Measurements of the Size Distribution of Aerosols Produced by Ultrasonic Humidification

Aerosol number and mass distributions produced by an ultrasonic room humidifier and an ultrasonic medical nebulizer were examined in a limited-scale study. Rapid droplet drying occurred at room humidities and under near saturated conditions. A model was tested describing the diameter of dried particles as a function of the dissolved mineral content of the water and the transducer frequency. Water containing 102 mg/L of dissolved minerals in a humidifier with a 1.6 MHz transducer produced droplets with a mass median diameter of 2.9 μm. The number median diameter of particles after drying was computed to be 0.11 μm. The distribution of particles in a nebulizer tube using a NaCl solution was shown to consist of a mixture of dried salt particles and droplets which included coagulated multiplets.     

Size Distribution Evolution of Fine Aerosols Due to Intercoagulation with Coarse Aerosols

Abstract

Controlling the emission of submicron particles of toxic metals in a combustion system poses a challenge. One possible mechanism for removing these fine particles is through intercoagulation with coarse particles. A bimodal lognormal model was applied to investigate the impact of intercoagulation rate on the size distributions of fine-mode aerosols. Fine-mode particle removal time was found to depend strongly on the number concentration of coarse-mode particles, but it was independent on the number concentration of fine-mode particles. An increase of geometric standard deviation of fine-mode particles from 1 to 1.6 significantly increased the dimensionless removal time 27 times. On the contrary, an increase of the deviation of coarse-mode particles in the same range only decreased 3% of the dimensionless removal time. The variation of geometric mean size ratio, meanwhile, had only insignificant effects on dimensionless removal time. For a constant mass concentration, removal time decreased as geometric standard deviation narrowed and mean size of coarse mode decreased. Fine-mode particles ultimately approached monodisperse when the dominant mechanism was intercoagulation; meanwhile, coarse-mode particles approached the asymptotic shape because intracoagulation was the dominant mechanism. The results show that on a constant mass basis, monodisperse coarse-mode particles with a high number concentration are the optimal condition for enhanced removal of fine-mode particles through intercoagulation.     

Size Distribution and Morphology of Liposome Aerosols Generated By Two Methodologies

Pulmonary delivery of sustained release formulations needs drug encapsulation in a suitable matrix, as well as the generation of aerosols with high lung penetration and suitable release characteristics. Nanometer sized liposomes offer the potential for biocompatibility, controlled release and easy internalization in the lung. For uniform dose delivery and drug release kinetics, it is of interest to understand generation techniques to obtain aerosols containing nearly monodispered nanometer sized dry particles. Two aerosolization techniques, air-jet atomization and electrohydrodynamic atomization (EHDA) were studied to identify conditions under which the inclusion of one-liposome-per-drop could be achieved. In air-jet atomization, low lipid concentrations resulted in a unimodal aerosol with a median mobility diameter of 94 (± 3.5) nm, while higher concentrations led to larger median diameters, implying possible inclusion of multiple liposomes per drop. In EHDA, tuning drop sizes in the range of 130 to 200 nm, as well as the use of high lipid concentrations, resulted in a bimodal aerosol distribution, with peaks at 35 and 100 nm mobility diameters. TEM images of the liposome aerosol from EDHA showed fused liposomes, resulting in cylindrical structures with different physical diameters. It was hypothesized that deformation of liposomes to cylindrical structures in the micro-capillary liquid tip of the electrospray, and interactions along the axial or cross sectional surfaces led to dry particles with different mobility sizes.     

