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).     

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.     

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.     

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.     

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.     

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     

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     

A note on the aerodynamic diameter and the mobility of non-spherical aerosol particles

The aerodynamic diameter of a non-spherical aerosol particle is primarily related to the final settling velocity of the particle. The aerodynamic diameter is not obtained directly from mobility measurements by formally calculating a sphere diameter from the mobility equation for a spherical particle. Instead, a correction factor involving the dynamic shape factor of the non-spherical particle must be applied.     

Modeling and measurement of aerosol deposition on a heated substrate

Formation of an inorganic film by chemical aerosol deposition has been investigated experimentally and theoretically. Carrier gas flow rate, nozzle-to-substrate distance and substrate temperature were chosen as major process variables. The experimental work has been carried out to find their effect on the deposition efficiency, film thickness and its distribution. Both the deposition efficiency and film thickness increased with the carrier gas flow rate and substrate temperature but decreased with the nozzle-to-substrate distance. Especially at higher deposition rates, the central part of the film has a concave surface like a bowl. Flow and temperature fields of the fluid phase in the region between the nozzle and substrate were calculated numerically. Particle trajectories and particle evaporation were simulated numerically. As a result, the evaporation of the aerosol particles occurred so abruptly that the aerosol-existing region has a clear boundary. The extent of the region was found to be a determining factor in the film deposition, which characterizes the process of the chemical aerosol deposition.     

Inversion to obtain aerosol size distributions from measurements with a differential mobility analyzer

To measure size distributions of submicrometer aerosols with an electrical differential mobility analyzer (DMA) requires an inversion procedure. The Knutson (1976) and the Hoppel (1978) inversion procedures were numerically investigated for the case of log-normal aerosol size distributions. It was found that the Hoppel procedure converges to the same result as that given by the Knutson procedure. The computational range for geometric mean diameter ( _g) was 0.025-0.25 μm, and for geometric standard deviation (σ_g) was 1.1–2.4. The inversion error was found to be greater than 10% in certain “forbidden zones” of _g and σ_g values. For the case of an ideal DMA having no lower mobility limit, only one forbidden zone exists, this consisting of small σ_g values. The boundary of this forbidden zone intercepts the computational range boundaries at σ_g = 1.25, and σ_g = 1.62, . These results also apply to an actual DMA when the size distribution of particles larger than the DMA singly charged mobility limit is available a priori. If such information is not available, the concentration of these larger particles is assumed to be zero in performing the inversion. This assumption adds a second forbidden zone, consisting of large σ_g values and having the intercepts σ_g = 2.44, and σ_g = 1.50, . The first forbidden zone remains nearly the same.     

A mobility spectrometer for measurement of initial properties of Po

Reports in the literature on the measurement of the diffusive properties of ²¹⁸Po in the so-called unattached state yield conflicting results. It is now accepted that the ²¹⁸Po ion does not remain a single atom but interacts with trace vapours in the atmosphere, forming larger radioactive particles in the 0.5–3 nm size range. The mechanisms through which these interactions occur are unexplained. Recent application of the theory of ion-induced nucleation to the ²¹⁸Po ion showed that this theory can explain the experimental results reported in the literature qualitatively. The initial phases of ion-induced nucleation involve the formation of pure water clusters around which molecules of other condensable trace vapours may nucleate. This happens within microseconds after the formation of the ion. Methods available to study this phenomenon include diffusion screens, which do not have very high resolution, and electric mobility spectrometers, which can only provide information on charged particles, as is the case for the initial stages of the ²¹⁸Po ion's life before neutralization. Ion-induced nucleation can furthermore only occur for charged particles. This paper describes the development of a mobility spectrometer with adequate resolution to study the clustering processes that may occur. The first results of measurements of pure water clustering around ²¹⁸Po ions using the spectrometer are presented. It is found that the radius of the ion increases with relative humidity. The results are compared with predictions of two representations of the clustering theory. The theory qualitatively explains the results obtained, but there are considerable differences between the experimental measurements and the theoretical predictions, as well as between the two representations of the theory.     

Near real-time measurement of carbonaceous aerosol using microplasma spectroscopy: Application to measurement of carbon nanomaterials

A sensitive, field-portable microplasma spectroscopy method has been developed for real-time measurement of carbon nanomaterials. The method involves microconcentration of aerosol on a microelectrode tip for subsequent analysis for atomic carbon using spark emission spectroscopy (SES). The spark-induced microplasma was characterized by measuring the excitation temperature (15,000–35,000 K), electron density (1.0 × 10¹⁷–2.2 × 10¹⁷ cm^?3), and spectral responses as functions of time and interelectrode distance. The system was calibrated and detection limits were determined for total atomic carbon (TAC) using a carbon emission line at 247.856 nm (C I) for various carbonaceous materials including sucrose, EDTA, caffeine, sodium carbonate, carbon black, and carbon nanotubes. The limit of detection for total atomic carbon was 1.61 ng, equivalent to 238 ng m^?3 when sampling at 1.5 L min^?1 for 5 min. To improve the selectivity for carbon nanomaterials, which mainly consist of elemental carbon (EC), the cathode was heated to 300°C to reduce the contribution of organic carbon to the total atomic carbon. Measurements of carbon nanotube aerosol at elevated electrode temperature showed improved selectivity to elemental carbon and compared well with the measurements from the thermal optical method (NIOSH Method 5040). The study shows the SES method to be an excellent candidate for development of low-cost, hand-portable, real-time instrument for measurement of carbonaceous aerosols and nanomaterials.