Yang Wang   

Research Associate
Department of Energy, Environmental and Chemical Engineering
Washington University in St. Louis

        My research focuses in particle science and technology. Following is a list of projects I am currently or was previously involved in: 

1. Rapid Measurements of Particle Hygroscopic Growth with a Humidity-Controlled Fast Integrated Mobility Spectrometer (HFIMS)

    2017-present, Environmental and Climate Sciences Department, Brookhaven National Laboratory

    PI: Dr. Jian Wang

        Hygroscopicity is a key parameter in determining the impact of atmospheric aerosols on global radiation and climate change. Tandem differential mobility analyzer (TDMA) system is the most widely used instrument for determining the aerosol hygroscopicity. However, TDMA measurements are relatively slow because the time needed for a full scan of DMA voltage is typically in the range of minutes. The slow measurement speed becomes a significant issue when unsteady sources of aerosols are being studied, for example, direct measurement of combustion aerosols and measurement of aerosols onboard mobile platforms.  

        The recently developed water-based fast integrated mobility spectrometry (WFIMS) allows rapid mobility-based measurement of particle size distribution.  It uses a parallel plate mobility system to separate charged particles with different mobilities. Upon exiting the mobility separator, the spatially separated particles are condensationally grown in a three-stage water-based growth channel and imaged onto a CCD array. The size distribution is obtained by counting particles located in each mobility bin shown on the image, providing a near instantaneous measurement (1 Hz) of mobility size distributions. The use of water vapor enables independent control of RH in the mobility separation region, which cannot be achieved with alcohol vapor due to their lower diffusivity. A humidity-controlled FIMS (HFIMS), consisting of a DMA, a relative humidity control unit, and a WFIMS coupled in series, was tested for measuring the hygroscopic growth of particles. In this study, a data inversion algorithm is developed to derive the growth factor distribution of the DMA-classified particles. The inversion algorithm uses the known transfer functions of the upstream DMA and the WFIMS, and calculates the growth factor distribution that reproduces the position distribution of particles measured by the WFIMS. The growth factor distributions of ambient particles at various RHs are analyzed with the inversion algorithm. Further optimization of the HFIMS system will be discussed.  

2.  Sub 2 nm Particle Characterization in Systems with Aerosol Formation and Growth (Ph. D. Dissertation) 

    2014 - 2017, Aerosol and Air Quality Research Laboratory,  Washington University in St. Louis

    PI: Professor Pratim Biswas

        Aerosol science and technology enable continual advances in material synthesis and atmospheric pollutant control.  Among these advances, one important frontier is characterizing the initial stages of particle formation by real time measurement of particles below 2 nm in size.  Sub 2 nm particles play important roles by acting as seeds for particle growth, ultimately determining the final properties of the generated particles.  Tailoring nanoparticle properties requires a thorough understanding and precise control of the particle formation processes, which in turn requires characterizing nanoparticle formation from the initial stages.  The knowledge on particle formation in early stages can also be applied in quantum dot synthesis and material doping.  This dissertation pursued two approaches in investigating incipient particle characterization in systems with aerosol formation and growth: (1) using a high-resolution differential mobility analyzer (DMA) to measure the size distributions of sub 2 nm particles generated from high-temperature aerosol reactors, and (2) analyzing the physical and chemical pathways of aerosol formation during combustion. 

        Part. 1.  Particle size distributions reveal important information about particle formation dynamics.  DMAs are widely utilized to measure particle size distributions.  However, our knowledge of the initial stages of particle formation is incomplete, due to the Brownian broadening effects in conventional DMAs.  The first part of this dissertation studied the applicability of high-resolution DMAs in characterizing sub 2 nm particles generated from high-temperature aerosol reactors, including a flame aerosol reactor (FLAR) and a furnace aerosol reactor (FUAR).  Comparison against a conventional DMA (Nano DMA, Model 3085, TSI Inc.) demonstrated that the increased sheath flow rates and shortened residence time indeed greatly suppressed the diffusion broadening effect in a high-resolution DMA (half mini type).  The incipient particle size distributions were discrete, suggesting the formation of stable clusters that may be intermediate phases between initial chemical reactions and downstream particle growth.  The evolution of incipient cluster size distributions further provided information on the gaseous precursor reaction kinetics, which matched well with the data obtained through other techniques.  

