Ultrafine Particle Collection Research Articles

Review on Sampling Methods and Health Impacts of Fine (PM2.5, ≤2.5 µm) and Ultrafine (UFP, PM0.1, ≤0.1 µm) Particles

Airborne particulate matter (PM) is of great concern in the modern-day atmosphere owing to its association with a variety of health impacts, such as respiratory and cardiovascular diseases. Of the various size fractions of PM, it is the finer fractions that are most harmful to health, in particular ultrafine particles (PM0.1; UFPs), with an aerodynamic diameter ≤ 100 nm. The smaller size fractions, of ≤2.5 µm (PM2.5; fine particles) and ≤0.1 µm (PM0.1; ultrafine particles), have been shown to have numerous linkages to negative health effects; however, their collection/sampling remains challenging. This review paper employed a comprehensive literature review methodology; 200 studies were evaluated based on the rigor of their methodologies, including the validity of experimental designs, data collection methods, and statistical analyses. Studies with robust methodologies were prioritised for inclusion. This review paper critically assesses the health risks associated with fine and ultrafine particles, highlighting vehicular emissions as the most significant source of particulate-related health effects. While coal combustion, diesel exhaust, household wood combustors’ emissions, and Earth’s crust dust also pose health risks, evidence suggests that exposure to particulates from vehicular emissions has the greatest impact on human health due to their widespread distribution and contribution to air pollution-related diseases. This article comprehensively examines current sampling technologies, specifically focusing on the collection and sampling of ultrafine particles (UFP) from ambient air to facilitate toxicological and physiochemical characterisation efforts. This article discusses diverse approaches to collect fine and ultrafine particulates, along with experimental endeavours to assess ultrafine particle concentrations across various microenvironments. Following meticulous evaluation of sampling techniques, high-volume air samplers such as the Chem Vol Model 2400 High Volume Cascade Impactor and low-volume samplers like the Personal Cascade Impactor Sampler (PCIS) emerge as effective methods. These techniques offer advantages in particle size fractionation, collection efficiency, and adaptability to different sampling environments, positioning them as valuable tools for precise characterisation of particulate matter in air quality research and environmental monitoring.

Characterizing the Chemical Profile of Incidental Ultrafine Particles for Toxicity Assessment Using an Aerosol Concentrator.

Incidental ultrafine particles (UFPs) constitute a key pollutant in industrial workplaces. However, characterizing their chemical properties for exposure and toxicity assessments still remains a challenge. In this work, the performance of an aerosol concentrator (Versatile Aerosol Concentration Enrichment System, VACES) was assessed to simultaneously sample UFPs on filter substrates (for chemical analysis) and as liquid suspensions (for toxicity assessment), in a high UFP concentration scenario. An industrial case study was selected where metal-containing UFPs were emitted during thermal spraying of ceramic coatings. Results evidenced the comparability of the VACES system with online monitors in terms of UFP particle mass (for concentrations up to 95 µg UFP/m3) and between filters and liquid suspensions, in terms of particle composition (for concentrations up to 1000 µg/m3). This supports the applicability of this tool for UFP collection in view of chemical and toxicological characterization for incidental UFPs. In the industrial setting evaluated, results showed that the spraying temperature was a driver of fractionation of metals between UF (<0.2 µm) and fine (0.2–2.5 µm) particles. Potentially health hazardous metals (Ni, Cr) were enriched in UFPs and depleted in the fine particle fraction. Metals vaporized at high temperatures and concentrated in the UF fraction through nucleation processes. Results evidenced the need to understand incidental particle formation mechanisms due to their direct implications on particle composition and, thus, exposure. It is advisable that personal exposure and subsequent risk assessments in occupational settings should include dedicated metrics to monitor UFPs (especially, incidental).

