Development of portable aerosol mobility spectrometer for personal and mobile aerosol measurement

We describe development of a portable aerosol mobility spectrometer (PAMS) for size distribution measurement of submicrometer aerosol. The spectrometer is designed for use in personal or mobile aerosol characterization studies and measures approximately 22.5×22.5×15 cm and weighs about 4.5 kg including the battery. PAMS uses electrical mobility technique to measure number-weighted particle size distribution of aerosol in the 10–855 nm range. Aerosol particles are electrically charged using a dual-corona bipolar corona charger, followed by classification in a cylindrical miniature differential mobility analyzer. A condensation particle counter is used to detect and count particles. The mobility classifier was operated at an aerosol flow rate of 0.05 L/min, and at two different user-selectable sheath flows of 0.2 L/min (for wider size range 15–855 nm) and 0.4 L/min (for higher size resolution over the size range of 10.6–436 nm). The instrument was operated in voltage stepping mode to retrieve the size distribution in approximately 1–2 min. Sizing accuracy and resolution were probed and found to be within the 25% limit of NIOSH criterion for direct-reading instruments. Comparison of size distribution measurements from PAMS and other commercial mobility spectrometers showed good agreement. The instrument offers unique measurement capability for on-person or mobile size distribution measurement of ultrafine and nanoparticle aerosol.     

Direct measurement of aerosol glass fiber alignment in a DC electric field

We report non-conducting aerosol fiber (i.e., glass fiber) alignment in a DC electric field. Direct observation of fiber orientation state is demonstrated and quantitative analysis of fiber alignment is made using phase contrast microscopy in four different conditions: (i) dry air and naturally charged fibers, (ii) humid and naturally charged, (iii) humid and neutralized (Boltzmann charge distribution), and (iv) humid and neutralized with an electrostatic precipitator upstream electrodes (i.e., non-charged). The glass fiber aerosols generated by a vortex shaking method were conditioned using a Po-210 neutralizer or humidifier and were provided into a test unit where cylindrical or parallel plate electrodes are used and high voltage is applied to them. Fibers were collected on a filter immediately downstream from the electrodes and their images were taken through an optical microscope to visualize the fiber orientation and measure the alignment angles and lengths of the fibers. The results showed that under all four conditions tested, airborne glass fibers could be aligned to the electric field with different alignment quality, indicating that the glass fibers can be polarized in a steady electric field. In humid air, the fiber alignment along the field direction was observed to be much better and the number of uniform background particles (i.e., randomly oriented fibers) in angular distributions is smaller than that in dry air. Also, it was found that charged fibers in humid air could be better aligned with negligible uniform background than neutralized and non-charged fibers. Possible mechanisms about humidity and charge effects on enhanced fiber alignment are discussed to support the observations. The results indicate that the enhancement of alignment in an electric field would be possible in humid air for other non-conducting fibrous particles having surface chemistry similar to glass fibers.     

Statistics of aerosol electric charging

The paper discusses a theoretical approach to the statistics of aerosol electric charging. It is based on the analysis of electric charge flows among charged aerosols. The steady-state aerosol charge distribution is described theoretically for normal environments, also for radioactive charging, charging by external irradiation and charging in external electric fields. Theoretical results are compared with experimental data taken from the literature (including early work by the authors (Emets et al., J. Aerosol Sci. 22, 389–394 (1991)). The agreement is good.     

Apparatus for measurement of the electrical mobility of aerosol particles: computer control and data analysis

The construction and computer control of a parallel plate mobility analyser for measuring the charge distribution of an aerosol is described in the paper. The experimental results obtained are in the form of a penetration/voltage curve, which contains the information required in a convoluted form. Methods of data reduction are described, comprising tangent-drawing, fitting of analytical functions or histograms to the data by least squares, and a maximum entropy method. Finally some generalisations about the suitability of the methods are made using data from a number of aerosols, and from a theoretical simulation.     

