Mass Range and Optimized Operation of the Aerosol Particle Mass Analyzer

We investigated, theoretically, the mass range in which an aerosol particle mass analyzer (APM) can be used for classification, and how the APM classification performance can be optimized. We listed factors that set limits to the APM, which were constraints of the rotation speed and the voltage, as well as requirements on the APM classification performance parameter, λ, that guarantee at least minimal performance in both resolution and penetration. We introduced the APM operation diagram, which is a tool to visualize the limits and mass range. We proposed to operate the APM that was considered in this study with the λ value set within the range from 0.25 to 0.5 for optimum classification performance by balancing both resolution and penetration. The mass range for the APM, with the λ value maintained between 0.25 and 0.5, was calculated to be from 0.003 to 2000 fg, which corresponds to the diameter range from 20 to 1600 nm for the density of 1 g/cm³. To verify the validity of the mass range and the idea of the optimized operation, we carried out experiments on an APM with polystyrene and sodium-chloride particles that were classified by electrical mobility. We found that the APM was able to provide bell-shaped spectra down to 12 nm, and was able to perform mass classification with an accuracy better than 5% down to 50 nm. Underestimation of mass and reduction of resolution and penetration were observed at sizes smaller than about 30 nm.     

Determination of Particle Effective Density in Urban Environments with a Differential Mobility Analyzer and Aerosol Particle Mass Analyzer

Effective densities of atmospheric aerosols in various locations of the Los Angeles Basin were determined by a DMA-APM technique. Effective density was calculated by comparing voltage distributions of sampled atmospheric aerosols with PSL particles of known density. The five sites chosen for field experiments were: (1) Interstate-710 Freeway, impacted by heavy-duty diesel vehicles; (2) State Route CA-110, open only to gasoline vehicles; (3) Riverside, a receptor site known for secondary particle formation; (4) University of Southern California, a typical urban and industrial environment; and (5) Coast for marine aerosol. The size range selected for this study was from 50 nm to 414 nm. While 50 nm particles exhibited a single effective density multiple effective densities were measured for each of the other particle sizes as significant fractions of these particles are transported from background sources. Regardless of location, 322–414 nm particle effective densities were considerably lower than unity. The lowest effective densities (～ 0.1 g cm ^{? 3} ) were reported for I-710, confirming that diesel combustion aerosols are rich in chain agglomerates with large void spaces. Riverside exhibited high effective densities (～ 1.2–1.5 g cm ^{? 3} ) for 50–202 nm particles, which we hypothesize is due to transformations that occur during advection from Los Angeles. Measurements of diurnal variation of effective density at Riverside support this hypothesis. Overall, our results suggest that effective density declines as the particle mobility diameter increases irrespective of location. Fractal dimensions calculated from average effective densities were lowest for I-710 ( D _f = 2.41) and CA-110 (D _f = 2.54) aerosols, presumably due to the influence of vehicular combustion emission on these sites. By contrast, average fractal dimensions at USC, Riverside and Coast were found to be 2.79, 2.83, and 2.92, respectively. High fractal dimensions at these sites may be the effects of aging, moisture absorption and/or organic vapor condensation on the particles, which fills void space and makes particles more spherical.     

Online Nanoparticle Mass Measurement by Combined Aerosol Particle Mass Analyzer and Differential Mobility Analyzer: Comparison of Theory and Measurements

A combination of a differential mobility analyzer (DMA) and aerosol particle mass analyzer (APM) is used to measure the mass of NIST Standard Reference Materials (SRM®) PSL spheres with 60 and 100 nm nominal diameter, and NIST traceable 300 nm PSL spheres. The calibration PSL spheres were previously characterized by modal diameter and spread in particle size. We used the DMA to separate the particles with modal diameter in a narrow mobility diameter range. The mass of the separated particles is measured using the APM. The measured mass is converted to diameter using a specific density of 1.05. We found that there was good agreement between our measurements and calibration modal diameter. The measured average modal diameters are 59.23 and 101.2 nm for nominal diameters of 60 and 100 nm (calibration modal diameter: 60.39 and 100.7 nm) PSL spheres, respectively. The repeatability uncertainty of these measurements is reported. For 300 nm, the measured diameter was 305.5 nm, which is an agreement with calibration diameter within 1.8%.