Comparison of Measured Particle Lung-Deposited Surface Area Concentrations by an Aerotrak 9000 Using Size Distribution Measurements for a Range of Combustion Aerosols

Surface area in addition to mass concentration is increasingly being emphasized as an important metric representing potential adverse health effects from exposure to inhaled particles. Lung-deposited surface area (SA) concentrations for a variety of aerosols: coal, biomass, cigarette, incense, candle, and TiO₂ were measured using an AeroTrak 9000 (TSI Incorporated) and compared with those calculated from number size distributions from a scanning mobility particle sizer (SMPS). Three methodologies to compute the SA concentrations using the International Commission on Radiological Protection's (ICRP) Lung Deposition model and an SMPS were compared. The first method calculated the SA from SMPS size distributions, while the second method used lognormal size distribution functions. A third method generated a closed-form equation using the method of moments. All calculated SMPS SA data against which the measured SA data were compared were generated using the first method only; however, the SA concentrations calculated from each of the three methods demonstrated strong correlations with each other. Overall, results between measured and calculated lung-deposited SA indicated strong positive linear associations (R ² 0.78 - >0.99), moderately dependent on the type of aerosol. In all cases, the measured SA concentrations slightly underestimated those calculated from the SMPS data, with the exception of coal combustion particles. Although some dependency on aerosol material exists, the instrument measuring lung-deposited SA demonstrated consistent reliability across a range of concentrations for a range of materials. For optimal results however, applying a correction factor (CF) before taking the instrument to the field is recommended.

Copyright 2013 American Association for Aerosol Research     

Measurements of Kelvin-Equivalent Size Distributions of Well-Defined Aerosols with Particle Diameters > 13 nm

During the 1979 workshop of the working group on ultrafine aerosols, different experimental techniques for measuring the number concentration and size of ultrafine aerosol particles were compared. In the present paper we report on a comparison of different particle size measuring techniques for ultrafine aerosols. Well-defined monodisperse aerosols with electrical mobility particle diameters ranging from 13 to 100 nm were generated using an electrical aerosol classifier. Kelvin-equivalent size distributions of these aerosols were determined by means of a process-controlled expansion chamber, the size-analyzing nuclei counter (SANC). To this end the considered aerosol was humidified and the number concentration of the droplets growing in the expansion chamber was measured for stepwise increase in supersaturation. At a quite well defined critical supersaturation, a significant increase in the measured droplet concentration, and thus the onset of heterogeneous nucleation, was observed. By means of the Kelvin-Gibbs equation this critical supersaturation is related to the Kelvin-equivalent diameter of the aerosol particles. Measurements were made on NaCl and dioctyl phthalate (DOP) aerosols. For NaCl particles the Kelvin diameter was found to be larger by a factor of about 4 than the electrical mobility diameter, as determined by the electrostatic aerosol classifier. This is explained by the solubility of the NaCl particles. For DOP particles, however, the Kelvin diameter agrees quite well with the electrical mobility diameter. The Kelvin size distributions were found to be quite narrow, indicating a high monodispersity of the generated aerosol as well as a satisfactory size resolution of the SANC. Thus different experimental techniques, based on completely different principles, yielded similar measurement results.     

Particle Size, Particle Size Distribution, and Related Measurements of Supported Metal Catalysts

It should be apparent from the contents of this review that no single method provides a full characterization of particle size and/or particle size distribution in supported metal catalysts without some inherent theoretical shortcoming, experimental difficulty, or ambiguities in interpretation of results. The message is clearly to use more than one technique whenever possible in order to obtain intercomparisons both of size/size distribution number themselves and internal consistency of the data. However, to be avoided are incorrect comparisons, such as sizes obtained from the Scherrer formula and chemisorption experiments, for example, which measure different things. We believe that high-resolution TEM has a bright future in catalyst characterization, but perhaps the most accessible and convenient combination at present is that of Fourier line profile x-ray analysis and chemisorption, albeit at the expense of some analysis in the former case.     

Influence of Air Pollutants in the 7Be Size Distribution of Atmospheric Aerosols

ABSTRACT

The atmospheric behavior of ⁷Be aerosols was studied by using 1-ACFM cascade impactors. The activity distribution of Be measured by gamma spectrometry (E _γ=477 keV), was largely associated with submicron aerosols in the accumulation mode (0.4–2.0 μm). The activity median aerodynamic diameter, AM AD ranged from 0.62 to 1.00 yam (average 0.80 μm). The geometric standard deviation, σ_g ranged from 1.87 to 2.50 (average 2.22). Low AMADs of ⁷Be aerosols have been observed at locations characterized with relatively low pollution. Some dependency of AMADs on height has been also observed. In near marine environment the ⁷Be activity size distribution was observed in higher size range of aerosol particles (AMAD 0.82 μm).     

