Impactor Collection Efficiency Research Articles

A PM2.5 Sensor Module Based on a TPoS MEMS Oscillator and an Aerosol Impactor

In this work, a real-time PM2.5 sensor module with high mass resolution and wide detection range is successfully demonstrated while the module consists of a two-stage aerosol impactor, a Thin-film Piezoelectric-on-Silicon (TPoS) MEMS oscillator, and a micro-pump. The aerosol impactor's collection efficiency of 51% for $2.54~\mu \text{m}$ and 50% for $1.03~\mu \text{m}$ is verified for stage 1 and stage 2, respectively. A length-extensional mode TPoS MEMS resonator is implemented, featuring a resonant frequency of 4.05 MHz and a quality factor (${Q}$ ) of 438 in air. To achieve a portable sensor module, a board-level sustaining circuit, which includes a transimpedance amplifier (TIA), a phase shifter, a band-pass filter, and a buffer circuit are integrated to fulfill the condition of Barkhausen criteria for oscillation. Phase noise of -111.92 dBc/Hz at 1 kHz offset was recorded under ambient pressure and room temperature for a carrier frequency of 4.05 MHz. The mass resolution for the PM2.5 sensor implemented in this work is 0.71 pg. Finally, an incense was used to produce the liquid type particles continuously for the aerosol measurement. The coefficient of determination (${R} ^{2}$ ) of 0.81 between the frequency slope of the proposed sensor and the aerosol concentration of the commercial optical aerosol sensor within a range of $10~\mu $ g/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> to $1000~\mu $ g/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> . This work successfully integrated MEMS, circuit, and air flow technologies to realize a complete mass sensing module, and has the potential to be implemented in the particulate matter concentration monitoring applications of air.

Effect of nozzle spacing in the formation of primary and secondary deposits in multi-nozzle inertial impactors part I: Experimental study

Particle deposits in an inertial impactor stage can be classified as primary or secondary. The primary deposits are directly downstream of each nozzle. Secondary deposits can be in the form of a halo around the primary deposit or of straight-lines between adjacent nozzles. Certainly, these additional secondary deposits lead to a deviation from the theoretical efficiency curve in which only a single primary deposit existed and was considered.The jet-to-jet interaction in multi-nozzle inertial impactors and the formation of secondary deposits has been analyzed and quantified as a function of the distance between nozzles and the Reynolds number. This work has been studied experimentally (Part I) and numerically (Part II), using CFD (Computational Fluid Dynamics) analysis to provide insight into the physical mechanisms for the formation of the secondary deposits and to support the experimental results.This paper provides detailed experimental impactor collection efficiency curves for impactor plates with 3 nozzles spaced in the range of 2.5W to 8W, where W is the nozzle diameter, and Reynolds numbers between 465 and 2210, which quantify the effects of the jet-to-jet interactions and the magnitude of the primary and secondary deposits. From the experimental results it may be concluded that the jet-to-jet interactions can produce four non-ideal effects: premature primary deposits with a half-moon shape, straight-line deposits between nozzles, nozzle plate deposits between nozzle exits (losses), and dispersion of the halo deposits.The secondary deposits and premature primary deposits reduce the slope of the efficiency curve, and/or displace the √Stk50 towards smaller values and/or generate tails at the low end. All of which result in the collection of small particles that otherwise would be collected at another stage with a lower cut point. With respect to the design criteria, to avoid the formation of the additional deposits generated by jet-to-jet interactions (premature primary deposits, straight-line and nozzle plate deposits), the nozzle-to-nozzle spacings have to be larger than 4W and/or the impactor should work at low Re (in this work Re≤465), even at 2.5W spacing.

