Final 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 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

Objective:

The adverse effect on human health from the exposure to ultrafine particles (UFPs) has been evidenced in recent epidemiologic studies. People living in the areas with high ambient UFP concentration levels were found to have high risk of asthma and cardiovascular disease. For continuous monitoring of spatial distribution of fine and ultrafine particles in cities and residential communities in close proximity to highways, cost-effective and miniature particle sensors with built-in networking function are required. This project aims to develop cost-effective miniature electrical ultrafine particle sizers (mini-eUPSs) based on the particle electrical mobility technology and a wireless sensor network using mini-eUPSs as the nodes.

Summary/Accomplishments (Outputs/Outcomes):

To develop a mini-eUPS, three core components (i.e., mini-plate aerosol particle charger, mini-plate differential mobility analyzer [DMA] and mini-Faraday cage particle electrometer) were designed and their performances were evaluated. Proposed mini-eUPSs then were assembled and tested upon the completion of core components. The following summarizes the findings during the course of this project:

(1) Development of a new mini-plate particle charger

The performance of a new DC-corona-based, unipolar aerosol charger had been investigated in this part of the project. The design of this new aerosol charger consists of two metal blocks. The corona chamber is embedded in one block. A tungsten wire (50 µm in diameter) was used in the corona chamber to produce positive ions via the DC corona discharging. The aerosol charging chamber was designed in the other metal block. A metal screen is used to separate two chambers. Aerosol particles were electrically charged by passing through the charging zone. No ion-driving voltage/flow was used in this charger to minimize the multiple charge issue (often encountered in unipolar chargers) and to reduce the accessories required to operate this charger.

The prototype charger’s performance was optimized by varying operational parameters (i.e., corona position, aerosol flowrate, corona current) to achieve high extrinsic charging efficiency. According to the experimental data collected, corona occurring at the chamber located nearest to the aerosol outlet provides the highest extrinsic charging efficiency. The corona current of 2 µA and aerosol flowrate of 0.6 lpm also was determined as the optimal operation condition for the new charger. Both intrinsic and extrinsic charging efficiencies of particles with mobility diameters ranging from 10 to 200 nm then were measured at the determined operation condition. As expected, the intrinsic and extrinsic charging efficiencies increased with the increase of mobility particle size. The highest extrinsic charging efficiency of this charger was about 85 percent. The extrinsic charge distributions of particles with the mobility sizes less than 300 nm also were measured. A Gaussian distribution function with the particle size as the independent variable was further used to fit measured charge distributions at different particle sizes. The fitted Gaussian function was later used in the data reduction scheme to retrieve the particle size distributions from the particle mobility distribution measured by mini-eUPSs.

(2) Development of new mini-plate particle classifiers

In this part of the work, both the mini-plate electrical aerosol analyzer (EAA) and mini-plate differential mobility analyzer (DMA) were designed and their performance characterized for fine and ultrafine particle sizing. The tandem DMA (TDMA) technique was applied to evaluate the performance of mini-plate classifiers. Studied EAA/DMAs in general consist of two metal plates separated in parallel by an insulation spacer (i.e., the spacer height is the height of particle classification zone). Both polydisperse and classified aerosol flow channels were built in both metal plates to minimize the electrostatic particle loss during the transport. To keep the aerosol flow away from the spacer walls of EAA/DMA (in order to minimize the wall effect) aerosol slits with the opening length less than the full width of classification zone were used for injecting and extracting aerosol streams in and out from the classification zone, respectively. The distance between the aerosol entrance and exit slits is the length for particle classification. A high DC voltage (positive/negative) was applied to one electrode plate while the other one was electrically grounded. A uniform electrical field was established in the particle classification zone. For the safety operation, two metal plates were insulated in the plastic enclosure. Particle-free sheath gas was directed into the EAA/DMA from the sheath flow inlet. The excess flow exited the classifiers from the excess flow outlet. The overall size of the mini-plate EAA/DMA is comparable to the size of an iPhone 6.