The effect of spread in particle size on the APM transfer function is investigated. Two sources of the spread in “mono-dispersed” particle size distributions are discussed: (a) spread due to the triangular DMA transfer function, and (b) spread in the calibration particle size. The APM response function is calculated numerically with parabolic flow through the APM and diffusion broadening. As expected from theory, the calculated APM response function and measured data followed a similar trend with respect to APM voltage. However, the theoretical APM transfer function is narrower than the measured APM response.     

The Effect of Nanoparticle Convection-Diffusion Loss on the Transfer Function of an Aerosol Particle Mass Analyzer

The existing theoretical response spectra of APM-3600 agree well with the experimental data for submicron particles larger than 100 nm in the electrical mobility diameter but not for nanoparticles. In this study, a 2-D numerical model was developed to predict the transfer function and response spectra of APM-3600 based on the detailed simulation of flow and particle concentration fields. It was found that recirculation flows existed in the annular classifying region and APM's inlet and outlet regions, which led to enhanced convection-diffusion loss of nanoparticles compared to that without considering flow recirculation. As a result, the APM underestimates the mass of naonoparticles due to the shift of the peak position of the transfer function to a larger diameter than the targeted diameter. The response spectra calculated with the simulated transfer function agree well both in shapes and peak values with the experimental data present in a previous study for both nanoparticles and submicron particles larger than 100 nm. The predicted particle masses also agree well with the PSL's experimental data of the article.

Copyright 2014 American Association for Aerosol Research     

Evaluation of a New Particle Trap in a Laser Desorption Mass Spectrometer for Online Measurement of Aerosol Composition

We have developed a new analyzer for the online measurement of aerosol composition: a particle trap laser desorption mass spectrometer (PT-LDMS). The main components of the instrument include an aerodynamic lens, a particle trap enclosed by a quartz cell, a quadrupole mass spectrometer (QMS), a vacuum chamber incorporating the above components, and a carbon dioxide (CO₂) laser (wavelength 10.6 μm). The aerodynamic lens generates a beam of submicron particles, which is focused on a small area on the particle trap. The particle trap consists of custom-made mesh layers, the structure of which was newly designed using engineering techniques for micro electro mechanical systems (MEMS). A large number of mesh frames are well arranged in the trap, and particles can be efficiently captured after multiple impactions on the frames. The CO₂ laser is used to vaporize aerosol compounds captured on the particle trap. The evolved gas confined within the quartz cell is analyzed using an electron impact ionization (EI) QMS to quantify the chemical composition of the particles. The concept of the PT-LDMS and first evaluation of its performance are presented, specifically focusing on the structure and performance of the particle trap.     

Performance of a Single Ultrafine Particle Mass Spectrometer

We report on the performance of a rapid single particle mass spectrometer (RSMS-II), designed to obtain the size and composition of individual ultrafine particles. Particles are sized aerodynamically at the inlet using a dynamic focusing mechanism to transmit particles to the source region of a time-of-flight mass spectrometer. Since the target particles are too small to be detected optically, an excimer laser is pulsed at high frequency so that data are acquired only when a particle coincides with a laser pulse within the source region. The instrument is tested with sodium chloride and oleic acid particle standards of various sizes and the hit rate efficiency is monitored as the normalized number of particle hits per unit time. The hit rate efficiency depends on the particle flux through the active region of the laser beam, in addition to the particle size and composition, and may thus be used to determine the relative transmission efficiency and size selectivity of the inlet.     

Comparison and Combination of Aerosol Size Distributions Measured with a Low Pressure Impactor,Differential Mobility Particle Sizer,Electrical Aerosol Analyzer,and Aerodynamic Particle Sizer

Data from a different mobility particle sizer (DMPS) or an electrical aerosol analyzer (EAA) has been combined with data from an aerodynamic particle sizer (APS) and converted to obtain aerosol mass distribution parameters on a near real-time basis. A low pressure impactor (LPI), a direct and independent measure of this mass distribution, provided information for comparison.

