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     

Real-time effective density monitor (DENSMO) for aerosol nanoparticle production

A new instrument, density monitor (DENSMO), for aerosol particle size distribution characterization and monitoring has been developed. DENSMO is operationally simple and capable of measuring the effective density as well as the aerodynamic and the mobility median diameters with a time resolution of 1 s, from unimodal particle size distributions. The characterization is performed with a zeroth order mobility analyzer in series with a low pressure impactor and a filter stage. The operation of DENSMO was investigated with sensitivity analysis and, based on the results, optimal operation parameters were determined. DENSMO was also compared, in lab test measurements, against a reference method with several particle materials with bulk densities from 0.92 to 10.5 g/cm³. The results show that the deviation from the reference method was less than 25% for suitable materials.

Copyright © 2016 American Association for Aerosol Research     

Comparative study: Correlating extraction efficiency for Hg(II), Cd(II), and Pb(II) metal ions with the chelate stability and total hardness in simple nitrogen donors

Four N-donors (PHDA, ATPH, APHO, and MTAN) containing NH₂ were used to extract Hg(II), Cd(II), and Pb(II). Their extraction capacity was determined by measurement of percentage extraction.

The chelates extract these metals differently: for example, efficiency of ATPH was the highest for Hg(II) compared to PHDA, APHO, and MTAN.

The extraction efficiency was found to depend on: donor atom hardness, chelate total hardness, metal: chelate mole ratio and substituent’s electronic effects. Among all, total hardness and chelate stability are key factors and molecule of small (E_HOMO – E_LUMO) is more reactive, where extraction efficiency increases as molecular stability decreases.     

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     

Aerosol Measurement by Induced Currents

We introduce a new electrical measurement technique for aerosol detection, based on pulsed unipolar charging followed by a non-contact measurement of the rate of change of the aerosol space charge in a Faraday cage. This technique, which we call “aerosol measurement with induced currents,” has some advantages compared to the traditional method of collecting the charged particles on either an electrode or with a particle filter. We describe the method and illustrate it with a simple and miniature (shirt-pocket-sized) instrument to measure lung-deposited surface area. Aerosol measurement by induced currents can also be applied to more complex devices.

Copyright 2014 American Association for Aerosol Research     

A practical set of miniaturized instruments for vertical profiling of aerosol physical properties

In situ atmospheric aerosol measurements have been performed from a Manta unmanned aircraft system (UAS) using recently developed miniaturized aerosol instruments. Flights were conducted up to an altitude of 3000 m (AMSL) during spring 2015 in Ny-Ålesund, Svalbard, Norway. We use these flights to demonstrate a practical set of miniaturized instruments that can be deployed onboard small UASs and can provide valuable information on ambient aerosol. Measured properties include size-resolved particle number concentrations, aerosol absorption coefficient, relative humidity, and direct sun intensity. From these parameters, it is possible to derive a comprehensive set of aerosol optical properties: aerosol optical depth, single scattering albedo, and asymmetry parameter. The combination of instruments also allows us to determine the aerosol hygroscopicity.

Copyright © 2017 American Association for Aerosol Research     

Comparative Testing and Evaluation of Nine Different Air Samplers: End-to-End Sampling Efficiencies as Specific Performance Measurements for Bioaerosol Applications

Accurate exposure assessments are needed to evaluate health hazards caused by airborne microorganisms and require air samplers that efficiently capture representative samples. This highlights the need for samplers with well-defined performance characteristics. While generic aerosol performance measurements are fundamental to evaluate/compare samplers, the added complexity caused by the diversity of microorganisms, especially in combination with cultivation-based analysis methods, may render such measurements inadequate to assess suitability for bioaerosols. Specific performance measurements that take into account the end-to-end sampling process, targeted bioaerosol and analysis method could help guide selection of air samplers.

Nine different samplers (impactors/impingers/cyclones/ electrostatic precipitators/filtration samplers) were subjected to comparative performance testing in this work. Their end-to-end cultivation-based biological sampling efficiencies (BSEs) and PCR-/microscopy-based physical sampling efficiencies (PSEs) relative to a reference sampler (BioSampler) were determined for gram-negative and gram-positive vegetative bacteria, bacterial spores, and viruses.