Improving the foundation for particulate matter risk assessment by individual nanoparticle statistics from electron microscopy analysis

Air pollution is one of the major contributors to the global burden of disease, with particulate matter (PM) as one of its central concerns. Thus, there is a great need for exposure and risk assessments associated with PM pollution. However, current standard measurement techniques bring no knowledge of particle composition or shape, which have been identified among the crucial parameters for toxicology of inhaled particles. We present a method for collecting aerosols via impaction directly onto Transmission Electron Microscopy (TEM) grids, and based on the measured impactor collection efficiency and observed impact patterns we establish a reproducible imaging routine for automated Scanning Electron Microscopy (SEM) analysis. The method is validated by comparison to scanning mobility particle sizer (SMPS) measurements, where a good agreement is found between the particle size distributions (PSD), ensuring a representative description of the sampled aerosol. We furthermore determine sampling conditions for achieving optimal particle coverage on the TEM grids, allowing for a statistical analysis. In summary, the presented method can provide not only a representative PSD, but also detailed statistics on individual particle geometries. If coupled with Energy-dispersive X-ray spectroscopy (EDS) analysis elemental compositions can be assessed as well. This makes it possible to categorize particles both according to size and shape e.g. round and fibres, or agglomerates, as well as classify them based on their elemental composition e.g. salt, soot, or metals. Combined this method brings crucial knowledge for improving the foundation for PM risk assessments on workplaces and in ambient conditions with complex aerosol pollution.

Design of a High-Throughput Chemical Trace Detection Portal That Samples the Aerodynamic Wake of a Walking Person

The aerodynamic wake behind a walking person is first investigated using flow visualization and spectrophotometry in order to characterize the plume-to-wake transition and the decay of a trace body contaminant downstream of a walking human. The buoyant human thermal plume is shown to transition to an aerodynamic wake at a walking speed of 0.2 m/s. At higher walking speeds, the human wake is highly turbulent, resulting in an exponential decay of a passive scalar downstream of the walking human subject. From these results, a high-throughput chemical trace detection portal that uses the natural momentum of the wake to assist its collection is designed. This wake collection portal is shown to be capable of substantial improvement in security-checkpoint throughput over previous trace-detection-screening technology. To collect and sample the trace-bearing wake for a chemical signal, air flow rates nearly an order of magnitude larger than those of the previous collection technology must be accommodated. A high-flow-rate particle impactor was designed and tested to effectively collect the entire human aerodynamic wake. This impactor accommodates a flow rate of 750 L/s with a particle cutoff diameter of 5 μm. An iterative CFD process is described which streamlined the impactor geometry to reduce the total pressure drop across it. Experimental results are presented on the impactor collection efficiency. Preliminary experimental results show that trace chemicals can be detected at a useful level from a range of locations across the body when a human subject passes through the system wearing a chemical particle patch source.

Enhancement of collection efficiency of inertial impactors using elliptical concave impaction plates

In this study, elliptical concave impaction plate was suggested for lowering cut-off size and therefore enhancing collection efficiency of the inertial impactor. Statistical Lagrangian Particle Tracking (SLPT) model was employed for calculating impactor collection efficiency and validated by comparing with the experimental data of Tsai, C.J., Cheng, Y.H. ((1995). Solid particle collection characteristics on impaction surfaces of different designs. Aerosol Science Technology, 23, 96–106), for three different shapes of impaction plates. Then, the effect of the ratio of major axis length ( A) to minor axis length ( B) for determining the curvature of elliptical concave impaction plate, on impactor collection efficiency was numerically investigated using the SLPT model, with nozzle Reynolds numbers ranging from 1440 to 2600. It was found that there existed an optimum range of the A/ B ratio for minimizing the cut-off size, i.e. the A/ B ratio ranged between 3.2 and 4.2 for the PM10 inertial impactor, or between 3.2 and 3.5 for the PM2.5 inertial impactor. When the elliptical concave impaction plates with the A/ B ratio of 4.0 and 3.5 were applied to the MST indoor air sampling impactor having PM10 and PM2.5 stages, the cut-off size was predicted to decrease from 10 to 6.5 μm and from 2.5 to 1.6 μm, respectively, while the impactor collection efficiency curves became less steep.

