TSI Aerodynamic Particle Sizer Research Articles

2 Low-Cost Pm Sensor Performance Testing

Abstract Low-cost particulate matters sensors (LCPMS) are widely used in indoor and outdoor air quality monitoring. Previous studies have explored the accuracy and precision of sensors, showing that LCPMS were most accurate for PM with diameters below 1 μm, and they poorly measured PM in the 2.5–5 μm range. The study aimed to investigate the aspiration and transmission efficiency of LCPMS, to set up a standard aerosol mass concentration generation system, to further comprehend the performance characteristics of LCPMS, and to examine the effect of aerosol loading. Three models of Plantower sensors were tested in this work. An ultrasonic atomizing nozzle was used to generate micrometer-sized NaCl aerosol particles. A TSI aerodynamic particle sizer was used to measure the aerosol concentrations and size distributions upstream and downstream of the LCPMS, to determine the aspiration and transmission efficiency. The mass concentration, from 20 µg/m3 to 200 mg/m3, could be varied by controlling the solution feeding rate and the solution concentration. The high mass concentration was mainly designed for aerosol loading study. The results showed that the aspiration efficiencies of the tested LCPMS were all almost 100% for particle smaller than 5 µm. The transmission efficiency of these sensors was function of particle size and strongly dependent on sampling flow. All sensors tested showed significant performance degrading when challenged with high mass concentration. The orientation of the sensor also played a role affecting the aerosol loading. These sensors performed better when mass concentration is below 100 µg/m3.

Aerosol insecticide distribution inside a flour mill: Assessment using droplet measurements and bioassays

The distribution of aerosol applications of pyrethrin + methoprene, generated from a mechanical fogger, and pyrethrin + pyriproxyfen, dispensed from a pressurized cylinder, were characterized inside a pilot-scale flour mill using measurements of particle size and concentration and effects on adult confused flour beetles, Tribolium confusum Jacqueline duVal, in bioassay arenas. Particle measurements were made using TSI Aerodynamic Particle Sizer (APS) units placed in an open straight line at distances of 4.3, 8.9, and 13.5 m from the aerosol release point (open configuration). Measurements were also made using a second configuration (termed obstructed), which was done by moving the APS unit at 8.9 m underneath a piece of equipment, and moving the APS unit at 13.5 m behind a support beam. Actual concentration and calculated deposition were about 4× greater for the pyrethrin + methoprene aerosol compared to the pyrethrin + pyriproxyfen aerosol. However, efficacy was similar since bioassays using adult T. confusum showed no difference in recovery after exposure to either insecticide. Concentration and calculated deposition of both aerosols decreased with increasing distance from the spray release point and when the APS units were in the obstructed configuration, and recovery of bioassay insects after exposure increased with increasing distance from the spray release point. Results from this field trial show how efficacy of aerosol applications is impacted by distance and obstacles, and how use of equipment that measures droplet size and concentration can help quantify the dispersion and spread of insecticidal aerosols. Results also provide guidance to develop relationships between aerosol deposition and efficacy and thus improve pest management programs for structural management of stored product insects.

Particle effective density and mass during steady-state operation of GDI, PFI, and diesel passenger cars

Particle effective density is an important physical property of vehicle exhaust, and is required for estimating particulate matter (PM) mass emissions using the Integrated Particle Size Distribution (IPSD) method. In this study, we measure particle effective density of five light-duty vehicles with PM emissions below the Low Emission Vehicle (LEV) III PM standards of 1 or 3mg/mi (0.62 and 1.86mg/km) using the Differential Mobility Analyzer (DMA) – Centrifugal Particle Mass Analyzer (CPMA) approach. Test vehicles included two gasoline direct injection (GDI) vehicles without particulate filters, and for the first time reported in the literature, two port-fuel injected (PFI) vehicles and a turbocharged direct injection (TDI) light-duty diesel vehicle with a diesel particulate filter (DPF). The particle effective density functions generally resemble previous work on GDI and diesel engines without particulate filters but, for many size ranges, the PFI and TDI vehicles produced emissions with higher particle effective densities than GDI vehicles. Good linear correlation was found between the gravimetric and IPSD methods when applying the new particle effective density functions to size distribution measured by the TSI Engine Exhaust Particle Sizer (EEPS, 5.6–560nm, R2=0.84); however, the IPSD method underestimated gravimetric mass by 64%. When using a TSI Aerodynamic Particle Sizer (APS, 0.54–2.5µm) to measure the contribution of larger particles, underestimation bias was virtually eliminated and the correlation improved dramatically (R2=0.96). Even stronger correlation between IPSD and gravimetric methods was achieved when using a Scanning Mobility Particle Sizer (SMPS, 8.7–365nm) and the APS (R2=0.97). A procedure for correcting EEPS measurements using the SMPS is presented and evaluated.

