Design,simulation, and characterization of a radial opposed migration ion and aerosol classifier (ROMIAC)

We present the design, simulation, and characterization of the radial opposed migration ion and aerosol classifier (ROMIAC), a compact differential electrical mobility classifier. We evaluate the performance of the ROMIAC using a combination of finite element modeling and experimental validation of two nearly identical instruments using tetra-alkyl ammonium halide mass standards and sodium chloride particles. Mobility and efficiency calibrations were performed over a wide range of particle diameters and flow rates to characterize ROMIAC performance under the range of anticipated operating conditions. The ROMIAC performs as designed, though performance deviates from that predicted using simplistic models of the instrument. The underlying causes of this non-ideal behavior are found through finite element simulations that predict the performance of the ROMIAC with greater accuracy than the simplistic models. It is concluded that analytical performance models based on idealized geometries, flows, and fields should not be relied on to make accurate a priori predictions about instrumental behavior if the actual geometry or fields deviate from the ideal assumptions. However, if such deviations are accurately captured, finite element simulations have the potential to predict instrumental performance. The present prototype of the ROMIAC maintains its resolution over nearly three orders of magnitude in particle mobility, obtaining sub-20 nm particle size distributions in a compact package with relatively low flow rate operation requirements.

© 2017 American Association for Aerosol Research     

Preparation and characterization of polycaprolactone microspheres by electrospraying

The ability to reproducibly produce and effectively collect electrosprayed polymeric microspheres with controlled morphology and size in bulk form is challenging. In this study, microparticles were produced by electrospraying polycaprolactone (PCL) of various molecular weights and solution concentrations in chloroform, and by collecting materials on different substrates. The resultant PCL microparticles were characterized by optical and electron microscopy to investigate the effect of molecular weight, solution concentration, applied voltage, working distance, and flow rate on their morphology and size. The work demonstrates the key role of a moderate molecular weight and/or solution concentration in the formation of spherical PCL particles via an electrospraying process. Increasing the applied voltage was found to produce smaller and more uniform PCL microparticles. There was a relatively low increase in the particle average size with an increase in the working distance and flow rate. Four types of substrates were adopted to collect electrosprayed PCL particles: a glass slide, aluminium foil, liquid bath, and copper wire. Unlike 2D bulk structures collected on the other substrates, a 3D tubular structure of microspheres was formed on the copper wire which could find application in the construction of 3D tumor mimics.     

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.     

Evaluation of the Alphasense optical particle counter (OPC-N2) and the Grimm portable aerosol spectrometer (PAS-1.108)

We compared the performance of a low-cost (～$500), compact optical particle counter (OPC, OPC-N2, Alphasense) to another OPC (PAS-1.108, Grimm Technologies) and reference instruments. We measured the detection efficiency of the OPCs by size from 0.5 to 5 µm for monodispersed, polystyrene latex (PSL) spheres. We then compared number and mass concentrations measured with the OPCs to those measured with reference instruments for three aerosols: salt, welding fume, and Arizona road dust. The OPC-N2 detection efficiency was similar to the PAS-1.108 for particles larger than 0.8 µm (minimum of 79% at 1 µm and maximum of 101% at 3 µm). For 0.5-µm particles, the detection efficiency of the OPC-N2 was underestimated at 78%, whereas PAS-1.108 overestimated concentrations by 183%. The mass concentrations from the OPCs were linear (r ≥ 0.97) with those from the reference instruments for all aerosols, although the slope and intercept were different. The mass concentrations were overestimated for dust (OPC-N2, slope = 1.6; PAS-1.108, slope = 2.7) and underestimated for welding fume (OPC-N2, slope = 0.05; PAS-1.108, slope = 0.4). The coefficient of variation (CV, precision) for OPC-N2 for all experiments was between 4.2% and 16%. These findings suggest that, given site-specific calibrations, the OPC-N2 can provide number and mass concentrations similar to the PAS-1.108 for particles larger than 1 µm.

Copyright © 2016 American Association for Aerosol Research     

Measurement of size-dependent dynamic shape factors of quartz particles in two flow regimes

Understanding and modeling the behavior of quartz dust particles, commonly found in the atmosphere, requires knowledge of many relevant particle properties, including particle shape. This study uses a single particle mass spectrometer, a differential mobility analyzer, and an aerosol particle mass analyzer to measure quartz aerosol particles mobility (d_m), vacuum aerodynamic, and volume equivalent diameters, mass, composition, effective density, and dynamic shape factor as a function of particle size, in both the free molecular and transition flow regimes. The results clearly demonstrate that dynamic shape factors can vary significantly as a function of particle size. For the quartz samples studied here, the dynamic shape factors increase with size, indicating that larger particles are significantly more aspherical than smaller particles. In addition, dynamic shape factors measured in the free-molecular (χ_v) and transition (χ_t) flow regimes can be significantly different, and these differences vary with the size of the quartz particles. For quartz, χ_v of small (d_m < 200 nm) particles is 1.25, while χ_v of larger particles (d_m ～ 440 nm) is 1.6, with a continuously increasing trend with particle size. In contrast, χ_t of small particles starts at 1.1 increasing slowly to 1.34 for 550 nm diameter particles. The multidimensional particle characterization approach used here goes beyond determination of average properties for each size, to provide additional information about how the particle dynamic shape factor may vary even for particles with the same mass and volume equivalent diameter.