Simulation of low pressure impactor collection efficiency curves

The collection efficiency of the low pressure impactors has been studied using numerical simulations. Flow field was modeled by solving the equations describing the time-average flow field (RANS) with a commercial CFD solver. Particle tracks were calculated separately using Lagrangian methods. Simulation results were verified against published experimental results. Effect of turbulent velocity fluctuations on the impactor resolution was investigated by comparing the ratio of the simulated to experimental impactor resolutions as a function of the turbulence level of the jet. It was found that the turbulence is the dominant mechanism reducing the resolution when the local Reynolds number is over 1800. Effect of jet-to-plate distance on the resolution of the low pressure impactor was studied in the case of low turbulence level. Highest resolution was achieved when the ratio of jet diameter to jet to plate distance ( S/ W) is 2. When the ratio is lower or higher, resolution is reduced because there is an increase in nonuniformity of the impaction conditions across the jet.

Quantitative sampling and analysis of trace elements in atmospheric aerosols: impactor characterization and Synchrotron-XRF mass calibration

Abstract. Identification of trace elements in ambient air can add substantial information to pollution source apportionment studies, although they do not contribute significantly to emissions in terms of mass. A method for quantitative size and time-resolved trace element evaluation in ambient aerosols with a rotating drum impactor and synchrotron radiation based X-ray fluorescence is presented. The impactor collection efficiency curves and size segregation characteristics were investigated in an experiment with oil and salt particles. Cutoff diameters were determined through the ratio of size distributions measured with two particle sizers. Furthermore, an external calibration technique to empirically link fluorescence intensities to ambient concentrations was developed. Solutions of elemental standards were applied with an ink-jet printer on thin films and area concentrations were subsequently evaluated with external wet chemical methods. These customized and reusable reference standards enable quantification of different data sets analyzed under varying experimental conditions.

배지 종류 및 저장 조건에 따른 impactor의 부유세균 시료 채취 효율 평가

The range of reduction rates of airborne bacteria concentration at 6 hrs, 12 hrs, 24 hrs and 48 hrs, which means a storage time until input of agar media into incubator after air sampling with an impactor. were 15-20%, 25-40%, 35-50% and 55-70%, respectively, compared to initial concentration. Types of agar media and storage thermal condition did not significantly affect a collection efficiency of impactor in terms of evaluating airborne bacteria level (p>0.05). To better improve the impactor's collection efficiency of airborne bacteria, based on the result of this study, it is recommended that the vicinity of <TEX>$25^{\circ}C$</TEX> should be sustained until input of agar media into incubator after air sampling.

Comparison of Various Methods for Processing Cascade Impactor Data

This study proposes a method of processing cascade impactor data, called the “calibration matrix method,” and compares it with usual methods. This method takes into account the real collection efficiency curves obtained from cascade impactor calibration, in contrast to the usual methods which assume that cascade impactor collection efficiency is ideal. The comparison between the new and usual methods was carried out for sodium fluoride aerosols generated with five different nebulizers (Multisonic®, LS260 kit2601®, Sidestream®, PariLC+® and Aeroneb Go®) and one dry powder (terbutaline, Astra Zeneca, France), sized with the 8-stage Marple Model 298 Personal cascade impactor. Particle size distributions were represented as differential and cumulative distributions. Statistical differences were highlighted between the calibration matrix method and the usual methods in terms of MMAD and respirable fractions for all aerosols. The largest differences were 28% in terms of MMAD and 64% in terms of respirable fraction.

