Inter-laboratory performance between two nanoparticle air filtration systems using scanning mobility particle analyzers

The performance of two aerosol testing systems, at two different laboratories (University of Nebraska Medical Center—UNMC and 3M Company), was compared to evaluate which calibration procedures minimized variability in filter testing of nanoparticles. Both charged electret and uncharged flat-web fibrous filters were used with Scanning Mobility Particle Sizers to give upstream and downstream size distributions and calculate filter penetration. Challenge aerosols were polydisperse nanoparticles of sodium chloride (NaCl) ranging from approximately 10–300 nm and monodisperse polystyrene latex (PSL) spheres of preselected sizes, including 40, 60, 100, and 200 nm. The implementation of optimized procedures resulted in comparable filtration performance at the two testing sites with challenges of NaCl particles and PSL spheres. The penetration results for the uncharged filter were nearly identical for both challenges, while lower penetration through the charged filter was observed with NaCl aerosol, probably due to differences in NaCl and PSL dielectric constants. Results showed that reproducible, comparable nanoparticle filtration data could be achieved between two separate laboratories when sources of error and proper calibration procedures were addressed.     

Estimation of the upper limit of aerosol nanoparticles penetration through inhomogeneous fibrous filters

The fully segregated flow model (FSFM) was formulated to describe filtration of aerosol nanoparticles in polydisperse fibrous filters made of fibers with different diameters. The model is capable of predicting significantly higher penetration of nanoparticles through polydisperse filters than it may be expected from the classical theory applied to a mean fiber diameter. The model was solved numerically in the case of the log-normal fiber size distribution, and a simple correlation between the actual penetration through a polydisperse filter and the one calculated for the geometric mean fiber diameter was proposed. Equivalent fiber diameter for deposition due to Brownian diffusion was determined and it was found to be dependent on particle size and filter’s polydispersity degree, being significantly greater than any mean fiber diameter. It was noted that it is impossible to select any one universal mean fiber diameter to describe penetration of nanoparticles with different sizes. It was also shown that in the case of a polydisperse fibrous filter the apparent exponent of the Peclet number based on the mean fiber diameter is greater than the expected value of −2/3 for diffusional deposition in a monodisperse filter. This prediction is in agreement with the available experimental data. The FSFM is expected to give the estimation of the upper limit of nanoparticles penetration in polydisperse fibrous filters.     

Experimental Studies of Electrically Active Fibrous Filter Loading

Electrically active fibrous filters, that is fibrous filters whose fibres carry a permanent electric charge, are an increasingly popular alternative to conventional fibrous filters in applications where low pressure drop and high collection efficiencies are critical. The advantage of these materials is the additional collection efficiency, due to electrostatic mechanisms, that can be achieved without pressure drop increase. The efficiency of these materials can fall as they are loaded with aerosol particles, so it is necessary that proper account be taken of this process during use. A complete understanding of the mechanisms responsible for this reduction in efficiency has not yet been reached. An experimental study has been undertaken at Loughborough University of Technology investigating the loading behaviour of a mixed fibre type electrically active material. The experimental techniques used in this work and experimental findings with a range of aerosol particle sizes are reported here.     

Single Fibre Collection and Adhesion Efficiency for Biological Particles

Good efficiency of fibrous filters requires not only single fibre collision efficiency but also single fibre adhesion efficiency. Especially at higher filtration velocities (>0.5 m/s) and for larger particles (>5 μm), particles may not adhere to the fibres after impact but rebound off the fibres. The particle bounce mechanisms on filter fibres are rarely investigated, particularly for biological particles (fungal spores, pollen) which are in the size range where particle bounce is dominant. With a special measuring device working with a modified optical particle counter, single fibre collection efficiencies could be determined for various biological and non-biological particles and fibre materials. The results reveal that the single fibre collection efficiencies for biological and non-biological materials are very similar and increase with increasing Stokes number to a maximum value of about 0.45. Beyond a Stokes number of about 2, the single fibre collection efficiency decreases with increasing Stokes number, indicating particles bouncing off the fibres. A comparison of the experimentally determined collection efficiencies with published theoretical considerations showed insufficient agreement, suggesting that the properties of the particle materials do not significantly affect the adhesion efficiency. Based on the experimental results, a mathematical correlation for the single fibre collection efficiency was derived which allows a more accurate prediction of the collection efficiency of fibrous filters.     