Significant differences were revealed among the samplers and shown to depend on the bioaerosol's stress–sensitivity and particle size. Samplers employing dry collection had lower BSEs for stress-sensitive bioaerosols than wet collection methods, while nonfilter-based samplers showed reduced PSEs for 1 μm compared to 4 μm bioaerosols. Several samplers were shown to underestimate bioaerosol concentration levels relative to the BioSampler due to having lower sampling efficiencies, although they generally obtained samples that were more concentrated due to having higher concentration factors.

Our work may help increase user awareness about important performance criteria for bioaerosol sampling, which could contribute to methodological harmonization/standardization and result in more reliable exposure assessments for airborne pathogens and other bioaerosols of interest.

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     

Thermal/optical reflectance equivalent organic and elemental carbon determined from federal reference and equivalent method fine particulate matter samples using Fourier transform infrared spectrometry

A fine particulate matter (PM_2.5) monitoring network of filter-based federal reference methods and federal equivalent methods (FRM/FEMs) is used to assess local ambient air quality by comparison to National Ambient Air Quality Standards (NAAQS) at about 750 sites across the continental United States. Currently, FRM samplers utilize polytetrafluoroethylene (PTFE) filters to gravimetrically determine PM_2.5 mass concentrations. At most of these sites, sample composition is unavailable. In this study, we present the proof-of-principle estimation of the carbonaceous fraction of fine aerosols on FRM filters using a nondestructive Fourier transform infrared (FT-IR) method. Previously, a quantitative FT-IR method accurately determined thermal/optical reflectance equivalent organic and elemental carbon (a.k.a., FT-IR organic carbon [OC] and elemental carbon [EC]) on filters collected from the chemical speciation network (CSN). Given the similar configuration of FRM and CSN aerosol samplers, OC and EC were directly determined on FRM filters on a mass-per-filter-area basis using CSN calibrations developed from nine sites during 2013 that have collocated CSN and FRM samplers. FRM OC and EC predictions were found to be comparable to those of the CSN on most figures of merit (e.g., R²) when the type of PTFE filter used for aerosol collection was the same in both networks. Although prediction accuracy remained unaffected, FT-IR OC and EC determined on filters produced by a different manufacturer show marginally increased prediction errors suggesting that PTFE filter type influences extending CSN calibrations to FRM samples. Overall, these findings suggest that quantifying FT-IR OC and EC on FRM samples appears feasible.

© 2018 American Association for Aerosol Research     

Using the WIBS-4 (Waveband Integrated Bioaerosol Sensor) Technique for the On-Line Detection of Pollen Grains

Primary biological aerosol particles (PBAP) such as pollen and fungal spores can induce allergenic responses and affect health in general. Conditions such as allergic rhinitis (hay fever) and asthma have been related to pollen concentrations. Likewise some pollen have been shown to induce ice nucleation and cloud condensation at higher temperatures than those associated with some chemical species, thereby affecting planet Earth's albedo and overall radiative balance. Hence, the near real-time (on-line) monitoring of airborne pollen and other PBAP using a variety of spectroscopic and light scattering techniques represents an area of growing development and consequence.

In this study, two separate field campaigns (one at a rural site in Ireland and the other at an urbanized location in Germany) were performed to detect and quantify pollen releases using a novel on-line fluorescence spectrometer (WIBS-4). The results were compared with results obtained using more traditional Hirst-type impactors. Size, “shape,” and fluorescence characteristics of ambient particles were used to determine the concentrations and identity of the PBAP likely to be pollen grains.

The concentration results obtained for both methodologies at both the Irish and German sites correlated very well, with R ² values >0.9 determined for both campaigns. Furthermore, the sizing data available from the WIBS-4 approach employed in Ireland indicated that pollen grains can be identified in appropriate conditions. WIBS-4 measurements of Yew pollen both in the laboratory and at the rural site indicated almost identical size ranges of 25 to 27 μm. Yew pollen is generally reported to be in this range, but the measurements reported here are the first of their type providing data on the size of in-flight Yew pollen.