Experimental study of a string-based counterflow wet electrostatic precipitator for collection of fine and ultrafine particles

ABSTRACT Wet electrostatic precipitators (WESP) have been widely studied for collecting fine and ultrafine particles, such as diesel particulate matter (DPM), which have deleterious effects on human health. Here, we report an experimental and numerical simulation study on a novel string-based two-stage WESP. Our new design incorporates grounded vertically aligned polymer strings, along which thin films of water flow down. The water beads, generated by intrinsic flow instability, travel down the strings and collect charged particles in the counterflowing gas stream. We performed experiments using two different geometric configurations of WESP: rectangular and cylindrical. We examined the effects of the WESP electrode bias voltage, air stream velocity, and water flow rate on the number-based fractional collection efficiency for particles of diameters ranging from 10 nm to 2.5 μm. The collection efficiency improves with increasing bias voltages or decreasing airflow rates. At liquid-to-gas (L/G) as low as approximately 0.0066, our design delivers a collection efficiency over 70% even for fine and ultrafine particles. The rectangular and cylindrical configurations exhibit similar collection efficiencies under nominally identical experimental conditions. We also compare the water-to-air mass flow rate ratio, air flow rate per unit collector volume, and collection efficiency of our string-based design with those of previously reported WESPs. The present work demonstrates a promising design for a highly efficient, compact, and scalable two-stage WESPs with minimal water consumption. Implications: Wet Electrostatic Precipitators (WESPs) are highly effective for collecting fine particles in exhaust air streams from various sources such as diesel engines, power plants, and oil refineries. However, their large-scale adoption has been limited by high water usage and reduced collection efficiencies for ultrafine particles. We perform experimental and numerical investigation to characterize the collection efficiency and water flow rate-dependence of a new design of WESP. The string-based counterflow WESP reported in this study offers number-based collection efficiencies >70% at air flow rates per collector volume as high as 4.36 (m3/s)/m3 for particles of diameters ranging from 10 nm – 2.5 μm, while significantly reducing water usage. Our work provides a basis for the design of more compact and water-efficient WESPs.

A novel semi-automatic method for measuring acidic ultrafine particles in the atmosphere

The adverse effects of acidic ultrafine particles (AUFPs) have been widely recognized in scientific communities. Two methods have previously been developed to measure AUFPs, but there are certain drawbacks. Thus, the aim of the study was to develop an easier, more rapid and more accurate measurement system for semi-automatic measurement of AUFPs in the atmosphere. The new measurement system was developed by integrating a diffusion sampler (DS) with three quartz crystal microbalances (QCM), namely QCM + DS system. The QCM detectors were coated with a nano-film of metal (metal-QCM detectors) and then placed inside the DS at three sampling spots for collection and detection of ultrafine particles (UFPs). The frequency changes obtained from the metal-QCM detectors were converted into the weights of deposited particles and used to determine the proportions of AUFPs in UFPs through the removal process of non-AUFP particles. Prior to sampling, the sensitive response of the QCM system and collection efficiencies of the QCM + DS system were calibrated using standard acidic and non-acidic particles. Reactions between the AUFPs and nano-film of metal were guaranteed by confirming much lower than one-layer deposition of particles on the detectors based on theoretical calculation and experimental results. Finally, the QCM + DS system was validated in a field measurement by comparing the results with those obtained from the previously developed method and a commercial measurement system (i.e. SMPS). All the three methods showed good agreements in measuring AUFPs and UFPs concentrations, indicating the reliability of the QCM + DS system for the quantification of ambient UFPs and AUFPs.

Fabrication of a microcantilever-based aerosol detector with integrated electrostatic on-chip ultrafine particle separation and collection