Characteristic of nanoparticles generated from different nano-powders by using different dispersion methods

A standard rotating drum with a modified sampling train (RD), a vortex shaker (VS), and a SSPD (small-scale powder disperser) were used to investigate the emission characteristics of nano-powders, including nano-titanium dioxide (nano-TiO2, primary diameter: 21 nm), nano-zinc oxide (nano-ZnO, primary diameter: 30–50 nm), and nano-silicon dioxide (nano-SiO2, primary diameter: 10–30 nm). A TSI SMPS (scanning mobility particle sizer), a TSI APS (aerodynamic particle sizer), and a MSP MOUDI (micro-orifice uniform deposit impactor) were used to measure the number and mass distributions of generated particles. Significant differences in specific number and mass concentration or distributions were found among different methods and nano-powders with the most specific number and mass concentration and the smallest particles being generated by the most energetic SSPD, followed by VS and RD. Near uni-modal number or mass distributions were observed for the SSPD while bi-modal number or mass distributions existed for nano-powders except nano-SiO2 which also exhibited bimodal mass distributions. The 30-min average results showed that the mass median aerodynamic diameter (MMAD) and number median diameter (NMD) of the SSPD ranged 1.1–2.1 μm and 166–261 nm, respectively, for all three nano-powders, which were smaller than those of the VS (MMAD: 3.3–6.0 μm and NMD: 156–462 nm), and the RD (MMAD: 5.2–11.2 μm and NMD: 198–479 nm). For nano-particles (electric mobility diameter < 100 nm), specific mass concentrations were nearly negligible for all three nano-powders and test methods. Specific number concentrations of nano-particles were low for the RD tester but were elevated when more energetic VS and SSPD testers were used. The quantitative size and concentration data obtained in this study is useful to elucidate the field emission and personal exposure data in the future provided that particle loss in the generation system is carefully assessed.

Penetration of charged particles through metallic tubes

Several theoretical studies have shown that charged particles increased the particle deposition efficiency in cylindrical tubes by image force. Some experimental investigations have also found that the deposition loss increased with increasing aerosol charge. However, the amount of charges on the aerosols or the electrical mobility was relatively low in these previous experimental studies. In order to extend to higher aerosol charge, a TSI vibrating orifice monodisperse aerosol generator was modified to generate 1-μm DEHS aerosols with charge up to 24,000 elementary units. Metallic tubes were employed to exclude the effect of Coulombic force. Tube diameter, tube length, and average velocity in tubes were among the major operating parameters. Aerosol charge was monitored using a TSI electrometer, while a TSI aerodynamic particle sizer was utilized to measure both aerosol concentrations and size distributions upstream and downstream of the tubes. Aerosol deposition loss in the sampling train of the TSI electrometer was measured to back-calculate the average aerosol charge. A closed-form theoretical model, showing the deposition efficiency as a function of particle charge, was validated by the experimental data produced.The aerosol deposition efficiency increased with increasing aerosol charge and tube length, due to stronger image force and longer retention time, respectively. The deposition efficiency decreased with increasing average velocity because of shorter retention time. Under the same average velocity, the deposition loss decreased with increasing tube diameter because the fraction of aerosols near the inner wall was higher for small-diameter tube than for large-diameter tube. The model developed from parabolic flow showed good agreement with most of the experimental data, except for data of highly charged particles. The discrepancy was probably due to the space charge effect.