New fast integrated mobility spectrometer for real-time measurement of aerosol size distribution—I: Concept and theory

A new instrument capable of measuring aerosol size distribution with high time and size resolution, and high signal-to-noise ratios is described. The instrument, referred to as Fast Integrated Mobility Spectrometer (FIMS), separates charged particles based on their electrical mobility into different trajectories in a uniform electric field. The particles are then grown into super-micrometer droplets, and their locations on the trajectories are recorded by a fast charge-coupled device (CCD) imaging system. Images captured by the CCD reveal mobility-dependent particle positions and their numbers, which are then used to derive a particle size distribution spectrum. By eliminating the need to scan over a range of voltages, FIMS significantly improves the measurement speed and counting statistics. A theoretical framework has been developed to quantify the measurement range, mobility resolution, and transfer function of FIMS. It is shown that FIMS is capable of measuring aerosol size distributions with high-time and size resolution.     

Photo electric nucleus counters for aerosol climatology

Photo electric nucleus counters are used as aerosol detectors at several observatories which are attempting to establish aerosol climatologies. The photo electric nucleus counter has an advantage over absolute nucleus counters in such applications, as it produces a nearly instantaneous “snap shot” of the integrated aerosol concentration in a relatively large volume of air.

The Pollak—Nolan type of photo electric nucleus counter is most frequently used in determining the aerosol concentration at these climatological observatories. Laboratory and field experiments indicate that the response of this instrument is extremely repeatable, even though questions may exist relative to its absolute calibration and the minimum size of particles it detects. The repeatability is sufficiently reliable to permit use of existing data if a more precise calibration is accomplished in the future.

Many components used in construction of replicas of the Pollak—Nolan counter have become difficult to obtain. A transfer of Pollak's calibration to a geometric replica of this instrument, using more modern components would insure continuance of high quality climatic aerosol data.     

An improved parallel plate mobility analyzer for aerosol particles

A high resolution, parallel plate mobility analyzer is presented, together with improvements to facilitate the measurements and the assessment of the deposition bands. With the aid of this instrument measurements have been performed of relative mobilities of latex aggregates and of number of elementary charges carried by unipolarly charged particles; also, monodisperse aerosol generators were characterized.     

Measuring aerosol size distributions with the fast integrated mobility spectrometer

A fast integrated mobility spectrometer (FIMS) has been developed for rapid aerosol size distribution measurements including those aerosols with low particle number concentrations. In this work, an inversion routine has been developed for the FIMS and it is demonstrated that the FIMS can accurately measure aerosol size distributions. The inversion routine includes corrections for the particle residence time in the FIMS and other factors related to the width of the response (or transfer) function and multiple charging of particles. Steady-state size distributions measured with the FIMS compared well with those measured by a scanning mobility particle sizer (SMPS). Experiments also show that the FIMS is able to capture the size distribution of rapidly changing aerosol populations. The total particle concentration integrated from distributions measured by the FIMS agrees well with simultaneous measurements by a condensation particle counter (CPC).     

Accuracy of recovered moments for narrow mobility distributions obtained with commonly used inversion algorithms for mobility size spectrometers

Aerosol mobility size spectrometers are commonly used to measure size distributions of submicrometer aerosol particles. Commonly used data inversion algorithms for these instruments assume that the measured mobility distribution is broad relative to the DMA transfer function. This article theoretically examines errors that are incurred for input distributions of any width with an emphasis on those with mobility widths comparable to that of the DMA's transfer function. Our analysis is valid in the limit of slow scan rates, and is applicable to the interpretation of measurements such as those obtained with tandem differential mobility analyzers as well as broader distributions. The analysis leads to expressions that show the relationship between the inverted number concentration, mean size, and standard deviation and true values of those parameters. For narrow distributions (e.g., for a mobility distribution produced by a DMA with a 1:10 aerosol:sheath air flow ratio) under typical operating conditions, number concentrations and mean mobility obtained with inversion algorithms are accurate to within 0.5% and 1.0%, respectively. This corresponds to mean diameter retrieval errors of 1.0% for large particles and 0.5% for small (kinetic regime) particles. The widths (i.e., relative mobility variance) of the inverted distributions, however, significantly exceed the true values.