The number distribution of particles within the electrical measurement range was obtained with the DMPS and EAA. Data from the APS for particles greater than that size were used to complete the number distribution. Two methods of obtaining mass distribution parameters from this number data were attempted. The first was to convert the number data, channel by channel, to mass data and then fit a log-normal function to this new mass distribution. The second method was to fit a log-normal function to the combined number distribution and then use the Hatch-Choate equations to obtain mass parameters.

Both the DMPS / APS and the EAA / APS systems were shown to successfully measure aerosol mass distribution as a function of aerodynamic diameter. Careful operation of the measurement equipment and proper data manipulation are necessary to achieve reliable results. A channel-by-channel conversion from number to mass distribution provided the best comparison to the LPI measurement. The DMPS / APS combination furnishes higher-size resolution and accuracy than the EAA / APS system. A small gap was observed in the EAA / APS combined data; however, this did not seem to adversely affect the determination of mass distribution parameters.     

Design and Performance of a Low-Cost Micro-Channel Aerosol Collector

Aerosol sampling and identification is vital for assessment and control of particulate matter pollution, airborne pathogens, allergens and toxins, and their effect on air quality, human health, and climate change. Assays capable of accurate identification and quantification of chemical and biological airborne components of aerosol provide very limited sampling time resolution and relatively dilute samples. A low-cost micro-channel collector (μCC) which offers fine temporal and spatial resolution, high collection efficiency, and delivers highly concentrated samples in very small liquid volumes was developed and tested. The design and optimization of this μCC was guided by computational fluid dynamics (CFD) modeling. Collection efficiency tests of the sampler were performed in a well-mixed aerosol chamber using aerosolized fluorescent microspheres in the 0.5–6 μm diameter range. Samples were collected in the μCC and eluted into 100 μL liquid aliquots; bulk fluorescence measurements were used to determine the performance of the collector. Typical collection efficiencies were above 50% for 0.5 μm particles and 90% for particles larger than 1 μm. The experimental results agreed with the CFD modeling for particles larger than 2 μm, but smaller particles were captured more efficiently than predicted by the CFD modeling. Nondimensional analysis of capture efficiencies showed good agreement for a specific geometry but suggested that the effect of channel curvature needs to be further investigated.

Copyright 2014 American Association for Aerosol Research     

Design and Performance of a New Personal Aerosol Monitor

Three particle size fractions of airborne dust are defined in Euro pean and U.S. standards for health-related dust measurements at the workplace: the respirable, the thoracic, and the inhalable fraction. We developed a novel instrument for personal, time-resolved concentration monitoring and sampling of these three fractions. The instrument combines inertial classification, filter sam pling, and photometric aerosol detection. It consists of a two-stage virtual impactor (cut-off diameters of 4 and 10 mu m), three filters, and three light scattering photometers. The virtual impactor serves as a particle size classifier and a coarse particle concentrator. This enrichment compensates for the decreasing particle mass-based photometric sensitivity with increasing particle diameter. The optical sensors are calibrated in-situ via the mass concentrations obtained gravimetrically from the filter samples. The device operates at a flow rate of 3.1 l min. There is strong agreement between the experimentally determined particle size-dependent collection efficiencies and the definition curves of the corresponding dust fractions. The size dependence of the sensitivity of the inertial concentrator and photometric detection units follow the definition curves qualitatively. Exposure data were obtained for different workplace environments characterized by temporally and spatially highly fluctuating concentrations. The field measurements have shown that the instrument is practicable under rough industrial conditions and that it enables a more comprehensive and more realistic characterization of the individ ual exposure of workers to health-endangering dusts than was previously possible.     

Improved Lower Particle Size Limit for Aerosol Time-of-Flight Mass Spectrometry

Laboratory and field tests have shown that electrostatic precharging can lead to a substantial decrease in the flow resistance of fabric filters. For lead smelter dust, the effectiveness of the precharger is dependent on the relative humidity of the suspending gas. For relative humidities below 5%, severe back corona occurs in the precharger, which results in poor particle charging and little improvement in filter performance. For relative humidities between 5% and 40%, little or no back corona occurs, precharging is effective and results in a reduction of up to 60% in the flow resistance of the fabric filter. The operational range of the precharger can be extended to lower relative humidities by cooling the earthed electrode. The improvement in filter performance can be achieved without the precipitation of large quantities of dust in the precharger.     