        Part 2.  The size distributions and their evolution measured by the DMAs help explain the physical pathways of aerosol formation.  The chemical analysis of the incipient particles is an important counterpart to the existing characterization method.  The chemical compositions of charged species were measured online with an atmospheric pressure interface time-of-flight mass spectrometer (APi-TOF).  The tandem arrangement of the high-resolution DMA and the APi-TOF realized the simultaneous measurement of the mobility and the mass of combustion-generated natively charged particles, which enabled their chemical and physical formation pathways to be derived.  The results showed that the initial stages of particle formation were strongly influenced by chemically ionized species during combustion, and that incipient particles composed of pure oxides did not exist.  The effective densities of the incipient particles were much lower than those of bulk materials, due to their amorphous structures and different chemical compositions.  Measuring incipient particles with high-resolution DMAs is limited because a DMA classifies charged particles only, while the charging characteristics of sub 2 nm particles are not well understood.  The charge fraction of combustion-generated incipient particles was measured by coupling a charged particle remover and a diethylene-based condensation particle counter (Airmodus A10).  A high charge fraction was observed, confirming the strong interaction among chemically ionized species and formed particles.  The combustion system was modeled by using a unimodal aerosol dynamics model combined with Fuchs’ charging theory, and showed that the charging process indeed affected particle formation dynamics during combustion.  

3. Laboratory evaluation, calibration, and application of low-cost particle sensors for particulate matter measurement and sensor network deployment
    Sept 2014 - present,  in collaboration with Jiayu Li, Aerosol and Air Quality Research Laboratory, Washington University in St. Louis
    PI: Professor Pratim Biswas

        Particulate matter (PM) is a crucial factor of air quality affecting visibility, human health, and global climate.PM is quantified using PM10, PM2.5, or PM1, according to inhalation and deposition properties in the human respiratory system, representing the mass concentration of particles below 10 μm, 2.5 μm, and 1 μm in aerodynamic size, respectively. The measurement of PM indices can rely on various instruments, among which impactors, cyclones, tapered element oscillating microbalances (TEOM), and beta attenuation monitors (BAM) are commonly used. Other instruments, such as Dusttrak and SidePak (TSI Inc.) use light-scattering to obtain particle mass concentrations, while scanning mobility particle sizers (SMPS) and aerodynamic particle sizer (APS) derive particle mass concentrations from measured particle size distributions. Temporal and spatial PM index may differ from each other significantly due to the limited transport coefficient of particles. In order to obtain accurate local PM indices down to streets or blocks, a higher density of measurement sites mapping out the entire area is required, while the expense of the complicated instruments mentioned above frustrates the plan. With prototypes first assembled in smoke detectors and air purifiers decades ago, particle sensors become popular in recent years with the usage of portable PM monitors, due to the remarkably low price of around $ 10 USD and the merging need for “big data”. Although cannot generate data for enacting air pollution regulations or for health studies due to the less accuracy compared to the advanced instruments, these low-cost sensors can be used in locating pollution hotspots or generating coarse 3-D map of PM concentrations for individuals, industries, and environmental agencies. In a broad sense, the usage of the low-cost particle sensors can also raise the awareness of air quality among the society. Hence, it is worthwhile to study and evaluate the performance of the particle sensors and further provide instructions to customers and manufacturers.

        The real-time and local measurement of PM concentration can be achieved by networking the particle sensors wirelessly. We are using Arduino and XBee boards to communicate among the sensors and the computers, weaving a network reporting indoor and outdoor PM concentraitons. 

4. Aerosol measurements in Exhaust Gas at the Abbott Power Plant, IL and Gaston Steam Plant, AL
    Feb 2016 - present,  in collaboration with Zhichao Li, Aerosol and Air Quality Research Laboratory, Washington University in St. Louis
    PI: Professor Pratim Biswas    

        The number concentration and size distribution of particles in the exhaust gas of a coal-fired power plant are important parameters that influence the design of the amine-based CO2 capture unit, which is to be placed downstream of the flue gas desulfurization (FGD) scrubber and reheat burner. In this study, the size distribution, number, and mass concentration of particles in flue gas were measured at University of Illinois Abbott Power Plant and Alabama Gaston Steam Plant, through which design recommendations on the future amine-based CO2 capture unit will be established.  This study was conducted by the Aerosol and Air Quality Research Laboratory (AAQRL) led by Professor Pratim Biswas, and his two doctoral students, Zhichao Li and Yang Wang from Washington University in St. Louis. State of the art research grade instrumentation was used to map out the aerosol particle size distributions at the exit of the electrostatic precipitator (ESP). 

