Numerical simulation of parallel-plate particle separator for estimation of charge distribution of PM2.5

A numerical simulation of an instrument that is used to measure the charging state of PM2.5 is conducted in order to clarify its measurement uncertainty and to improve its performance. The instrument, a parallel-plate particle separator (PPPS), is designed to classify aerosol particles according to their charging states and measure their quantities. The trajectories of submicron particles in the PPPS are numerically analyzed using the Lagrangian particle tracking method, taking into account the Brownian force and the electrostatic force. First, it is confirmed that the deterioration in the classification accuracy observed in the experiment is due to Brownian diffusion. The optimal condition that improves the accuracy is investigated through a parametric study by varying the balance of flow rates at the inlets, the geometry of the inlet and exit sections, and the applied voltage. It is found that decreasing the flow rate of the central inlet for aerosol or narrowing the central inlet improves the accuracy. The dependence of the accuracy on the flow rate is found to be in accordance with the experimental results. For charged particles, an optimum voltage that maximizes the classification accuracy is found. On the basis of the simulation results, we propose a method to determine the charge distribution of aerosol from the number of particles counted at each exit of the PPPS. In the test assuming aerosol in the air, the charge distribution determined from the number count at the exits is found to perfectly agree with the charge distribution specified at the inlet. Copyright © 2019 American Association for Aerosol Research</p

Development of an openable small cyclone for atmospheric particulate matter sampling for toxicological experiments

The chemical components and mechanisms underlying the toxicity and adverse health effects of particulate matter (PM) in the atmosphere have not been fully elucidated. After designing a small, openable, stainless steel cyclone to collect PM samples effectively in powder form for use in toxicological experiments, we evaluated its performance. We compared it with a commercially available aluminum cyclone of similar dimensions, but which is unopenable. The aerodynamic cutoff diameter of the openable cyclone was found by experimentation to be approximately 0.2 µm at a flow rate of 90 L min−1, which is comparable to the unopenable commercial cyclone. The sampling yields, representing the fraction of obtained sample mass relative to the total mass of PM with aerodynamic diameter smaller than 2.5 μm (PM2.5) drawn into the sampler, were approximately 1.3 times higher, on average, for the openable cyclone than for the unopenable cyclone. The openable design of the cyclone might contribute to a marked increase in the finally obtained amounts of PM samples. Analyses of metal concentrations in the PM samples collected simultaneously using the stainless steel openable cyclone and aluminum unopenable cyclone suggest that the stainless steel cyclone is less likely than the aluminum cyclone to cause sample contamination from its material. The openable cyclone developed for this study facilitates the effective collection of powder-form PM samples suitable for use in toxicological experiments.</p

Technique for estimating the charge number of individual radioactive particles using Kelvin probe force microscopy

The Fukushima Daiichi Nuclear Power Plant accident in Japan resulted in the emission of many radioactive cesium (Cs)-containing particles that have charges on the surface due to self-charging. Charged aerosol particles are efficiently deposited inside human airways, leading to adverse health effects. To evaluate these effects, we developed a technique for estimating the charge number (np) of radioactive particles by measuring the surface potentials (Vp) of individual radioactive particles using Kelvin probe force microscopy. The Vp values of the individual CsCl particles were highly correlated with the surface np, indicating that Vp is a measure of the charged aerosol state. To further examine the Vp–np relationship, a simplified capacitance model was developed to estimate the ratio of Vp to np per unit area of particles. Although the calculated Vp was proportional to the np, consistent with our experiment, the calculated ratio was higher than those determined experimentally. The magnitude of this ratio may depend on the conductivity, microphysical properties and chemical composition of the particles. Despite these uncertainties, the experimentally determined Vp–np relationship of the CsCl particles was used to estimate the np of the radioactive and non-radioactive particles from the measurement of the Vp of these particles. It was demonstrated that the np of the radioactive particles was much higher than that of the non-radioactive particles, suggesting that radioactive particles are efficiently charged by self-charging. These charged radioactive particles may strongly cause adverse human health effects owing to their efficient deposition in human airways.</p