The studied EAA/DMA was experimentally evidenced to have satisfactory performance for particle sizing. In this part of study, a new correction factor (ษณ) was proposed to modify the equations derived in the 2-D model for better calculation of the voltage required to size particles with a given electrical mobility. A piecewise linear deconvolution scheme was applied to recover the transfer function of mini-plate EAA/DMA from measured TDMA data. It was found that typical transfer functions of mini-plate DMA are in the triangular shape for less-diffusive particles (i.e., particles of large sizes). In the cases of three studied flowrates (i.e., aerosol flowrate of 0.3 lpm; sheath flowrates of 1.5, 3.0, and 6.0 lpm), the height of transfer function for the mini-plate DMA increased and the full width at half maximal height (FWHM) decreased as the particle size increased. The discrepancy between the measured FWHMs and calculated ones (by the 2-D model) also was observed in all studied flowrate cases. It is because the aerosol slits were not fully opened along the entire width of the classification zone, resulting in the partial use of total sheath flow. The above reasoning was supported by the observed fact that the increase of aerosol slit opening length in the mini-plate DMA yielded measured FWHM values closer to those calculated by the 2-D model.

(3) Development of a new mini-Faraday cage

To measure the particle size distribution by the DMA/EAA technique, the classifiers must be coupled with a particle concentration detector. In this part of the study, a new miniature Faraday cage with a sensitive electrometer was designed and evaluated. The performance of the mini-Faraday cage operated at both 0.3 and 1.5 lpm aerosol flowrates was evaluated using single-charged particles. Good linear relationships between measured electrical current and number concentration of test particles were found. The slopes of the linear relationship were close to the value of one elementary charge.

(4) Final assembly of mini-eUPSs

Prototype mini e-UPSs were assembled with the mini-plate aerosol particle charger, mini-plate DMA/EAA, and mini-Faraday cage with a sensitive electrometer, as well as all the accessories (i.e., two mini-pumps, Honeywell mass flow meters, two HEPA tube filters and electronic components) required to run the core components. A mini-eUPS has the final package of 6" in length, 5" in width and 4" in height and weighs about 3 lbs. A computer code also was developed in Raspberry Pi (an embedded computer) to operate the mini e-UPS (including the data acquisition, recording, analysis, and display). A constrained least-squares data reduction scheme was programmed in the code for in situ retrieval of the size distributions of particles from measured raw data. The final size distributions of measured particles could be obtained offline using a more sophisticated data reduction scheme. In addition to the measurement of ultrafine particles, mini-eUPSs also measure and record the temperature, relative humidity, pressure, timing, position, and altitude of each particle sample taken. The wireless functions (i.e., wireless data download/upload, sensor networking, and data hopping) also are built into the mini-eUPSs. A special electronic circuit board was developed to fulfil the above tasks of measurement and wireless functions. 

To reduce the interference from large particles, a multi-inlet miniature cyclone was further designed and its performance was evaluated. The mini-cyclone was able to remove particles with sizes larger than 300 nm at the flow rate of 0.6 lpm.  

Laboratory-generated aerosol particles were used to evaluate the performance of assembled mini-eUPSs. Good agreement between the size distribution data measured by mini-eUPSs and the Scanning Mobility Particle Sizer was achieved. The preliminary field testing also evidences the success of mini-eUPS development.


Journal Articles on this Report : 4 Displayed | Download in RIS Format

Other project views: All 14 publications 4 publications in selected types All 4 journal articles
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Journal Article Liu D, Hsiao T-C, Chen D-R. Performance study of a miniature quadru-inlet cyclone. Journal of Aerosol Science 2015:90:161-168. R835132 (2015)
R835132 (Final)
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  • Journal Article Liu D, Zhang Q, Jiang J, Chen D-R. Performance calibration of low-cost and portable particular matter (PM) sensors. Journal of Aerosol Science 2017;112:1-10. R835132 (Final)
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  • Journal Article Liu Q, Chen D-R. Experimental evaluation of miniature plate DMAs (mini-plate DMAs) for future ultrafine particle (UFP) sensor network. Aerosol Science and Technology 2016;50(3):297-307. R835132 (2015)
    R835132 (Final)
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  • Journal Article Liu Q, Chen D-R. Performance evaluation of a miniature plate Electrostatic Aerosol Analyzer (mini-plate EAA). Journal of Aerosol Science 2016;95:30-42. R835132 (2015)
    R835132 (Final)
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  • Supplemental Keywords:

    ultrafine particle, particle sizer, electrical mobility, miniature, cost-effective

    Progress and Final Reports:

    Original Abstract
    2013 Progress Report
    2014 Progress Report
    2015 Progress Report