Effect of axial eccentricity on the performance of a cylindrical differential mobility classifier

A differential mobility classifier (DMC) is one of the core components in electrical mobility particle sizers for sizing sub-micrometer particles. A DMC in the cylindrical configuration (i.e., constructed by axial aligning of the inner and outer cylinders) is typically included in the sizers. The knowledge of construction tolerance is required in the design of a cylindrical DMC. The numerical approach was applied in this study. Our study shows that the DMC transfer function deteriorated as the axial eccentricity was increased (i.e., the peak is reduced and the width at the half peak height is broaden). At high axial eccentricity, the transfer function peak would split into two. In addition to the flow parameters such as the sheath-to-aerosol flow rate ratio and total flow rate, the effect of geometrical parameters (i.e., the length and aspect ratio of the particle classification channel, and the ratio of outer-to-inner cylinder radii) on the transfer function of an eccentric DMC were also investigated. It is found that the classification length and the sheath-to-aerosol flow rate ratio have obvious impact on the transfer function of an eccentric DMC. Furthermore, the particle diffusivity reduced the effect of axial eccentricity on DMC transfer function, especially for particles with the sizes less than 10?nm.

Copyright © 2019 American Association for Aerosol Research     

Determination of particle diameter and lumped density and shape factor using a three-beam time-of-flight (TOF) instrument

A mathematical scheme was developed to calculate particle diameter as well as particle lumped density and shape factor using the information obtained from particle trajectory calculations using a 3-beam aerosizer (a time-of-flight particle size determination instrument). A suitable drag coefficient for the flow regime present in the aerosizer was selected by comparison of the calculated trajectories with experimental data (Numerical study on the simultaneous determination of particle physical diameter, lumped density and shape factor using time-of-flight sizers, Illinois Institute of Technology, Chicago, 1996, Aerosol Sci. Technol 29 (1998) 433). A model for particle trajectory in a one-dimensional supersonic compressible flow (Oskouie et al., 1998) was extended to provide the density and shape factor for the particles. The velocity of a particle was calculated along the centerline of three equidistant laser beams downstream of a sonic nozzle, and the physical diameter of this particle, and its lumped density and shape factor were theoretically calculated and compared to experimental results.     

Intercomparison of a portable and two stationary mobility particle sizers for nanoscale aerosol measurements

During occupational exposure studies, the use of conventional scanning mobility particle sizers (SMPS) provides high quality data but may convey transport and application limitations. New instruments aiming to overcome these limitations are being currently developed. The purpose of the present study was to compare the performance of the novel portable NanoScan SMPS TSI 3910 with that of two stationary SMPS instruments and one ultrafine condensation particle counter (UCPC) in a controlled atmosphere and for different particle types and concentrations.

The results show that NanoScan tends to overestimate particle number concentrations with regard to the UCPC, particularly for agglomerated particles (ZnO, spark generated soot and diesel soot particles) with relative differences >20%. The best agreements between the internal reference values and measured number concentrations were obtained when measuring compact and spherical particles (NaCl and DEHS particles). With regard to particle diameter (modal size), results from NanoScan were comparable < [± 20%] to those measured by SMPSs for most of the aerosols measured.

The findings of this study show that mobility particle sizers using unipolar and bipolar charging may be affected differently by particle size, morphologies, particle composition and concentration. While the sizing accuracy of the NanoScan SMPS was mostly within ±25%, it may miscount total particle number concentration by more than 50% (especially for agglomerated particles), thus making it unsuitable for occupational exposure assessments where high degree of accuracy is required (e.g., in tier 3). However, can be a useful instrument to obtain an estimate of the aerosol size distribution in indoor and workplace air, e.g., in tier 2.     

