber, trap buffer, cyclone, and mesh filter, are also monitored using the above in-situ monitoring system. The resulting in-situ measurement data can be used for design modifications of equipment, as well as for investigating the sources of nanopowder release to the workplace environment.     

Characterization of a Vortex Shaking Method for Aerosolizing Fibers

Generation of well-dispersed, well-characterized fibers is important in toxicology studies. A vortex-tube shaking method is investigated using glass fibers to characterize the generated aerosol. Controlling parameters that were studied included initial batch amounts of glass fibers, preparation of the powder (e.g., preshaking), humidity, and airflow rate. Total fiber number concentrations and aerodynamic size distributions were typically measured. The aerosol concentration is only stable for short times (t < 10 min) and then falls precipitously, with concomitant changes in the aerosol aerodynamic size distribution; the plateau concentration and its duration both increase with batch size. Preshaking enhances the initial aerosol concentration and enables the aerosolization of longer fibers. Higher humidity strongly affects the particle size distribution and the number concentration, resulting in a smaller modal diameter and a higher number concentration. Running the vortex shaker at higher flow rates (Q > 0.3 lpm), yields an aerosol with a particle size distribution representative of the batch powder; running the vortex shaker at a lower aerosol flow rate (Q ～ 0.1 lpm) only aerosolizes the shorter fibers. These results have implications for the use of the vortex shaker as a standard aerosol generator.

Copyright 2013 American Association for Aerosol Research     

Assessment of a Cylindrical and a Rectangular Plate Differential Mobility Analyzer for Size Fractionation of Nanoparticles at High-Aerosol Flow Rates

An existing differential mobility analyzer (DMA) of cylindrical electrodes and a novel DMA of rectangular plate electrodes are demonstrated for size fractionation of nanoparticles at high-aerosol flow rates in this work. The two DMAs are capable of delivering monodisperse size selected nanoparticles (SMPS σ_g < 1.1) at gas flow rates ranging from 200 slm to 500 slm. At an aerosol flow rate of 200 slm, the maximum attainable particle mean size is of about 20 nm for the cylindrical DMA and of nearly 50 nm for the rectangular plate DMA. The number concentration of the monodisperse nanoparticles delivered by the high-flow DMAs spans from 10⁴ cm^?3 to 10⁶ cm^?3 depending upon the particle mean size and particle size dispersion.

Copyright 2014 American Association for Aerosol Research     

An Ion Generator for Neutralizing Concentrated Aerosols

An ion generator was developed to neutralize concentrated streams of large, highly charged particles in a low-velocity wind tunnel. The aerosol stream tested consisted of 30 mu m aluminum oxide particles (aerodynamic diameter 52 mu m) at a flow rate of 9.6 m³/h (160 L min) and a mass concentration of 43 g/m³. The average number of excess charges per particle was 240,000 (positive), which corresponds to a neutralizing current requirement of 0.11 mu A. Neutralization to < +/- 10,000 charges per particle was necessary to prevent electrostatic sampling artifacts. Neutralization with radioactive sources would have required an impractically large source. The ion generator, constructed from 21 and 32 mm PVC pipe, has 4 peripheral radial electrodes of 0.5 mm tungsten wire and a 2.0 mm diameter central electrode. The aerosol flowed through the ion generator along its axis. The ion generator was powered by an adjustable (0-8.5 kV) power supply. Performance of the ion generator was monitored with an isokinetic Faraday-cup sampler connected to a Keithley Model 6512 electrometer capable of 0.1 fA resolution. The sampler used a stainless steel 47 mm filter holder as the Faraday cup. The cup was insulated with Teflon inside a 90 mm diameter stainless steel enclosure with a 21 mm diameter inlet. This setup gave near real-time measure ment of the charge state of the aerosol in the wind tunnel. By adjusting the ion generator power supply, particle charge could be reduced to < 2% of its original charge. Ion generator output was sufficiently stable to maintain the particle charge within +/- 2% of the original charge over a 1 h period. These reduced charge levels are comparable to charge levels found on workplace aerosols.     