Determination of Particle Size Distribution of Ultra-Fine Aerosols Using a Differential Mobility Analyzer

The size analysis of ultrafine aerosol particles using a differential mobility analyzer combined with a CNC is discussed from two standpoints: (1) particle loss caused by Brownian diffusion in the analyzer, and (2) data reduction procedure where Fuchs' charging theory is applied. As a result, it has been found that (1) particle loss becomes significant when particle size is smaller than about 15 nm, and (2) a simple and practical data reduction procedure may be used, where the stationary bipolar charge distribution given by Boltzmann's law is modified by using Fuchs' charge distribution in the smaller size range.     

Size Distribution Measurements of Ultrafine Aerosols,d p > 1.8 nm,Formed by Radiolysis in a Diameter Measurement Analyzer Aerosol Charger

A University of Vienna differential mobility analyzer-electrometer arrangement was used to measure the size distribution of charged ultrafine aerosols (d _p > 1.8 nm) generated from a N₂-SO₂-H₂O mixture by radiolysis in a bipolar diffusion charger loaded with a 2.5 mCi Am-241 α-source. Despite the short residence time in the charger, t_r < 1s, nucleation mode diameters near 2 nm were found for dry gas mixtures with water vapor concentrations < 32 ppm. At 20% relative humidity number concentrations increased and modal diameters shifted to larger sizes with increasing SO₂ concentration. The addition of trace concentrations of nitric oxide or ethanol vapor to the gas mixtures resulted in a near complete suppression of particle formation.     

Determination of the Nanoparticle Size Distribution in Media by Turbidimetric Measurements

For incorporation of nanoparticles into matrices, it is necessary to control the particle size distribution during processing. In this paper, a turbidimetric method was applied that uses spectroscopic data to determine the nanoparticle size distribution and, consequently, to control the material properties with a non‐contact measurement method. This method is based on the scattering and absorption of light by the particles. Unlike conventional turbidimetric methods, this method uses not only a few wavelengths but a whole spectral range for evaluation. To determine the distribution parameters, a nonlinear numerical least squares fit routine was established. It was validated by comparing data of water‐silica nanosuspensions with the results from the sedimentation method. The results show that this method is an accurate and easy‐to‐use analysis instrument for the characterization of nanosuspensions, emulsions, and aerosols.     

Particle Size Distribution of Organic Compounds in Aqueous Aerosols Collected from Above Sewage Aeration Tanks

Several classes of organic compounds were analyzed in aqueous aerosols collected in June, November, and December 1996 from above the sewage aeration tanks of a treatment plant (Prato, Italy). Particle size distribution of organic compounds and their enrichment ratio (E_r) with respect to the magnesium ion were determined to infer the extent to which various species were aerosolized. Organic components were found to be predominantly enriched in fine and large particles of the aerosol and their transfer may be attributed to the 1) adsorption of surfactant organic matter at the air/water interface (such phenomenon is particularly evident for the fine and ultra-fine fractions) and 2) flotation of colloidal matter from wastewater to the largest particles with consequent transport of the adsorbed organic compounds. In addition, the interaction of surfactants with hydrophobic compounds explained the enrichment of the latter in the finest fractions.     

Characterization of Pore Size Distribution by Infrared Scattering in Highly Dense ZnS

A model based upon Mie theory was developed to calculate the amount of spectral infrared scattering caused by the presence of a porous second phase. Various in-line transmission curves were calculated and used to characterize the scattering effects of pore size and concentration. The in-line transmission from 2.5 to 10 fjim of ZnS samples hot-pressed at 137.8,172.3, and 206.7 MPa was measured using Fourier transform infrared spectrophotometry and compared with calculated transmission curves. Good agreement with measured results was obtained only when a size distribution effect was included. From the analysis, a bimodal distribution of pores was found to give the best agreement which duplicates the measured transmission.     