Copyright © 2018 American Association for Aerosol Research     

Chemically resolved aerosol dynamics for internal mixtures by the quadrature method of moments

The quadrature method of moments (QMOM), a promising new tool for aerosol dynamics simulation, is extended to multicomponent, internally mixed particle populations. A new moment closure method, the Jacobian matrix transformation (JMT), is introduced and shown to provide an efficient procedure for evolving quadrature abscissas and weights directly and in closed form. For special growth laws where analytic results are available for comparison, the QMOM is also found to be exact. The JMT implementation of the QMOM is used to explore the asymptotic behavior of coagulating aerosols at long time. Nondimensional reduced moments are constructed, and found to evolve to constant values in excellent agreement with estimates derived from ‘self-preserving’ distributions previously obtained by independent methods. Our findings support the QMOM as a new tool for rapid, accurate simulation of the dynamics of an evolving internally mixed aerosol population, including the approach to asymptotic behavior at long time, in terms of lower-order moments.     

Accurate measurement of fracture toughness in structural ceramics

The measured values of fracture toughness for ceramics are closely correlated with the sharpness of notch tips, which in turn influences the accurate measurement of fracture toughness. Here, typical structural ceramics, i.e., 3 mol% yttria-stabilized tetragonal zirconia polycrystal (3Y-TZP), ZrB₂, ZrB₂-SiC and ZrB₂-SiC-Grapite, were used for the measurement of fracture toughness, and the effect of notch tip radius on the fracture toughness values of these typical structural ceramics was investigated. Ultra-sharp notches with a tip radius less than 1 μm can be fabricated by laser, lower than the critical notch tip radius in ceramics below which the fracture toughness value almost remains constant, and improved accuracy and consistency of fracture toughness measurement can be obtained by this method compared with traditional method.     

Brownian diffusion effect on nanometer aerosol classification in electrical mobility spectrometer

A multi-channel differential mobility analyzer (MCDMA) or aerosol spectrometer is widely used for classifying and measuring nanometer aerosol particles in the size range from 1 nm to 1 μm because of its better time response than a typical differential mobility analyzer. In the present study, the effect of Brownian diffusion on electrical mobility classification and trajectory of nanometer aerosol particles in an electrical mobility spectrometer developed at Chiang Mai University has been analytically investigated. Th Brownian diffusion of particles inside the spectrometer increased with decreasing particle size and flow rates of aerosol and clean sheath air, and with increasing inner electrode voltage, and also decreased with decreasing operating pressure. The particle trajectories considering Brownian diffusion motion inside the spectrometer were found to be broader than those under no Brownian diffusion. Smaller particles were found to have higher degree of broadening of trajectory than the larger particles. Brownian diffusion effect was found to be significant for particles smaller than 10 nm.     

Experimental characterization of a short electrical mobility spectrometer for aerosol size classification

A prototype of a short column electrical mobility spectrometer (EMS) for size measurement of aerosol particle was design, constructed, and experimentally characterized. The short EMS consists of a particle charger, a size classifier column, and a multi-channel electrometer. Its particle size resolution is derived from a 10 channel electrometer detector. The short EMS is capable of size measurements in the range between 10 nm to 1,000 nm with a time response of about 50 s for full up and down scan. Particle number concentration in which the short EMS can measure ranges from 10¹¹ to 10¹³ particles/m³. The operating flow rate of the short EMS is set for the aerosol flow rate of 1.0–2.0 l/min and the sheath air flow rate fixed at 10.0 l/min. The inner electrode voltage of the classifier can be varied between 500–3,000 VDC. The short EMS operates at sub-atmospheric pressure, typically at 526 mbar. Validation of the short EMS performance was performed against a scanning electron microscope (SEM). Good agreements were obtained from comparison between sizes determined from the short EMS classifier and the SEM analysis. Signal current from the detector was also analyzed to give rise to number concentration of particles. Experimental results obtained appeared to agree well with the theoretical predictions.     

Improving the accuracy of the moments method for solving the aerosol general dynamic equation

The General Dynamic Equation for aerosol evolution is converted into a set of ordinary differential equations for the moments M_m by multiplying by v^m and integrating over particle volume, v. Closure of these equations is achieved by assuming a functional form for the moments, instead of the usual assumption of a functional form for the size distribution itself. Specifically, it is assumed that In(M_m) can be expressed as a pth-order polynomial in m. The time-dependent coefficients in the polynomial are found by solving (p + 1) differential equations numerically. The case p = 2 corresponds to the assumption that the size distribution is always log-normal but comparison with accurate solutions shows that increasing p increases the accuracy of the method for all processes considered (removal, condensation and Brownian coagulation). Particle loss during evaporation and achievement of a self-preserving form for Brownian coagulation are also considered. Inversion of the moment expression to obtain the size distribution using the Mellin inversion formula is discussed.     