087 Comparison of methods for cascade impactor data analysis to predict aérosol déposition into patient airways

Cascade impactors are the most widely encounte-red means to measure aerosol particle size distribution produced by nebulizers. The usual method used to treat particle size data was compared to a new method (calibration method) which is not based on a specifie hypothesis, contrary to the usual method which assumes that cascade impactor collection efficiency is ideal. Four nebulizers (Multisonic ® , Sidestream ® , Aeroneb Go ® , PariLC+ ® ) were used to produce NaF aerosols, and particles size was measured with the 8-stage Marple Model 290 Personal cascade impactor. The usual method consists in plotting the mass fraction (F j ) deposi-ted on each stage us. the cut-offdiameter (De j ) corresponding to the stage. According to the European Standard (EN13544-1), a best-fit curve links the points De j , F j . This work develops another method of particle size data processing based on cascade impactor calibration, which takes into account imperfect stage collection efficiency. To calibrate the impactor, monodisperse aerosols of diameter Dcalj (i = 1 to m) were generated, and the mass fraction deposited on each stage was measured. The mass fraction of monodisperse aerosols of diameter Dcalj deposited on impactor stage De j is f i,j ;. Values of F j and f i,j j can be represented by mathematical matrix. To obtain the corrected distribution of aerosol, matrix containing f-: values is multiplied by matrix contai-ning Fj values. The curve linking the points (X-axis = Monodisperse aerosol diameters Dcal j ; Y-axis = corrected distribution) is then plotted. Values of Mass Median Aerodynamic Diameters and Respiratory Fractions (%l-5 μm) for the usual and calibration methods are summarized in the table below. The highest differences were observed for Sidestream and PariLC+ ® nebulizers. Impactor calibration must be done with many monodisperse aerosols so that calibration method gives more precise calculations of MMAD and respiratory fractions than usual method.

Sampling atmospheric carbonaceous aerosols using a particle trap impactor/denuder sampler.

A particle trap impactor/denuder system has been developed and tested for the sampling of ambient carbonaceous aerosols. Use of a particle trap impactor allows for a reduction of particle bounce and re-entrainment at high particle loadings, and operation at high volumetric flow rates is achieved without the use of oiled impaction substrates, thus facilitating the chemical and physical analysis of the organic compounds comprising the collected gas (G) and particle (P) phases. Honeycomb denuders have a greater density of channels for a given denuder cross-sectional area than parallel plate or annular denuders; for a given sampling flow rate, honeycomb denuders can be fabricated in more compact shapes and will have a greater amount of surface area for the collection of gases. Field testing of the sampler was conducted primarily at night to minimize the evaporation of organic carbon (OC) from collected particles, which can result from the heating of collected particles as ambient temperatures rise during the day. In side-by-side testing with an open-face filter pack sampler, the denuder system was found to minimize positive gas adsorption artifacts caused by the adsorption of gaseous OC to quartz filter fiber (QFF) surfaces. In the denuder sampler, negligible amounts of OC were observed on a QFF placed downstream of a particle-loaded QFF, suggesting that OC detected on the backup QFF in the filter pack sampler resulted primarily from the adsorption of ambient G-phase OC rather than OC evaporated from particles collected on the front filter. Equations are presented for the evaluation of the magnitude of positive and negative sampling artifacts. Analysis of these equations indicates that the mass of OC evaporated from filter-bound particles present downstream of a denuder depends on (i) the volume of OC-free gas passed through the filter, (ii) the P-phase concentration and the P/G partition coefficients (Kp) of the compounds comprising the P-phase OC, (iii) the temperature (T) (values of Kp are inversely proportional to T), and (iv) the mass fraction of carbon in the compounds comprising P-phase OC. For these reasons, the magnitude of evaporative losses of OC in denuder samplers may vary among different sampling events. In addition, a method utilizing gas chromatography/mass spectrometry has been developed for determination of inertial impactor collection efficiency and denuder particle transmission efficiency. Using this method, only a single extraction of the sampler components is necessary, thereby reducing the number of extractions and analyses over conventional approaches by at least a factor of 2.

The effect of varying impaction plate temperature on impactor performance: experimental studies

This study concerns the effect of impaction plate temperature change on impactor performance. The impaction plate was cooled or heated with a thermoelectric module, and the temperature of the impaction plate was monitored by the thermocouple. The size of used particles ranged from 1 to 10 μm . Finally, a collection efficiency curve was obtained by employing a TSI Aerosizer. The results reveal that cooling the impaction plate makes the impactor collection efficiency higher and the re-entrainment lower. Therefore, cooling gives a more desirable collection efficiency curve. The increased adhesion effect on the impaction plate explains this. Based on the present investigations, cooling the impaction plate enhances impactor performance without using coating materials. A new type of impactor with a thermoelectrically cooled plate is demonstrated to be useful in this experimental study.