© 2016 American Association for Aerosol Research     

Using a new inversion matrix for a fast-sizing spectrometer and a photo-acoustic instrument to determine suspended particulate mass over a transient cycle for light-duty vehicles

Integrated particle size distribution (IPSD) is a promising alternative method for estimating particulate matter (PM) emissions at low levels. However, a recent light-duty vehicle (LDV) emissions study showed that particle mass estimated using IPSD (M_IPSD) with the TSI Engine Exhaust Particle Sizer (EEPS) Default Matrix was 56–75% lower than mass derived using the reference gravimetric method (M_Grav) over the Federal Test Procedure (FTP). In this study, M_IPSD calculated with a new inversion matrix, the Soot Matrix, is compared with M_Grav and also photoacoustic soot mass (M_Soot), to evaluate potential improvement of the IPSD method for estimating PM mass emissions from LDVs. In addition, an aerodynamic particle sizer (APS) was used to estimate mass emission rates attributed to larger particles (0.54–2.5 µm in aerodynamic diameter) that are not measured by the EEPS. Based on testing of 10 light-duty vehicles over the FTP cycle, the Soot Matrix significantly improved agreement between M_IPSD and M_Grav by increasing slopes of M_IPSD/M_Grav from 0.45–0.57 to 0.76–1.01 for gasoline direct injected (GDI) vehicles; however, for port-fuel injection (PFI) gasoline vehicles, a significant discrepancy still existed between M_IPSD and M_Grav, with M_IPSD accounting for 34 ± 37% of M_Grav. For all vehicles, strong correlations between M_IPSD and M_Soot were obtained, indicating the IPSD method is capable of capturing mass of soot particles. The discrepancy between the M_IPSD and M_Grav for PFI vehicles, which have relatively low PM emissions (0.22 to 1.83 mg/mile), could be partially due to limited size range of the EEPS by not capturing larger particles (0.54–2.5 µm) that accounts for ～0.08 mg/mile of PM emission, uncertainties of particle effective density, and/or gas-phase adsorption onto filters that is not detected by in situ aerosol instrumentation.

Copyright © 2016 American Association for Aerosol Research     

Detachment of droplets by air jet impingement

Surface cleaning using air jets is an appropriate method to remove particles from surfaces especially when cleaning by mechanical methods is not suitable. The detachment behavior of droplets using an air jet is not necessarily the same as solid particles and there is a lack of studies regarding this behavior. In this article, the detachment of droplets on a plastic substrate by air jet impingement was investigated experimentally. Droplets of two different size ranges were impinged by an air jet with different impinging angles. For micrometer-sized droplets, a smaller horizontal velocity was required to detach large droplets. Moreover, the horizontal velocity required to detach 50% number fraction of droplets decreased when the air jet impinging angle increased. Millimeter-sized droplets split into many portions. Most portions remained on the substrate and only a few were resuspended. The remaining portions were distributed in a fan shape, with larger droplets traveling further on the substrate. A linear lower bound of traveled distance was observed. Due to the splitting and the small fraction of resuspension, it should not be expected that air jet cleaning of droplets is the same as that for solid particles.

Copyright © 2017 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     

Characteristics of particles deposited on a single free-fall charged droplet

Particles deposited on a free-fall charged droplet were experimentally studied. A droplet, charged under 40% Rayleigh limit, fell through the particle chamber to capture particles by electrostatic attractions. The velocity of the droplet was smaller than 2.1 m/s. The particle-laden droplet eventually spread on a glass slide, which was further analyzed using optical microscope. It was found that the equivalent number of particles captured by the charged droplet were larger than that of uncharged ones by one order of magnitude at least. Remarkably, particles on the charged droplet agglomerated into a large cluster, which indicates that the agglomerated cluster can be actively precipitated due to the gravity force if the droplet completely evaporates. The front side of the charged droplet was the predominant region to capture the particles. However, the actual area of capture was smaller than hemispheric surface.