Stratospheric Aerosol Sampling: Effect of a Blunt-Body Housing on Inlet Sampling Characteristics

During a campaign to study ozone loss mechanisms in the Arctic stratosphere (SOLVE), several instruments on NASA's ER-2 aircraft observed a very low number density (0.1 I^?1) of large, nitric-acid-containing particles that form the polar stratospheric clouds (PSCs). For effective physical and chemical characterization of these particles, the measurements from these instruments have to be intercompared and integrated. In particular, proper interpretation requires knowledge of the sampling characteristics of the particles into the instruments. Here, we present the calculation of the sampling characteristics of the one of the instruments on the ER-2, the NOAA NOy instrument. This instrument sampled ambient particles and gas from two forward-facing inlets located fore and aft on a particle-separation housing (the football) and measured total NOy in the sample. In recent studies, ambient aerosol mass has been estimated by the difference of the measurements of the two inlets with the assumption that the rear inlet observations represent the gas-phase NOy and small particles and the front inlet samples represent gas-phase NOy and all particle sizes with varied efficiency (anisokinetic sampling). This knowledge was derived largely from semiempirical relations and potential flow studies of the housing. In our study, we used CFD simulations to model the compressible flow conditions and considered noncontinuum effects in calculating particle trajectories. Our simulations show that the blunt body housing the inlets has a strong and complex interaction with the flow and particles sampled by the two inlets. The simulations show that the front inlet characteristics are influenced by the effect of the blunt body on the upstream pressure field. The rear inlet sampling characteristics are influenced both by the shape and size of the inlet and its location on the blunt body. These interactions result in calculated inlet characteristics that are significantly different from previously assumed values. Analysis of the SOLVE data, considering the ambient conditions and the calculated inlet sampling characteristics, in conjunction with thermodynamic growth modeling of super-cooled ternary solution (STS) particles, provides validation of the CFD results.     

Sampling diced almonds for aflatoxin

To ensure that diced almonds meet the current FDA guideline limit for total aflatoxin, it is necessary to have a sampling plant that will allow representative sampling with defined precision-i.e., with confidence limits on the average aflatoxin found. A sequential sampling plan using 4.54-kg samples of diced almonds or 150-g samples of meal by-product (fines screened from diced nuts during production) was constructed with data from a study of aflatoxin distribution among samples of 2 selected lots of almonds. These 2 lots of whole nuts, estimated to have 400 and 25 ppb aflatoxin, were diced and boxed with normal processing equipment and procedures to approximate the distribution of aflatoxin in the product during commercial production. With a square root trans-formation of the data from 4.54-kg samples of diced nuts, the aflatoxin in samples of both lots approximated a normal distribu-tion and the within-lot variances were not significantly different, which allowed the statistical plan described. A supplemental study was made of aflatoxin distribution in the meal by-product. The lack of a significant difference between the results for diced nuts and those for the corresponding meal suggests that diced almonds can be monitored for aflatoxin indirectly by sampling the meal, which will allow the use of fewer analyses of 150-g samples of less expen-sive product to reach a decision.     

Inter-Comparison of a Fast Mobility Particle Sizer and a Scanning Mobility Particle Sizer Incorporating an Ultrafine Water-Based Condensation Particle Counter

An Ultrafine Water-based Condensation Particle Counter (UWCPC), a Scanning Mobility Particle Sizer (SMPS) incorporating an UWCPC, and a Fast Mobility Particle Sizer (FMPS) were deployed to determine the number and size distribution of ultrafine particles. Comparisons of particle number concentrations measured by the UWCPC, SMPS, and FMPS were conducted to evaluate the performance of the two particle sizers using ambient particles as well as lab generated artificial particles. The SMPS number concentration was substantially lower than the FMPS (FMPS/SMPS = 1.56) measurements mainly due to the diffusion losses of particles in the SMPS. The diffusion loss corrected SMPS (C-SMPS) number concentration was on average ～ 15% higher than the FMPS data (FMPS/C-SMPS = 0.87). Good correlation between the C-SMPS and FMPS was also observed for the total particle number concentrations in the size range 6 nm to 100 nm measured at a road-side urban site (r² = 0.91). However, the particle size distribution measured by the C-SMPS was quite different from the size distribution measured by the FMPS. An empirical correction factor for each size bin was obtained by comparing the FMPS data to size-segregated UWCPC number concentrations for atmospheric particles. The application of the correction factor to the FMPS data (C-FMPS) greatly improved the agreement of the C-SMPS and C-FMPS size distributions. The agreement of the total particle concentrations also improved to well within 10% (C-FMPS/C-SMPS = 0.95).     

