Effect of convergence angle on impactor performance

Two nozzles, modified and original, were tested in a sampler that was placed in a wind tunnel and penetration efficiencies, √Stk50, and slope of the performance curve were determined by challenging the sampler with fluorescent-tagged monodisperse test aerosol particles having known concentration. It was shown that a change in convergence angle of the modified nozzle can affect impactor performance. The √Stk50 for original and modified nozzles were 0.57 and 0.49, respectively. The slope of the efficiency curve for original and modified nozzles was 1.52 and 1.36, respectively.

© 2017 American Association for Aerosol Research     

Real-time measurement of size-resolved elemental composition ratio for flame synthesized composite nanoparticle aggregates using a tandem SMPS-ICP-OES

Composite nanoparticles find application in catalysis, drug delivery, and energy storage and require increasingly fine control of their physical properties and composition. While composite nanoparticles have been widely synthesized and characterized, little work has systematically correlated the initial concentration of precursors and the final composition of flame synthesized composite nanoparticles. This relationship is explored in a diffusion flame aerosol reactor by coupling a scanning mobility particle sizer (SMPS) with an inductively coupled plasma optical emission spectrometer (ICP-OES). A framework for studying the relationship between the initial precursor concentrations of different elements and the final nanoparticle composition is explored. The size-resolved elemental composition was measured by directly injecting size-selected fractions of aggregated magnetite and silicon dioxide composite nanoparticles into the ICP-OES plasma. This work showed a correlation between precursor molar ratio and the measured elemental ratio in the mobility size range of 50 to 140 nm. Building on previous work studying size resolved elemental composition of engineered nanoparticles, the analysis is extended to flame synthesized composite nanoparticle aggregates in this work.

Copyright © 2017 American Association for Aerosol Research     

Assessment of indoor bioaerosols using a lab-made virtual impactor

To assess indoor bioaerosols, a virtual impactor having 1 µm cutoff diameter was designed, fabricated, and evaluated with computational fluid dynamics simulation and also with laboratory test using polystyrene latex particles. Two other cutoff diameters of 635 nm and 1.5 µm were obtained by changing the inlet flow rate and the ratio of minor channel-to-inlet flow rates. In field test, the virtual impactor was operated with varying cutoff diameter and field-emission scanning electron microscope (FE-SEM) analysis was performed for each cutoff diameter to observe morphologies of indoor aerosol particles sampled at the major and minor outlet channels. Particles were sampled at both outlet channels using the SKC Button Aerosol sampler and subsequently cultured. By colony counting, it was found that 56% of cultured fungal particles and 63% of cultured bacterial particles had aerodynamic sizes smaller than 1 µm. MALDI-TOF analysis and visual inspection of culture samples were used to identify indoor bacterial and fungal species, respectively. Nearly same species of bacteria and fungi were detected both in the major and minor flow channels.

© 2017 American Association for Aerosol Research     

Aerosol Generation by a Spray-Drying Technique Under Coulomb Explosion and Rapid Evaporation for the Preparation of Aerosol Particles for Inhalation Tests

In this study, nanosized (<100 nm) aerosol particles with high mass concentrations for inhalation tests were generated by a spray-drying technique with combining Coulomb explosion and rapid evaporation of the droplets. Under typical spray-drying conditions, aerosol particles with average diameter of 50–150 nm were prepared from a suspension of NiO nanoparticles with a primary diameter of 15–30 nm. Under the Coulomb explosion method, the sprayed droplets were charged by being mixed with unipolar ions to break up the droplets, which resulted in the generation of smaller aerosol particles with diameters of 15–30 nm and high number concentrations. Under the rapid evaporation method, the droplets were heated immediately after being sprayed to avoid inertial impaction on the flow path due to shrinkage of the droplet, which increased the mass concentration of the aerosol particles. The combination of the Coulomb explosion and rapid evaporation of droplets resulted in the generation of aerosol particles with sizes less than 100 nm and mass concentrations greater than 1 mg/m³; these values are often necessary for inhalation tests. The aerosols generated under the combined method exhibited good long-term stability for inhalation tests. The techniques developed in this study were also applied to other metal oxide nanoparticle materials and to fibrous multiwalled carbon nanotubes.