Comparison of the SidePak™ personal monitor with the Aerosol Particle Sizer (APS)

The aim of this study was to compare the performance of the TSI Aerodynamic Particle Sizer (APS) and the TSI portable photometer SidePak to measure airborne oil mist particulate matter (PM) with aerodynamic diameters below 10 μm, 2.5 μm and 1 μm (PM(10), PM(2.5) and PM(1)). Three SidePaks each fitted with either a PM(10), PM(2.5) or a PM(1) impactor and an APS were run side by side in a controlled chamber. Oil mist from two different mineral oils and two different drilling fluid systems commonly used in offshore drilling technologies were generated using a nebulizer. Compared to the APS, the SidePaks overestimated the concentration of PM(10) and PM(2.5) by one order of magnitude and PM(1) concentrations by two orders of magnitude after exposure to oil mist for 3.3-6.5 min at concentrations ranging from 0.003 to 18.1 mg m(-3) for PM(10), 0.002 to 3.96 mg m(-3) for PM(2.5) and 0.001 to 0.418 mg m(-3) for PM(1) (as measured by the APS). In a second experiment a SidePak monitor previously exposed to oil mist overestimated PM(10) concentrations by 27% compared to measurements from another SidePak never exposed to oil mist. This could be a result of condensation of oil mist droplets in the optical system of the SidePak. The SidePak is a very useful instrument for personal monitoring in occupational hygiene due to its light weight and quiet pump. However, it may not be suitable for the measurement of particle concentrations from oil mist.

Combined Single-Drop and Rotating Drum Dustiness Test of Fine to Nanosize Powders Using a Small Drum

A dustiness test has been developed that performs both a single-drop and a continuous rotation test using a 6-g sample. Tests were completed on pigment-grade and ultrafine TiO2, two grades of corundum (Aloxite), yttrium-stabilized zirconia (Y-zirconia) granules, fumed silica, goethite, talc and bentonite. The generated particles were quantified by counting and sizing at 1-s time resolution using the TSI Fast Mobility Particle Sizer and the TSI Aerodynamic Particle Sizer and by collecting the particles on a filter for weighing. The method generated reproducible amounts and size distributions of particles. The size distributions had two more or less separated size modes >0.9 microm and in addition all materials except TiO2 pigment-grade and Aloxite F1200 generated a size mode in the range from approximately 100 to approximately 220 nm. Pigment-grade TiO2 had the lowest dustiness and ultrafine TiO2 the highest dustiness as measured by particle number for both the single-drop and rotation test and as measured by mass for both tests combined. The difference was a factor of approximately 300. Three types of dust generation rate time profiles were observed; brief initial burst (talc, both grades of corundum), decaying rate during rotation period (fumed silica, TiO2 ultrafine and pigment grade, bentonite) and constant rate (Y-zirconia, goethite). These profile types were in agreement with the differences in the ratio of amount of particles generated during the single drop to the amount generated during the single-drop and rotation test combined. The ratio ranged a factor approximately 40. The new test method enables a characterization of dustiness with relevance to different user scenarios.

A Universal Calibration Curve for the TSI Aerodynamic Particle Sizer

A Universal Calibration Curve for the TSI Aerodynamic Particle Sizer Chuen-Jinn Tsai a , Sheng-Chieh Chen a , Cheng-Hsiung Huang b & Da-Ren Chen c a Institute of Environmental Engineering, National Chiao Tung University , Hsin Chu, Taiwan b Department of Environmental Engineering and Health, Yuanpei University of Science and Technology , Hsin Chu, Taiwan c Department of Mechanical Engineering, Joint Program in Environmental Engineering, University of Washington in St. Louis , St. Louis, Missouri Published online: 17 Aug 2010.

A Universal Calibration Curve for the TSI Aerodynamic Particle Sizer

A universal calibration curve for the accurate determination of particle aerodynamic diameter by the TSI APS (TSI, Inc., St. Paul, MN, USA) operating at various temperature, pressure, particle, and gas properties is proposed. The previous dimensionless APS response function proposed by Chen et al. (1985) uses the Stokes number as the governing parameter and is valid in the Stokesian regime only. In the non-Stokesian regime, an additional non-Stokesian correction factor is needed to correct for the indicated aerodynamic diameters of the APS. The universal curve in the present study is based on the relationship between V p * and Stm, whereV p * = V p /V g and V p is particle velocity, V g is gas velocity at the nozzle exit, and Stm is modified Stokes number. Stm incorporates the non-Stokesian effect and is defined as 24St/(Re*C D ), where Re is the flow Reynolds number and C D is the particle drag coefficient. We find that the new calibration curve can predict the particle aerodynamic diameter accurately wi...