Differential mobility analyser transfer functions in scanning mode

A novel method is described for the calculation of the differential mobility analyser (DMA) transfer function in scanning mode, which is based on the derivation of an analytical solution for the non-diffusive particle trajectories inside the DMA, under exponentially varying electrical field and fully developed laminar flow. The scanning mode transfer functions can substantially improve the measurement accuracy of fast scanning mobility particle sizers (SMPS) as shown by Collins, Cocker, Flagan, and Seinfeld [(2004). The scanning DMA transfer function. Aerosol Science and Technology, 38, 833–850]. Compared to the Monte Carlo simulations described by Collins et al., the method developed here is much more accurate and sufficiently fast to be employed in advanced DMA inversion algorithms.     

静安区PM_(2.5)浓度手工法与自动监测法的比对分析

分别运用传统的手工采样重量法和基于微量振荡天平加膜动态测量系统方法(FDMS)的自动监测法测量PM2.5的质量浓度。通过2013年10月1日至2014年1月31日期间两种方法的监测数据比对显示,自动监测法检测系统误差偏高,自动监测法与手动法的监测结果具有很高的相关性,其变化趋势具有较高的一致性。     

Long-Term Sampling of Viable Airborne Viruses

A novel bioaerosol sampling technique, which utilizes the bubbling process in the collection fluid, has recently been developed and found feasible for a long-term personal sampling of airborne bacteria and fungal spores as it maintained high physical collection efficiency and high microbial recovery rate for robust and stress-sensitive microorganisms. Further tests have shown that the new technique also has potential to collect viable airborne viruses, particularly when utilized for a short-term sampling of robust strains. As the short-term sampling has a limited application for assessing personal exposure in bioaerosol-contaminated environments, the present study was undertaken to investigate the feasibility of the “bubbler” for a long-term monitoring of viable airborne viruses. Liquid droplets containing Vaccinia virions (that simulate Variola, a causative agent of smallpox) were aerosolized with a Collison nebulizer into a 400-liter test chamber, from which the droplets were collected by three identical prototype personal samplers in the liquid medium during different time periods ranging from 1 to 6 hours. The viral content was measured in the collection fluid of the sampler and in the initial suspension of the nebulizer using the fluorescence-based method and by enumerating plaque-forming units per milliliter of the fluids. The relative recovery of viruses after the sampling act was determined. The results show that the “bubbling” technique has consistent collection efficiency over time and is capable of maintaining the viability of Vaccinia, for at least 6 hours, with a loss in recovery rate of about 10%. The data demonstrate a good potential of the new technique for measuring personal exposure to robust airborne viruses over a long period.     

Field Evaluation of Sampling Bias with Plastic Petri Dishes for Size-Fractionated Bioaerosol Sampling

Reusable glass dishes are recommended for use with the six-stage viable impactor for size-fractionated bioaerosol sampling. However, it is not convenient to use glass dishes because they are fragile and heavy, not to mention the time-consuming preparation process prior to bioaerosol sampling. On the other hand, disposable plastic dishes have been widely used in microbiology laboratories. However, plastic materials can retain electrostatic charges and may lead to sampling bias. The objective of this study was to evaluate the sampling bias with the use of plastic dishes when a multistage viable impactor is used for airborne fungi and bacteria sampling for field sampling. Two six-stage viable impactors were placed side-by-side 1 m apart in a 147-m³ room. One was used with plastic dishes and the other with glass dishes. Compared with the concentration data obtained with glass dishes, those collected with the plastic dishes demonstrated a significant difference for both fungi and bacteria. However, there was a strong correlation between the data obtained using glass and plastic dishes, which can be estimated by C_plastic = 0.88 C_glass for airborne fungi and C_plastic = 0.86 C_glass for airborne bacteria. When using plastic dishes fungi and bacteria counts were underestimated by 12% and 14%, respectively.

