Sang-Gu Lee’s research while affiliated with Yonsei University and other places

Airborne particulate matter is an important factor in the quality of an indoor environment. In this study, a miniaturized particle sensor was developed to detect submicron-sized aerosols based on number counting. This particle number (PN) sensor was designed and fabricated for real-time measurement of total aerosol number concentration and geometric mean diameter. The sensor (hereafter called as PN1sensor) comprised a particle-classification unit, a particle-charging unit, and a particle-detection unit. After integrating all the three units, the total number concentration and the geometric mean diameter of test aerosol particles were determined and the results were compared with those obtained using commercial instruments. First, the PN1sensor was compared with a condensation particle counter (CPC) in lab-test. For this, different groups of monodisperse sodium chloride particles between 20 and 700 nm in diameter were used. Then the PN1sensor was compared with a scanning mobility particle sizer (SMPS) and an aerodynamic particle sizer (APS). For this, the PN1sensor data were obtained by varying the combination of two corona voltage and test particles (sodium chloride and polystyrene latex) size distribution. In addition to lab-test, field test was carried out with indoor aerosols in different places. The number concentration and geometric mean diameter of indoor aerosols were measured by PN1sensor and compared with SMPS data. The number concentration was also compared with the results of CPC and Pegasor AQ Indoor (Pegasor, Finland) measurements.

For rapid and effective detection of airborne microorganisms, it is preferable to remove dust particles during the air sampling process because they can reduce the detection accuracy of measurements. In this study, a methodology of real-time separation of aerosolized Staphylococcus epidermidis (S. epidermidis) and polystyrene latex (PSL) particles of similar size was investigated. These two species represent biological and non-biological particles, respectively. Due to their different relative permittivities, they grasp different numbers of air ions under corona discharge. After these charged particles enter a mobility analyzer with airflow, in which an electric field is applied perpendicular to the airflow, the S. epidermidis and PSL particles separate, due to the difference in their electric mobilities, and exit through two different outlets. Purities and recoveries for S. epidermidis and PSL at their respective outlets were determined with measurements of aerosol number concentrations and ATP bioluminescence intensities at the inlet and two outlets. The results were that purities for PSL and S. epidermidis were 70% and 80%, respectively. This methodology provides a rapid and simple way to increase the detection accuracy of bacterial agents in air.

The effect of corona discharge-generated air ions on the filtration of aerosolized bacteriophage MS2 was studied. A carbon-fiber ionizer was installed upstream of a medium-efficiency air filter to generate air ions, which were used to charge the virus aerosols and increase their filtration efficiency. After the virus aerosols were captured by the filter for a certain time interval, they were exposed to a newly incoming air ion flow. Captured virus particles were detached from the filter by sonication, and their antiviral efficiency due to air ions was calculated by counting the plaque-forming units. The antiviral efficiency increased with ion exposure time and ion concentration. When the concentration of positive air ions was 10⁷ ions/cm³, the antiviral efficiencies were 46.1, 78.8, and 83.7% with exposure times of 15, 30, and 45 min, respectively. When the ionizer was operated in a bipolar mode, the number concentrations of positive and negative ions were 6.6×10⁶ and 3.4×10⁶ ions/cm³, respectively, and the antiviral efficiencies were 64.3, 89.1, and 97.4% with exposure times of 15, 30, and 45 min, respectively. As a quantitative parameter for the performance evaluation of air ions, the susceptibility constant of bacteriophage MS2 to positive, negative, bipolar air ions was calculated as 5.5×10⁻³, 5.4×10⁻³ and 9.5×10⁻³, respectively. These susceptibility constants showed bipolar ion treatment was more effective about 1.7 times than unipolar ion treatment.