Copyright 2014 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     

Recent Advances in Flame Retardancy of Polymeric Materials (Materials,Applications, Industry Developments,Markets)

The Fifth Annual Conference on Flame Retardancy was held May 24–26, 1994, at the Ramada Plaza Hotel, Stamford, Connecticut, USA. The conference was organized by Business Communications Company, Inc., Norwalk, Connecticut (Company President, Mr. Louis Naturman; Conference Coordinator, Mrs. Sharon D. Faust). New materials (polymers, blends, composites), their applications, industry developments, and markets were considered. Specifically, the most important topics were:

Introduction of new technological achievements and development in the field of flame retardancy (FR)

Review of the current state of science and technology in FR

Review of applications and markets for FR products

Presentation of recent developments in local and global standardization and in testing technology

Discussion of toxicity and environmental issues

Provision of a unique opportunity for newcomers to FR research technology and marketing to become acquainted with the FR field in all its aspects

Discussion of halogen-based and non-halogen-based flame retardant chemicals, syngergism, intumescence, FR mechanisms, modeling, flame parameters, inherently FR polymers, and polymer blends     

Modification and laboratory evaluation of a TSI ultrafine condensation particle counter (Model 3776) for airborne measurements

A butanol-type ultrafine condensation particle counter (UCPC, Model 3776, TSI, Inc., Shoreview, MN, USA), which can achieve a 50% detection efficiency diameter (d₅₀) of 2.5 nm using a capillary-sheath structure, was modified and tested in the laboratory for airborne measurements. The aerosol flow rate through the capillary is a key factor affecting the quantification of aerosol particle number concentrations. A pressure-dependent correction factor for the aerosol flow rate was determined using a newly added mass flow meter for the sheath flow and the external calibration system. The effect of particle coincidence in the optical sensing volume was evaluated using an aerosol electrometer (AE, Model 3068B, TSI, Inc.) as a reference. An additional correction factor for the coincidence effect was derived to improve the quantification accuracy at higher concentrations. The particle detection efficiency relative to the AE was measured for mobility diameters of 3.1–50 nm and inlet absolute pressures of 101–40 kPa. The pressure dependence of the d₅₀ value, asymptotic detection efficiency, and shape of the particle detection efficiency curve is discussed, along with simple theoretical calculations for the diffusion loss of particles and the butanol saturation ratio in the condenser.

© 2017 American Association for Aerosol Science     

Intercomparison of a portable and two stationary mobility particle sizers for nanoscale aerosol measurements

During occupational exposure studies, the use of conventional scanning mobility particle sizers (SMPS) provides high quality data but may convey transport and application limitations. New instruments aiming to overcome these limitations are being currently developed. The purpose of the present study was to compare the performance of the novel portable NanoScan SMPS TSI 3910 with that of two stationary SMPS instruments and one ultrafine condensation particle counter (UCPC) in a controlled atmosphere and for different particle types and concentrations.

The results show that NanoScan tends to overestimate particle number concentrations with regard to the UCPC, particularly for agglomerated particles (ZnO, spark generated soot and diesel soot particles) with relative differences >20%. The best agreements between the internal reference values and measured number concentrations were obtained when measuring compact and spherical particles (NaCl and DEHS particles). With regard to particle diameter (modal size), results from NanoScan were comparable < [± 20%] to those measured by SMPSs for most of the aerosols measured.

The findings of this study show that mobility particle sizers using unipolar and bipolar charging may be affected differently by particle size, morphologies, particle composition and concentration. While the sizing accuracy of the NanoScan SMPS was mostly within ±25%, it may miscount total particle number concentration by more than 50% (especially for agglomerated particles), thus making it unsuitable for occupational exposure assessments where high degree of accuracy is required (e.g., in tier 3). However, can be a useful instrument to obtain an estimate of the aerosol size distribution in indoor and workplace air, e.g., in tier 2.     