Copyright 2014 American Association for Aerosol Research     

Crossover from Ballistic to Epstein Diffusion in the Free-Molecular Regime

We investigate, through simulation, a system of aggregating particles in the free molecular regime that undergoes a crossover from ballistic to diffusive motion. As the aggregates grow, the aggregate mean free path becomes smaller and the motion between collisions becomes more diffusive. From growth kinetics, we find that when the ratio of the aggregate mean path to the mean aggregate nearest neighbor separation reaches of the order of unity, a crossover to diffusive motion occurs. This ratio, called the nearest neighbor Knudsen number, becomes an important parameter in understanding aerosol aggregation in the free molecular regime.

Copyright 2014 American Association for Aerosol Research     

Aerosol Process for the In Situ Coating of Nanoparticles with a Polymer Shell

This article presents a novel method to encapsulate gas-borne nanoparticles with a polymeric shell. This method implies heterogeneous condensation of monomer vapor around the surface of nanoparticles as nuclei and polymerization is then subsequently started by addition of NH₃ as aerosol initiator. Ag and SiO₂ nanoparticles were generated as inorganic core by spark discharge and nebulization, respectively, and glycidyl methacrylate (GMA) was used as organic monomer. The effect of several parameters, including vapor pressure of monomer and properties of inorganic core such as morphology, material, particle size, and production method on the thickness of polymeric shell and morphology of resulting nanocomposites has been investigated. The particle size distribution and morphology of the resulting core-shell nanoparticles have been studied via scanning mobility particle sizer (SMPS) and transmission electron microscope (TEM). Finally, the coating efficiency was determined by aerosol photoemission (APE) and the results show that monomer and polymer coating efficiency are 99% and 60%, respectively.

Copyright 2014 American Association for Aerosol Research     

Sampling and dilution of nanoparticles at high temperature

Sampling and dilution of flame-generated, fractal-like ZrO₂ aerosols is investigated by aerosol mass/mobility measurements and microscopy. Two broadly used sampler configurations, a straight-tube (ST) and a hole-in-a-tube (HiaT), at three different in-flow orientations and hole diameters are evaluated. The mobility size distributions, mass-mobility exponent, D_fm, prefactor, k_fm, and average primary particle diameter are obtained at 10–60 cm height above the burner (HAB) of fuel-rich (hot) and fuel-lean (cold) spray flames by differential mobility analyzer (DMA) and aerosol particle mass (APM) measurements using a recent power law for fractal-like particles. The primary particle diameter, agglomerate size distributions, and corresponding standard deviations from aerosol measurements are compared to those by counting images of particles collected by thermophoretic sampling along the flame centerline. Once new particle formation is completed in the flame, both sampler configurations result in nearly identical particle size distributions. Furthermore, all HiaT samplers result in similar mobility size distributions at all orientations, regardless of hole size. Sampling using a downstream in-flow hole orientation results in slightly larger Sauter mean diameters than those obtained by upstream or sidestream ones, especially for the cold flame. Additionally, a correlation is developed by Discrete Element Modeling (DEM) for the agglomerate D_fm evolution to its asymptotic value of 2.2 as function of the average number of primary particles per agglomerate, n_va, or the relative particle density with pre-exponential constant k_fm = 1.18, regardless of primary particle size. This is in good agreement with an experimentally obtained correlation in terms of relative particle density as well as with experimental data for ZrO₂, Ag, and Cu nanoparticles.

© 2016 American Association for Aerosol Research     

Opto-aerodynamic focusing of aerosol particles

We describe a new method for focusing and concentrating a stream of moving micron-sized aerosol particles in air. The focusing and concentrating process is carried out by the combined drag force and optical force that is generated by a double-layer co-axial nozzle and a focused doughnut-shaped hollow laser beam, respectively. This method should supply a new tool for aerosol science and related research.