In this paper, fabrication and testing of a miniaturized microcantilever-based particulate matter detector with integrated electrostatic on-chip ultrafine particle (UFP) separation and collection are presented. Mass added to the sensor causes a resonance frequency shift. To attract naturally charged particles, the cantilever is equipped with a collection electrode. In addition, a µ-channel is integrated, to improve the particle collection efficiency and to enable a size/mass-related particle separation. For electrical read-out, piezo-resistive struts are attached to the cantilever sidewalls near its clamping. This design offers high miniaturization potential, since no integration of transducing electronics on the cantilever beam is needed. The sensors are fabricated using Si bulk material and standard micromachining technology; the cantilevers have a thickness of 3 ± 0.5 µm, a width of 3.1 ± 0.3 µm, 5.9 ± 0.4 µm or 10.5 ± 0.4 µm and a length of 118.7 ± 0.8 µm, 168.8 ± 0.8 µm or 171.2 ± 1 µm, respectively. To this end, a front-side release process using cryogenic inductive-coupled plasma reactive ion etching was developed, which does not require additional sidewall passivation steps. Testing of the resonator function by operating the sensor inside a scanning electron microscope and reference measurements inside a temperature-controlled test chamber using synthetic carbon UFPs (~160 nm average mass concentration distribution) and a fast mobility particle sizer as a reference instrument were carried out. Here, the ability to detect low UFP mass concentrations in the range <10 µg m−3 could be shown with a limit of detection of ~1 µg m−3 and a collection time of ~10 min. In addition, a voltage dependence of the collection efficiency was found at constant UFP-concentration conditions, which is an indication of size-selective UFP collection.

Multi-staged granular beds applied to the filtration of ultrafine particles: An optimization of collector diameters

Granular beds exhibit interesting performances, in terms of ultrafine particle collection efficiency, hot gas cleaning, low cost and robustness. Nevertheless, some authors highlighted that ultrafine particle deposit is essentially located in the first layers of the granular bed. Thus, only a small depth is useful thereby reducing both the operation time between two uncloggings and the process yield. Our improvement approach consists in reducing the pressure drop increase while maintaining high collection efficiency by using a granular bed presenting a collector diameter gradient. Performances of this improved granular bed and of a conventional one were compared during their clogging with particles generated by thermal spraying. Since the collection efficiency is a decreasing function of the collector size, this improved configuration permits to firstly clog the downstream layers of the granular bed. Although there is a 20% decrease of the initial efficiency for this new type of bed compared to the conventional one, this gap is rapidly reduced during the clogging process. At the same time, the results highlight a lower pressure drop increase of the modified granular bed. A predictive clogging model was also developed in order to find an optimized configuration of collector diameter gradient to ensure a contribution of the whole granular bed depth.

Removal of ultrafine particles by porous nanomaterials with varied morphologies

The removal characteristics of sub-25 nm ultrafine particles (UFPs) by different porous nanomaterials with different morphologies were studied based on efficiency curves obtained with specially designed filtration system. The material morphological effects on UFPs collection were investigated. The results demonstrate a complex and synergic mechanism of material structural framework, pore size distribution, and porosity order in terms of diffusion and interception dependent on flow velocity. Both porosity and fiber-like tubular structures promoted the UFPs removal. Microporosity and disordered mesoporosity could enhance the roughness of the internal surface and intensify the interaction with UFPs. Specifically, the large and ordered mesoporosity was able to render selectivity for UFPs at certain sizes and exhibited the best particle collection performance which can be largely associated with significant pore size and volume. The dependence of the ratio of diffusion to interception on flow velocity and the comparison of different pores in terms of the matching sizes with particles derived from single sphere efficiency model fitting results were also discussed. This work is significant to deeper understanding of interactions between airborne nanoparticles and porous surfaces.

Aerosol sampler for analysis of fine and ultrafine aerosols

A new aerosol sampler based on the original version of Aerosol Counterflow Two-Jets Unit (ACTJU) is described. The ACTJU collector, connected with a water-based Condensation Growth Unit (CGU) placed upstream of the ACTJU, accomplished the quantitative collection of fine and ultrafine aerosol particles down to a few nanometers in diameter. Condensation of water vapor in the CGU enlarges nanometer sized particles to larger sizes in the supermicrometer range and the formed droplets are then collected into water in the ACTJU collector.The continuous collection of aerosols with the CGU-ACTJU sampler allows for the time-resolved measurement of changes in the concentration of particulate constituents. Coupling of the CGU-ACTJU sampler with on-line detection devices allows in-situ automated analysis of water-soluble aerosol components with high time resolution of 1 s (e.g., FIA detection for nitrite or nitrate) or 1 h (e.g., IC detection with preconcentration step for inorganic anions). Under the optimum conditions (the air flow rate of 10 L min−1 and water flow rate of 1.5 mL min−1), the limit of detection (IC including the preconcentration) for particulate fluoride, chloride, nitrite, nitrate, sulphate and phosphate is 2.53, 6.64, 24.2, 16.8, 0.12 and 5.03 ng m−3, respectively. The apparatus is sufficiently robust for its application at routine monitoring of aerosol composition in real-time.