Development and Validation of a High-Volume, Low-Cutoff Inertial Impactor

A low-cutoff, high-volume conventional impactor has been designed. This sampler uses a slit-shaped acceleration jet and operates at 1100 L/min. The impaction substrate is polyurethane foam (PUF). The impactor collection efficiency was characterized using polydisperse particles, and the 50% size cutoff point was 0.12 ¡m. Losses within the sampler were also characterized and were less than 10%. The use of polyurethane foam (PUF) as a substrate has the following advantages: (I) PUF has a very high particle collection efficiency over a large range of particle sizes, even under conditions of heavy particle loading, as compared to other impaction substrates, such as flat plates and less porous membranes, which typically are subject to significant bounce-off and reentrainment; (2) no oil or grease coating is required, so potential interferences of impurities within such coatings are avoided when chemical, biological, and toxicological tests are performed on the collected particles; (3) PUF itself is chemically inert, minimizing interference with any of these tests; (4) because of the high flow rate of 1100 L/min, a large amount of particles can be collected in a short period of time on a relatively small surface of substrate, facilitating recovery of the collected particles for the different tests; and (5) a large amount of particles can be collected on a relatively small collection surface and easily extracted with small amounts of water or organic solvents. This method will be suitable for the collection of large amounts for toxicological studies and analysis of organic aerosols, which is not possible with other high-volume samplers that utilize large filtration surfaces.

DEVELOPMENT AND VALIDATION OF A HIGH-VOLUME, LOW-CUTOFF INERTIAL IMPACTOR

A low-cutoff, high-volume conventionalimpactor has been designed. This sampler uses a slitshaped acceleration jet and operates at 1100 L/min. The impaction substrate is polyurethane foam (PUF). The impactor collection efficiency was characterized using polydisperse particles, and the 50% size cutoff point was 0.12 mu m. Losses within the sampler were also characterized and were less than 10%. The use of polyurethane foam (PUF) as a substrate has the following advantages: (1) PUF has a very high particle collection efficiency over a large range of particle sizes, even under conditions of heavy particle loading, as compared to other impaction substrates, such as flat plates and less porous membranes, which typically are subject to significant bounce-off and reentrainment; (2) no oil or grease coating is required, so potential interferences of impurities within such coatings are avoided when chemical, biological, and toxicological tests are performed on the collected particles; (3) PUF itself is chemically inert, minimizing interference with any of these tests; (4) because of the high flow rate of 1100 L/min, a large amount of particles can be collected in a short period of time on a relatively small surface of substrate, facilitating recovery of the collected particles for the different tests; and (5) a large amount of particles can be collected on a relatively small collection surface and easily extracted with small amounts of water or organic solvents. This method will be suitable for the collection of large amounts for toxicological studies and analysis of organic aerosols, which is not possible with other high-volume samplers that utilize large filtration surfaces.

On the shape of impactor efficiency curves

The effect of impactor stage geometry on impactor collection efficiency curves is explored numerically. It is found that the geometry of the impaction stage where the gas is accelerated does not affect the 50% cut size but has a strong effect on the sharpness and shape of the efficiency curve. This phenomenon is explained from the aerodynamic focusing effect of the particles in the nozzle (gas acceleration) section. The flat-plate orifice configuration, commonly used in cascade impactors, tends to focus particles closer to the centerline than the angled nozzle. When the real nozzle configuration is used, the prediction shows a characteristic S shape, in good agreement with the experimental results in the literature.