Theory and design of a new miniature electrical-mobility aerosol spectrometer

Theory and design of a new electrical-mobility based instrument for measurement of aerosol particle size distributions in real-time is presented. Miniature electrical aerosal spectrometer (MEAS) has a rectangular cross-section with two main regions: the electrostatic precipitator (ESP) and classifier sections. The ESP section enables charged particle injection into the classifier section in a narrow range of streamlines at the desired location. The injected charged particles are then segregated based on their electrical mobility in the classifier section and collected on a series of plates that are connected to electrometers. Real-time particle size distribution measurements can be inferred from the electrometer signal strengths with the knowledge of the instrument transfer function. A theoretical approach is developed to calculate MEAS transfer function considering the non-uniformity in the electric and flow fields inside the instrument, and accounting for the instrument dimensions and its operating conditions. The theoretical predictions of size classification characteristics are seen to compare well with numerical results. The modeling results suggest that an optimal operational domain exists for MEAS.     

Ultrafine Particle Sampling with the UNC Passive Aerosol Sampler

A model is presented to describe the collection of ultrafine particles by the UNC passive aerosol sampler. In this model, particle deposition velocity is calculated as a function of particle size, shape and other properties, as well as a function of sampler geometry. To validate the model, deposition velocities were measured for ultrafine particles between 15 and 90 nm in diameter. Passive aerosol samplers were placed in a 1 m ³ test chamber and exposed to an ultrafine aerosol of ammonium fluorescein. SEM images of particles collected by the samplers were taken at 125 kX magnification. Experimental values of deposition velocity were then determined using data from these images and from concurrent measurements of particle concentration and size distribution taken with an SMPS. Deposition velocities from the model and from the experiments were compared and found to agree well. These results suggest that the deposition velocity model presented here can be used to extend the use of the UNC passive aerosol sampler into the ultrafine particle size region.     

A Differential Mobility Analyzer (DMA) for Size Selection of Nanoparticles at High Flow Rates

This article demonstrates the feasibility of scaling-up the technique for particle size selection in the gas phase based on differential mobility analysis. Nano-DMAs used to select the particle size in processes for the synthesis of nanomaterials in the laboratory operate at aerosol flow rates of a few liters per minute. A new DMA capable of classifying nanoparticles of up to 30 nm in size at aerosol flow rates as high as 100 l· min ^?1 will be presented (HF-DMA). A major advantage of the HF-DMA over current nano-DMAs covering the same particle size is that its resolution is almost unaffected by Brownian diffusion for particles as small as 3 nm. Monodisperse nanoparticles in the 5 to 25 nm size range have been produced at flow rates of up to 90 l· min ^?1 . The spread in particle size and the particle number concentration were studied with respect to their dependency on the flow rates in the HF-DMA. The measurements reflect the behavior predicted by the theory. The HF-DMA makes it possible to deliver nanoparticles of a well-defined size at yields two orders of magnitude higher than with current nano-DMAs.     

Counting Efficiency of the TSI Model 3020 Condensation Nucleus Counter

The counting efficiency of the TSI model 3020 condensation nucleus counter (CNC) was determined as a function of aerosol flow rate and trigger level using aerosols of known size and an aerosol electrometer. When the aerosol flow rate dropped from 300 to 200 mL/min, counting efficiencies increased significantly in the single-particle counting mode for particles with diameter < 20 nm while those for larger particles remained constant. However, the photometric mode counting efficiency for particles with diameter > 20 nm increased and exceeded unity. When the aerosol flow rate was reduced to 100 mL/min, the counting efficiencies for both counting modes decreased regardless of particle size. Varying the trigger level of the CNC did not influence the photometric mode counting efficiency. However, the counting efficiency of the single-particle counting mode increased with decreasing trigger level, especially for particles < 20 nm in diameter. Characteristics for individual instruments need to be measured because counting efficiencies of two CNCs with the same trigger level and flow rate were not identical.     