Copyright © 2017 American Association for Aerosol Research     

Characterization of a Herrmann-type high-resolution differential mobility analyzer

Aerosol instrument characterization and verification for nanometer-sized particles requires well-established generation and classification instruments. A precise size selection of sub-3-nm charged aerosol particles requires a differential mobility analyzer (DMA), specially designed for the sub-3-nm size range. In this study, a Herrmann-type high-resolution DMA developed at Yale University was characterized in various operation conditions. A relation between sheath flow rate and tetraheptylammonium ion (C₂₈H₆₀N⁺, THA⁺, 1.47 nm, mobility equivalent diameter) was established. The maximum particle size that the DMA was able to classify was 2.9 nm with the highest sheath flow rate of 1427 liters per minute (Lpm), and 6.5 nm with the lowest stable sheath flow rate of 215 Lpm, restricted by the maximum and minimum flow rates provided by our blower. Resolution and transmission of DMA are reported for tetrapropylammonium (C₁₂H₂₈N⁺, TPA⁺, 1.16 nm), THA⁺, and THA₂Br⁺ (1.78 nm) ions measured with two different central electrodes and five different sheath flow rates. The transmission varied between 0.01 and 0.22, and the resolution varied between 10.8 and 51.9, depending on the operation conditions.

Copyright © 2016 American Association for Aerosol Research     

Capture of submicrometer particles in a pressurized electrostatic precipitator

This study investigated the influence of gas pressure on the submicrometer particle capture performance of an electrostatic precipitator (ESP). Current-voltage characteristics and particle capture performance of the ESP were studied in air and in simulated flue gas (SFG) under 1, 2, and 3 atm. Using negative corona and air as the feed gas, the penetration of most particles of 40–400 nm in diameter decreased from 8 × 10^?4 ? 2 × 10^?2 to 2 × 10^?4 ? 1 × 10^?2 as pressure increased from 1 atm to 3 atm at constant current; and increased from 3 × 10^?5 ? 1 × 10^?3 to 2 × 10^?4 ? 1 × 10^?2 as pressure was elevated when the voltage was held roughly constant. Similar type of disparity under different pressures was also observed for positive corona and for SFG. Experiments set up to capture fly ash in the ESP showed that with constant current, higher pressure resulted in a higher initial charge fraction of the particles from the furnace, which could facilitate the penetration of fly ash particles. A semiempirical model was developed based on the Deutsch–Anderson equation and experimental data under 1, 2, and 3 atm to calculate the particle penetrations under high pressure. The total charge number on a particle (n') is calculated by incorporating the effects of current (I) and pressure (P) on relative weights of the diffusion charging number (n_diff) and field charging number (n_field), that is, n' = B₁(I,P)n_diff + B₂(I,P)n_field, where B₁(I,P) and B₂(I,P) are both empirical coefficients dependent on current and pressure. Experimental penetrations under 1.5 and 2.5 atm validated this model over the particle diameter range in 100–400 nm.

Copyright © 2016 American Association for Aerosol Research     

Experimental evaluation of miniature plate DMAs (mini-plate DMAs) for future ultrafine particle (UFP) sensor network

Two iPhone-sized differential mobility analyzers (DMAs) in the parallel-plate configuration (i.e., mini-plate DMAs) were designed and their performance was calibrated in this study in order to gain the instructive knowledge for the future mini-plate DMA design and to have a well-calibrated mini-plate DMA for the ultrafine particle (UFP) sensor network. The performance of mini-plate DMAs was calibrated using the tandem DMA (TDMA) technique. The experimental transfer functions of prototypes at different particle sizes and under various combinational conditions of aerosol and sheath flow rates were derived from the TDMA data. It is concluded that mini-plate DMAs performed reasonably well for UFP sizing. It was also found that the sizing resolution of mini-plate DMAs is closer to the aerosol-to-sheath flow rate ratio when the percentage of aerosol slit opening in length was increased (relative to the width of aerosol classification zone). A new concept of “effective sheath flow rate” was introduced to better interpret the experimental observation on the area and FWHM (full width at half maximum) data of measured DMA transfer functions. Based on the experimental data, we proposed a modified equation for mini-plate DMAs to better calculate the voltage required to size particles of a given electrical mobility.

Copyright © 2016 American Association for Aerosol Research     

Optimized detection of particulates from liquid samples in the aerosol phase: Focus on black carbon