A New Method for Retrieving Particle Refractive Index and Effective Density from Aerosol Size Distribution Data

As part of the Big Bend Regional Aerosol Visibility and Observation study (BRAVO) in July-October 1999, dry aerosol size distributions were measured over the size range of 0.05 < Dp < 20 w m using a TSI differential mobility analyzer (DMA), a PMS LASAIR 1003 optical particle counter (OPC), and a TSI aerodynamic particle sizer 3320 (APS). Extensive calibrations were performed to characterize the response of the OPC and APS to particles of different size and composition. This paper describes a new method that was developed to align size distributions in the instrument overlap regions, allowing for the retrieval of aerosol real refractive index and effective density. To validate the method, retrieved particle real refractive index was compared with volumeweighted model estimates based on measured PM 2.5 chemical composition. The study average retrieved real refractive index was m r = 1.566 - 0.012, and the average computed PM 2.5 refractive index was m r = 1.56 - 0.02; the agreement is well within experimental uncertainties. The average value of computed PM 2.5 bulk density was 1.85 - 0.14 g cm -3 . The average value of retrieved effective density, a function of particle dynamic shape factor, was 1.56 - 0.12 g cm -3 . The comparisons of effective density to computed bulk density suggested an average particle dynamic shape factor of h = 1.2. Sensitivity studies showed that real refractive index could be retrieved with uncertainties on the order of 2-3%, and effective density was retrieved with uncertainties on the order of 20-30%.

Design and validation of a high-flow personal sampler for PM2.5.

A high-flow personal sampler (HFPS) for airborne particulate matter has been developed and fully characterised, and validation tests have been carried out. The sampler is a low-cost gravimetric instrument designed to collect particulate matter with a 50% cut point at 2.5 microm aerodynamic equivalent diameter (PM2.5), where size selection is achieved by the use of porous polyurethane foam. Development of a porous foam selector was chosen over a cyclone or impactor due to the lightweight, low-cost, and compact design that could be achieved. The sampler flow rate of 16 1/min is achieved using a portable, flow-controlled pump; this flow rate is far higher than that of conventional personal samplers and the HFPS can therefore be used for personal sampling in the ambient environment over short sampling periods of much less than 24 h. The HFPS is currently being used in a study of particle exposure of urban transport users (cyclists, car drivers, bus and Underground rail passengers) where personal sampling over short time periods representing typical commuter journey times is required. The HFPS was fully characterised in chamber studies with a TSI aerodynamic particle sizer (APS). The sampler was then validated against a co-located U.S. EPA Federal Reference PM2.5 Well Impactor Ninety Six (WINS) and a KTL cyclone, and parallel testing was performed. Initial testing showed some penetration of particles through the porous foam structure; applying an oil coating to the foam eliminated this problem. Chamber testing was carried out on a number of different selector prototypes, with the final design giving a 50% penetration diameter (i.e., d50) of 2.4 microm at 16 1/min. The new sampler exhibited good agreement in three sets of co-located tests with established samplers, and parallel testing showed excellent agreement between paired HFPS samplers.

Light scattering from deformed droplets and droplets with inclusions I Experimental results

We provide experimental results from the scattering of light by deformed liquid droplets and droplets with inclusions. The characterization of droplet deformation could lead to improved measurement of droplet size as measured by commercial aerodynamic particle-sizing instruments. The characterization of droplets with inclusions can be of importance in some industrial, occupational, and military aerosol monitoring situations. The nozzle assembly from a TSI Aerodynamic Particle Sizer was used to provide the accelerating flow conditions in which experimental data were recorded. A helium-neon laser was employed to generate the light-scattering data, and an externally triggered, pulsed copper vapor laser provided illumination for a droplet imaging system arranged orthogonal to the He-Ne scattering axis. The observed droplet deformation correlates well over a limited acceleration range with theoretical predictions derived from an analytical solution of the Navier-Stokes equation.