A performance study of a nonlinear all Fibre Michelson interferometer,add-drop multiplexer,based in Fibre Bragg grating mirrors

In this paper we report a numerical investigation of an all-fibre narrowband transmission filter in the arrangement of two identical photo-imprinted Bragg gratings symmetrically located in each arm of a Michelson Interferometer, simulating, thus, an add-drop filter. The transmission characteristics and in addition the crosstalk level and extinction ratio, were studied. The coupled mode theory and the fourth-order Runge-Kutta method were applied, respectively, to solve the coupled differential equations. This is the first study, as far as we now, of the performance of this device considering the nonlinearity of the fibre coupler and linearity of the fibre Bragg gratings. The device is presenting a highly nonlinear behavior dependence in dephasing and pump power.     

Fractional Penetrations for Electrostatically Charged Fibrous Filters in the Submicron Particle Size Range

Particle penetrations through commercially available, electrically charged fibrous filters have been measured. The particles were monodisperse and in charge equilibrium. Tests were conducted for several particle sizes in the range 0.02 μm ≤ x ≤ 1 μm. The face velocity was varied in the range of 2 cm/s to 30 cm/s. The penetration has a bimodal dependence on particle size. This behaviour is not found for uncharged filters. The reduction in penetration and bimodal dependence is attributed to electrical collection effects. As the face velocity is increased, the electrostatic collection effects are significantly reduced. The results agree quantitatively with a model for particle penetration through charged fibrous filters recently presented in the literature by the authors.     

Experimental study of nanoparticles penetration through commercial filter media

In this study, nanoparticle penetration was measured with a wide range of filter media using silver nanoparticles from 3 nm to 20 nm at three different face velocities in order to define nanoparticle filtration characteristics of commercial fibrous filter media. The silver particles were generated by heating a pure silver powder source via an electric furnace with a temperature of 870°C, which was found to be the optimal temperature for generating an adequate amount of silver nanoparticles for the size range specified above. After size classification using a nano-DMA, the particle counts were measured by an Ultrafine Condensation Particle Counter (UCPC) both upstream and downstream of the test filter to determine the nanoparticle penetration for each specific particle size. Particle sampling time continued long enough to detect more than 10⁵ counts at the upstream and 10 counts at the downstream sampling point so that 99.99% efficiency can be detected with the high efficiency filter. The results show a very high uniformity with small error bars for all filter media tested in this study. The particle penetration decreases continuously down to 3 nm as expected from the classical filtration theory, and together with a companion modeling paper by␣Wang et al. in this same issue, we found no significant evidence of nanoparticle thermal rebound down to 3 nm.     

Numerical simulation of a gradient-index fibre probe and its properties of light propagation

In order to verify the properties of the light propagating through a gradient-index (GRIN) fibre probe for optical coherence tomography (OCT), numerical simulation using the optical software GLAD is carried out. Firstly, the model of the GRIN fibre probe is presented, which is consisted of a single mode fibre (SMF), a no-core fibre (NCF), a GRIN fibre lens and an air path. Then, the software GLAD is adopted to numerically investigate how the lengths of the NCF and the GRIN fibre lens influence the performance of the Gaussian beam focusing through the GRIN fibre probe. The simulation results are well consistent with the experimental ones, showing that the GLAD based numerical simulation technique is an intuitive and effective tool for the verification of the properties of the light propagation. In this paper, we find that on the conditions of a constant GRIN fibre lens length of 0.1 mm and an NCF length of 0.36 mm, the working distance of the probe will be 0.75 mm and the focus spot size is 32 μm.     

Off-centre impact on a horizontal fibre

Fibre filters that consist of a network of randomly oriented thin fibres are commonly used to recover the liquid phase from an aerosol. During the filtration process, drops of the aerosol impact the solid fibres of the filter. If the impact velocity is smaller than a threshold velocity V_c, the drop is entirely captured by the fibre whereas if the velocity is larger than V_c, only a small portion of fluid remains trapped on the solid fibre. While this threshold velocity has been determined when the impact is centred – i.e. when the trajectory of the drop and the axis of the fibre do intersect – fewer studies are related to off-centre impact. Here, we focus on the influence of the relative position of the drop and the fibre on the impact. For velocity larger than V_c, we vary the eccentricity and measure the ability of the fibre to retain the liquid phase. Quite surprisingly, we show that off-centre impact situation enhances the ability of the fibre to capture a portion of the drop.     

