Predicting Cutoff Aerodynamic Diameter and Sharpness of Single Round-Nozzle Impactors with a Finite Impaction Plate Diameter

Abstract

This investigation numerically examined the cutoff aerodynamic diameter (da₅₀) and the sharpness (GSD) of the particle collection efficiency curve of impactors with a finite impaction plate diameter. Results revealed that the inertial impactors have a limited cutoff aerodynamic diameter at different air velocities. The extreme value of the cutoff aerodynamic diameter increases with the nozzle diameter (W)/the plate diameter (D_c). The computed da₅₀/D_c values of the impactors increase with W/D_c at various Reynolds numbers (Re) and with the nozzle-toplate distance (S)/D_c when Re is 100. The value of GSD slightly increases with W/D_c for Re of 10 and 100, although the effect of S/D_c on GSD is not evident at various Res. The particle collection efficiency curve of the impactor with a lower Re is less sharp than that with a high Re at various W/D_c and S/D_c values. Statistical equations closely fitted the obtained numerical results for Res of 10–3000. The equations are useful for directly calculating the cutoff aerodynamic diameter and the sharpness of the particle collection efficiency curve for single round-nozzle impactors with a finite impaction D_c.     

Experimental studies on particle impaction and bounce: effects of substrate design and material

This paper presents an experimental investigation of the effects of impaction substrate designs and material in reducing particle bounce and reentrainment. Particle collection without coating by using combinations of different impaction substrate designs and surface materials was conducted using a personal particle sampler (PPS) developed by the University of Southern California. The PPS operates at flow rate of 4 l min^-1 with a 50% cutpoint of approximately 0.9 μm in aerodynamic diameter. The laboratory results showed that the PPS collection efficiency for particles larger than 50% cutpoint is strikingly low (e.g., less than 50%) when an uncoated open cavity made of aluminum was used as an impaction substrate. The collection efficiency gradually increased when Teflon tape, Nuclepore, and glass fiber filters were used as impaction surfaces, respectively. Conical or partially enclosed cavity substrate designs increased collection efficiency of particles of 9 μm up to 80–90%. A conical cavity with glass fiber filter used as impaction surface was identified as the optimum configuration, resulting in a collection efficiency of 92% at Stokes numbers as high as 15.4 (corresponding to 9 μm in aerodynamic diameter). Particle losses were low (less than 10%) and relatively independent of particle size in any design with glass fiber filter. Losses seemed to increase slightly with particle size in all other configurations. Finally, outdoor PM₁ concentrations obtained with the PPS (in its optimum configuration) and a modified micro-orifice uniform deposit impactor (MOUDI) with coated impaction stages were in excellent agreement. The mean ratio of the PPS-to-MOUDI concentration was 1.13(±0.17) with a correlation coefficient R²=0.95. Results from this investigation can be readily applied to design particle bounce-free impaction substrates without the use of coating. This is a very important feature of impactors, especially when chemical analysis of the collected particulate matter is desirable.     

Calibration of sharp cut impactors for indoor and outdoor particle sampling

A low-flow rate, sharp cut point inertial impaction sampler was developed in 1986 that has been widely used in PM exposure studies in the United States and several other countries. Although sold commercially as the MS&T Area Sampler, this sampler is widely referred to as the Harvard Impactor, since the initial use was at the Harvard School of Public Health. Impactor nozzles for this sampler have been designed and characterized for flows of 4, 10, 20, and 23 L/min and cut points of 1, 2, 5, and 10 microns. An improved method for determining the actual collecting efficiency curve was developed and used for the recent impactor calibrations reported here. It consists of placing a multiplet reduction impactor inline just downstream of the vibrating orifice aerosol generator to remove the multiplets, thus allowing only the singlet particle s to penetrate through to the impactor being calibrated This paper documents the techniques and results of recent nozzle calibrations for this sampler and compares it with other size-selective inertial impactors. In general, the impactors were found to have sharp cutoff characteristics. Particle interstage losses for all of the impactors were very low, with the exception of the 10-micron cut size 20 L/min impactor, which had greater losses due to the higher flow rate. All of the 2.5-micron cut nozzle laboratory calibrations compare favorably to the U.S. Environmental Protection Agency (EPA) WINS-96 fine particle mass (PM2.5) impactor calibration data.     