Inhalation Toxicology Session: Breathing Zone Particle Size and Lead Concentration from Sanding Operations to Remove Lead Based Paints

ABSTRACTThe relationship between lead concentration in the dry film of lead based paints applied to steel bulkheads aboard ship, the lead concentration found in the air when the paint is removed by mechanical means, and blood lead concentrations of workers involved in lead based paint removal has not been well characterized. Intuitively a direct relationship must exist but confounding factors confuse the issue. Simultaneous sampling procedures from the same paint removal operation may differ by several orders of magnitude. The process from dried film to aerosol (airborne dust) exposure, and on to dose can be separated into two major phases; (1) generation of the dust and its transport through the air to the worker and (2) uptake and dose related factors within the body. Both phases involve complex interactions and there are a number of factors within each phase that significantly affect the potential lead dose for the worker. This study attempts to clarify the mechanisms involved in the generation and transportation of the dust to the worker by evaluating the relationship of a number of key factors on particle size and lead distribution within the aerosol dust generated when lead based paint is removed by sanding. The study examined the relationship between particle size in the dust and grit size of the abrasive. It also examined the distribution of lead within selected particle sizes. The Mass Median Aerodynamic Diameter (MMAD) was used as an indicator of change in the particle size distribution. Particle size distributions were evaluated using a TSI Aerodynamic Particle Sizer, a five stage cyclone and scanning electron microscopy. Lead distribution was determined using the five stage cyclone, and personal or area samples analyzed using inductively coupled plasma (ICP). Mass concentrations were evaluated using a MIE Mass Concentration Analyzer and gravimetric analysis of filter samples collected in the breathing zone. Student's t-tests were used to evaluate changes in MMADs, mass concentrations and other indices for inter and intra-grit size samples. Correlation coefficients (Pearson's r) were used to determine the relationship between factors. Findings of the research indicated that the particle size distribution in the dust is directly related to the grit size of the abrasive (i.e. inversely related to the abrasive grit number). Particulate mass concentrations of dust varied directly with abrasive grit number. The distribution of lead did not appear to be affected by grit size of the abrasive in that the lead distribution within the particle size ranges remained homogeneous and consistent with the lead concentration in the dried film. Mass concentrations of lead in air samples varied directly with lead concentration in the bulk coating. Results of this project, coordinated with deposition modeling and bioavailability studies will be useful in the development of a model to characterize lead dose to workers based on known parameters within the work specifications.

Sampling Efficiency of the Tsi Aerodynamic Particle Sizer

The Aerodynamic Particle Sizer (APS) has been tested and used in a variety of laboratory and field situations for indoor air quality research of both industrial and livestock buildings. The APS is capable of providing rapid, real-time measurement of particle size distribution. However, no literature was found in which the sampling efficiency of the APS was addressed. This paper presents a method to evaluate the sampling efficiency of the APS, considering both the aspiration efficiency and the penetration efficiency. Six tests were conducted and measurements were taken by a cascade impact sampler and the APS simultaneously. The results obtained by the APS were compared with the results of the cascade impact sampler.

The Effect of Sampling Time and Flow Rates on the Bioefficiency of Three Fungal Spore Sampling Methods

ABSTRACT The influences of sampling time and flow rates on total recovery rate of airborne fungal spores were evaluated in the laboratory test chamber by three sampling methods: AGI-30 all-glass impingers, the Nuclepore filtration and elution method, and the gelatin filters. In the test system, a Pitt-3 generator was used to generate the spores of Penicillium citrinum. The real-time number concentration of fungal spores was measured by a TSI aerodynamic particle sizer to determine the total recovery. The results demonstrated that total recovery of spores collected by the AGI-30 impinger was in the range of 4% to 24%. Moreover, the average total recoveries of spores collected by Nuclepore and gelatin filters were found to be in the range of 50% to 75%, and 50% to 90%, respectively. The observed low total recovery for the AGI-30 impinger may be due primarily to the biological stress during sampling process. The relative survival of fungal spores collected in AGI-30 impingers became lower as the sampling tim...

Determination of Cyclone Model Variability Using a Time-of-Flight Instrument

ABSTRACT A test method for the measurement of the internal penetration of personal cyclones using a modified TSI Aerodynamic Particle Sizer (APS) is presented. A new method to generate a polydisperse polystyrene aerosol has been developed. The effects of phantom particle generation in the recorded size distributions determined were reduced mainly by using the Large Particle Processor (LPP) data and extending the particle-size range of the LPP by reducing the threshold voltage of the LPP. The performance of five commercially available versions of the Higgins-Dewell-type cyclone has been tested using this method. The cyclones have virtually identical internal geometric dimensions but are machined from different materials and have different internal surfaces. It was found that the measured penetration curve for each model has a unique combination of cutoff diameter and slope. It was also shown that accurate determinations of cyclone penetration require particle loading to be considered.

