Performance of ventilation filtration technologies on characteristic traffic related aerosol down to nanocluster size

Near traffic routes and urban areas, the outdoor air particle number concentration is typically dominated by ultrafine particles. These particles can enter into the nearby buildings affecting the human exposure on ultrafine particles indoors. In this study, we demonstrate an aerosol generation system which mimics the characteristic traffic related aerosol. The aerosol generation system was used to determine the size-resolved particle filtration efficiencies of five typical commercial filters in the particle diameter range of 1.3–240 nm. Two different HEPA filters were observed to be efficient in all particle sizes. A fibrous filter (F7) was efficient at small particle sizes representing the nucleation mode of traffic related aerosol, but its efficiency decreased down to 60% with the increasing particle size. In contrast, the filtration efficiency of an electrostatic precipitator (ESP) increased as a function of the particle size, being more efficient for the soot mode of traffic related aerosol than for the nucleation mode. An electret filter with a charger was relatively efficient (filtration efficiency >85%) at all the observed particle sizes. The HEPA, F7 and electret filters were found to practically remove the particles/nanoclusters smaller than 3 nm. All in all, the filtration efficiencies were observed to be strongly dependent on the particle size and significant differences were found between different filters. Based on these results, we suggest that the particulate filter test standards should be extended to cover the ultrafine particles, which dominate the particle concentrations in outdoor air and are hazardous for public health.

Copyright © 2017 American Association for Aerosol Research     

Clogging of Industrial High Efficiency Particulate Air (HEPA) Filters in Case of Fire: From Analytical to Large-Scale Experiments

The Institute of Radioprotection and Nuclear Safety (IRSN in French) is conducting research on the impact of a fire on the behaviour of containment devices such as high efficiency particulate air (HEPA) pleated filters for radioactive materials. This work aims to study the clogging of HEPA filters in case of fire involving realistic materials (polymers making up gloves boxes, waste treatment solvent, hydraulic oil, solid material mixtures making up a trash bin, electrical cables, and cabinets) used in nuclear facilities, from the medium to large scale. The clogging kinetics of industrial pleated HEPA filters is monitored by measuring the pressure drop of the filters and the filtered air temperature at a given filtration velocity (from 0.23 to 2.1 cm/s). Upstream HEPA filters, combustion aerosols are characterized in terms of size distribution, mass concentration, composition, and particle morphology using, respectively, a DMS500 (Cambustion^LTD), glass fiber filter sampling, and transmission electron microscope analysis of particles deposited on TEM grids. Particles emitted denote well-known fractal morphology, are composed of carbonaceous primary particles with diameters ranging from 31 nm to 48 nm and showing an high clogging efficiency. An empirical relationship has been successfully applied to the obtained results for a larger range of fuels, filtration velocities and fire conditions.

Finally, experiments have been performed on a large-scale facility, using full-scale fire scenarios (electrical cabinet, constant, and variable filtration velocity) and a reasonable agreement was observed with our empirical relationship. At this scale, particles appear to be compact, with a complex composition and diameters close to 220 nm with a lower clogging efficiency.

Copyright 2014 American Association for Aerosol Research     

Evaluation of metallic filter media for sub-micrometer soot particle removal at elevated temperature

Soot particle removal performance of two types of metallic filter media, sintered metal powder and sintered metal fiber, is experimentally evaluated as potential improvements to conventional ceramic filtration media for gasoline direct injection (GDI) engine PM after-treatment application. Soot collection efficiency and flow resistance of several grades of metallic media are measured at temperatures of 25, 350, and 650°C and a range of representative filtration velocities for sub-micrometer soot particles generated from a propane flame. Theoretical collection efficiency based on single fiber efficiency theory shows good agreement with experimental data for nearly spherical KCl particles at 350°C. Improved collection efficiency is observed for soot particles in the interception-dominated size range above ～100 nm due to enhanced interception length. Soot collection is slightly enhanced at higher temperature, which is consistent with model predictions. Sintered metal fiber media are found capable of removing ～75% of soot particles by mass with an incremental flow resistance of less than 1.5 kPa under 10 cm/s and 350°C, which is promising for gasoline particulate filter (GPF) application. The media level figure of merit (FOM) is used to quantify the soot collection efficiency versus flow resistance tradeoff of all media tested. It is found that due to their more open structure (higher porosity) sintered metal fiber media have FOMs nearly one order of magnitude higher than those of sintered metal powder media, and by analogy those of conventional wall flow ceramic media. This suggests that sintered metal fiber media represents an attractive alternative to ceramic media for designing GPFs; however, further research into creating comparable surface area to the honeycomb structures used for wall flow filters is needed to extract the full potential of metal fiber media.