Operational parameters for a single particle soot photometer (SP2) and a CETAC Marin-5 nebulizer were optimized for detection of particulates aerosolized from liquid samples. The sensitivity of nebulization efficiency on nebulizer input gas pressure, liquid sample flow rate, and alcohol doping of the sample were explored. The nebulization efficiency of the Marin-5 was found to be roughly independent of applied gas pressure once above a minimum pressure. The nebulization efficiency changed by ～50% for an order of magnitude change in liquid sample flow. Doping the sample with isopropyl alcohol at a 1:1 ratio results in a ～50% relative increase in nebulization efficiency over a broad range of liquid flows. These results should apply to all particulate materials in the size range studied. SP2 operational parameters including sheath and sample flow were explored to optimize detection of refractory black carbon (rBC) specifically via coupling to the nebulizer. The SP2 tested samples up to 5 cc s?1 with 100% detection of rBC in its size range of detection, with increased sample jet spread and corresponding lack of detected rBC in the air at higher flows, leading to a total undetected rBC mass fraction of ～15% at 16 cc s?1. Varying sheath flow does not improve this result, which is significant because under reasonable Marin-5 operating conditions, the SP2 only samples a fraction of the total air flow out of the nebulizer. Recommended operational parameters for cases of sample with low rBC loadings are presented: first, when very little liquid sample is available; second, when considerable sample is available.

© 2017 American Association for Aerosol Research     

Triboelectric charging of carbon-black-impregnated suspension-polymerized copolymers

Carbon-black/copolymer-based toner particles have been made by fragmentation of suspension-copolymerized styrene-n-butyl methacrylate copolymer leads. The latter were impregnated with carbon black during their formation from the liquid comonomers. This procedure eliminates the costly technology associated with conventional melt-mix processes. Triboelectric coefficients of such toners are generally lower than conventionally formed species, as a result of the different nature of the carbon-black dispersion achieved. Coefficients pass through a maximum value as the carbon-black content is increased, and good correlation with conventionally produced species can be achieved in copolymerizations that are made highly alkaline. This results in simultaneous hydrolysis of some of the butyl methacrylate groups, yielding sodium methacrylate residues. The latter have a very significant effect on the nature of the carbon-black dispersion achieved, which appears to correspond much more closely to that achieved by melt mixing.     

Triboelectric charging of powders: A review

Particles are often electrostatically charged by frictional contact during powder-handling operations. This phenomenon is called ‘triboelectric charging’ or ‘contact electrification’. The charged particles cause problems such as particle deposition and adhesion. In addition, if particles are excessively charged, an electrostatic discharge may occur, which can pose a risk of fire and explosion hazards; thus, to mitigate the adverse effects, it is important to elucidate the underlying triboelectric charging mechanisms. The electrostatics is, on the other hand, very useful in a number of applications that have been developed using the principles. In this review, the basic concepts and theories of charge transfer between solid surfaces are summarized, and chemical factors depending on materials and environmental effects are described. To theoretically analyze the process of particle charging, relevant models are discussed. Using the models, particle charging by repeated impacts on a wall is formulated. To experimentally evaluate particle charging, measurement and characterization methods are outlined. Furthermore, important applications and computer simulations are described.     

Triboelectric charging of monodisperse particles in fluidized beds

To investigate the interplay between particle charging and hydrodynamics in fluidized beds, models for triboelectric charging and electrostatic forces were built into a computational fluid dynamics‐discrete element method model. Charge transfer was governed by the difference in effective work function between contacting materials as well as the electric field at the point of contact. Monodisperse particles were fluidized with an effective work function difference between the particles and the conducting walls. For smaller work function differences, hydrodynamics were not changed significantly as compared with an uncharged case. In these simulations, the average charge saturated at a value much lower than the value anticipated based on the work function difference, and a unimodal distribution of charges was observed. For larger work function differences, particles stuck to walls and bed height oscillations due to slugging were less pronounced. For these cases, a bimodal distribution of charges emerged due to effects from strong electric fields. © 2016 American Institute of Chemical Engineers AIChE J, 63: 1872–1891, 2017     

The structure of fossil fungal spores based on the results of ion etching

New data on the structure of fossil fungal spores in coal beds were obtained with the use of the previously developed method of ionic etching (sputtering). The multilayer structure of spore shells was found for the first time, and considerations of the biochemical composition of layers were presented. The morphology of spores and their internal cytochemical elements was characterized. The reasonability of a further petrographic study of the spores of fungi in the coal beds is discussed.     

Comparison of concentrations and size distributions of fungal spores in buildings with and without mould problems

Concentrations and types of airborne fungal spores and their aerodynamic sizes were studied in nine buildings with mould problems and in matched reference buildings in order to characterize the exposure in buildings with mould problems. Samples were taken with Andersen six-stage impactors in the autumn and winter. In winter, total concentrations of fungal spores in buildings with mould problems were twice those in reference buildings, even though all concentrations were quite small. In the autumn, the differences were less obvious due to the masking effect of outdoor air fungi. Also, the compositions of fungal genera differed in buildings with mould problems from that in reference buildings. The difference was most significant in the size range 2.1-4.7 μm. The size distribution of fungal spores in dwellings with mould problems skewed towards larger particles than in reference dwellings. The observed differences in size distribution indicate differences in airborne behaviour of fungal spores and, consequently, in respiratory exposure.