Aerodynamic and Mobility Size Distribution Measurements to Reveal Biodiesel Effects on Diesel Exhaust Aerosol

This article examines the effect of biodiesel blends on the exhaust aerosol from a Euro 3 passenger car. Five different feedstock oils (soybean, palm, sunflower, rapeseed, and used frying oil) were used to produce fuels with 10% vol. content in biodiesel (B10). Use of the B10 blends led to a systematic reduction of PM mass emissions in the range of ～9% (rapeseed) to 23% (used frying oil) on average. The combination of particle size distributions based on the aerodynamic and the mobility diameters led to the estimation of the fractal dimension (DF) for non-volatile particles. This was found to range from 2.52 for the baseline (fossil) fuel to 2.62 for the palm oil blend, suggesting that biodiesel can affect the particle morphology, even at this low blending ratio. The differences were statistically significant. The increase of the DF is translated to more compact particle structure, which in turn denotes lower specific surface area. The volatile fraction of PM lies within a range of 1–9% when fossil diesel fuel is employed. Use of palm, sunflower and rapeseed B10 blends results to PM that contain up to 28% volatile particulate mass. The higher emissions of volatile components together with the lower specific area of non-volatile particles, promotes the formation of volatile particles, especially at high speed conditions. This increases the total particle population under motorway driving by up to three times over the baseline levels.     

Refrigerant Droplet Size Measurements in Conjunction with a Novel Method for Improving Flow Distribution in Evaporators

Maldistribution of two-phase flow in evaporators is known to significantly affect their performance. To reduce refrigerant maldistribution, a pressure-swirl atomizer is used to distribute R-134a in the header of a typical evaporator. A phase-doppler particle analyzer is used to characterize the spray along the centerline of the header for 1, 2, and 3 g/s total mass flow rate. The thermodynamic quality is varied in the header from 0% to 15%. The centerline droplet size is found to decrease with increasing quality and to decrease with increasing distance downstream of the nozzle. Flow distribution is found to be more uniform when the atomizer is used as the expansion device when compared to a typical expansion valve and pipe inlet.     

Calibration and Intercomparison of Filter-Based Measurements of Visible Light Absorption by Aerosols

Data on light absorption by atmospheric particles are scarce relative to the need for global characterization. Most of the existing data come from methods that measure the change in light transmission through a filter on which particles are collected. We present a calibration of a recently developed filter-based instrument for continuous measurement of light absorption (model PSAP, Radiance Research, Seattle, WA) that has been incorporated in several measurement programs. This calibration uses a reference absorption determined as the difference between light extinction and light scattering by unaltered (suspended) particles. In addition, we perform the same calibration for two other common filter-based methods: an Integrating Plate and the Hybrid Integrating Plate System. For each method, we assess the responses to both particulate light scattering and particulate light absorption. We find that each of the instruments exhibits a significant response to nonabsorbing aerosols and overestimates absorption at 550 nm by suspended particles by about 20-30%. We also present correction factors for the use of the PSAP.     

Exhaust Aerosol of a Plasma Enhanced CVD System: I. Size Distribution Measurements

Understanding the origin and fate of plasma-enhanced chemical vapor deposition (CVD) contaminant particles is a critical issue in semiconductor manufacturing in order to improve thin film deposition on wafer surfaces. Several competing external forces will affect a particle's motion in the plasma field prior to either landing on the wafer or entering the exhaust line. Electrical forces dominate during plasma radio frequency (RF) activation creating regions of potential wells. If trapped, the nucleated particles can continue to grow and gain electron charges until gas or ion drag forces can overcome the potential barrier. Mutual electrostatic repulsion between particles can also cause the traps to "leak" out contaminants into the exhaust line. In this way, contaminants formed solely in the plasma volume are hypothesized to possess a distinctive size and charge distribution independent of condensation particles originating from gas compression by the oil-based rotary pump. For these reasons, a novel experimental aerosol sampling system was designed to continuously monitor submicron particles carried during a thin film deposition cycle without disturbing the internal operation of the plasma. Sampling from the plasma enhanced chemical vapor deposition (PECVD) process exhaust gases using an oil-free mechanical piston pump parallel to the main vacuum line is considered to be an effective alternative to in situ probe measurement. Concentration and size distribution data were continuously measured using a condensation nucleus counter and an optical spectrometer. Results show that the particles in the reactor exhaust line are bimodal or made up of fine and coarse sizes divided near 100 nm. Experimental results show the fine fraction increases in the exhaust line after a certain time interval. This delay is hypothesized to be the initial period the nucleated particles were trapped inside the plasma's potential wells. Once trapped, reactor particles can continue to grow in agreement with free molecular coagulation models. A larger particle will experience greater gas drag to eventually overcome the electrical forces. The delay, or critical transport time, depends upon the reactor pressure and plasma power, which also affect the size of the trapping field. The second paper in the series "Exhaust Aerosol of a Plasma Enhanced CVD System" compares a computational charging model of the plasma sheath with experimental charge distribution measurements of contaminant particles carried through the CVD exhaust.     