Calibration of Sharp Cut Impactors for Indoor and Outdoor Particle Sampling

ABSTRACT

A low-flow rate, sharp cut point inertial impaction sampler was developed in 1986 that has been widely used in PM exposure studies in the United States and several other countries. Although sold commercially as the MS&T Area Sampler, this sampler is widely referred to as the Harvard Impactor, since the initial use was at the Harvard School of Public Health. Impactor nozzles for this sampler have been designed and characterized for flows of 4,10, 20, and 23 L/min and cut points of 1, 2, 5, and 10 |im. An improved method for determining the actual collecting efficiency curve was developed and used for the recent impactor calibrations reported here. It consists of placing a multiplet reduction impactor inline just downstream of the vibrating orifice aerosol generator to remove the multiplets, thus allowing only the singlet particle s to penetrate through to the impactor being calibrated.

This paper documents the techniques and results of recent nozzle calibrations for this sampler and compares it with other size-selective inertial impactors. In general, the impactors were found to have sharp cutoff characteristics. Particle interstage losses for all of the impactors were very low, with the exception of the 10-|im cut size 20 L/ min impactor, which had greater losses due to the higher flow rate. All of the cut nozzle laboratory calibrations compare favorably to the U.S. Environmental Protection Agency (EPA) WINS-96 fine particle mass (PM_{2 5}) impactor calibration data.     

Low Flow Rate Sharp Cut Impactors for Indoor Air Sampling: Design and Calibration

Two single round nozzle impactors have been developed for use in Harvard’s indoor air pollution health study. Both impactors operate at flow rates of 4 L/m and are nearly identical, differing only in their cut sizes of 2.5 μm and 10 μm aerodynamic diameters. Two identical cascaded stages of the same cut size are used to obtain sharp cut-off characteristics. The particles are deposited on impaction plates made of oil impregnated, porous material to reduce particle bounce and are discarded. Only the particles collected on the afterfilter are analyzed. Special care has been taken to collect the particles uniformly on the afterfilter to aid in particle analysis.

The jmpactors were calibrated with a vibrating orifice monodisperse aerosol generator. However, due to the sharp cut of the impactors, doublets and triplets in the calibration aerosols, even in small quantities, gave erroneous calibration curves. Therefore, the number of doublets and triplets in the challenging aerosols were measured and appropriate corrections made to the calibration curves.     

A Cyclone for Size-Selective Sampling of Ambient Air

A cyclone with a 47 mm after-filter has been developed for ambient air size-selective monitoring. It has been extensively evaluated with laboratory-generated aerosol. Variation of the pressure drop and 50% cut point with flow rate show that the cyclone operates in a single flow regime with a vortex in the outlet flow. The particle size cutoff curve is comparable in sharpness to a cascade impactor and is the same for solid or liquid particles. At 21.7 L/min, D ₅₀ is 2.5μm and at 15.4 L/min, D ₅₀ is 3.5 μm. Collection efficiency data for flow rates from 8 to 27 L/min fit a universal curve when plotted vs. the normalized particle diameter, (D-D ₅₀)/D ₅₀ Reentrainment of previously deposited particles is less than 1 % of the loading per day. In field tests the cyclone has proved to be a very satisfactory size-selective sampler.     