Copyright © 2018 American Association for Aerosol Research     

Estimation of acoustic forces on submicron aerosol particles in a standing wave field

The net acoustic force acting on submicron particles suspended in a gas and exposed to a standing wave field is investigated as a function of particle size, by measuring both the aerosol number density and size distribution in a flow-through resonator. By taking into account all contributions relevant to the net force, this experimental study provides a first estimate for the acoustic radiation force in a size range where molecular effects are expected to be significant. The experiment consists of an electrostatic transducer generating a standing wave in the 50–80 kHz frequency range, with the submicron aerosol particles concentrated at pressure antinodes located across the height of a rectangular channel. A section of the flow is sampled isokinetically and analyzed using a Scanning Mobility Particle Sizer (SMPS), while the nodal patterns are visualized simultaneously using light scattering. The net acoustic force is calculated from their measured displacement along the axis of the 1D standing wave field. The component of this force resulting from radiation pressure is estimated by subtracting contributions from other forces. The results provide the first experimental estimation of the size dependence of the acoustic contrast factor for submicron aerosol particles, demonstrating the possibility of performing acoustic separation for diameters as small as 150 nm.

Copyright © 2018 American Association for Aerosol Research     

Sensitivity of Aerosol Refractive Index Retrievals Using Optical Spectroscopy

Accurate refractive index values are required to determine the effects of aerosol particles on direct radiative forcing. Theoretical retrievals using extinction data alone or extinction plus absorption data have been simulated to determine the sensitivity of each retrieval. A range of aerosol types with a range of different refractive indices were considered. The simulations showed that the extinction-only retrieval was not able to accurately or precisely retrieve refractive index values, even for purely scattering compounds, but the addition of a simulated absorption measurement greatly improved the retrieval.

Copyright 2014 American Association for Aerosol Research     

Impacts of Aerosol Aging on Laser Desorption/Ionization in Single-Particle Mass Spectrometers

Single-particle mass spectrometry (SPMS) has been widely used for characterizing the chemical mixing state of ambient aerosol particles. However, processes occurring during particle ablation and ionization can influence the mass spectra produced by these instruments. These effects remain poorly characterized for complex atmospheric particles. During the 2005 Study of Organic Aerosols in Riverside (SOAR), a thermodenuder was used to evaporate the more volatile aerosol species in sequential temperature steps up to 230°C; the residual aerosol particles were sampled by an aerosol mass spectrometer (AMS) and a single-particle aerosol time-of-flight mass spectrometer (ATOFMS). Removal of the secondary species (e.g., ammonium nitrate/sulfate) through heating permitted assessment of the change in ionization patterns as the composition changed for a given particle type. It was observed that a coating of secondary species can reduce the ionization efficiency by changing the degree of laser absorption or particle ablation, which significantly impacted the measured ion peak areas. Nonvolatile aerosol components were used as pseudo-internal standards (or “reference components”) to correct for this LDI effect. Such corrected ATOFMS ion peak areas correlated well with the AMS measurements of the same species up to 142°C. This work demonstrates the potential to accurately relate SPMS peak areas to the mass of specific aerosol components.

Copyright 2014 American Association for Aerosol Research     

Application of fractal theory to estimation of equivalent diameters of airborne carbon nanotube and nanofiber agglomerates

Understanding transport characteristics of airborne nanotubes and nanofibers is important for assessing their fate in the respiratory system. Typically, diffusion and aerodynamic diameters capture key deposition mechanisms of near-spherical particles such as diffusion and impaction in the submicrometer size range. For nonspherical particles with high aspect ratios, such as aerosolized carbon nanotubes, these diameters can vary widely, requiring their independent measurement. The objective of this study was to develop an approach to provide approximate estimates of aerodynamic- and diffusion-equivalent diameters of airborne carbon nanotubes (CNTs) and carbon nanofibers (CNFs) using their morphological characteristics obtained from electron micrographs. The as-received CNT and CNF materials were aerosolized using different techniques such as dry dispersion and nebulization. Mobility and aerodynamic diameters of test aerosol were directly deduced from tandem measurement of particle mobility and mass. The same test aerosol was mobility-classified and subsequently collected on a microscopy grid for transmission electron microscopy (TEM) analysis. TEM micrographs were used to obtain projected area, maximum projected length, and two-dimensional (2-D) radius of gyration of test particles. Estimates of the aerodynamic diameter and the diffusion diameter were obtained by applying the fractal theory developed for aerosol agglomerates of primary spherical particles. After accounting for the particle dynamic shape factor, estimated aerodynamic diameters agreed with those from the direct measurements (using tandem mobility-mass technique) within 30–40% for the agglomerates with relatively open structures while the diffusion diameters agreed within 40–50%. The uncertainty of these estimates mainly depends on degree of overlapping structures in the microscopy image and nonuniformity in tube diameter. The approach could be useful in calculating approximate airborne properties from microscopy images of CNT and CNF agglomerates with relatively open structures.