Ultrafine Particle Collection Performance of a Two-Stage ESP With a Novel Mixing-Type Charging Stage Using Different Geometries and Electrical Conditions

We improved the two-stage electrostatic precipitator (ESP) developed for information technology manufacturing with highly corrosive and explosive gases by adding a novel mixing-type charger to generate unipolar ions outside of the gas duct and inject them into the main gas flow using an additional electric field. The charging stage (100 × 100 × 400 mm3) is composed of a brush-type charger located outside of the upper part of the charging stage and two parallel plates with an additional electric field. Downstream of the charging stage is a typical collection stage with parallel plates and a collection area of 0.1 m2 at 50 L/min. Electrical and particle removal characteristics of the ESP were evaluated using ultrafine particles and varying the thickness, hole size, and electrical connections in the charging stage. The total air flow rate in the ESP during tests was approximately 50 L/min, with a 10:1 ratio of the main and mixing flows. Using our novel mixing-type charging stage and electrical connections, a high voltage of −5 kV was applied between the brush charger and the grounded upper plate, and a positive high voltage was applied to the lower plate in the charging stage. Collection efficiency ranged from 20% to 90% of 60 nm particles over a wide range of electrical potentials between the upper and lower plates of 0–15 kV.

Performance of Ultrafine Particle Collection of a Two-Stage ESP Using a Novel Mixing Type Carbon Brush Charger and Parallel Collection Plates

A novel two-stage electrostatic precipitator (ESP) for information technology manufacturing industries was developed. It uses a mixing type carbon brush charger located outside of the main gas flow duct to completely prevent contact of the polluted gases with the chargers, and applies an additional electric field between the upper and bottom plates of the charging stage to enhance collisions between ions and particles, thereby increasing the particle charges. The performance of the ESP was evaluated experimentally for ultrafine particles with a mean diameter of 100 nm based on number, and of 200 nm based on surface area by varying the voltages of the chargers and for generating the additional electric field, and by varying the number of the chargers. Finally, the collection efficiencies of the ESP were compared to those obtained from Deutch's collection theory using the average experimental particle charges. The total air flow rate used in the tests was approximately 110 L/min with a 10:1 ratio of the main and mixing flows in the ESP, and the applied voltages of the charger and additional electric field were –10 to –22 kV, and –5 to –10 kV, respectively. Collection efficiencies in the ESP were linearly proportional to the increase in the number of chargers and the increase in the applied voltage to the chargers, which in turn were inversely proportional to the increase in the applied voltage used to generate the additional electric field. A collection efficiency of the ESP of > 90% was obtained at a flow rate of 110 L/min, corresponding to flow speeds of approximately 0.19 m/s through the charging stage and 0.23 m/s through the collection cell based on the total particle number at an applied voltage to the triple chargers of –15 kV and an additional electric field between the upper and bottom plates in the charging stage of –6 kV/100 mm.

Modeling of the deep granular bed clogging by nanoparticles

Abstract Nanoparticle aerosols are a potential health hazard and are increasingly used in industry. Effective means for their filtration to avoid occupational exposure are necessary. Granular bed filtration exhibits interesting properties in terms of ultrafine particle collection efficiency or retention capacity, which could make them a good alternative to the fiber filters commonly used in industry today. Being able to predict changes to their collection efficiency and pressure drop during clogging appears essential for the design, optimization and control of the filtration process. To meet these needs, this study presents a predictive model for the different phases of deep-bed clogging. The model developed successfully describes changes to collection efficiency and pressure drop using equivalent collector diameters. Microscopic observations showed these equivalent diameters to be compatible with considering the deposit as a uniform spherical layer all over the collector’s surface. The predictions of the model agreed well with data from several clogging experiments for which various operating conditions were used, i.e., different collector’s diameters, particulate materials or superficial velocities.

