2014 Progress Report: Development of Cost-effective, Compact Electrical Ultrafine Particle (eUFP) Sizers and Wireless eUFP Sensor Network

EPA Grant Number: R835132
Title: Development of Cost-effective, Compact Electrical Ultrafine Particle (eUFP) Sizers and Wireless eUFP Sensor Network
Investigators: Chen, Da-Ren , Lu, Chenyang
Institution: Washington University , Virginia Commonwealth University
EPA Project Officer: Chung, Serena
Project Period: September 1, 2012 through August 31, 2015 (Extended to August 31, 2016)
Project Period Covered by this Report: September 1, 2013 through August 31,2014
Project Amount: $499,130
RFA: Developing the Next Generation of Air Quality Measurement Technology (2011) RFA Text |  Recipients Lists
Research Category: Air Quality and Air Toxics , Air


Specific objectives of this EPA project are to: (1) develop a cost-effective, portable electrical ultrafine particle (eUFP) sizer, enabling the spatial and temporal monitoring of UFP size distribution in the ambient; (2) develop a cost-effective, pocket eUFP sizer for measuring the UFP exposure at the personal level; (3) develop a wireless mesh network using proposed portable sizers as the nodes, enabling to monitor the working status of deployed sizers and to acquire data being collected via internet; and (4) evaluate and validate proposed technologies in planned field testing. 

The primary objectives in Year 2 of this project is to continue developing the core components for miniature electrical Ultrafine Particle Sizers (mini- eUFP sizers) and to assemble the first prototype mini- eUFP sizer. The sizer’s core components to be developed in Year 2 are mini-plate aerosol charger and mini-plate differential mobility classifier (mini- plate DMC).


Progress Summary:

A mini-plate aerosol charger and differential mobility classifier (DMC), key components of a mini-eUFP sizer, have been developed in Year 2 of this project. Detailed experiments had been carried out to evaluate the performance of a mini-plate aerosol charger and DMC. The calibration results show that both components provide satisfactory performance for a mini-eUFP sizer. The semi-empirical models have also been developed to mathematically predict the performance of the above two components. Further, the first mini-eUFP sizer has been assembled and past the preliminary testing. The full calibration of mini- eUFP sizer is ongoing. In addition to the measurement of ultrafine particle size distribution, the temperature and relative humidity of the aerosol stream are monitored in this fisrt-assembled mini-sizer. 

In Year 2 we had successfully developed a mini-plate differential mobility classifier (mini-plate EAC) and mini-plate aerosol charger. Two prototype mini-plate DMCs were constructed. The performance of mini-plate DMCs had been experimentally evaluated. The experimental evaluation of mini-plate DMC evidences its superior performance, following the DMA performance theory. The mini-plate aerosol charger has also been constructed and its performance has been experimentally evaluated. The evaluation of mini-aerosol charger shows that it offers satisfactory performance for electrically charging ultrafine aerosol. With the developed key components the first mini-eUFP sizer has been assembled in Year 2. The associated computer software required to operate the sizer has also been developed. In addition to the measurement of ultrafine particles, the assembled prototype also measures the temperature, pressure and relative humidity of aerosol stream. The calibration of the first prototype is currently ongoing. Finally, the sizer’s interface and circuit board for proposed wireless sensor network is under the development (at the 80% completion level).
Ultrafine particles (UFPs) are defined as ones with diameters smaller than 0.1 µm. They either derive directly from combustion processes and vehicle emission or are formed when compounds with lower vapor pressures spontaneously nucleate/condense on other small particles (Kulmala et al., 2004). The concerns regarding the environmental impact, toxicity and health effect of UFPs are emerging these days. Nanoparticles with high surface-to-volume ratios often have high bio-availability and toxicity (Nel et al., 2006; Heinlaan et al., 2008; Park et al., 2011). Recent studies have been reported that nanomaterials with the particle sizes between 10 to 50 nm could easily enter the human body and deposit in the alveolar region of a human lung, even entering in the blood stream and being transported to vital organs (Kreyling et al., 2002; Takenaka et al., 2001; Oberdörster et al., 2004; Paur et al., 2011). Epidemiologic research also found UFPs were particularly relevant to pulmonary diseases, cancer and mortality (Hoek et al, 2002; Peters et al. 2004; Delfino et al, 2005; Oberdörster et al, 2005; Bräuner et al, 2007; Shah et al, 2008; Li et al, 2010; Stewart et al, 2010). In order to better investigate UFPs, better instrumentation and measurement techniques for UFPs are thus in need.
Instruments and techniques for aerosol characterization have been developed and applied for decades. All of them are designed for scientific studies and laboratory usage, with high resolution and sensitivity. However, they are expensive, heavy and large in size. Because of the tempo-spatial distribution of UFPs in the ambient, the demand for cost-effective aerosol sensors in compact packages for the portability and ease of use is increasing. In this study, we advanced our current techniques for the UFP monitoring via the development of a cost-effective and compact electrical mobility particle sizer (mini-eUFP sizer), which included a mini-plate aerosol charger, a mini-plate differential mobility classifier (DMC) and a mini-Faraday cage aerosol electrometer. The following summarizes the development made in Year 2 of this project:

1. Development of New Mini-Plate Aerosol Charger

The schematic diagram of the prototype DC-corona-based mini-plate aerosol charger is shown in Figure 1. The construction of the prototype consists of two brass blocks. Single tubing hard pressed into the upper block is used as aerosol inlet. An aerosol charging channel, two corona discharging modules and an aerosol outlet are designed in the button block. Two tungsten wires of 50 µm in diameter (Alfa Aesar, A Johnson Matthey Company, MA, USA) welded to four metal pins are parallel installed inside the corona discharge zone of the prototype to provide positive/negative ions for particle charging, when positive/negative high voltage is applied. According to the combination of wire to be used and polarity of voltage to be applied, four different conditions can be set up in the prototype: (1) one wire closed to aerosol inlet operated at positive/negative high voltage; (2) one wire closed to aerosol outlet operated at positive high/negative voltage; (3) 2 wires operated at high positive/negative voltage; and (4) one wire operated at positive high voltage and the other wire operated at negative high voltage. A perforate plate above with about 30 percent opening is grounded, the same as the entire body of the charger. When the electrical field strength at the surface of the wires is raised to a sufficiently high level, surrounding air undergoes electrical breakdown process, resulting in a production of a large amount of ions. Ions produced in corona zone diffuse through the metal screen into the charging zone with a spacing of 0.125". For simplicity, no sheath air is used in this prototype. Particles in the charging zone are electrically charged by random collision with positive/negative ions. The amount of ion captured on the particle surface will be evaluated to determine particle charging efficiency and charge distribution of particles, which are of importance in the DMA operation and calculation of particle size distribution.
The experimental setup in Figure 2 was used to characterize the performance of the prototype mini-plate charger, including measuring the charging efficiency and particle charge distribution. Both intrinsic and extrinsic charging efficiencies were measured. Two different aerosol generation techniques (atomization method and evaporation-and-condensation method) were used to produce test aerosols. At the downstream of the aerosol generation system, a differential mobility analyzer (TSI model 3085) and TSI 3080 classifier was used to classify monodisperse particles with sizes from 10 nm to 200 nm. A set of Po210 radioactive particle neutralizer and an electrostatic precipitator was used to obtain electrically neutral particles for evaluating mini plate charger. To measure the charging efficiencies of particles exiting the prototype charger, a second electrostatic precipitator and an ultrafine condensation particle counter (UCPC, TSI model 3776) were used. To measure particle charge distributions, a tandem DMA (TDMA) technique was used.
As discussed the prototype charger can be operated at four different configurations. The optimization of operational settings is required to maximize the particle charging performance, especially to maximize the extrinsic charging efficiency. The DMA-classified NaCl particles with the size of 35 nm were tested. By measuring the charging efficiencies of 4 configurations in different the aerosol flow rate and corona current conditions, the optimal operational setting of studied prototype charger was finally determined as one wire used at 2.7 µA corona current and aerosol flow rate of 0.3 lpm. Figure 3 shows the intrinsic positive charging efficiencies and extrinsic positive charging efficiencies for particles from 20nm to 60nm when operating the mini plate charger at this optimal configuration. The charging efficiencies go increasing when particle size becomes larger. The extrinsic charging efficiency of 20 nm particles is about 48 percent, evidencing a good charging performance of studied charger. For 60 nm particles, the intrinsic charging efficiency is 98.52 percent, closed to 100 percent. The ratio of extrinsic charging efficiency to intrinsic charging efficiency increases with increase of particle size, from 67 percen (for 20 nm particles) to 78 percen (for 60 nm particles), indicating that smaller particles are more likely lost inside the charger.