Evaluation of Particle Bounce in Various Collection Substrates to be Used as Vaporizer in Aerosol Mass Spectrometer

The determination of the collection efficiency (CE) of particles during transport, vaporization, and ionization in the aerosol mass spectrometer (AMS), which uses vaporizer to evaporate non-refractory particles with subsequent ionization, is important for accurately quantifying the concentrations of chemical constituents. Particle bounce in the vaporizer can be considered as one of the most important parameters influencing the CE of particles. Substrates with various shapes (flat, cylindrical, reverse-conical, cup, trapezoidal, and reverse-T), materials (stainless steel, copper, tungsten, and molybdenum), pores with average sizes of 0.2, 1, 5, 20, and 100 μm, and mesh with a size of 79 μm, which can be a possible candidate for the vaporizer in the AMS, were constructed. Bounce fractions of sub-micrometer particles (polystyrene latex, oleic acid, and dioctyl phthalate) were determined using the differential mobility analyzer (DMA)-impactor technique under a constant impact velocity. For the porous substrate, the particle bounce fraction significantly decreased with increasing pore size and porosity, but there was an upper limit for the pore size above which the particle bounce fraction no longer decreased significantly (i.e., the rebounded particles successfully escaped from the pores). The mesh substrate also had a lower particle bounce fraction than the flat substrate. Among the tested materials, the copper substrate having the lowest hardness and elasticity had the lowest particle bounce fraction. In addition, the reverse-T shape substrate having more available surfaces for particle entrapment led to the reduction of particle bounce fraction. In terms of phase, the liquid particles had lower particle bounce fractions than the solid particles. Our results suggest that the vaporizer in the AMS should provide traps for multiple collisions of the rebounding particles with an appropriate porosity or mesh and should be made of low-hardness materials to minimize particle bounce.

Copyright 2015 American Association for Aerosol Research     

Design,Modeling, Optimization,and Experimental Tests of a Particle Beam Width Probe for the Aerodyne Aerosol Mass Spectrometer

Aerodynamic lens inlets have revolutionized aerosol mass spectrometry by allowing the introduction of a very narrow particle beam into a vacuum chamber for subsequent analysis. The real-time measurement of particle beam width after an aerodynamic lens is of interest for two reasons: (1) it allows a correction to be made to the measured particle concentration if the beam is so broad, due to poor focusing by non-spherical particles, that some particles miss the detection system; and (2) under constant lens pressure it can provide a surrogate particle non-sphericity measurement. For these reasons, a beam width probe (BWP) has been designed and implemented for the Aerodyne Aerosol Mass Spectrometer (AMS), although this approach is also applicable to other instruments that use aerodynamic lens inlets. The probe implemented here consists of a thin vertical wire that can be precisely positioned to partially block the particle beam at fixed horizontal locations in order to map out the width of the particle beam. A computer model was developed to optimize the BWP and interpret its experimental data. Model assumptions were found to be reasonably accurate for all laboratory-generated particle types to which the model was compared. Comparisons of particle beam width data from a number of publications are also shown here. Particle losses due to beam broadening are found to be minor for the AMS for both laboratory and ambient particles. The model was then used to optimize the choice of the BWP dimensions, and to guide its use during continuous operation. A wire diameter approximately 1.55 times larger than the beam width to be measured provides near optimal sensitivity toward both collection efficiency and surrogate non-sphericity information. Wire diameters of 0.62 mm and 0.44 mm (for the AMS “long” and “short” chambers, respectively) provide reasonable sensitivity over the expected range of particle beam widths, for both spherical and non-spherical particles. Three other alternative BWP geometries were also modeled and discussed.     

Overall Performance Evaluation of Aerosol Number Samplers

At present, there is neither an officially accepted size-selective fiber (aerosol number) sampler, nor are there established performance criteria. In this work, a prototype preclassifier (multihole impactor) was used to connect a conventional asbestos sampler so that the aerosol penetration test and particle counting process could be performed. The bias, as a function of particle size, was defined as the difference between the measured penetration curve and the target ISO/ACGIH/CEN respirable convention. The imprecision was the standard error with reference to the mean aerosol penetration curve. A statistical term, one standard error shift (OSES) was used in a previous study to combine the sampling bias and imprecision. The bias and imprecision could be for aerosol number, aerosol mass, or even surface area. In this work, an additional step was taken by introducing another statistical term, maximum sampling shift (MSS), to further combine the OSES with the counting imprecision. For the surrogate sampler tested, the particle counting imprecision increased with increasing particle diameter and decreased with increasing geometric standard deviation. The particle counting imprecision was comparable with the OSES, and the resultant MSS map was actually the summation of imprecision and OSES.     