Filtration characteristics of fibrous filter following an electrostatic precipitator

This study presents the results of investigations of a hybrid electrostatic filtration system (HEFS), which combines an electrostatic precipitator (ESP) and a fibrous filter installed downstream of the ESP. The particles escaping from the ESP carry large amount of charge and this can increase the filtration efficiency of the fibrous filter. The filtration characteristics, including the efficiency, pressure drop and ozone generation, were investigated experimentally. The influence of system parameters, including the filter type, applied voltage, and distance between the ESP and fibrous filter on the overall efficiency were also studied. The measured results show that utilizing the non-high-efficient fibrous filter to remove the charged particle could provide a much higher efficiency without adding the pressure drop due to the electrostatic force. If the efficiency was similar, the ozone generated by HEFS was much lower than that of the single ESP. The results proved that filter efficiency increased with a higher applied voltage and higher initial mechanical filtration efficiency. The distance between the filter and ESP had no influence on the system filtration efficiency. The efficiency of filter in HEFS supplied with the positive voltage was slightly lower than for the negative voltage. In addition, the mathematical model was utilized to model the air filter efficiency in HEFS. The modeled and measured results agreed reasonably. Overall conclusion is that the HEFS could operate at a high efficiency with the lower applied voltage, ozone generation and pressure drop.     

Erbium-doped twin-core fibre narrow-band filter for fibre lasers

Theoretical analysis of a recently experimentally demonstrated erbium-doped twin-core (ErTCF) fibre saturable-absorber-based narrow-band tracking filter is presented. Basic qualitative characteristics of the filter are deduced from an analogy with an ideally inhomogeneously broadened saturable absorber while the effect of wide band saturation of absorption due to spatial overlap of individual spectral components of the radiation is studied in detail using a numerical model based on coupled mode theory, resolved in space and frequency. The effect of the input power, twin-core fibre length and laser operating wavelength on the spectral characteristics of the filter is investigated.     

Morphology of single-wall carbon nanotube aggregates generated by electrospray of aqueous suspensions

Airborne single-wall carbon nanotubes (SWCNTs) have a high tendency to agglomerate due to strong interparticle attractive forces. The SWCNT agglomerates generally have complex morphologies with an intricate network of bundles of nanotubes and nanoropes, which limits their usefulness in many applications. It is thus desirable to produce SWCNT aerosol particles that have well-defined, unagglomerated fibrous morphologies. We present a method to generate unagglomerated, fibrous particles of SWCNT aerosols using capillary electrospray of aqueous suspensions. The effects of the operating parameters of capillary electrospray such as strength of buffer solution, capillary diameter, flow rate, and colloidal particle concentration on the size distributions of SWCNT aerosols were investigated. Results showed that electrospray from a suspension of higher nanotube concentration produced a bimodal distribution of SWCNT aerosols. Monodisperse SWCNT aerosols below 100 nm were mostly non-agglomerated single fibers, while polydisperse aerosols larger than 100 nm had two distinct morphologies: a ribbon shape and the long, straight fiber. Possible mechanisms are suggested to explain the formation of the different shapes, which could be used to produce SWCNT aerosols with different morphologies.     

Heat transfer by radiation and conduction in fibrous media without axial symmetry

Coupled radiative and conductive heat transfer in a fibrous medium formed by silica fibres is investigated in this paper by not taking account of the axial symmetry for the distribution of fibres or the boundary conditions. Radiative properties of the medium are calculated by using the Mie theory. The model obtained depends only on optical parameters (indices of silica) and on morphological parameters (diameter and orientation of the fibres, density of the medium). Simulations make it possible to study the strongly anisotropic behaviour of the scattering of the radiation by a fibre and to study the influence of various parameters on the radiative properties of the medium. The results of the Mie theory make possible the simulation of the heat transfer coupled by radiation and conduction. To do this, we introduce a new numerical scheme able to simulate heat transfer in the lack of axial symmetry. With this model, we can show the effects of distribution of fibres and temperature on the thermal behaviour of the medium as well as showing the importance of the phenomenon of scattering in fibrous media.     