Deposition of Ultrafine Particles in Human Tracheobronchial Airways of Adults and Children

Inhalation exposure to ultrafine particles, including radon progeny and other combustion aerosols, has been implicated in potential health risks of ambient and indoor environments. These particles deposit in the respiratory tract mainly by diffusion. The purpose of this study was to determine the deposition pattern of nanometer-sized particles in the human tracheobronchial (TB) airways of children and young adults. The deposition was determined for 1.75, 10, and 40 nm ²¹²Pb particles at flow rates corresponding to respiratory minute volumes at rest and during moderate exercise. The 1.75 nm particles were unattached clusters, whereas the 10 and 40 nm particles were silver particles with attached ²¹²Pb clusters. Replicate casts of the upper TB airways of 3, 16, and 23 year old humans were used, including the larynx, trachea, and bronchial airways down to generations 5-8. Deposition in each generation and total deposition were measured by counting the ²¹²Pb gamma photopeak in a NaI (Tl) detector. The effects of airway geometry, particle size, and flow rate on deposition efficiency were studied. The deposition of the 1.75 nm particle, corresponding to unattached indoor radon progeny, was substantially higher than that of the 10 and 40 nm particles. The dependence of particle deposition on the flow rate was relatively weak, and deposition efficiencies were only slightly higher at the lower flow rates. The deposition models for diffusion from parabolic flow underestimated aerosol deposition, whereas the diffusion deposition predicted for plug flow overestimated the TB deposition. The deposition models resulting from this study can be used for developing lung deposition models and in the risk assessment of radon progeny and ultrafine ambient particles.     

Design and Characterization of a Fluidized Bed Aerosol Generator: A Source for Dry,Submicrometer Aerosol

A fluidized bed aerosol generator has been designed and built for the purpose of generating a constant output of dry, submicrometer particles with a large number density. The output of the fluidized bed for generating aerosol particles from dry soot powder has been characterized using a differential mobility analyzer and a condensation particle counter. The particle size distribution is bimodal, with a mode in the submicrometer diameter size range and a mode in the supermicrometer diameter size range. The larger diameter mode is fully separated from the smaller mode and can thus be easily removed from the aerosol flow using impaction techniques. The distribution in the submicrometer size range is nearly log-normal, with a count median diameter falling between 0.1 and 0.3 micrometers. A number density of greater than 10⁵ particles cm^-3 of soot particles in the submicrometer range can be produced, constant to within 25% (1 standard deviation) over a 4 h time period. The number density of particles produced in the submicrometer range was found to vary with the ratio of soot to bronze beads in the bed mixture, whether or not a feed system was used, and nitrogen flow rate through the fluidized bed and feed system.     

气溶胶溶剂萃取技术制备β-谷甾醇亚微粒

The submicroparticles of β-sitosterol were produced by using an aerosol solvent extraction system (ASES) and characterized by scanning electronic microscope (SEM), X-ray diffraction (XRD), and Fourier transform infrared (FT-IR) analysis. The effects of operational parameters including pressure, temperature, solution concentration, and ratio of flow rate (CO2/solution, r) on particle size (PS), yield, and morphology were investigated. The results showed that microparticles of β-sitosterol (less than 1000 nm size and larger than 70% yield) could be obtained at 10-15 MPa, 35-50°C, 15 mg&#8226;ml－1, 10/1(r); β-sitosterol particles were found to occur as three mophologies: flakes, rods, and spheres by varying ratio of flow rate or solution concentration. In contrast, the crystallinity of β-sitosterol decreased, whereas its molecular structure remained almost unchanged after being ASES-treated. Therefore, ASES was an effective method to produce submicroparticles of β-sitosterol.     