Evaluation of Artifacts Generated during Collection of Ultrafine Particles Using an Inertial Filter Sampler

ABSTRACTTwo artifacts were observed during collection of atmospheric ultrafine particles (UFPs; < 100 nm diameter) using an inertial filter (INF) sampler recently developed for this purpose. These artifacts were evaluated. First, the adsorption of organic and ionic gas onto a quartz fiber filter installed in the INF sampler was evaluated to provide information on the positive artifact; this information is important for accurate analysis of the UFP components. Gas adsorption by the INF sampler was similar to or less than that of other devices for collecting UFPs. The gas adsorption of organic carbon (OC) fractions, and ionic components of NH4+, NO3– and SO42– was confirmed, and the adsorption of OC1 and ionic components were highly dependent on the concentration of the gas and the UFP concentration under ambient conditions. These results suggest that it is necessary to know the concentration of the UFPs on the filter in order to evaluate the exact concentrations of UFP components under high flow rate conditions. Second, the efficiency of UFP collection on a polytetrafluoroethylene (PTFE) filter installed in the INF sampler was evaluated to clarify the negative artifact. The data confirmed that the efficiency of collection changed with changing structure such as pore size, porosity and thickness of the filter. The structure of these filters and UFPs collected on there were observed by a scanning electron microscopy. The highest particle collection efficiency (almost 100%) was obtained by installing two thick membrane PTFE filters. The collection of UFPs using a sampler comprising several filter stages is a convenient and useful method for evaluating positive and negative artifacts and for quantifying the concentration of components of UFPs.

Ionizer assisted air filtration for collection of submicron and ultrafine particles-evaluation of long-term performance and influencing factors.

Previous research has demonstrated that unipolar ionization can enhance the filter performance to collect airborne particles, aeroallergens, and airborne microorganisms, without affecting the filter pressure drop. However, there is a lack of research on the long-term system performance as well as the influence of environmental and operational parameters. In this paper, both field and laboratory tests were carried out to evaluate the long-term particle collection efficiency of a synthetic filter of class M6 with and without ionization. The effect of air velocity, temperature, relative humidity, and particle concentration were further investigated in laboratory tests. Results showed that ionization enhanced the filtration efficiency by 40%-units during most of the operation time. When the ionization system was managed by periodically switching the ionizer polarity, the filtration efficiency against PM0.3-0.5 was maintained above 50% during half a year. Furthermore, the pressure drop of the ionizer-assisted M6 filter was 25-30% lower than that of a filter of class F7. The evaluation of various influencing factors demonstrated that (1) air moisture reduced the increase of filtration efficiency; (2) higher upstream particle concentration and air velocity decreased the filtration efficiency; and (3) the air temperature had very limited effect on the filtration efficiency.

The effects of leaf size and microroughness on the branch‐scale collection efficiency of ultrafine particles

AbstractWind tunnel experiments were performed to explore how leaf size and leaf microroughness impact the collection efficiency of ultrafine particles (UFP) at the branch scale. A porous media model previously used to characterize UFP deposition onto conifers (Pinus taeda and Juniperus chinensis) was employed to interpret these wind tunnel measurements for four different broadleaf species (Ilex cornuta, Quercus alba, Magnolia grandiflora, and Lonicera fragrantissima) and three wind speed (0.3–0.9 ms−1) conditions. Among the four broadleaf species considered, Ilex cornuta with its partially folded shape and sharp edges was the most efficient at collecting UFP followed by the other three flat‐shaped broadleaf species. The findings here suggest that a connection must exist between UFP collection and leaf dimension and roughness. This connection is shown to be primarily due to the thickness of a quasi‐laminar boundary layer pinned to the leaf surface assuming the flow over a leaf resembles that of a flat plate. A scaling analysis that utilizes a three‐sublayer depositional model for a flat plate of finite size and roughness embedded within the quasi‐laminar boundary layer illustrates these connections. The analysis shows that a longer leaf dimension allows for thicker quasi‐laminar boundary layers to develop. A thicker quasi‐laminar boundary layer depth in turn increases the overall resistance to UFP deposition due to an increase in the diffusional path length thereby reducing the leaf‐scale UFP collection efficiency. It is suggested that the effects of leaf microroughness are less relevant to the UFP collection efficiency than are the leaf dimensions for the four broadleaf species explored here.