Soot Particle Aerosol Mass Spectrometer: Development,Validation, and Initial Application

The Soot Particle Aerosol Mass Spectrometer (SP-AMS) was developed to measure the chemical and physical properties of particles containing refractory black carbon (rBC). The SP-AMS is an Aerodyne Aerosol Mass Spectrometer (AMS) equipped with an intracavity laser vaporizer (1064 nm) based on the Single Particle Soot Photometer (SP2) design, in addition to the resistively heated, tungsten vaporizer used in a standard AMS. The SP-AMS can be operated with the laser vaporizer alone, with both the laser and tungsten vaporizers, or with the tungsten vaporizer alone. When operating with only the laser vaporizer, the SP-AMS is selectively sensitive to laser-light absorbing particles, such as ambient rBC-containing particles as well as metal nanoparticles, and measures both the refractory and nonrefractory components. When operated with both vaporizers and modulating the laser on and off, the instrument measures the refractory components of absorbing particles and the nonrefractory particulate matter of all sampled particles. The SP-AMS design, mass spectral interpretation, calibration, and sensitivity are described. Instrument calibrations yield a sensitivity of greater than 140 carbon ions detected per picogram of rBC mass sampled, a 3σ detection limit of less than 0.1 μg·m^?3 for 60 s averaging, and a mass-specific ionization efficiency relative to particulate nitrate of 0.2 ± 0.1. Sensitivities were found to vary depending upon laser-particle beam overlap. The utility of the instrument to characterize ambient rBC aerosol is demonstrated.

Copyright 2012 American Association for Aerosol Research     

Aerosol Light Extinction Measurements by Cavity Attenuated Phase Shift (CAPS) Spectroscopy: Laboratory Validation and Field Deployment of a Compact Aerosol Particle Extinction Monitor

We present laboratory and field measurements of aerosol light extinction ( σ_ep ) using an instrument that employs Cavity Attenuated Phase Shift (CAPS) spectroscopy. The CAPS extinction monitor comprises a light emitting diode (LED), an optical cavity that acts as the sample cell, and a vacuum photodiode for light detection. The particle σ_ep is determined from changes in the phase shift of the distorted waveform of the square-wave modulated LED light that is transmitted through the optical cell. The 3-σ detection limit of the CAPS monitor under dry particle-free air is 3 Mm^–1 for 1s integration time. Laboratory measurements of absolute particle extinction cross section ( σ_ext ) using non-absorbing, monodisperse polystyrene latex (PSL) spheres are made with an average precision of ± 3% (2-σ) at both 445 and 632 nm. A comparison with Mie theory scattering calculations indicates that these results are accurate within the 10% uncertainty stated for the particle number density measurements. The CAPS extinction monitor was deployed twice in 2009 to test its robustness and performance outside of the laboratory environment. During these field campaigns, a co-located Multi Angle Absorption Photometer (MAAP) provided particle light absorption coefficient ( σ_ap) at 635 nm: the single scattering albedo ( ω) of the ambient aerosol particles was estimated by combining the CAPS σ_ep measured at 632 nm with the MAAP σ_ap data. Our initial results show the high potential of the CAPS as lightweight, compact instrument to perform precise and accurate σ_ep measurements of atmospheric aerosol particles in both laboratory and field conditions.     