An Evaluation of Mass-Weighted Size Distribution Measurements with the Model 3320 Aerodynamic Particle Sizer

The ability of the Model 3320 aerodynamic particle sizer (APS) to make accurate mass-weighted size distribution measurements was investigated. Significant errors were observed in APS size distribution measurements with measured mass median aerodynamic diameters (MMADs) as much as 17 times higher than from cascade impactor measurements. Analysis of APS correlated time-of-flight and light scattering data indicated that the MMAD distortions were due to a few anomalous large particle measurements (～0.1% of the total measurements) with surprisingly low scattered light. Computational fluid dynamics modeling indicated that these anomalous measurements were due to particles that deviated from the intended aerosol pathway and recirculated through the APS measurement volume at low velocities leading to erroneous large particle measurements. A technique for removing erroneous measurements based on correlated aerodynamic diameter and light scattering values is presented. When this technique was used, APS and cascade impactor size distribution measurements agreed well.     

A Technique for Rapid Estimation of the Charge Distribution of Submicron Aerosols under Atmospheric Conditions

A portable technique is presented for rapid estimation of the charge distribution of submicron aerosols under atmospheric conditions, using two Differential Mobility Analyzer (DMA) systems in parallel. Simultaneous measurement of the aerosol mobility and size distributions are made by using one DMA with a neutralizer and the other without. An estimate of the aerosol charge is obtained by a fitting procedure, in which the size distribution and an expression for the charge distribution are used to calculate the mobility distribution of the sample. The parameters in the theoretical charge distribution are varied iteratively until the calculated and measured mobility distributions match. Validation was undertaken with separate measurements of ion mobility and concentration used in the charging expression. Results are presented for ambient indoor air, unipolar ion production by an ionizer and downwind of a high-voltage overhead AC powerline.     

A Study on the Size Effect Via Spectrophotometry and Impedence Measurements

The optical and dielectric properties of polycarbonate films have been studied as a function of sample thickness. The observed optical energy gaps were determined from the absorption spectra. The dielectric constant was determined from impedance data collected in the frequency range of 30 Hz–40 kHz. Four polycarbonate thin sheets of different thickness (0.17, 0.25, 0.30, and 0.55 mm) were used to assess the size effect on the physical behavior of this material. It was found that the measured properties as optical energy gap, dielectric constant, refractive index, and glass transition temperature are nearly independent of the specimen thickness. In general, it can be seen that the processing conditions of the glassy polymer sheets are quite identical and not producing structural changes leading to detectable size effect.     

A Moment Method for Evaporation Using a Logarithmic Size Distribution with the Smallest Size

A moment method of the log-normal distribution with the smallest size is applied to evaporation by newly introducing correction factors obtained from the error function. In this article, the improved moment method is tested for the evaporation, and is compared with the exact solutions calculated by the CIP semi-Lagrangian (CIP-SL) method. In small particle regime, the size distributions and the time histories of the total number and volume per unit volume are reproduced by the moment method for the evaporation near the smallest size. In large particle regime, however, the differences between the moment and exact solutions are larger with time, after the exact distribution spreads to the smallest size. This new moment solver can reproduce the evaporation near the smallest size even when large time step size is given, and thus is expected to be used for the parameterization of the evaporation of small particles in aerosol-transport model.