Characteristics of droplets generated by bubble bursting from chromic acid solutions

The effects of electrolyte concentration and gas flow rate on the characteristics of droplets generated from bubbles bursting on the surface of CrO₃ solution were studied with an experimental bubbling system. The experimental conditions included two electrolyte concentrations, 125 and 250 g l^-1 of CrO₃, and three flow rates of sparging air in the range of 4–8 l min^-1. A cascade impactor collected droplet samples for chemical analysis. A laser aerosol spectrophotometer and an aerodynamic particle sizer were employed simultaneously to measure the number concentration and size distribution of the droplets. A layer of foam formed on the liquid surface under all experimental conditions studied except at the gas flow rate of 4 l min^-1 in 125 g l^-1 CrO₃ solution. Foams had a significant effect on the characteristics of droplets generated from bursting bubbles. At identical gas flow rate and electrolyte concentration, the formation of foams led to a reduction in number concentration of droplets larger than 10 μm in aerodynamic diameter and a lower concentration of airborne Cr(VI). In the ranges of gas flow rate and electrolyte concentration tested, the results showed that the airborne Cr(VI) mass concentration increased significantly with gas flow rate and slightly with electrolyte concentration in the presence of foams. The results obtained in the present study should have applications in the emission control of Cr(VI)-containing droplets in chromium electroplating processes.     

Single Charging of Nanoparticles by UV Photoionization at High Flow Rates

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.     

Improving the signal-to-noise ratio of Faraday cup aerosol electrometer based aerosol instrument calibrations

This study introduces a new bipolar measurement routine for particle number concentration calibrations. In the new routine, singly-charged particles of opposite polarities are measured sequentially with a Faraday cup aerosol electrometer (FCAE). We compared the bipolar routine to the traditional FCAE routine, where particle signal and electrometer offset are measured in turns, by calibrating a single CPC on a wide particle number concentration range (from 1000 to 77,000 cm^?3) with both routines. By increasing the signal-to-noise ratio, the bipolar routine decreases the type A uncertainty of the calibration especially at low particle concentrations. In practice, the new routine enables shortening the measurement times by 80% at the lowest particle concentrations which, in practice, corresponds to hours.

Copyright © 2016 American Association for Aerosol Research     

Deposition of Ultrafine Aerosols and Thoron Progeny in Replicas of Nasal Airways of Young Children

The deposition efficiencies of ultrafine aerosols and thoron progeny were measured in youth nasal replicas. Clear polyester-resin casts of the upper airways of 1.5-yr-old (Cast G), 2.5-yr-old (Cast H), and 4-yr-old (Cast I) children were used. These casts were constructed from series of coronal magnetic resonance images of healthy children. The casts extended from the nostril tip to the junction of the nasopharynx and pharynx. These casts were similar in construction to those used in previous studies (Swift et al. 1992; Cheng et al. 1993). Total deposition was measured for monodisperse NaCl or Ag aerosols between 0.0046 and 0.20 (Jim in diameter at inspiratory and expiratory flow rates of 3, 7, and 16 L min^?1 (covering a near-normal range of breathing rates for children of different ages). Deposition efficiency decreased with increasing particle size and flow rate, indicating that diffusion was the main deposition mechanism. Deposition efficiency also decreased with increasing age at a given flow rate and particle size. At 16 L min^?1, the inspiratory deposition efficiencies in Cast G were 33% and 6% for 0.008- and 0.03-μm particles, respectively. Nasal deposition of thoron progeny with a mean diameter of 0.0013 μm was substantially higher (80%-93%) than those of the ultrafine aerosol particles, but still had a similar flow dependence. Both the aerosol and thoron progeny data were used to establish a theoretical equation relating deposition efficiency to the diffusion coefficient (D in cm² s^?1) and flow rate (Q in L min^?1) based on a turbulent diffusion process. Data from all casts can be expressed in a single equation previously developed from an adult nasal cast: E = 1 - exp(-aD ^0.5 Q ^?0.125). We further demonstrated that the effect of age, including changes to nasal airway size and breathing flow rate, on nasal deposition can be expressed in the parameter “a” of the fitted equation. Based on this information and information on minute volumes for different age groups, we predicted nasal deposition in age groups ranging from 1.5- to 20-yr-old at resting breathing rates. Our results showed that the nasal deposition increases with decreasing age for a given particle size between 0.001 to 0.2 μm. This information will be useful in deriving future population-wide models of respiratory tract dosimetry.