Determination of submicron aerosols effective density using two-stage low-pressure impactor and aerosol electrometer based on pre-measured particle size distribution.

A novel methodology was presented for determining the representative effective density of aerosols of a given size distribution, using a lab-made two-stage low-pressure impactor and an aerosol electrometer. Electrical currents upstream (Imeasured, up) and downstream (Imeasured, down) of the 2nd stage of the impactor were measured using a corona charger and the aerosol electrometer. In addition, the electrical currents upstream (Icalculated, up) and downstream (Icalculated, down) of the 2nd stage of the impactor were calculated using the aerosol charging theory. Then, the difference between the ratio of Imeasured,down to Imeasured,up and the ratio of Icalculated,down to Icalculated,up was iterated with varying the presumed effective density until the difference was smaller than 0.001. The methodology was validated using poly-disperse sodium chloride (NaCl) particles. The effective densities of ambient aerosols were then obtained from indoor and outdoor environments and compared with those calculated from a relation between mobility (scanning mobility particle sizer (SMPS) measurement) and aerodynamic (electrical low-pressure impactor (ELPI) measurement) diameters. Compared to the effective densities obtained with SMPS and ELPI measurements, the effective densities obtained using the methodology introduced in this paper differed within 10 % deviation, depending on measurement location. After an averaged effective density for given size distribution is obtained at a measurement location, the number-based size distribution can be easily converted to mass-based size distribution using the representative effective density.

MEMS-based condensation particle growth chip for optically measuring the airborne nanoparticle concentration.

To monitor airborne nanoparticles at a particular point of interest sensitively and accurately, we developed a compact and inexpensive but highly-precise nanoparticle detection system. The proposed system, based on nucleation light-scattering, consists of two components: a microelectromechanical system (MEMS)-based particle growth chip that grows nanoparticles to micro-sized droplets through condensation and a miniaturized optical particle counter (mini-OPC) that detects individual grown droplets using a light-scattering method. To minimize the dimensions and cost of this system, all elements of the particle growth chip were integrated onto a glass slide through simple photolithography and 3D printing. Moreover, a passive cooling technique was adopted, which eliminated the need for an active cooling system. Thus, our system was much more compact, inexpensive, and power-efficient than conventional nanoparticle detection instruments. Through quantitative experiments using Ag nanoparticles in the size range of 5 to 70 nm, it was found that our system could count extremely small nanoparticles (12.4 nm) by growing them to micrometer-sized droplets. Furthermore, our system could provide an accurate number concentration of nanoparticles (the maximum difference was within 15% compared to the reference instrument), regardless of high (3500 N cm-3) and low (0.05 N cm-3) concentration environments. These results indicate that our system can be applied successfully to the monitoring of nanoparticles in various kinds of fields including not only indoor and outdoor environments but also high-tech industries utilizing cleanrooms, air filtration systems, etc.

Particle size spectrometer using inertial classification and electrical measurement techniques for real-time monitoring of particle size distribution.

To achieve real-time monitoring of aerodynamic submicron particle size distributions at a point-of-interest, we developed a high-performance particle size spectrometer that is compact, low-cost, and portable. The present system consists of four key components: a unipolar mini-discharger for electrically charging particles, an inertial size-separator for classifying charged particles into five size fractions in terms of their aerodynamic sizes, a portable multi-channel electrometer for detecting femto-ampere currents carried by charged particles at each stage, and a retrieval algorithm for converting the current data into a smooth particle size distribution. The unipolar mini-discharger and inertial size separator were quantitatively characterised by using standard polystyrene latex (PSL) particles. The experimentally determined cut-off diameters at each stage in the inertial size separator were 1.17, 0.94, 0.71, 0.54, and 0.23 µm, respectively. Then, the system was compared with a commercial reference aerodynamic particle sizer (APS) in the environment where the number concentration and the average size of TiO2 particles were changing. The present system resolved peak size and geometric standard deviation of particles to within 11.2%, and 6.3%, respectively, indicating that the system can be used to accurately monitor submicron particle size distributions in real time.