5. Aerosol Generation During Patient Care Activities
    May 2015 - Dec 2015,  in collaboration with Jiayu Li, Anna Leavey, Carrie ONeil, Meghan Wallace, Aerosol and Air Quality Research Laboratory, Medical School, Washington University in St. Louis
    PI: Professors Pratim Biswas, Carey-Ann Burnham, and Hilary Babcock,     

        Background:  Questions remain about the degree to which aerosols are generated during patient care activities and whether generated aerosols could transmit viable pathogens to healthcare personnel or  to other patients.  This project measured aerosol production during patient care and took samples for pathogen recovery.
        Methods: Seven patient care activities were each sampled five times: patient bathing, changing bed linens, pouring and flushing liquid waste, bronchoscopy (with and without negative pressure ventilation), non-invasive ventilation and nebulized medication administration.  Five real time aerosol characterization instruments were used for each sampling episode.  A biosampler was used for pathogen recovery.  Cultures were performed of all collection media and any growth identified.  Patients on contact precautions for drug resistant organisms were selected for routine activities.  Any patient undergoing bronchoscopy was eligible.  Baseline samples were performed when possible.
        Results:  There were 35 sampling episodes.  Only two procedures generated a significant change in particle presence over baseline: bronchoscopy with nebulized medication administration and nebulized medication administration.  Bronchoscopy without nebulized medication and non-invasive ventilation did not generate significant aerosols.  Only nebulized medication administration generated a significant increase in the total surface area concentration of particles that could deposit in the alveolar region of the lung.  Of 78 impinger samples cultured, only 18 were positive.  Of baseline samples, 6/28 (21%) and of procedure samples 12/50 (24%) were positive. No targeted organisms were recovered. The most frequently isolated was coagulase-negative staphylococcus (12), followed by micrococcus (6).
        Conclusion:  In this small pilot study, significant aerosol generation was only seen with nebulized medication administration. Minimal viable bacteria were recovered, mostly common environmental contaminants.  Limitations include small numbers, lack of clinical data, one sampling location for each procedure, and lack of viral pathogen recovery.

6. Building of a Tandem Differential Mobility Analyzer System for Particle Volatility and Hygroscopicity Measurement 
    Jan 2013 - Sept 2013, in collaboration with Christopher Oxford, Atmospheric Chemistry and Technology Lab, Washington University in St. Louis
    PI: Professor Brent Williams

        Tandem Differential Mobility Analyzer (TDMA) systems are utilized to investigate the size change of sub-micron particles under certain environments, including heating, humidifying, and reaction conditions. In a TDMA, monodisperse particles generated by the first DMA is introduced into the environment for investigation. Then the size of the grown or shrinked particles is further measured with the second DMA. By determining the extent of growth or shrinkage, the effect of environment on particle shape could be evaluated. The study on TDMA system is mainly focused on measurement of hygroscopicity and volatility of sub-micron particles. The existence of sub-micron particles can greatly influence global climate forcing due to their optical properties. On the other hand, the formation of these particles is largely dependent on the humidity in atmosphere, since particle growth is mainly a result of condensation of water vapor on newly formed nanometer range particles. Therefore, by knowing how size of particles with different chemical composition will react in environment with certain humidity, the growth or shrinkage of particles can be determined. Further evaluation of optical property and climate forcing can be estimated and modeled. A Labview program was designed to obtain particle size distribution information in three modes, including SMPS mode, heating mode for volatility study, and humidifying mode for hygroscopicity study. 

7. Premixed Stagnation Swirl Flame Synthesis of nanostructured TiO2 films for Dye Sensitized Solar Cells 
    Sept 2010 - May 2012, Particle and Combustion Engineering Laboratory, Tsinghua University, China
    PI: Professor Shuiqing Li and Qiang Yao

        A uniform, high-quality nanostructured TiO2 film used in Dye Sensitized Solar Cells (DSSCs) is synthesized by a novel premixed stagnation swirl flame synthesis system, followed by a short-time sintering and densification process. By simply tuning synthesizing and annealing parameters, we could prepare films with different morphology and characteristics, i.e. particle size, surface area and packing density. The whole process for film preparation costs around 1 hour in time, which is significantly less than the time needed (~12 hours) using traditional sol-gel method. The cell reaches an efficiency of 5.7% under AM 1.5G incident irradiance of 22mW/cm2 without using a back scattering layer or an anti-reflective layer. The role of TiO2 film annealing time in cell performance is investigated. It was found that there exists an optimum value of annealing time. As annealing time increases, cell efficiency increases at first which is caused by a decrease of crystal deficiency; then, cell efficiency drops down because annealing decreases film surface area, which results in a decrease of dye loading. In our research, we also found that the procedure of densification determines the performance of DSSCs in shrinking film thickness and adding electron path ways.