Copyright 2015 American Association for Aerosol Research     

Sampling dynamics for pressurized electrochemical cells

A model describing the gas distribution within a constant pressure electrolysis system and how the distribution impacts electrochemical efficiencies is presented. The primary system of interest is the generation of syngas (CO and H₂) associated with the co-electrolysis of H₂O and CO₂. The model developed for this system takes into account the primary process variables of operation including total system pressure, applied current, and the in-flow of reactant gases. From these, and the chemical equilibria within the system, the impact on electrochemically generated gases is presented. Comparison of predicted and measured faradaic efficiency of an electrode's processes reveals significant disagreement under certain conditions. Methods to minimize and account for the discrepancy are presented with the goal of being able to discern, in a real-time manner, degradation of electrode performance. Comparison of the model to experimental data shows a strong correlation between the two with slight variation in experimental data, which is attributed to reversible system dynamics such as wetting of the gas diffusion electrode used as the cell cathode.     

Wind Tunnel Evaluation of an Aircraft-Borne Sampling System

The U.S. Environmental Protection Agency (EPA), the Florida Department of Environmental Protection (FLDEP), and Texas A&M University collaborated in the design, construction, and testing of a unique, highly crosslinked, Teflon-coated inlet and manifold gas and aerosol sampling system that is being used in EPA aircraft atmospheric pollution characterization studies. The aircraft-borne ambient sampling system, which consists of a Teflon-coated shrouded probe coupled to a Teflon-coated aluminum manifold, is designed to collect reactive gases (e.g., mercury and halide species) and aerosols for subsequent analysis and characterization. The shrouded inlet probe was tested for particle transmission ratios in a high-speed aerosol wind tunnel. An existing wind tunnel was upgraded from a maximum wind speed of 13.4 m/s (48 km/h or 30 miles/h) to 50.5 m/s (182 km/h or 113 miles/h) to test this probe. The wind tunnel was evaluated for compliance with the criteria of ANSI 13.1 to establish the acceptability of its use in testing probes. The results demonstrated that the velocity and tracer gas concentration profiles were within the specified limits. A wellcharacterized ThermoAndersen Shrouded Probe (Model RF-2-112) was also tested to check tunnel performance and test methodology. The results obtained from these tests are in close agreement with earlier published data.

When operated at a sampling flow rate of 90 L/min, the aircraft-borne shrouded probe showed a transmission ratio of about 0.76 at 45 m/s (162 km/h or 100 miles/h) for 10 μ m aerodynamic diameter particles. To improve the transmission ratio of the sampling probe, the sampling flow rate was reduced to 80 L/min and the air speed increased to 50.5 m/s, which increased the transmission ratio to about 0.9 for 10 μ m particles. Further reduction of the flow rate to 60 L/min increased the transmission to 1.2. The Teflon-coated manifold, which is located downstream of the shrouded probe, was statically tested for transmission ratio at flow rates of 90 L/min and 30 L/min. The results were a transmission ratio of about 0.80 for 10 μ m aerodynamic diameter particles. The combination of the shrouded probe operated at 60 L/min with a transmission ratio of 1.2 and the manifold with its transmission of 0.8 will give an overall transmission of about unity for 10 μ m aerodynamic diameter particles at a flight speed of 50.5 m/s.

These findings suggest that shrouded probes can be used for low speed (～ 100 miles/h) aircraft applications. The transmission ratio of these probes is a significant improvement over the conventional aircraft-mounted, sharp-edged isokinetic diffuser-type inlets.     

Deposition Uniformity and Particle Size Distribution of Ambient Aerosol Collected with a Rotating Drum Impactor

Rotating drum impactors (RDI) are cascade type impactors used for size and time resolved aerosol sampling, mostly followed by spectrometric analysis of the deposited material. They are characterized by one rectangular nozzle per stage and are equipped with an automated stepping mechanism for the impaction wheels. An existing three-stage rotating drum impactor was modified, to obtain new midpoint cutoff diameters at 2.5 μm, 1 μm, and 0.1 μm, respectively. For RDI samples collected under ambient air conditions, information on the size-segregation and the spatial uniformity of the deposited particles are key factors for a reliable spectrometric analysis of the RDI deposits. Two aerodynamic particle sizers (APS) were used for the determination of the RDI size fractionation characteristics, using polydisperse laboratory room air as quasi-stable proxy for urban ambient air. This experimental approach was suitable for the scope of this study, but was subject to numerous boundary conditions that limit a general use. Aerodynamic stage penetration midpoint diameters were estimated to be 2.4 and 1.0 μm for the first two RDI stages. Additionally, the spatial uniformity and geometrical size distribution of the deposited aerosol were investigated using micro-focus synchrotron radiation X-ray fluorescence spectrometry (micro-SR-XRF) and transmission electron microscopy (TEM), respectively. The size distribution of the particles found on the TEM samples agreed well with the results from the APS experiments. The RDI deposits showed sufficient uniformity for subsequent spectrometric analysis, but in the 2.5–10 μm size range the particle area density was very low. All of the applied methods confirmed the theoretical cutoff values of the modified RDI and showed that compared to other cascade impactors, the determined stage penetration sharpness was rather broad for the individual impactor stages.     

