Although the basic principles of fibrous filters have been well understood for capture of micron and submicron sized particles, questions arise when they are applied to nanoscale particles. In the first part of this review, the classical theory of fibrous filters is described with focus on the principles that are applicable to nanoparticle collection. The areas of recent developments reviewed include thermal rebound of nanoparticles and the effects of particle shape, aggregate morphology, flow regime, humidity, fiber size, and particle loading. One of the outstanding questions in nanoparticle collection is the particle size at which the effect of thermal rebound on collection efficiency can be observed. Theoretical calculations indicate that the effect probably can be observed only for particles smaller than 1 nm, but experimental confirmation is difficult at present because of lack of instruments for classifying and counting subnanoscale particles. Two promising devices based on filtration principles have been studied in recent years: multilayer filters and inertial fibrous filters. Multilayer filters, which are composed of nanofiber and microfiber mats, have potential to become an efficient and economical device for removing nanoparticles from gas streams. The inertial fibrous filter operates at high flow rates and relatively low pressure drop, thereby offering an attractive alternative to lowpressure impactors for nanoparticle sampling. Further development of these two types of filtration devices is needed to make them simple and reliable.

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Collection performance of an electret filter with rectangular fibers was studied experimentally for cases in which electrostatic effect and Brownian diffusion are predominant by using particles from 0.02 to 0.4 ym in diameter and at different charging states. A single fiber collection efficiency qEl, was found to be expressible as a function of dimensionlegs parameters of Peclet number Pe, and Coulombic and induced force parameters, K c and K I,, as, here, A, B, C, and 1) are the numerical constants depending upon the charging density of electret fiber. Indices of each dimensionless parameter determined through the experiment coincided with the previous theory. A maximum penetration of particles appeared in the transition region of predominant collection mechanisms, i.e., between Brownian diffusion and induced force effect, ranging smaller than 0.1 pm in diameter for uncharged particles, and between Coulombic and induced force effects, ranging larger than 0.1 pm in diameter, for charged particles. Semiempirical expressions for a single electret fiber collection efficiency and a most penetrating particle size, applicable to particles in any charging state, were obtained taking account of Brownian diffusion, and induced and Coulombic force effects simultaneously. Undulation of the penetration observed in the filtration of particles in charge equilibrium was explained by using the semiempirical expression for a single fiber efficiency and charge distribution on a particle.

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Filtration efficiency of multi-walled carbon nanotube (MWCNT) aerosol by fibrous filter was evaluated experimentally. Mono-mobility test aerosols with electrical mobility diameter of 100, 200, and 300 nm were generated by the atomization of MWCNT aqueous suspension followed by mobility classification with a differential mobility analyzer (DMA). By analyzing the shape of classified aerosol particles under a scanning electron microscope, it was found that the DMA-classified 300 nm particles were fibrous in shape and had uniform diameter of about 60 nm and length of 2.1 micrometer. On the other hand, 100 nm and 200 nm particles contained a fairly large amount of multiply charged fibrous particles with a larger diameter. These test aerosols were challenged to a medium performance fibrous filter at various filtration velocities. As a result, fibrous particles were captured by fibrous filter at a higher collection efficiency than the spherical particles with the same mobility. By analyzing the single fiber capturing efficiency, interception incorporating the rotation of fibrous particles is found to be the dominant capturing mechanism for the fibrous particles in the studied size range.

Physical and chemical characteristics of particles from rubber-wood combustion in a natural rubber sheet smoking process were studied. Experimental parameters include wood moisture content and wood-burning period. The size distribution of smoke particles was measured by using an 8-stage Andersen air sampler. Total smoke particle concentration was determined by collecting particles using a modified high volume sampler. Polycyclic Aromatic Hydrocarbons (PAHs) were extracted with Benzene-Ethanol by using ultrasonic technique and analyzed by HPLC/UV detection. Results show that the size distribution of smoke particles is single-mode in which the mass median aerodynamic diameter (MMAD) is 0.68 μm and the average smoke concentration is 15.806 mg/m 3 . This is equivalent to a mass emission to workplace of 4.33 kg/month/room. The smoke particle concentration and associated PAH concentration clearly depend on the wood moisture content and burning period. The highest PAH concentration and smoke particle concentrations were found to be 60.59 to 118.06 μg/m 3 and 23.35 to 47.54 mg/m 3 , respectively, for a wood moisture content of 37.4 to 73.6% d.b. (dry basis) at the initial period. Smoke particle-bound PAHs are dominated by 4-6 ring PAH compounds that contribute to more than 60% of the total PAHs.

The aim of this study is to investigate the effect of the intermediate frequency (1-10 kHz) of the sinusoidal driving voltage on the characteristics of a developed surface dielectric barrier discharge (SDBD)-based reactor having spikes on its discharge electrode. Moreover, its influence on the production of ozone and nitrogen oxide byproducts is evaluated. The results show that SDBD is operated in the filamentary mode at all the frequencies. Nevertheless, the pulses of the discharge current at high frequencies are much denser and have higher amplitudes than those at low frequencies. The analysis of the power consumed in the reactor shows that a small portion of the input power is dissipated in the dielectric material of SDBD source, whereas the major part of the power is consumed in the plasma discharge. The results of the ozone production show that higher frequencies have a slightly adverse effect on the ozone production at relatively high energy density values, where the ozone concentration is slightly decreased when the frequency is increased at the same energy density. The temperature of the discharge channels and gas is not a crucial factor for the decomposition of ozone in this reactor, while the results of the measurements of nitrogen oxides characteristics 2 indicate that the formation of NO and NO 2 has a significant adverse effect on the production efficiency of ozone due to their oxidation to another nitrogen oxides and their catalytic effect.