The Hamilton Study: Estimating Exposure to Ambient Suspended Particles

In the industrial city of Hamilton, Ontario, Canada, we recently carried out an epidemiological study of the effect of environmental factors on respiratory health in 3500 elementary, schoolchildren. The level and size distribution of suspended particles in ambient air was measured from 24-h samples taken at 6-day intervals from a network of 29 hivol TSP samplers, and nine Andersen 2000 4-stage cascade impactors. Exposure was computed by generating a 3-dimensional response surface (in TSP and in easting and northing geographical coordinates) using a linear regression model of the form: TSP = (1 + E + N)², based on monthly geometrical mean data for all sites. From the response surface generated for a given month, TSP levels were predicted by the model for all schools by specifying their geographical coordinates. The yearly exposure for a given child was determined from the arithmetic mean of the predicted values for 12 monthly TSP levels. A similar procedure was employed for calculation of the exposure to the "fine" (≤3.3 μm) and "coarse" (>3.3 μm) size fraction, as well as the aerodynamic mass median diameter of particles from the network of cascade impactors. Results of the measurements showed that gradients for TSP up to approximately 10 μg/m³/km exist over the city covering distances from 5 to 10 km. The range of 1 yr mean exposure values calculated for each child was from 30.5 μg/m³ to 74.5 μg/m³. Comparable figures for particle size were up to 0.3 μm AMMD (aerodynamic mass median diameter)/km and annual mean particle size exposure from 2.69 to 3.53 μm AMMD.     

Mass and Elemental Composition of Fine and Coarse Particles in Six U.S. Cities

Inhalable particulate matter (IP) samples have been collected in six U.S. cities in conjunction with an air pollution health study. The IP were collected using dichotomous virtual impactors in two size ranges: fine particles (FP) having aerodynamic diameter (d_a) <2.5 μm, and coarse particles (CP) with 2.5 μm < da < 15 μm. The mass measurements were determined by beta-gauge attenuation. The elemental composition of the FP and CP were determined by X-ray fluorescence. The means and distributions for FP and CP and selected elemental data highlight the similarities and differences that exist among these cities in the health study. Examining the temporal variations gives additional information on the meteorology and sources influencing the FP and CP fractions of Inhalable particle mass.

Differences in the concentration (and ratios) of selected elements have indicated the varying presence of crustal, steel industry, automotive, oceanic and fuel combustion sources in these cities. The noted variation in the concentrations and character of ambient aerosols in these cities are pertinent to interpreting differences in population exposures.     

Development and laboratory performance evaluation of a personal cascade impactor

This paper presents the design and laboratory evaluation of a personal cascade impactor. The system is compact, lightweight, and uses a single battery-operated sampling pump. It operates at a flow rate of 5 L/min and consists of four impaction stages, each equipped with slit-shaped acceleration nozzles, and a backup filter. The impactor was calibrated using polydisperse particles. The 50% cut points of the four stages were 9.6, 2.6, 1.0, and 0.5 microm, respectively. The backup filter is placed downstream of the fourth stage and is used to collect the particles with an aerodynamic diameter smaller than 0.5 microm (dp < 0.5 microm). The major feature of this novel sampler is its ability not only to fractionate the particles with an aerodynamic diameter smaller than 10 microm to the various size fractions, but also to collect them onto relatively small polyurethane foam substrates without using adhesives. Although the impaction substrates are not coated with adhesives such as grease or mineral oil, particle bounce and re-entrainment losses were found to be insignificant. Interstage losses of particles smaller than 0.5 microm were less than 10%; for fine particles, less than 5%; and for coarse particles, less than 12%. The pressure drop across the four stages and the backup filter were 0.015 kPa (0.153 cm H2O),0.025 kPa (0.255 cm H2O), 0.274 kPa (2.794 cm H2O), 0.323 kPa (3.294 cm H2O), and 0.370 kPa (3.773 cm H2O), respectively. Particles can be easily recovered from the foam substrates using aqueous extraction.     

The Prediction of Overall Collection Efficiency of Air Pollution Control Devices from Fractional Efficiency Curves

The application of air pollution control devices requires the prediction of overall collection efficiency from the particle size distribution of the dust and the fractional efficiency of the air pollution control device. The cumulative particle size distribution of dust resulting from industrial processes can usually be represented by a straight line on logarithmic probability paper or a log normal function. The fractional efficiency curves of many air pollution control devices such as cyclones or wet scrubbers can also be adequately represented by a log normal function. Only two parameters are required to define a log normal function, a median diameter and a geometric standard deviation. Both of these can easily be obtained from a plot on logarithmic probability paper. The overall collection efficiency has been found to be Very simply related to the four parameters required to define the log normal functions representing the particle size distribution and the collector fractional efficiency. These four parameters are: the mass median diameter and the geometric standard deviation of the dust size distribution, the cut diameter (50% efficiency diameter), and geometric standard deviation of collector fractional efficiency curve. Using this relationship the prediction of overall collection efficiency is greatly simplified with no loss of accuracy.     