Inlet Efficiency Study for the TSI Aerodynamic Particle Sizer

AbstractThe paper reports on the particle. sampling efficiency of the inlet system for the Aerodynamic Particle Sizer (TSI, Inc., St. Paul MN). Large particles are depleted from the sampled aerosol by two mechanisms: super‐isokinetic sampling at the entrance of the inlet, and inertial impaction on the inner nozzle. A fluorometric technique was used to separately characterize these mechanisms. Numerical studies were also performed. The experimental results show that the inlet's overall efficiency drops from around 90% for 3 μm particles to less than 45% for particles larger than 10 gm. Several high efficiency inlets were developed and tested. These inlets provide higher sampling efficiencies, but reduce the instrument's sizing resolution. Measurements of 7.3 μm oleic acid particles with a high efficiency inlet showed a 5% spread in measured diameter at 50% count, while less than a 1076 spread was observed using the standard inlet. It was also found that the super‐isokinetic condition reduces particle losses on the inner nozzle. The standard inlet is recommended for verifying test aerosol monodispersity. An alternative to the standard inlet is suggested for measurement of size distributions.

Behavior of Compact Nonspherical Particles in the TSI Aerodynamic Particle Sizer Model APS33B: Ultra-Stokesian Drag Forces

The aerodynamic behavior of aggregates consisting of uniform polystyrene latex (PSL) spheres and unaggregated cuboidal Natrojarosite particles in a TSI aerodynamic particle sizer (Model APS33B) has been studied. In initial tests, monodisperse PSL micro-spheres ranging from 0.3 to 7 μm in geometric diameter were generated from aqueous suspensions using a Lovelace nebulizer. APS33B responses for these uniform-sized particles showed multiple peaks. The major (primary) peak, which resulted from the smallest particle, corresponded to the unaggregated single spheres (singlets); the second, third, and fourth peaks were identified as doublets, triangular triplets, and tetrahedral quadruplets, respectively. Both doublets and triplets moved with their long axes in perpendicular (maximum drag) orientation to the flow direction in the APS33B. In contrast, the tetrahedral particles were isometric and had the same dynamic shape factor (drag resistance) for all three primary orientations. The particle Reynolds numbers (...

The behaviour of spheroidal particles in time-of-flight aerodynamic particle sizers

Previous studies (Marshall et al. , 1991; Marshall and Mitchell, 1991) have shown that time-of-flight aerodynamic particle sizers, such as the TSI Aerodynamic Particle Sizer and the Amherst Process Instruments Aerosizer, underestimate the aerodynamic size of cubeshaped particles, since they operate under ultra-Stokesian flow conditions. The present work has extended the investigation to include an assessment of the behaviour of two sizes of monodisperse, prolate spheroidal, ferric oxide particles close to 2 μm aerodynamic diameter in both instruments. Both techniques undersized the smaller particles significantly, but the larger particles (more elongated) were oversized by the Aerodynamic Particle Sizer and undersized marginally by the Aerosizer. Similar behaviour was reported by Griffiths et al ., (1984) with micron-sized glass fibres sampled by an earlier version of the Aerodynamic Particle Sizer.

The behaviour of regular-shaped non-spherical particles in a TSI aerodynamic particle sizer

The TSI Aerodynamic Particle Sizer (APS) is known to underestimate the size of liquid droplets larger than a few micrometers aerodynamic diameter. It is believed that the cause is shape related, since the liquid spheres will be distorted as the droplets pass through the region of strong acceleration in the tapered nozzle prior to size classification by a time-of-flight technique. The influence of particle shape on APS performance has been investigated using a range of monodisperse, regular-shaped and non-porous solid particles in the size range from about 6 to 14 μm aerodynamic diameter. These particles were first size-classified under Stokesian conditions using a Timbrell aerosol spectrometer, so that their true aerodynamic diameters could be determined. The size-classified particles were collected on glass slides, and a technique was developed to resuspend them for further size analysis in an APS. Deviations from true aerodynamic diameter were caused by the ultra-Stokesian environment as the particles were accelerated in the APS and this behaviour was assessed as a function of size. The dynamic shape factor of these particles was almost constant with varying size and was determined to be 1.19±0.06. Their true aerodynamic diameters were underestimated by an average of 25% when using the APS.