2. Development of New Prototype Mini-Plate Differential Mobility Classifier (DMC)

To measure ultrafine particles based on electrical mobility, a new mini-plate DMA is designed. Figure 4 is the schematic diagram of the prototype mini-plate DMA. As shown in the Figure 4, the overall size of the mini-plate DMA, with a length of 3.875", a width of 1.75" and a height of 0.875", is comparable to that of an iPhone 5S. Two metal plates will be installed in parallel inside the device with a spacing of 0.125", to establish an aerosol classification zone with a length of 1 15/32" and a width of 1". Particle free sheath gas is going to be directed into the DMA through the right tubing, passing the aerosol classification zone and finally exiting from the left tubing. Two identical flow laminarizers will be installed right after and before the sheath flow channels, respectively, to guarantee the laminar flow status inside the classification zone. Both of the polydisperse aerosol flow channel and monodisperse aerosol flow channel are designed inside two metal plates to eliminate charged particle loss due to the space charging effect. The two aerosol slits on the plates will have the same width of 1/2", for protecting aerosol stream away from the walls and eliminating aerosol loss due to the diffusion effect and flow expansion effect during classification. A high DC voltage (positive/negative) will be applied on to one electrode plate while the other one will be grounded, so that a uniform electrical field will be built up for particle classification. In the classification zone, charged particles will be separated according to their electrical mobilities. Only particles with the appropriate charge and size will travel to the grounded electrode and exit from the sample air outlet as monodisperse aerosol. By varying the voltage applied on the DMA and measuring the concentration of classified aerosol, the electrical mobility distribution of sampled particles can be directly obtained. The maximum particle size is design to be reached at 220 nm when operating aerosol flow at 0.1 lpm and sheath flow at 1 lpm.
The performance of mini-plate DMA was evaluated carefully in this study, by measuring the transmission probabilities of particles with a specific electrical mobility as a function of applied voltage at different operational conditions. The same aerosol generation system as that used in mini-plate charger evaluation was set up in this study (Figure 5). After the aerosol generation system, a TDMA system for evaluating the transfer functions described by Hummes et al. (1996) was used to study the mini-plate DMA performance. The TDMA scans were obtained by fixing the voltage on the first DMA (TSI 3081or TSI 3085) and varying the voltage on the second DMA (i.e., mini-plate DMC). A piecewise-linear function deconvolution scheme (Li et al., 2006), in which the true transfer function consists of N linear subsections, was used to recover the true transfer function of the mini-plate DMC.
The sizing accuracy of the prototype mini-plate DMA was evaluated by comparing the measured and calculated central voltages for different particles. As shown in Figure 6, the ratios of measured voltage at the peak particle concentration to the calculated central voltage are all close to 1.0 (within the 5% deviation range), indicating the mini-plate DMA operated as designed. The deviations may be the result of the errors from flow rate controlling and 3D effect (expansion of aerosol stream).
By best fitting the calculated TDMA curve with the experimental one, a typical transfer function for 30 nm particles is shown in Figure 7 (a), for mini-plate DMA operated at aerosol and sheath flow rates of 0.3 and 3 lpm, respectively. The good agreement between the predicted and experimental TDMA curves is evident in Figure 7(b). The shape of the transfer function is nearly triangular. The maximum transmission probability of the function is 0.92 and the half width of the transfer function is 0.19.
Figure 8 shows the comparison of (a) the height and (b) the half width of the de-convoluted transfer functions for the case of 0.3 lpm aerosol and 3.0 lpm sheath flow rates, as a function of particle diameter. Due to the particle Brownian diffusion, the height of the transfer function was reduced and the width was increased as the particle size decreased. For particles larger than 45nm, the heights all reached the ideal value of 1.0. In general, the sizing resolution of a DMC for a given particle size is presented by the half width of the experimental DMA transfer function. As expected, the resolution of mini-plate DMC varies among different particle sizes. The ideal resolution of the studied DMC should be further estimated by modeling work when carefully considering 3D effect inside the device.