Dynamic Calibration of an Acid Aerosol Analyzer

A method is described for dynamic calibration of an acid aerosol analyzer based on a commercial modification of the Thomas Autometer and manufactured by the Instrument Development Company. This automated instrument removes acid aerosol from an air stream by sonic impaction, and the sulfuric acid collected is determined conductometrically. An all-glass aerosol generator based on the reaction of water vapor with sulfur trioxide vapor released from fuming sulfuric acid was built for the calibration. Air samples were withdrawn for instrument calibration before and after the concentration of the acid aerosol was determined by titration. The apparent particle size as determined by an Andersen sampler ranged from 2.0 microns to less than 0.68 micron and exhibited a sharp peak with mass median diameter at 1.3 microns in the distribution curve. The size of the aerosol, within certain limits, could be controlled by humidity. Data indicated a linear response with an aerosol collection efficiency of 80 percent in the important respirable size range.     

Size and composition distributions of particulate matter emissions: part 2--heavy-duty diesel vehicles

Particulate matter (PM) emissions from heavy-duty diesel vehicles (HDDVs) were collected using a chassis dynamometer/dilution sampling system that employed filter-based samplers, cascade impactors, and scanning mobility particle size (SMPS) measurements. Four diesel vehicles with different engine and emission control technologies were tested using the California Air Resources Board Heavy Heavy-Duty Diesel Truck (HHDDT) 5 mode driving cycle. Vehicles were tested using a simulated inertial weight of either 56,000 or 66,000 lb. Exhaust particles were then analyzed for total carbon, elemental carbon (EC), organic matter (OM), and water-soluble ions. HDDV fine (< or =1.8 microm aerodynamic diameter; PM1.8) and ultrafine (0.056-0.1 microm aerodynamic diameter; PM0.1) PM emission rates ranged from 181-581 mg/km and 25-72 mg/km, respectively, with the highest emission rates in both size fractions associated with the oldest vehicle tested. Older diesel vehicles produced fine and ultrafine exhaust particles with higher EC/OM ratios than newer vehicles. Transient modes produced very high EC/OM ratios whereas idle and creep modes produced very low EC/OM ratios. Calcium was the most abundant water-soluble ion with smaller amounts of magnesium, sodium, ammonium ion, and sulfate also detected. Particle mass distributions emitted during the full 5-mode HDDV tests peaked between 100-180 nm and their shapes were not a function of vehicle age. In contrast, particle mass distributions emitted during the idle and creep driving modes from the newest diesel vehicle had a peak diameter of approximately 70 nm, whereas mass distributions emitted from older vehicles had a peak diameter larger than 100 nm for both the idle and creep modes. Increasing inertial loads reduced the OM emissions, causing the residual EC emissions to shift to smaller sizes. The same HDDV tested at 56,000 and 66,000 lb had higher PM0.1 EC emissions (+22%) and lower PM0.1 OM emissions (-38%) at the higher load condition.     

Size and composition distributions of particulate matter emissions: part 1--light-duty gasoline vehicles