Time-resolved characterization of particle associated polycyclic aromatic hydrocarbons using a newly-developed sequential spot sampler with automated extraction and analysis

A versatile and compact sampling system, the Sequential Spot Sampler (S3) has been developed for pre-concentrated, time-resolved, dry collection of fine and ultrafine particles. Using a temperature-moderated laminar flow water condensation method, ambient particles as small as 6 nm are deposited within a dry, 1-mm diameter spot. Sequential samples are collected on a multiwell plate. Chemical analyses are laboratory-based, but automated. The sample preparation, extraction and chemical analysis steps are all handled through a commercially-available, needle-based autosampler coupled to a liquid chromatography system. This automation is enabled by the small deposition area of the collection. The entire sample is extracted into 50–100 μL volume of solvent, providing quantifiable samples with small collected air volumes. A pair of S3 units was deployed in Stockton (CA) from November 2011 to February 2012. PM2.5 samples were collected every 12 h, and analyzed for polycyclic aromatic hydrocarbons (PAHs). In parallel, conventional filter samples were collected for 48 h and used to assess the new system's performance. An automated sample preparation and extraction was developed for samples collected using the S3. Collocated data from the two sequential spot samplers were highly correlated for all measured compounds, with a regression slope of 1.1 and r2 = 0.9 for all measured concentrations. S3/filter ratios for the mean concentration of each individual PAH vary between 0.82 and 1.33, with the larger variability observed for the semivolatile components. Ratio for total PAH concentrations was 1.08. Total PAH concentrations showed similar temporal trend as ambient PM2.5 concentrations. Source apportionment analysis estimated a significant contribution of biomass burning to ambient PAH concentrations during winter.

Optimization of bubble column performance for nanoparticle collection

Fibrous media embody the most effective and widely used method of separating ultrafine particles from a carrier fluid. The main problem associated with them is filter clogging, which induces an increasingly marked pressure drop with time and thus imposes regular media cleaning or replacement. This context has prompted the idea of investigating bubble columns, which operate at constant pressure drop, as alternatives to fibrous filters. This study examines the influence of different operating conditions, such as liquid height, air flow rate, bubble size and presence of granular beds on ultrafine particle collection. Experimental results show that bubble columns are characterised by high collection efficiency, when they feature a large liquid height and small diameter bubbling orifices, while their efficiencies remain lower than those of fibrous filters. Gas velocity does not greatly influence collection efficiency, but the inclusion of a granular bed, composed of beads, increases the bubble residence time in the column, thereby increasing the column collection efficiency.

Vegetation collection efficiency of ultrafine particles: From single fiber to porous media

A number of parameterization schemes are available to determine the collection efficiency of ultrafine particles (UFP) onto vegetated surfaces. One approach represents the vegetated elements as a fibrous filter with a characteristic fiber size that is difficult to a priori determine, while the other, a more conventional approach, represents vegetation as a porous medium. To date, no attempts have been made to compare the performance of these two distinct approaches or bridge them so as to show the necessary conditions leading to their potential equivalence. In a wind tunnel study, the UFP collection efficiencies of pine branches at five different wind speeds, two branch orientations, and two packing densities were measured and analyzed using these two vegetation representations. This vegetation type was selected because pines are a dominant species in the Southeastern United States and pine needles geometrically resemble fibrous material with a well‐defined foliage diameter. The porous media and the fibrous filter representations described well observed UFP deposition at the branch scale. Conditions promoting their equivalence are thus explored. The difficult to determine effective fiber diameter was recovered from conventional canopy attributes such as the leaf area index by matching the collection efficiencies of UFP for the two vegetation representations. These results provide a working “aerodynamic” definition of the effective single‐fiber diameter thereby rendering the simplified single‐fiber formulation usable in large‐scale atmospheric deposition models. Furthermore, the aerodynamic correction factor allows upscaling of pine needles to an effective leaf area index and provides some quantification of the effect of needle spatial clustering on UFP deposition. The applicability of the results to other vegetation species remains to be verified.