Application of centrifugal filter to aerosol size distribution measurement

In the present work, the centrifugal filter proposed by the authors was applied to classify aerosol particles followed by the detection of total mass or number concentrations so as to measure the size distribution of aerosol particles. The structure and operating condition of the centrifugal filter were optimized in order to attain sharp separation curves with various cut-off sizes between 0.3 and 10 μm. The aerosol penetrating the centrifugal filter at various rotation speeds was measured with a photometer to determine the total mass concentration. The virtue of this system is that the cut-off size is varied just by scanning the rotation speed of filter and that it can be applied to the measurement of high concentration aerosols without dilution by choosing an appropriate filter medium. As a result, the centrifugal filter was successfully applied to measure the size distribution of solid particles in size ranging from 0.3 to 10 μm.

Copyright © 2017 American Association for Aerosol Research     

Principles of aerosol radioactivity measurement using the electrodynamic balance

A charged microparticle levitated in an electrodynamic balance (EDB) undergoes charge loss due to scavenging of the opposite-polarity ions when ionizing radiation is present in the EDB chamber. The rate of charge loss of the suspended particle can be used to measure the level of radioactivity. A theoretical analysis of ion generation and transport inside the EDB chamber is developed which reveals the physical parameters which control the sensitivity and performance of the device. Model predictions of charge-loss rates with radioactive material deposited on the bottom electrode are compared to previously reported experimental measurements. The modelling principles are adaptable to any configuration of radioactive source and the detection sensitivity for a suspended radioactive microparticle in the EDB is also estimated. The modelling indicates that detection of radioactivity levels near 1 decay min⁻¹ (dpm) in a suspended particle should be possible. The EDB may also function as a detector of individual decay events.     

Accurate size measurement of needle-shaped particles using digital holography

In this paper, a digital holography based method is proposed to accurately measure the length and orientation of the needle-shaped particles in solution. The method involves recording of the hologram and numerical reconstructions (focusing) of the hologram at several depths. An image analysis routine is then used to determine the length, location and orientation of the particles from the reconstructed images without any a priori information about the orientation of particles. The performance of the method is verified using a single fiber with known size and orientation. Subsequently, the proposed method is applied to a suspension of fibers, where the length measurements are found to be in good agreement with the true values. This proposed technique can overcome the shortcoming of existing 2D imaging tools, which can provide only the projected lengths of randomly oriented particles.     

The effect of alignment on the electric mobility of soot

The effect of an aligning electric field on the mobility of both flame generated smoke and smolder generated smoke was studied. A pulsed differential mobility analyzer was used to study the alignment without changing the DMA flows. No detectable change in the mobility was observed for the smolder smoke, while a small but detectable effect of up to 5% decrease in the mobility diameter with increasing field was observed for the largest aggregates with a mobility diameter of 200 nm. We modeled the friction coefficient tensor of soot fractal aggregates as an equivalent prolate spheroid to obtain the field induced mobility as a function of aspect ratio. The alignment probability distribution function was determined by computing the polarizability tensor for simulated fractal aggregates. One interesting result was the smallness of the prolate sphere aspect ratio of 1.2 to 1.3 compared to the much larger aspect ratio from TEM analysis and from the polarizability ratio. An explanation for the low value based on the contribution to the friction coefficient from the individual spheres for a fractal is given. Another interesting observation is the broadening of the mobility size distribution with decreasing field. This is shown to be related to the polydispersity of the aspect ratio. The fact that all three aggregate sizes appear to fit the same spheroid aspect ratio is interesting, and offers a first-order approach to describing transport properties of aggregates. An estimate of the rotation relaxation time of the fractal aggregate was made to verify that the rotation time was much shorter than the duration of the zero electric field period during each cycle.

Copyright © 2016 American Association for Aerosol Research