Toshiyuki Fujimoto’s research while affiliated with Muroran Institute of Technology and other places

Highly charged aerosol nanoparticles of a molecular size (< 5 nm) exhibit complex behaviors caused by random (Brownian) motion and electrostatic interactions (image force) with solid surfaces. The classical single fiber theory, however, can only partially describe the complex filtration mechanism of highly charged nanoparticles. We began this study by employing the Langevin Dynamics (LD) Method to simulate the random motion and deposition of highly charged nanoparticles on a single fiber. The single fiber efficiencies predicted by the LD simulation did not agree with data from experiments using electrosprayed highly charged molecular ions. To explain this mismatch and search for other factors that could potentially affect the capturing mechanism, we investigated the influence of the mesh structure. While the image force exerted a significant influence as the packing density decreased, the influence tended to oppose the theoretical predictions. These results suggested that the fiber-fiber interaction in a wire screen with high packing density significantly influences the collection of multiply charged molecular ions. A new theoretical model describing the influence of inter-fiber interaction will be required to clarify how highly charged molecular ions are captured by image force.

The slip flow effect is often brought out to explain the reduction in pressure drop for nanofiber filters. Kirsch, Stechkina, and Fuchs (1973) studied the slip flow effect on the pressure drop of fibrous filters consisting of micron fibers, and proposed an empirical equation to predict the dependence of the dimensionless drag, F, on the Knudsen number, Kn, with considering non-uniformity of fiber packing. However, their empirical equation was derived based on the experiments with filters consisting of micron fibers so that the empirical equation is not yet verified for nanofiber filters. In the present work, we used various commercially available nanofiber filters with various physical properties, and the pressure drop was measured at low pressures in order to examine the validity of the empirical equation. As a result, we found that the empirical equation is valid even for nanofiber filters with a large inhomogeneity factor at a large Kn up to 20.

We investigated the capturing mechanisms of highly charged macromolecular ions of polyethylene glycol electrosprayed onto a metal screen. Our experiments assessed how the charge state, size of the macromolecular ions, and filtration velocity affected the penetration of the ions through the metal screen. The single fiber efficiencies were plotted as functions of the Peclet number and image force parameter. Highly charged molecular ions had much higher collection efficiencies than neutralized macromolecules, suggesting the presence of a strong image force between the ions and metal surface. The single fiber efficiency by image force was proportional to the square root of an image force parameter predicted by theory. When using the prefactor of 9.7 proposed by Alonso et al. (2007), we found fair agreement between the experimental data and theoretical predictions on the collection efficiency of highly charged molecular ions with mobility diameters from 2.6 to 4.8 nm and numbers of electrical charges from 2 to 7. The experimental evidence from our study reveals that image force contributes strongly to the collection of multicharged macromolecular ions by a metal wire screen.

There has been only limited information about the vertical chemical structure of the atmosphere, so far. We conducted aerial observations on 11, 12, and 14 December 2010 over the northern part of the East China Sea to analyze the spatial distribution of atmospheric pollutants from East Asia and to elucidate transformation processes of air pollutants during the long-range transport. On 11 December, a day on which Asian dust created hazy conditions, the average PM10 concentration was 40.69 μg m−3, and we observed high concentrations of chemical components such as Ca2+, NO3–, SO42−, Al, Ca, Fe, and Zn. The height of the boundary layer was about 1200 m, and most species of pollutants (except for dust particles and SO2) had accumulated within the boundary layer. In contrast, concentrations of pollutants were low in the boundary layer (up to 1000 m) on 12 December because clean Pacific air from the southeast had diluted the haze. However, we observed natural chemical components (Na+, Cl–, Al, Ca, and Fe) at 3000 m, the indication being that dust particles, including halite, were present in the lower free troposphere. On 14 December, peak concentrations of SO2 and black carbon were measured within the boundary layer (up to 700 m) and at 2300 m. The concentrations of anthropogenic chemical components such as NO3–, NH4+, and Zn were highest at 500 m, and concentrations of both anthropogenic and natural chemical components (SO42−, Pb, Ca2+, Ca, Al, and Fe) were highest at 2000 m. Thus, it was clearly indicated that the air above the East China Sea had a well-defined, layered structure below 3000 m.

Time-resolved data for size distribution, number concentration and chemical composition of atmospheric aerosols were taken from March 9 to 16, 2012 during Fukue Site Atmospheric Aerosol Observation Campaign (128.7 E, 32.8 N) organized by ASEPH (Impacts of Aerosols in East Asia on Plants and Human Health) project. A scanning mobility particle sizer was employed to continuously measure the particle size distribution from 14 nm to 670 nm and the total number concentration of particles (>3 nm) was measured by a condensation particle counter. Very large amount of polluted air mass transportation (PM2.5 > 65 mg m-3) was observed on March 11, associated with a sudden increase in particle number concentration having a mobility diameter from 100 to 300 nm. The transported air mass contained anthropogenic pollutants such as sulfate (>20 mg m-3), nitrate (>15 mg m-3) and black carbon (>3 mg m-3). Typical new particle formation and growth events were also observed on March 12e13. The onset of particle formation and nucleation bursts was identified at around noon and it is considered that these were nucleated species near the island area. It was found that some new particle formation events were associated with the long range transportation of polluted air mass from East Asian region.

Clarification of aerosol formation and growth mechanisms largely depends upon the development of measuring instruments for sub 10 nm particles (especially smaller than 3 nm). In the present work, by investigating the effect of nucleation temperature on minimum detectable size of a particle size magnifier (PSM), we developed a PSM-CPC system. Through the course of development, we found that a lower nucleation temperature is preferable in hindering homogeneous nucleation in PSM and detecting smaller particles. Newly developed PSM was capable of detecting 1 nm particles with the counting efficiency of 0.5 % at the nucleation temperature of 16.7 degC and the supersaturation of 14.3. The PSM-CPC system was applied to the study of nucleation of sulfuric acid aerosol via SO2 photochemical reaction, and it was shown to measure the size distribution of particles down to 2 nm.

In order to evaluate the potential of surface-discharge microplasma devices (SMDs) as ion sources for high-efficiency electrical charging of nanoparticles, a long SMD was installed in a surface-discharge microplasma aerosol charger (SMAC), and the charging performance of the SMD for aerosol nanoparticles was measured. The intrinsic charging efficiency for 5-nm-diameter particles was approximately 60%, and that for 10-nm-diameter particles was 90% at an aerosol flow rate of 1.5 L · min -1 because a long SMD is capable of generating a uniform concentration of ions over a wide area. The size-dependent charging efficiencies for these aerosol nanoparticles with diameters ranging between 3 nm and 30 nm were in good agreement with those predicted using the diffusion charging theory. The product of ion concentration (N) and charging time (t), estimated via the diffusion charging theory, was 3.0 10 13s ·m -3, which is one order of magnitude higher than that for previously reported chargers. These results confirm the high potential of SMDs as ion sources for charging aerosol nanoparticles.