Copyright © 2017 American Association for Aerosol Research     

Filtration performances of HVAC filters for PM10 and microbial aerosols— Influence of management in a lab-scale air handling unit

Filtration performances of air handling unit (AHU) filters for particles and microbial aerosols were investigated. The influence of the AHU operational conditions on the behavior of microorganisms collected on the filters was also studied. A lab-scale AHU with two filtration stages was developed and validated for the study of downsized filters with industrial geometries. Three types of filters of different efficiency were considered: G4, F7, and F9, according to European standard EN 779. Two configurations of filters were studied: G4 pleated/F7 bag and F7/F9 bag. Filters were sequentially clogged by alumina particles, which provided a mineral fraction in the particulate cake, and then by micronized rice particles, which provided the fungus Penicillium chrysogenum and an organic fraction that acts as a substrate for microorganisms. Finally, a microbial aerosol composed of endospores of Bacillus subtilis and spores of Aspergillus niger was nebulized to contaminate filters. After clogging, periods of 5 days on and 2 day weekend stops with restarts of ventilation were simulated for 6 weeks. The results showed that the filter efficiency for particles was quite comparable to that for microbial aerosols expressed in cultivable concentration. The particulate cake composed of alumina and micronized rice particles enabled the growth of the endogenous species P. chrysogenum and the survival of exogenous species B. subtilis and A. niger on filters. During restarts of ventilation, low particle concentrations were detected downstream of the second filtration stages by release but the microbial concentration from the fraction of air sampled was below the detection limit.

© 2016 American Association for Aerosol Research     

Development of an aerosol mass spectrometer lens system for PM2.5

The aerodynamic lens system of the Aerodyne Aerosol Mass Spectrometer (AMS) was analyzed using the Aerodynamic Lens Calculator. Using this tool, key loss mechanisms were identified, and a new lens design that can extend the transmission of particulate matter up to 2.5 μm in diameter (PM2.5) was proposed. The new lens was fabricated and experimentally characterized. Test results indicate that this modification to the AMS lens can significantly improve the transmission of large sized particles, successfully achieving a high transmission efficiency up to PM2.5 range.

© 2016 American Association for Aerosol Research     

Design and evaluation of an aerodynamic focusing micro-well aerosol collector

Aerosol sampling and identification is vital for the assessment and control of particulate matter pollution, airborne pathogens, allergens, and toxins and their effect on air quality, human health, and climate change. In situ analysis of chemical and biological airborne components of aerosols on a conventional filter is challenging due to dilute samples in a large collection region. We present the design and evaluation of a micro-well (µ-well) aerosol collector for the assessment of airborne particulate matter (PM) in the 0.5–3 µm size range. The design minimizes particle collection areas allowing for in situ optical analysis and provides an increased limit of detection for liquid-based assays due to the high concentrations of analytes in the elution/analysis volume. The design of the collector is guided by computational fluid dynamics (CFD) modeling; it combines an aerodynamic concentrator inlet that focuses the aspirated aerosol into a narrow beam and a µ-well collector that limits the particle collection area to the µ-well volume. The optimization of the collector geometry and the operational conditions result in high concentrations of collected PM in the submillimeter region inside the µ-well. Collection efficiency experiments are performed in the aerosol chamber using fluorescent polystyrene microspheres to determine the performance of the collector as a function of particle size and sampling flow rate. The collector has the maximum collection efficiency of about 75% for 1 µm particles for the flow rate of 1 slpm. Particles bigger than 1 µm have lower collection efficiencies because of particle bounce and particle loss in the aerodynamic focusing inlet. Collected samples can be eluted from the device using standard pipettes, with an elution volume of 10–20 µL. The transparent collection substrate and the distinct collection region, independent of particle size, allows for in situ optical analysis of the collected PM.

© 2017 American Association for Aerosol Research     

Comparison of the relative performance efficiencies of melt-blown and glass fiber filter media for managing fine particles

The purpose of this study was to compare the performance efficiency of melt-blown and currently used glass fiber filter media under the same environmental conditions. To evaluate filter efficiency, filter class was determined according to ISO and European standards (EN 1822-1:2009) using an automated filter tester (0.3 μm size), taking into account particle filtration, fractional efficiency for negative pressure devices, and consumption of electrical power. The average fractional efficiency, quality factor (QF), fractional efficiency by dust loading amount, pressure by dust loading amount, and QF by dust loading amount were higher in the case of melt-blown media than in the case of glass fiber filters. The fractional efficiency of hydrocharged melt-blown filters was higher than that of uncharged media. Based on performance efficiency, melt-blown filters are more effective high efficiency particulate air filters than glass fiber media.