Size-resolved particulate matter (PM) emitted from light-duty gasoline vehicles (LDGVs) was characterized using filter-based samplers, cascade impactors, and scanning mobility particle size measurements in the summer 2002. Thirty LDGVs, with different engine and emissions control technologies (model years 1965-2003; odometer readings 1264-207,104 mi), were tested on a chassis dynamometer using the federal test procedure (FTP), the unified cycle (UC), and the correction cycle (CC). LDGV PM emissions were strongly correlated with vehicle age and emissions control technology. The oldest models had average ultrafine PM0.1 (0.056- to 0.1-microm aerodynamic diameter) and fine PM1.8 (< or =1.8-microm aerodynamic diameter) emission rates of 9.6 mg/km and 213 mg/km, respectively. The newest vehicles had PM0.1 and PM1.8 emissions of 51 microg/km and 371 microg/km, respectively. Light duty trucks and sport utility vehicles had PM0.1 and PM1.8 emissions nearly double the corresponding emission rates from passenger cars. Higher PM emissions were associated with cold starts and hard accelerations. The FTP driving cycle produced the lowest emissions, followed by the UC and the CC. PM mass distributions peaked between 0.1- and 0.18-microm particle diameter for all vehicles except those emitting visible smoke, which peaked between 0.18 and 0.32 microm. The majority of the PM was composed of carbonaceous material, with only trace amounts of water-soluble ions. Elemental carbon (EC) and organic matter (OM) had similar size distributions, but the EC/OM ratio in LDGV exhaust particles was a strong function of the adopted emissions control technology and of vehicle maintenance. Exhaust from LDGV classes with lower PM emissions generally had higher EC/OM ratios. LDGVs adopting newer technologies were characterized by the highest EC/OM ratios, whereas OM dominated PM emissions from older vehicles. Driving cycles with cold starts and hard accelerations produced higher EC/OM ratios in ultrafine particles.     

Filter Beds: Energy-Efficient Packing Diameter

Factors which help minimize pressure drop at a given level of collection efficiency for a bed packed with roughly spherical collectors are studied here by using the quality factor, the ratio of the single collector collection efficiency to the force per unit area on the single collector. This analysis indicates that energy-efficient designs can be obtained as follows: choose a representative particle size; if impaction predominates, design for an impaction parameter near one; if interception predominates, design for the smallest packing diameters feasible; if gravitational settling predominates, design for the largest packing diameters possible; if diffusion predominates, design for collector Reynolds numbers near 10². Some more general cases are also discussed.     

Aerosol Aerodynamic Size Conventions For Inertia! Sampler Calibration

The body of information presented in this paper is directed to investigators using inertial samplers for precise and accurate studies of respirable aerosols. The conventions commonly used for aerodynamic size for aerosol particles are discussed including the definition popularized by the "Task Group on Lung Dynamics" of the ICRP, and the "Lovelace" definition. To emphasize the distinction, the Task Group definition (unit density sphere equivalent) is called the aerodynamic equivalent diameter, D_ae, and the Lovelace definition (characteristic expression based upon viscous resistance) is called the aerodynamic resistance diameter, D_ar. The implications and efficacy of these conventions are related to procedures for calibration of cascade impactors, cyclones, and spiral centrifuges. The calibration of a spiral centrifuge at different altitudes is used as an example of the potential problems associated with the use of the different conventions for describing aerodynamic size. The aerodynamic resistance diameter is recommended for calibration of inertial samplers to be used to collect aerosols in the respirable size range.     

advances in particle sampling and measurement

Seventeen papers were presented on improved instruments and techniques for measuring the size, number, and composition of particles in process streams. Recent studies on cascade impactors, cyclones, and diffusion batteries were reported. Several papers discussed methods for making in situ particle size or mass determinations through light scattering. Systems that have been developed to determine the collection efficiency of electrostatic precipitators, scrubbers, and bag filters were described.     