This article not subject to US copyright law     

Highly sensitive nano-aerosol detection based on the whispering-gallery-mode in cylindrical optical fiber resonators

Highly sensitive detection of nanoscale aerosols, or nano-aerosols, is a difficult challenge. Here, we report a fiber optical technique that is capable of detecting trace-level nano-aerosols. Our method is based on monitoring the nano-aerosol-induced resonance shift due to the optical Whispering-Gallery-Mode (WGM) in a cylindrical optical fiber resonator. A nearly linear relationship between the WGM resonance shift and the aerosol coverage ratio of silica nanoparticles (40–50 nm dia.) on the fiber resonator was identified in the low coverage regime. Our experimental results imply sensitivity at the level of ～2 nanoparticles per μm² deposited on the fiber resonator, which corresponds to pg-level sensitivity in the total aerosol mass within the effective detection area. The response of this fiber optical sensor is further confirmed by using silica nanoparticles deposited on the fiber surface via electrostatic self-assembly. The fiber optical technique for nanoparticle detection may ultimately lead to an instrument capable of real-time in situ aerosol detection with ultrahigh sensitivity.

Copyright © 2016 American Association for Aerosol Research     

Loss processes affecting submicrometer particles in a house heavily affected by road traffic emissions

The fraction of outdoor aerosol that penetrates into indoor environments plays an important role in determining the contribution of outdoor particles to the total lung dose of particles in human exposure. The objective of this study was to investigate the physical processes affecting migration of outdoor traffic particles into indoor environments. Particle number size distributions were measured by a fast mobility particle sizer system in both indoor and outdoor environments of a house located in close proximity to a busy street in Bologna (Italy) in the period February–April 2012. Indoor to outdoor (I/O) ratios for submicron particle number concentrations showed strong dependence on particle size and meteorological conditions. The loss rates of particles due to deposition, coagulation, and evaporation were determined using dynamic mass balance and coagulation models. Higher loss rates were found for small particles (nucleation and Aitken mode) indoors than for larger particles (accumulation mode). The coagulation and evaporation processes made a significant contribution to the loss of traffic nanoparticles indoors, especially during the day time. Application of positive matrix factorization to the indoor and outdoor particle size distributions showed a substantial loss of traffic-generated nucleation mode particles in the indoor environment, with evaporation playing a major role.

Copyright © 2017 American Association for Aerosol Research     

On the Feasibility of a Number Concentration Calibration Using a Wafer Surface Scanner

A new primary standard method for calibrating optical particle counters (OPC) has been developed based on quantitative gravitational deposition on a silicon wafer and accurate counting of the particles by a wafer surface scanner (WSS). The test aerosol consists of 3-μm diameter monodisperse polystyrene latex (PSL) spheres at concentrations in the range of 0.1 cm^?3 to 1 cm^?3. A key element to the calibration is the ability to generate monodisperse PSL spheres without residue particles by use of a virtual impactor and differential mobility analyzer. The use of these devices reduced the percentage of residue particles from more than 99.98% to about 5%. The expanded relative uncertainty (95% confidence level) in the number concentration determined with a WSS for a deposition of 200 particles is 17.8%. The major uncertainty component arises from the Poisson fluctuations in the aerosol concentration because of the low concentration. This methodology has advantages of a fast scanning time by the WSS of minutes compared to hours or days by microscopy and of counting every particle deposited compared to often only a small fraction via microscopy.

The WSS was used in the calibration of an OPC based on 12 depositions with concentrations ranging from 0.1 cm^?3 to 1 cm^?3 for each deposition. Make-up air was added to the aerosol entering the OPC so that the lowest achievable concentration for the OPC measurement is about 0.01 cm^?3 in this study. The detection efficiency of the OPC was measured to be 0.984 with an expanded uncertainty of 13.4%.