Copyright © 2018 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     

Preparation of Granular Ceramic Filter and Prediction of Its Collection Efficiency

The silicon carbide (SiC) ceramic filter is the most favorable component to remove particulate matter from hot flue gas due to its high filtration efficiency and high thermal durability. The effect of SiC powder size on the physical properties and filtration performance to prepare high-performance granular ceramic filter media was investigated in this study. Disk-type filter media were prepared by mixing ceramic components followed by physical compression and sintering. The porosity and average pore diameter in the filter media increased with increasing powder size. However, the mechanical strength decreased with increasing powder size, while it increased with increasing physical compressive force. The filter performance factor, q_FM was introduced to evaluate the ceramic filter properties, and the SiC50 filter was the best of the ceramic filters prepared in this study. We also found that diffusion was a dominant collection mechanism for particles smaller than 0.7 μm, and direct interception and inertia were dominant collection mechanisms for particles larger than 0.7 μm in the SiC50 filter based on a single collector efficiency model. In addition, the predicted collection efficiencies showed reasonably good agreement with the experimental ones.

Copyright 2014 American Association for Aerosol Research     

Assessing the accuracy of complex refractive index retrievals from single aerosol particle cavity ring-down spectroscopy

Cavity ring-down spectroscopy (CRDS) of single, optically manipulated aerosol particles affords quantitative retrieval of refractive indices for particles of fixed or evolving composition with high precision. Here, we quantify the accuracy with which refractive index determinations can be made by CRDS for single particles confined within the core of a Bessel laser beam and how that accuracy is degraded as the particle size is progressively reduced from the coarse mode (>1 μm radius) to the accumulation mode (<500 nm radius) regime. We apply generalized Lorenz–Mie theory to the intra-cavity standing wave to explore the effect of particle absorption on the distribution of extinction cross section determinations resulting from stochastic particle motion in the Bessel beam trap. The analysis provides an assessment of the accuracy with which the real, n, and imaginary, κ, components of the refractive index can be determined for a single aerosol particle.

Published with license by American Association for Aerosol Research     

Characterization of filter performance under low-pressure operation

Particulate gas filters are a critical element in the purification systems used to ensure defect-free manufacturing in semiconductor industry. In atomic layer deposition (ALD) processes, these filters are typically operated under sub-atmospheric pressure conditions, but their filtration characteristics are, often, only known at atmospheric pressure. In this study, performance of a metal filter that is typically used in low-pressure ALD precursor delivery systems is studied experimentally and theoretically down to 4.5 kPa. The experimental procedure was designed to minimize the presence of multiply charged particles in the test aerosol for different operating pressures and flowrates. The experimental results suggest that most penetrating particle size only slightly varies with pressure, but the shape of the penetration curve and the maximum value of the penetration changes significantly with pressure. The experimental data are used to test predictions of filter performance at low pressures made using classical theory. The comparison results suggest that the combination of classical theory and manufacturer-specified parameters results in large errors in calculated penetration values at low pressures. Accurate predictions are seen to be possible for particle Stokes numbers less than 0.1, when an inhomogenous filtration model is used in combination with effective filter parameters that are obtained from experimental measurements of filter efficiency and pressure drop at atmospheric pressures.

Copyright © 2016 American Association for Aerosol Research     

Differential diffusion analyzer

In this article, a proof of concept of a new measurement instrument, differential diffusion analyzer (DDA), is established. The DDA enables the measurement of the size distribution of sub-10 nm aerosol particles, and it can also be used as a size classifier to separate a certain particle size from a size distribution for subsequent analysis. The developed technique is based on the diffusion separation of different size particles. Thus, the main advantage of the DDA compared to other methods is that particle charging is not required. Simulated and experimentally measured transmission efficiencies show that the diffusion-based differential size classification is a feasible concept, and moreover, shows that particle size is inversely proportional to the square root of the total flow rate.