Sampling efficiencies of two modified viable cascade impactors

Prevention of airborne contagious diseases depends on successful characterization of aerosols in the environment. The use of cascade impactors to characterize ambient aerosols is one of the most commonly used methods, providing data on both particle size and concentration. In this study, the use of a cascade impactor recently described in the literature using 8 mL of liquid in Petri dishes (CI-L) was compared with a new method that uses wet membrane filters on top of wax filled Petri dishes (CI-WWMF). Sampling efficiencies of the cascade impactors were evaluated using 0.5, 1, 3, and 5 μm polystyrene latex (PSL) microspheres and aerosol consisting of single spores of Bacillus atrophaeus var. globigii (BG). The sampling efficiency of the CI-L was 6%, 11%, 17%, 21%, and 58% for 0.5, 1, 3, 5 μm PSL microspheres and BG spores, respectively. Higher overall sampling efficiencies of 71%, 91%, 60%, 64%, and 104% were observed for the same size and type of particles for the CI-WWMF. This study indicates that using wet filters on top of wax-filled Petri dishes (CI-WWMF) in a viable cascade impactor is more efficient than the CI-L method for size-selectively collecting biological aerosols from the environment. The CI-WWMF method is useful when a liquid medium is required for identifying and quantifying organisms using polymerase chain reaction (PCR) and immuno-assay techniques.

Copyright © 2017 American Association for Aerosol Research     

Implementing Residual‐Based KPSS Tests for Cointegration with Data Subject to Temporal Aggregation and Mixed Sampling Frequencies

We show how different data types (stocks and flows) and temporal aggregation affect the size and power of the dynamic ordinary least squares residual‐based Kwiatkowski–Phillips–Schmidt–Shin (KPSS) test of the null of cointegration. Size may be more effectively controlled by setting the minimum number of leads equal to one – as opposed to zero – when selecting the lag/lead order of the dynamic ordinary least squares regression using aggregated data, but at a cost to power. If high‐frequency data for one or more series are available – that is, the model has mixed sampling frequencies – we show how to effectively utilize the high‐frequency data to increase power while controlling size.     

Sampling Errors in Cylindrical Nozzles

This paper reviews publications on aerosol aspiration by axisymmetric tubes, a widely used form of practical sampler. Axisymmetric tubes are widely used, as a rule, in stack sampling and sometimes in other areas of aerosol sampling as well (e.g., workplaces, ambient atmosphere). Numerous reports on aspiration coefficients for particles sampled from disperse flows contain two contradictory viewpoints on the sampling efficiency at suction velocities exceeding that of wind: although some authors claim that the sample representativeness worsens, others maintain that it is improved. Aerosol aspiration from calm or weakly turbulent air has not been investigated fully, despite the fact that the problem of determining sampling errors under such conditions is important in relation to occupational hygiene and environmental monitoring. Along with the analysis of the results published by other investigators (Davies et al., Vincent et al., etc.), this paper contains the axisymmetric sampler aspiration data obtained by us during the last 5-year period.

Experimental evidence is given for the secondary aspiration of particles after their bounce or blow-off, not only from the front face of the sampling tube but also from its external side surface. This effect is responsible for the qualitative discrepancy between the aspiration coefficient values obtained by different methods. The sampling conditions, for which aspiration distortions can be compensated for by using the inertial aspiration coefficient calculated from conventional theory, have been determined for axisymmetric samplers. The aspiration coefficient dependences on the anisokinetic coefficient, Stokes number, sampler wall thickness, and yaw angle have been analyzed for the aerosol sampling from steady-state flows. Possibilities of using these dependences to estimate errors in sampling aerosols from flows with the wind vector fluctuating in direction and magnitude are discussed. The poorly predictable secondary aspiration and flow turbulence effects observed with thick-walled samplers are shown to invariably influence the aspiration coefficient, making correction for sampling errors extremely difficult.