Development and Evaluation of a Compact,Highly Efficient Coarse Particle Concentrator for Toxicological Studies

A high-efficiency coarse-mode particle concentrator (CPC) has been developed and evaluated in the laboratory as well as validated in the field experiments at the University of Southern California, in Los Angeles, CA, and in Bilthoven, the Netherlands. The CPC operates with a total intake flow of 1000 LPM. The minor flow rate, containing the concentrated coarse-mode particles (2.5-10 w m), can be adjusted from 33 to 120 LPM in order to enrich ambient coarse PM concentrations by a factor of 8-30, depending on the desirable exposure level and flow rate needed. The laboratory evaluation of the virtual impactors at 3 minor flow rates (3.3, 7, and 10 LPM, respectively) indicated that extremely efficient concentration enrichment was obtained for 2.5-10 w m particles. In the field tests, the CPC operated at a minor flow rate of 33 LPM and the mass obtained was compared to the mass collected by a reference sampler, a (rotating) micro-orifice uniform deposit impactor (MOUDI), which sampled at 30 LPM. Concentration enrichment factors in the range of 26 to 30 were achieved based on particle mass, sulfate, and nitrate as well as selected trace element and metal concentrations (Al, Si, Ca, Fe, K, Mn, Cu, Zn, Ti). CPC and MOUDI concentrations were highly correlated for all species, with R 2 in the range of 0.74 to 0.89. The use of round (compared to rectangular geometry) nozzle virtual impactors in the CPC results in a high concentration efficiency, which reduces the CPC size as well as the power requirement that is required for its operation. The compact size of the CPC makes it readily transportable to desired locations for exposure to coarse-mode particles derived from different sources and thus of a varying chemical composition.     

Extending the Capabilities of Single Particle Mass Spectrometry: II. Measurements of Aerosol Particle Density without DMA

Particle density is an important and useful property that is difficult to measure because it usually requires two separate instruments to measure two particle attributes. As density measurements are often performed on size-classified particles, they are hampered by low particle numbers, and hence poor temporal resolution. We present here a new method for measuring particle densities using our single particle mass spectrometer, SPLAT. This method takes advantage of the fact that the detection efficiency in our single particle mass spectrometer drops off very rapidly as the particle size decreases below 100 nm creating a distinct sharp feature on the small particle side of the vacuum aerodynamic size distribution. Thus, the two quantities needed to determine particle density, the particle diameter and vacuum aerodynamic diameter, are known. We first test this method on particles of known compositions and densities to find that the densities it yields are accurate. We then apply the method to obtain the densities of particles that were characterized during instrument field deployments. We illustrate how the method can also be used to measure the density of chemically resolved particles. In addition, we present a new method to characterize the instrument detection efficiency as a function of particle size that relies on measuring the mobility and vacuum aerodynamic size distributions of polydisperse spherical particles of known density. We show that a new aerodynamic lens used in SPLAT II improves instrument performance, making it possible to detect 83 nm particles with 50% efficiency.     

Aerosol Particle Deposition in a Recirculation Region

Digital simulation results concerning aerosol particle transport and deposition in a recirculation region are presented. It is assumed that the particles are shed from sources near the back face of a block in a turbulent duct flow. The results show that a large number of particles may be captured by the block and the upper wall of the channel due to impaction and interception. The capture efficiencies increase as the source distance from the wall decreases. The gravitational effects on the particle deposition rate are also studied.     

A Numerical Calculation of the Transfer Function of the Fluted Centrifugal Particle Mass Analyzer

A new particle mass classifier called the Fluted Centrifugal Particle Mass Analyzer (CPMA) is presented and compared to the Aerosol Particle Mass (APM) Analyzer and the Couette CPMA. These particle mass classifiers use electrostatic and centrifugal forces to classify particles by their mass-to-charge ratio. The Fluted CPMA uses reversed ‘scooped’ sections to create a stable system of forces in the radial direction but not in the angular direction. The stable forces in the radial direction improve the transfer function of the classifier. A Lagrangian model (neglecting diffusion) of the Fluted CPMA is derived from which the transfer function of the classifier is found. The model shows that, like the APM and Couette CPMA, a single non-dimensional number determines the shape of the transfer function. The comparison of the classifiers shows that the Couette CPMA outperforms the Fluted CPMA and APM. However, the Fluted CPMA outperforms the APM over a wide range of operating conditions and may be easier to manufacture than the Couette CPMA.     

A Microscope-Based Particle Counting System for Determining Collected Aerosol Mass During the Calibration of a Cascade Impactor

It is shown that a simple optical particle-counting system can be efficiently and accurately used for determining the mass of collected particles during the calibration of a cascade impactor. For particles larger than 1 mu m in diameter, the limit of detection was enhanced by a factor 5 compared to the traditional spectrophotometer-based method.