Copyright © 2017 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     

Modeling of submicron particle filtration in an electret monolith filter with rectangular cross-section microchannels

Electret monolith filters have the advantage of low pressure drop and high filtration efficiency. In such filters, the filtration of submicron aerosol particles occurs as air passes through millions of microchannels. This article investigates the flow and filtration mechanisms in a representative rectangular microchannel of an electret monolith filter. An improved incompressible lattice Boltzmann method with Bhatnagar–Gross–Krook (traditionally shortened as LBGK) and lattice velocity D3Q15 model is employed to simulate no-slip and slip flows in the rectangular microchannels of a monolith filter. We considered mono-disperse submicron particles and one-way coupling (particle motion was affected by the flow, but the presence of particles did not affect the flow). Based on flow computations, the effects of key dimensionless parameters (Reynolds number, Knudsen number, Stokes number and the dimensionless length of the channel) on the total capture efficiency of mono-disperse submicron particles were investigated. Our results indicate that the optimal monolith filter should be characterized by a Knudsen number between 0.022 and 0.044, and that the dimensionless length of the channel should be between 4 and 8.

Copyright © 2016 American Association for Aerosol Research     

To Sonicate or Not to Sonicate PM Filters: Reactive Oxygen Species Generation Upon Ultrasonic Irradiation

In aerosol research, a common approach for the collection of particulate matter (PM) is the use of filters in order to obtain sufficient material to undertake analysis. For subsequent chemical and toxicological analyses, in most cases the PM needs to be extracted from the filters. Sonication is commonly used to most efficiently extract the PM from the filters. Extraction protocols generally involve 10–60 min of sonication. The energy of ultrasonic waves causes the formation and collapse of cavitation bubbles in the solution. Inside the collapsing cavities the localized temperatures and pressures can reach extraordinary values. Although fleeting, such conditions can lead to pyrolysis of the molecules present inside the cavitation bubbles (gases dissolved in the liquid and solvent vapors), which results in the production of free radicals and the generation of new compounds formed by reactions with these free radicals. For example, simple sonication of pure water will result in the formation of detectable levels of hydroxyl radicals. As hydroxyl radicals are recognized as playing key roles as oxidants in the atmosphere the extraction of PM from filters using sonication is therefore problematic. Sonication can result in significant chemical and physical changes to PM through thermal degradation and other reactions. In this article, an overview of sonication technique as used in aerosol research is provided, the capacity for radical generation under these conditions is described and an analysis is given of the impact of sonication-derived free radicals on three molecular probes commonly used by researchers in this field to detect reactive oxygen species (ROS) in PM.

Copyright 2014 American Association for Aerosol Research     

Preliminary investigation of a water-based method for fast integrating mobility spectrometry

A water-based condensational growth channel was developed for imaging mobility-separated particles within a parallel plate separation channel of the Fast Integrated Mobility Spectrometer (FIMS). Reported are initial tests of that system, in which the alcohol condenser of the FIMS was replaced by a water-based condensational growth channel. Tests with monodispersed sodium chloride aerosol verify that the water-condensational growth maintained the laminar flow, while providing sufficient growth for particle imaging. Particle positions mapped onto particle mobility, in accordance with theoretical expectations. Particles ranging in size from 12 nm to 100 nm were counted with the same efficiency as with a butanol-based ultrafine particle counter, once inlet and line losses were taken into account.

Copyright © 2017 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     

Investigation of vehicle exhaust sub-23 nm particle emissions

A solid particle number limit was applied to the European legislation for diesel vehicles in 2011. Extension to gasoline direct injection vehicles raised concerns because many studies found particles below the lower size limit of the method (23 nm). Here we investigated experimentally the feasibility of lowering this size. A nano condensation nucleus counter system (nCNC) (d_50% = 1.3 nm) was used in parallel with condensation particle counters (CPCs) (d_50% = 3 nm, 10 nm and 23 nm) at various sampling systems based on ejector or rotating disk diluters and having thermal pre-treatment systems consisting of evaporation tubes or catalytic strippers. An engine exhaust particle sizer (EEPS) measured the particle size distributions. Depending on the losses and thermal pre-treatment of the sampling system, differences of up to 150% could be seen on the final detected particle concentrations when including the particles smaller than 23 nm in diameter. A volatile artefact as particles with diameters below 10 nm was at times observed during the cold start measurements of a 2-stroke moped. The diesel vehicles equipped with the Diesel Particulate Filter (DPF) had a low solid sub-23 nm particles fraction (<20%), the gasoline with direct injection vehicles had higher (35–50%), the gasoline vehicles with port fuel injection and the two mopeds (two and four-stroke) had the majority of particles below 23 nm. The size distributions peaked at 60–80 nm for the DPF equipped vehicles, at 40–90 nm for the gasoline vehicles with a separate nucleation mode peak at approximately 10 nm sometimes. Mopeds peaked at sizes below 50 nm when their aerosol was thermally pre-treated.

© 2017 American Association for Aerosol Research