Measuring electrostatic properties of fibrous materials: A review and a modified surface potential decay technique

Electrostatic charging of fibrous materials are of concern for the performance of electret filters and comfort issues of textiles. However there is a huge controversy in the characterization techniques of ion beam irradiated or corona charged fibrous materials. In this study we reported a reliable, simple surface potential measurement method. Large variations in potential measurements were found to be mostly due to structural nonuniformity such as packing density, thickness and fiber–fiber proximity. Test samples were prepared after optimizing those parameters and we were able to reduce coefficient of variation below 15%. Methods that were developed so far were also reviewed.     

Aerosol light absorption and its measurement: A review

Light absorption by aerosols contributes to solar radiative forcing through absorption of solar radiation and heating of the absorbing aerosol layer. Besides the direct radiative effect, the heating can evaporate clouds and change the atmospheric dynamics. Aerosol light absorption in the atmosphere is dominated by black carbon (BC) with additional, significant contributions from the still poorly understood brown carbon and from mineral dust. Sources of these absorbing aerosols include biomass burning and other combustion processes and dust entrainment.For particles much smaller than the wavelength of incident light, absorption is proportional to the particle volume and mass. Absorption can be calculated with Mie theory for spherical particles and with more complicated numerical methods for other particle shapes.The quantitative measurement of aerosol light absorption is still a challenge. Simple, commonly used filter measurements are prone to measurement artifacts due to particle concentration and modification of particle and filter morphology upon particle deposition, optical interaction of deposited particles and filter medium, and poor angular integration of light scattered by deposited particles. In situ methods measure particle absorption with the particles in their natural suspended state and therefore are not prone to effects related to particle deposition and concentration on filters. Photoacoustic and refractive index-based measurements rely on the heating of particles during light absorption, which, for power-modulated light sources, causes an acoustic signal and modulation of the refractive index in the air surrounding the particles that can be quantified with a microphone and an interferometer, respectively. These methods may suffer from some interference due to light-induced particle evaporation. Laser-induced incandescence also monitors particle heating upon absorption, but heats absorbing particles to much higher temperatures to quantify BC mass from the thermal radiation emitted by the heated particles. Extinction-minus-scattering techniques have limited sensitivity for measuring aerosol light absorption unless the very long absorption paths of cavity ring-down techniques are used. Systematic errors can be dominated by truncation errors in the scattering measurement for large particles or by subtraction errors for high single scattering albedo particles. Remote sensing techniques are essential for global monitoring of aerosol light absorption. While local column-integrated measurements of aerosol light absorption with sun and sky radiometers are routinely done, global satellite measurements are so far largely limited to determining a semi-quantitative UV absorption index.     

Chemical composition and mass closure for PM10 aerosols during the 2005 dry season at a rural site in Morogoro,Tanzania

An intensive aerosol field campaign was carried out from 11 July to 11 August 2005 (dry season) at a rural site in Morogoro, Tanzania. The objectives were to determine the chemical composition of the atmospheric particulate matter (PM) and to examine to which extent the gravimetric PM mass can be explained by the measured aerosol components. Two low‐volume filter samplers were deployed, a PM₁₀ filter holder with two Whatman QM‐A quartz fibre filters in series and a Gent PM₁₀ stacked filter unit (SFU) sampler with coarse and fine Nuclepore polycarbonate filters. The samplers operated in parallel and a total of 51 parallel collections were made. All samples were analysed for the PM mass by weighing. Depending on the sampler type and/or collection substrate, further analyses were performed for 25 elements by particle‐induced x‐ray emission spectrometry, for major water‐soluble inorganic ions by ion chromatography, and for organic carbon and elemental carbon by a thermal—optical transmission technique. The PM₁₀ mass, as derived from the SFU samples, was, on an average, 46 ± 12 µg/m³. Aerosol chemical mass closure calculations were made for this PM₁₀ mass; eight aerosol components were considered and they explained 93% of the average PM₁₀ mass. Organic matter (OM) and crustal matter were the dominant aerosol components; they accounted for, on an average, 44% and 33%, respectively, of the PM₁₀ aerosol. The large contribution from OM is thought to originate mainly from the burning of biomass, especially of charcoal and agricultural residues. Copyright © 2009 John Wiley & Sons, Ltd.     