The most efficient and widely used technique for monitoring aerosol particles is essentially an electrical method. For the development of any miniaturized aerosol classifier based on electrical techniques the miniaturized aerosol charger must provide a sufficient and stable charging efficiency. We designed and fabricated a ZnO nanowire charger (4 cm length × 2 cm width × 1 cm height) and then carried out an aerosol particle charging performance test. To test the electrical characteristics of this charger, corona currents were measured according to various applied voltages to determine the maximum stable ion number concentration. The average particle charge and wall loss of the charger were evaluated with monodispersed NaCl aerosol particles with diameters of 15–80 nm. The particle charge and wall loss were also obtained utilizing FLUENT (version 6.3), a commercial computational fluid dynamics (CFD) software, with an external user-defined function (UDF) code, by solving equations for the electric field, flow field, and particle trajectories. The measured data for particle loss and particle charge were in good agreement with the results calculated by FLUENT.

While heating, ventilating, and air-conditioning (HVAC) systems can provide healthy and comfortable indoor environments, virtually any part of an HVAC system can support active microbial growth if sufficient nutrients are present. In this study, we introduced a methodology to enhance the one-pass antibacterial performance of air ions against aerosolized bacteria in a ventilation duct flow. Staphylococcus epidermidis (S. epidermidis) was aerosolized, mixed with the duct flow, and exposed to air ions generated by carbon fiber ionizers installed inside the duct. The S. epidermidis was then sampled at the exit of the duct and incubated to evaluate their cultivability as functions of the ion exposure time, ion concentration, and ion polarity. When the ionizers produced bipolar air ions for 2 s, a high antibacterial efficiency of 85% was obtained when four ionizers were positioned both at the top and bottom walls of the duct in a configuration in which there were three changes in ion polarity (one positive ionizer, one negative ionizer, one positive ionizer, and one negative ionizer in series along the flow direction). When the ion exposure time was decreased to 0.2 s, an antibacterial efficiency of 50% was realized by applying a configuration with seven changes in ion polarity. By using a scanning electron microscope (SEM), cell contraction of S.epidermidis caused by the bipolar ion treatment was observed.

We theoretically investigated the momentum transfer and energy conversion process of ion-neutral and ensuing neutral-neutral collisions in the ion drift region of electrohydrodynamic flow. Our results are presented in explicit equations with physical interpretations of the phenomena. The unit conversion process was estimated to sustain for 1.0 nano-second in a very tiny 0.5-μm-sized volume in the air. Also, the continuum-based equation formulations are presented according to the microscopic energy conversion phenomena. Numerical simulations reflecting those formulations are performed to verify the theoretical results and experimentally supported by an air corona discharge.

Characterization of particulate matter (PM) emitted from diesel vehicle exhaust requires a real-time measurement sensor to record particle concentrations under transient tests. Recently, a micro-machined unipolar charger (MUC) based on a micro-electromechanical system (MEMS) was introduced and evaluated to test aerosol particles on a laboratory scale. We present the performance characteristics of the MUC for its potential use as a sensor for diesel PM emissions. A correlation equation was derived from particle loss experiments and tandem differential mobility analyzer (TDMA) measurements in the laboratory, which was used to convert the current measurement datum into a total particle number concentration. Under various idling and driving conditions of a diesel vehicle, the electrical signals from the MUC were verified to have followed the trend of the total number concentrations of diesel PM measured using a condensation particle counter (CPC). When the diesel PM concentrations measured using the CPC were within the range of 2×104–2×105#/cm3, the total number concentrations, estimated using a correlation equation, were in agreement with the CPC data.

For this study, a 4 stage electrical low pressure impactor was designed to measure the real‐time size distribution of diesel particulate matter (DPM). For the performance evaluation, sodium chloride (NaCl) particles and dioctyl sebacate (DOS) particles were used. After evaluating the collection efficiency of each stage of the impactor, the size distributions of test particles were estimated using electrical current data and their inversion algorithm, and this was found to agree with the results obtained by a scanning mobility particle sizer (SMPS). For measurement of DPM, a common‐rail direct injection (CRDI) diesel engine, for engine speeds of 1,200 rpm and 1,500 rpm at 2.7 kgf·m, was used. Therefore, it was found that the size distribution of the DPM could be easily obtained, with the currents measured by the impactor and the data inversion algorithm, in less than 5 seconds. Furthermore, the effective density of the DPM could be obtained using the calculated results and the SMPS data.