The Effects of Leaf Area Density Variation on the Particle Collection Efficiency in the Size Range of Ultrafine Particles (UFP)

Carbonaceous particles were generated during a "sooting burn" experiment to explore how heterogeneity in horizontal leaf area density (LAD) within the canopy impacts the ultrafine particle (UFP) collection efficiency at the branch-scale. To address this goal, wind tunnel experiments and a particle-size resolving model, which couples the turbulent flow field within the vegetated volume and the collection efficiency, were presented. Three scenarios were examined in a wind-tunnel packed with Juniperus chinensis branches: An LAD that was uniformly distributed, linearly increasing and linearly decreasing along the longitudinal or mean wind direction. The concentration measurements were conducted at multiple locations within the vegetated volume to evaluate the performance of the proposed model needed in discerning the role of LAD heterogeneity on UFP collection. Differences not exceeding 20% were found between modeled and measured concentration for all particle sizes across a wide range of wind speeds. The overall particle collection efficiency was found to be primarily governed by the spatially integrated LAD when differences in aerodynamic attributes (e.g., foliage drag) were accounted for. When combined with earlier studies, the results suggest that one parameter linking the laminar boundary layer conductance to the Schmidt number depends on particle size.

Experimental study of granular bed filtration of ultrafine particles emitted by a thermal spraying process

Ultrafine particles (UFP) emitted by metal thermal spraying processes are responsible for rapid and partially irreversible clogging of filters. Granular filter beds could offer an alternative solution to media filters based on their capacity for efficient UFP collection and surface collection layer renewal. The influence of the electric charges carried by particle and grains on filtration efficiency was revealed and there was a close agreement between the model and measured values without electric charges. Variation in the efficiency and pressure drop during filtration, hence gradual clogging of the bed, was studied using two types of column. The influence in filtration rate on clogging kinetics was also investigated. The column composed on three distinct one centimetre stages showed that clogging occurred primarily in the bed surface layer. The regular elimination of cake during filtration would enable the pressure drop to be maintained within acceptable limits.

A branch scale analytical model for predicting the vegetation collection efficiency of ultrafine particles

Abstract The removal of ultrafine particles (UFP) by vegetation is now receiving significant attention given their role in cloud physics, human health and respiratory related diseases. Vegetation is known to be a sink for UFP, prompting interest in their collection efficiency. A number of models have tackled the UFP collection efficiency of an isolated leaf or a flat surface; however, up-scaling these theories to the ecosystem level has resisted complete theoretical treatment. To progress on a narrower scope of this problem, simultaneous experimental and theoretical investigations are carried out at the “intermediate” branch scale. Such a scale retains the large number of leaves and their interaction with the flow without the heterogeneities and added geometric complexities encountered within ecosystems. The experiments focused on the collection efficiencies of UFP in the size range 12.6–102 nm for pine and juniper branches in a wind tunnel facility. Scanning mobility particle sizers were used to measure the concentration of each diameter class of UFP upstream and downstream of the vegetation branches thereby allowing the determination of the UFP vegetation collection efficiencies. The UFP vegetation collection efficiency was measured at different wind speeds (0.3–1.5 m s−1), packing density (i.e. volume fraction of leaf or needle fibers; 0.017 and 0.040 for pine and 0.037, 0.055 for juniper), and branch orientations. These measurements were then used to investigate the performance of a proposed analytical model that predicts the branch-scale collection efficiency using conventional canopy properties such as the drag coefficient and leaf area density. Despite the numerous simplifications employed, the proposed analytical model agreed with the wind tunnel measurements mostly to within 20%. This analytical tractability can benefit future air quality and climate models incorporating UFP.