3. Assemble of mini eUFP sizer and Its Software Development

In addition to the development of the mini-plate aerosol charger and mini-plate DMA, a mini-Faraday cage (developed by Huang and Chen, 2010) combined with a sensitive electrometer was used to measure the ultrafine particle concentration in the prototype mini-eUFP sizer. The mini-Faraday cage has been carefully calibrated prior to being assembled into the mini-eUPS.
Figure 9 is the schematic diagram of the assembled mini- eUFP sizer. The inlet of the mini-eUFP sizer was made of a small Tee. Aerosol flow enters in mini- eUFP sizer through the center tube of the tee. The two channels on both sides of tee were used to install both temperature and humidity sensors. After measuring the temperature and humidity, the aerosol flow was directed into the mini-plate aerosol charger for particle charging, mini-plate DMA for sizing and finally the mini-Faraday cage electrometer for measuring electrical charges carried by DMC-classified particles. Two micro air pumps (Sensidyne micro air pump, model AA, 3A) were used for pumping in aerosol flow and recycling the sheath air for DMC operation, respectively. Two small HEPA tube filters were installed in the upstream and downstream of sheath air pump for protecting pumps from particle contamination. The aerosol and sheath flow rates were monitored by two miniature flow meters (Honeywell microbridge mass airflow sensor, AWM 3000 series). For mini-plate charger operation, two mini high voltage power modules (positive and negative, EMCO Q50, Q50N) were set at fixed voltages to conduct corona discharge. Another regulated high voltage power supply (EMCO C50) for the DMC operation was controlled by the developed software. The circuit board for controlling and providing the power for various components were also developed for mini- eUFP sizer.
A user-friendly software enabling the unattended operation, data storage and processing were further developed based on the Visual Basic language. Via the Data Acquisition card (DAQ, Advantech USB-4702) the software is able to control and communicate with the hardware. Furthermore, the function and interface for the data transfer among sensors or between the sensor and computer has also been under development (by co-PI of this project).
Figure 10 shows the pictures of assembled mini- eUFP sizer, including the aerosol inlet, mini-plate charger, mini-plate DMC, mini-Faraday cage electrometer, micro pumps, flow meters, filters and the circuits. The size of the first sizer is of 5" (L)  5" (W)  4" (H). The total weight of the prototype is slightly less than 1.0 kg. The only connections to the sizer are an AC to DC adapter and a USB cable for signal transmission between the sizer and the computer. Further reduction in the package size and weight of mini- eUFP sizer is expected in its final version.

Future Activities:

The primary activities for Year 3 of this project are to: (1) complete the calibration of the newly assembled mini-eUFP sizer; (2) equip the mini-eUFP sizers with the GPS and wireless functions; (3) assemble six mini-eUFP sizers for proposed field testing; and (4) test the wireless network function of mini- eUFP sizers. The calibration of mini-eUFP sizer will involve the comparison of particle size distributions measured by both mini-eUFP sizer and TSI SMPS. A Collison atomizer to produce polydisperse particles with the mean diameters in the nanometer size range, aerosol dryer and neutralizer and flow controllers will be used in the setup for the calibration. The interface and circuit board for wireless network function are currently under the development (by co-PI) and are expected to complete in the first half of Year 3. The inclusion of GPS and wireless network functions into the mini-eUFP sizers will be carried out during the assembly of six prototype sizers. The last 4 months of Year 1 will be focused on the reliability testing of the sensor network using six mini-eUFP sizers as the nodes. The last task will be performed with the collaboration of the co-PI.


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Journal Articles:

No journal articles submitted with this report: View all 14 publications for this project

Supplemental Keywords:

Mini-plate, Differential mobility classifier (DMC), aerosol charger, mini-eUFP sizer, mini-UPS 

Progress and Final Reports:

Original Abstract
2013 Progress Report
2015 Progress Report
Final Report