Fine and Ultrafine Emission Dynamics from a Ferrous Foundry Cupola Furnace

Abstract

Aerosol size distributions from ferrous foundry cupola furnaces vary depending on semicontinuous process dynamics, time along the tap-to-tap cycle, dilution ratio, and the physical and chemical nature of the charge and fuel. All of these factors result in a highly time-dependent emission of particulate matter (PM) 2.5 µm or less in aerodynamic diameter (PM_2.5)—even on a mass concentration basis. Control measures are frequently taken on the basis of low-reliability parameters such as emission factors and loosely established mass ratios of PM_2.5 to PM 10 µm or less in aerodynamic diameter (PM₁₀). The new environmental requirements could entail unexpected and undesired drawbacks and uncertainties in the meaning and effectiveness of process improvement measures. The development of process-integrated and flue-gas cleaning measures for reduction of particle emissions requires a better knowledge of generation mechanisms during melting. Available aerosol analyzers expand the range of control issues to be tackled and contribute to greatly reduce the uncertainty of engineering decisions on trace pollutant control. This approach combines real-time size distribution monitoring and cascade impactors as preseparators for chemical or morphological analysis. The results allow for establishing a design rationale and performance requirement for control devices. A number size distribution below 10 µm in aerodynamic equivalent diameter was chosen as the main indicator of charge influence and filter performance. Size distribution is trimodal, with a coarse mode more than 12 µm that contributes up to 30% of the total mass. A temporal series for these data leads to identification of the most relevant size ranges for a specific furnace (e.g., the most penetrating size range). In this cupola, this size range is between 0.32 and 0.77 µm of aerodynamic equivalent diameter and defines the pollution control strategy for metals concentrating within this size range. Scrap quality effect is best monitored at less than 0.2 µm in aerodynamic equivalent diameter and has been confirmed as strongly dependent on the physical state of the charge.     

Programmable Instrument for Controlling Atmospheric Sampling

The University of Arizona and the Pima County Air Pollution Control District conducted a comparison study of the following aerosol samplers: a standard high-volume sampler, a high-volume sampler fitted with a size selective inlet, and a dichotomous virtual impactor. Over sixty samples were collected with the colocated samplers during the first six months of 1981. The concentration (μg/m³) of suspended particulate matter and of sulfate was determined for all the samples, while the concentration of four lithophilic elements (Ca, Fe, Mg, and K) was determined on one third of the samples. Well-defined linear relationships for suspended particulate matter and sulfate were found to exist between each of the three sample collection methods over the concentrafion range encountered in this study. For these samples, there were significant differences in the particulate mass and large particle lithophilic element concentrations collected by each device. However, sulfate values obtained from the three samplers were in excellent agreement with each other. This suggests that the inlet collection efficiency for large particles differs significantly for these three sampling devices. Since the size selective inlet and the dichotomous virtual impactor samplers are each designed for collection of inhalable particles (particles of 15 μm aerodynamic diameter and smaller), they would have been expected to measure approximately equivalent particle mass concentrations. Thus, these differences are important to those interested in selecting a method for measuring airborne particle mass concentrations.     

FF/C Scrubber Demonstration on a Secondary Metals Recovery Furnace

A flux force/condensation (FF/C) scrubbing system was built to control participate emissions from a secondary metals recovery furnace. Total mass penetration and fractional penetration measurements were made under several different operating modes. The performance of the demonstration scrubber was consistent with the results of previous studies on FF/C scrubbing. The system was generally capable of 90 to 95 % efficiency on particles with a mass median aerodynamic diameter of 0.75 µA. This efficiency was achieved with a 68 cm (27 in.) W.C. gas-phase pressure drop.     

Development and evaluation of an impactor for a PM2.5 speciation sampler

A conventional impactor for a particle speciation sampler was developed and validated through laboratory and field tests. The speciation sampler consists of the following components: a PM2.5 conventional impactor that removes particles larger than 2.5 microns, an all-glass, coated honeycomb diffusion denuder, and a 47-mm filter pack. The speciation sampler can operate at two different sampling rates: 10 and 16.7 L/min. An experimental characterization of the impactor's performance was conducted. The impactor's collection efficiency was examined as a function of critical design parameters such as Reynolds number, the distance from the nozzle exit to the impaction plate, and the impaction substrate coating method. The bounce of particles larger than the cut point was successfully minimized by using a greased surface as the impaction substrate. Additionally, a series of field intercomparison experiments were conducted at both 10 and 16.7 L/min airflow. PM2.5 mass and SO4(2-) concentrations were measured and compared with the Federal Reference Method (FRM) and found to be in good agreement. Results of the laboratory chamber tests also indicated that the impactor's performance was in good agreement with the FRM.