Copyright 2014 American Association for Aerosol Research     

The First Combined Thermal Desorption Aerosol Gas Chromatograph—Aerosol Mass Spectrometer (TAG-AMS)

To address the critical need for improving the chemical characterization of the organic composition of ambient particulate matter, we introduce a combined thermal desorption aerosol gas chromatograph—aerosol mass spectrometer (TAG-AMS). The TAG system provides in-situ speciation of organic chemicals in ambient aerosol particles with hourly time resolution for marker compounds indicative of sources and transformation processes. However, by itself the TAG cannot separate by particle size and it typically speciates and quantifies only a fraction of the organic aerosol (OA) mass. The AMS is a real-time, in-situ instrument that provides quantitative size distributions and mass loadings for ambient fine OA and major inorganic fractions; however, by itself the AMS has limited ability for identification of individual organic compounds due to the electron impact ionization detection scheme used without prior molecular separation.

The combined TAG-AMS system provides real-time detection by AMS followed by semicontinuous analysis of the TAG sample that was acquired during AMS operation, achieving simultaneous and complementary measurements of quantitative organic mass loading and detailed organic speciation. We have employed a high-resolution time-of-flight mass spectrometer (HR-ToF-MS) to enable elemental-level determination of OA oxidation state as measured on the AMS, and to allow improved compound identification and separation of unresolved complex mixtures (UCM) measured on the TAG. The TAG-AMS interface has been developed as an upgrade for existing AMS systems. Such measurements will improve the identification of organic constituents of ambient aerosol and contribute to the ability of atmospheric chemistry models to predict ambient aerosol composition and loadings.

Copyright 2014 American Association for Aerosol Research     

Measurement of the Nanoparticles Distribution in Flat and Pleated Filters During Clogging

It is currently admitted that for each filtration process using pleated filters, at least three steps can be distinguished: depth and surface filtration, which are common to flat filters, and surface reduction. This step is caused by inefficient filling of the pleat due to the filter geometry. For combustion aerosol, it has been proved that this third step strongly depends on the filtration velocity resulting in an increase of the resistance when air flow decreases. This observation leads one to think that Brownian diffusion, higher for low velocities, could influence the clogging dynamic of a pleated filter.

In this article, a protocol derived from the dust cake preparation method published by Schmidt is developed. The aim of this study is to measure the aerosol penetration inside a filter media as well as in a pleat using a scanning electronic microscope and energy dispersive X-ray spectroscopy elementary detection. This method has also been extended to the study of pleated filters to measure the particle distribution inside the pleat. Filters were loaded with nanoparticles in order to evaluate the specificity of the diffusional regime on the clogging of pleated HEPA filters. For pleated filters, two filtration velocities were investigated: 2.5 and 0.2 cm/s.

Copyright 2014 American Association for Aerosol Research     

Absence of any effect of the electric charging state of particles below 10 nm on their penetration through a metal grid

The effect of image force on the penetration of nanometer particles through metal grids remains a controversial issue. Experimental evidence of the existence and of the absence of such effect have both been reported in the past. A careful experimental work to measure penetration of particles in the mobility equivalent diameter range between 3.4 and 10 nm has been carried out. The possible particle size change between the aerosol generator and the filter has been considered, as well as the possible effect of particle number concentration on the filtration efficiency. The geometric dimensions of the filter allowed attainment of the fully developed parabolic flow velocity profile upstream the grid. Measurements were done at two values of the fiber Reynolds number, 0.09 and 0.12, much smaller than 1, as demanded by the currently accepted filtration theory. Penetration of charged particles, measured in three alternative ways, has been compared with penetration of uncharged and neutral particles (the latter consisting of a mixture of positive, negative, and uncharged particles). Two main conclusions have been reached: (1) the charging state of the particles does not affect their penetration through the metal grid and (2) the experimentally measured penetrations are fairly well predicted by the fan filter model of Cheng and Yeh.

Copyright © 2018 American Association for Aerosol Research     

Rectangular Slit Atmospheric Pressure Aerodynamic Lens Aerosol Concentrator

A rectangular slit micro-aerodynamic-lens (μADL) aerosol concentrator operating at atmospheric pressure has been developed. A single stage version has shown concentration ratios of up to 40:1 for 1 μm aerosol particles while particles larger than 2 μm can be concentrated by more than 100:1 in a single stage. The design of this device has been guided by unsteady 3D CFD modeling using detached eddy simulations (DES), and has been validated experimentally using polystyrene spheres and salt crystals of known aerodynamic diameters. The pressure drop in the device does not exceed 1.5 kPa in the major flow and 0.3 kPa in the minor flow at a total flow of 10 slpm.

Copyright 2014 American Association for Aerosol Research