A Novel Method for Inhalation Control of Workplace Anthropogenic Pollutant Particles

This in vitro study investigated electrically charging effect on the deposition of inhaled workplace anthropogenic pollutant particles (APP) in a hollow throat cast model. Many occupational lung diseases are associated with exposure to workplace dust particles and other pollutants. Since the human throat is an effective filter, this study devised a novel idea of charging particles, and studying their deposition in the throat. Simulated workplace aerosol particles were generated from a commercially available nebulizer, and charged by a corona charger. Charged and uncharged particles were allowed to pass through a polyester resin cast of cadaver based throat, a replicate of a human oropharyngeal region. The aerosol particles' size and charge distribution were characterized by an Electronic Single Particle Aerodynamic Relaxation Time (ESPART) analyzer before and after passing the throat cast. The ESPART operates on the principle of Laser Doppler Velocimetry to measure simultaneously aerodynamic diameter and electrostatic charge on a single particle basis and in real time. The study results revealed that electrically charging increased agglomeration of smaller particles and increased deposition. Deposition of charged particles increased with increasing particle size which can be explained as the effect of inertial impaction.     

Efficiency of protective dermal equipment against silver nanoparticles with water aerosol

Protective dermal equipment (PDE) should be provided for protecting against the penetration of nanomaterials into the skin in the workplace. It is important that workers utilize appropriate PDE with characteristics to accomplish this. During the liquid-phase process, nanomaterials are released with water aerosol, which can easily affect the health of workers. The efficiency of PDE in protecting workers against silver nanoparticles (AgNPs) aerosolized with water aerosol was evaluated. The rate of penetration of AgNPs with water aerosol through cleanroom wear was faster than that for a lab coat. This can be attributed to differences in the filling rate of water, as the result of differences in capillary force. Therefore, humidity appears to be a major factor in the rate of penetration of nanomaterials in the presence of water aerosol. Although no penetration was observed when disposable protective gloves were observed, the presence of AgNPs on the surface of gloves was clearly found. Based on these findings, recommendations for the safe use of PDE can now be made.     

Experimental Investigation of Particle Collection on Single Fibres of Different Configurations

This presentation is part of a fundamental investigation into the influence of structural characteristics on particle collection in fibrous filters. The collection on single fibres has been experimentally determined, whereby different fibre configurations were set. For this purpose, particle fluxes upstream and downstream of the fibres were measured simultaneously using an optical particle counter which operates with two purely optically delineated measuring volumes and two detectors. The experimental results are compared with published values.     

Treatment of submicron particles using an electrostatic agglomerator in DC-negative voltage: Re-entrainment experimental study and modeling

The aim of this study is to highlight the re-entrainment phenomenon encountered with an electrostatic agglomerator having a fibrous collecting electrode and treating submicron particles. The idea is to propose a process to control the particle number emitted by automotive diesel engines. Rather than trying to directly measure agglomerates of diesel particles exiting the agglomerator, we propose working with a synthetic submicron aerosol dispersed in ambient filtered air as a representative exhaust gas. The study will contribute to build a numerical modeling of the behavior of particles in such a process. The particular point that will be treated here is the collected particle re-entrainment as micron-sized agglomerates. We propose a joint experimentation/modeling approach to approximate the re-entrained aerosol size distribution in controlled conditions. From the modeling point of view, a local approach which uses the method of the balance of moments on agglomerates provides the re-entrained particle size in the agglomerator, according to the filtration conditions. The experimental approach confirms the clearly micron-sized character of the re-entrained agglomerates. This is unambiguously shown by measuring a greater micron particle numerical concentration downstream from the agglomerator than upstream. We show that the fluorescein submicron particles use can greatly simplify the characterization of an electrostatic agglomerator by allowing the use of a commercial laser granulometer to measure the size and the number of the generated agglomerates.