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     

Modeling of nonstationary processes during separation of multicomponent isotope mixtures

The article presents a mathematical model for calculation of nonstationary hydraulic and separation processes in a gas centrifuge (GC) cascade for separation of multicomponent isotope mixtures. The model has been applied to calculate the parameters of nonstationary processes in a GC cascade for separation of krypton, germanium and tungsten isotopes. As a result, the specifics of the excess holdup distribution along the cascade stages has been identified, and variations of the isotope concentrations in a nonstationary process have been revealed. The data obtained show that the proposed mathematical model is able to adequately describe nonstationary hydraulic processes in GC cascades for separation of multicomponent isotope mixtures.

Highlights:

Mathematical model of cascade for separation of multicomponent isotope mixture has been developed.

The model verification has been done.

The isotope transient regularities into cascade during nonstationary processes has been identified.     

Inter-comparison of low-cost sensors for measuring the mass concentration of occupational aerosols

Low-cost sensors are effective for measuring the mass concentration of ambient aerosols and second-hand smoke in homes, but their use at concentrations relevant to occupational settings has not been demonstrated. We measured the concentrations of four aerosols (salt, Arizona road dust, welding fume, and diesel exhaust) with three types of low-cost sensors (a DC1700 from Dylos and two commodity sensors from Sharp), an aerosol photometer, and reference instruments at concentrations up to 6500 µg/m³. Raw output was used to assess sensor precision and develop equations to compute mass concentrations. EPA and NIOSH protocols were used to assess the mass concentrations estimated with low-cost sensors compared to reference instruments. The detection efficiency of the DC1700 ranged from 0.04% at 0.1 µm to 108% at 5 µm, as expected, although misclassification of fine and coarse particles was observed. The raw output of the DC1700 had higher precision (lower coefficient of variation, CV = 7.4%) than that of the two sharp devices (CV = 25% and 17%), a finding attributed to differences in manufacturer calibration. Aerosol type strongly influenced sensor response, indicating the need for on-site calibration to convert sensor output to mass concentration. Once calibrated, however, the mass concentration estimated with low-cost sensors was highly correlated with that of reference instruments (R²= 0.99). These results suggest that the DC1700 and Sharp sensors are useful in estimating aerosol mass concentration for aerosols at concentrations relevant to the workplace.

© 2016 American Association for Aerosol Research     

Toward the Determination of Joint Volatility-Hygroscopicity Distributions: Development and Response Characterization for Single-Component Aerosol

This work presents the development and characterization of a thermodenuder for the study and interpretation of aerosol volatility. Thermodenuder measurements are further combined with a continuous-flow streamwise thermal gradient CCN counter to obtain the corresponding aerosol hygroscopicity. The thermodenuder response function is characterized with monodisperse aerosol of variable volatility and hygroscopicity. The measurements are then interpreted with a comprehensive instrument model embedded within an optimization framework to retrieve aerosol properties with constrained uncertainty. Special attention is given to the interpretation of the size distribution of the thermodenuded aerosol, deconvoluting the effects of impurities and multiple charging, and to simplifications on the treatment of thermodenuder geometry, temperature, the cooling section, and the effects of curvature and accommodation coefficient on inferred particle volatility. Retrieved vapor pressures are consistent with published literature and shown to be most sensitive to uncertainty in the accommodation coefficient.

Copyright 2014 American Association for Aerosol Research     

A Moment Method of the Log-Normal Size Distribution with the Critical Size Limit in the Free-Molecular Regime

The present work proposes new formulations of the moment in the free-molecular size regime involving (1) boundary equations at the critical size for evaporation, condensation, and nucleation, and (2) b coefficient functions for coagulation that are improved by two parameters (standard deviation and nondimensional critical size). Using these formulations based on the error function, the critical particle size is readily introduced into the log-normal moment method for applications in general aerosol dynamics. In the situation that the particle size distribution is located near the critical size, the proposed moment method (which considers the critical size limit) improves predictions of total particle number and particle volume concentrations as compared with previously well-used log-normal moment methods for sizes ranging from 0 to ∞. However, as the size distribution approaches to the continuum size regime, the influence of the critical size becomes smaller. Thus, the new formulations are expected to improve microphysical parameterization in the free-molecular regime in aerosol-transport models.

Copyright 2014 American Association for Aerosol Research