Measurements of relative humidity-dependent bounce and density for atmospheric particles using the DMA-impactor technique

The dependence of particle bounce on relative humidity in an inertial impactor was measured for atmospheric aerosols in Meadview, AZ, and Minneapolis, MN. An aerosol composed of 0.25 μm atmospheric particles, selected by a differential mobility analyser, was drawn through a single stage of a MOUDI impactor having an aerodynamic cut point of 0.112 μm. The impactor collection efficiency was determined by using two condensation nucleus counters to simultaneously count particles upstream and downstream of the impactor. Bounce measurements were done using uncoated aluminum substrates, and provision was made to control relative humidity of the sampled aerosol. We found that for both locations bounce increased sharply as relative humidities decreased below 60–70%, but little bounce occurred when the relative humidity was maintained above 70 or 80%. Because errors caused by particle bounce and flow-induced relative humidity changes are minimal for relative humidities from 70 to 80%, this is the optimal range for atmospheric sampling with cascade impactors having uncoated stages. Surface loading was found to have a negligible effect on particle bounce for atmospheric particles. A similar apparatus was also used to measure densities of particles in the 0.06–0.18 μm diameter range using the technique described by Kelly and McMurry (Aerosol Sci. Technol.17, 199–212, 1992). We found that the average density varied between 1.60 and 1.79 gcm−3 (typical uncertainty 4%), and tended to decrease with increasing relative humidity. Measured dry particle densities' exceeded the value of 1.48 gcm−3 that was estimated from size-resolved chemical composition measured with MOUDIs.

A Numerical Study of the Effect of a High Aerosol Concentration on Impactor Performance

The effect of a high aerosol concentration on the collection efficiency of impactors has been studied. The flow was assumed to be dilute and two-way coupled. A particle source in cell model for the computational scheme was used. Results show that the effect of the particle interactions with the air on the collection efficiency is enlarged as the aerosol concentration is increased. It was also found that when the value of the Reynolds number or the overall particle sizes increased, the change of the collection efficiency by the change of the aerosol concentration also increased. The effect of the high aerosol concentration on the collection efficiency was found to be considerable even in slow flows when particles are large.

Design and evaluation of new low-pressure impactor. I

An eight-stage, low-pressure impactor is developed with 50% efficiency cutoffs of 4.0, 2.0, 1.0, 0.05, 0.26, 0.11, 0.075, and 0.05-µm aerodynamic diameter. Particles larger than 0.5 µm are sampled at near atmospheric pressure; four stages with stagnation pressures below the jet of 8-140 mm Hg absolute, size segregate the smaller aerosol. The 0.50, 0.26, and 0.11-µm cutoff stages are calibrated with monodisperse polystyrene latex spheres. The final two stages are compared with electrical aerosol analyzer measurements of a laboratory-generated sulfuric acid mist. Studies of impactor collection efficiency show that particle bounce and reentrainment are not important when the collection surfaces are Vaseline coated. The impactor has one jet per stage, and thus is especially suited for aerosol sulfur and nitrate determinations by the method of flash volatilization and subsequent gas-phase analysis.

Influence of boundary layers on the performance of rectangular impactors

The collection efficiency of impactors is affected by boundary layers along the nozzle walls and the impaction plate. A simplified hydrodynamic model which takes account of these boundary layers is presented for rectangular impactors. The model operates with a set of new parameters which characterize the flow field between the nozzle and the impaction plate. In this flow field the motion of the particles along the impactor axis is considered and a method is presented for calculating the particle cutoff diameter, which is the diameter of the largest particle with zero collection efficiency. The design and performance of a five-stage cascade impactor are described and it is shown how measurements of flow rate and pressure drops over the impactor stages can be used for calculating these parameters. Measurements of the collection efficiencies using polystyrene PSL spheres are described and it is demonstrated that the calculated cutoff diameters agree well with the experimentally determined cutoff diameters.

Nonideal collection characteristics of single stage and cascade impactors.

Experimental studies of single stage and cascade impactors showed that bounce was severe for dry particles on dry solid surfaces. Oil drops on dry surfaces or dry particles on oil coated surfaces were collected as predicted by theory. While glass fiber media collection surfaces reduced particle bounce, the impactor collection efficiency characteristics were changed.