The inertial aspiration coefficient values measured for low-velocity wind and calm air have been analyzed. These results point to the not-so-obvious dependence of this coefficient on the sampling conditions. Experimental data are included, which make it possible to determine aspiration distortions at the orifices of samplers used with commercial aerosol analyzers.     

In-service performance assessment of electrostatic precipitators serving a rubber vulcanization process

We examined two emission abatement systems of some vulcanization ovens, serving a unit producing small rubber-based parts for automotive application. Each emission control unit treats the gases exhausted by three to five ovens. A heat exchanger cools down the fumes to a temperature suitable for the correct operation of a couple of two-stage electrostatic precipitators in series. We performed quantitative analysis of concentrations and size distributions in these rubber fumes using aerosol technology instrumentation, namely optical particle spectrometers and electrical mobility particle sizers. The size of sampled particles was mainly between 100?nm and 1000?nm. We evaluated the performance of the exhaust fume abatement units, with focus on the electrostatic precipitator. Concerning batch ovens, the quantitative trend of the emissions follows the thermal cycle of the post-curing process. Time interval since the last maintenance operation causes a gradual reduction in the removal efficiency. The measured data demonstrate the reliability and the adequacy of aerosol instrumentation for the characterization of the emissions from rubber vulcanization ovens. The pair of electrostatic precipitators was shown to be effective in removing most of the particles detected in the fumes stream. The measurement protocol developed in this study allows assessing the influence of the maintenance schedule on the performance of the emission control units. New technologies for treating organic vapors can be evaluated in a reliable and effective way.

Copyright © 2019 American Association for Aerosol Research     

Design and Evaluation of a Portable Dilution Sampling System for Measuring Fine Particle Emissions

The size and complexity of current dilution samplers is a mrajor barrier to more wide-spread application of these systems for source characterization. A new, more portable dilution sampler has been designed to provide masurements consistent with the widely cited Caltech di1ution sampler. Intercomparison experiments were performed using a diesel engine and wood stove to evaluate the comparability of the new design with a sampler based on the Caltech design. These experiments involved simultaneouos operation of mutiple dilution sampies from the same source. Filter based measurements included PM_2.5 mass, organic Carbon and elemeanta1 carbon emissions. Particle size distributions in the range from 10–480 nm were measured using a scanning mobility particle sizer. The filter based and integrated-total volume measurements made with the two designs are in good agreement. FOr example, the average relative bias between the two sampler of PM_2.5 mass emission rate measured with Teflon filters is 1%. Nucleation. was intermittently observed in the sampler based on the Caltech design, but rarely observed in the new design. Significant discrepancies in total number emissions between the two samplers occurred during periods of nucleation. Ewperiments were also conducted to examine the effects of residence time on the diluted emissions. No changes in the filter based or integrated colame measurements were observed with an additional 40-s residence time, indicating that phase equilibrium is established in the 2.5 s of residence time provided by the dilution tunnel, This conclusion is consistent with theoretical analysis. These results provide new insight into the effects of dilution sampling on measurements of fine particle emissions, providing important data for the ongoing effort of the EPA and ASTM to define a standardized dilution sampling methodology for characterizing emissions from stationary combustion sources.     

Meal and Grain Sampling

A short history of grain sampling is given and the recent U.S. Grain Standards Act. per U.S.D.A. publi-cation of 1968, is reviewed briefly. Inspection tech-nique depends on the type of sample being used: official sample, type sample or warehouseman’s sample. Terms, such as, permissive inspection and exception, are explained. Mechanical grain and meal sampling methods and the equipment used are dis-cussed. A close look is given to their practical application and the mathematics of operation. Facts that point to the future of sampling are given. One of four papers being published from the Symposium “Sampling and Process Control in the Oilseed Industry,” presented at the AOCS Meeting, New Orleans, April 1970.