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Airborne particles detection and sizing at single particle level by a novel electrical current pulse sensor
Measurement 92 (2016) 58–62
Contents lists available at ScienceDirect
Measurement journal homepage: www.elsevier.com/locate/measurement
Airborne particles detection and sizing at single particle level by a novel electrical current pulse sensor Hang Zhou a, Yongxin Song a, Xinxiang Pan a, Dongqing Li b,⇑ a b
Department of Marine Engineering, Dalian Maritime University, Dalian 116026, China Department of Mechanical and Mechatronics Engineering, University of Waterloo, Waterloo, ON N2L 3G1, Canada
a r t i c l e
i n f o
Article history: Received 10 March 2016 Received in revised form 30 May 2016 Accepted 7 June 2016 Available online 8 June 2016 Keywords: Airborne particle Detecting and sizing Current pulse
a b s t r a c t A novel method for detecting and sizing airborne particles at single particle level is presented in this paper. When a particle hits a metal electrode which is grounded, electrostatic charges will be transferred between the particle and the electrode. As a result, an electrical current pulse will be generated in the measurement system. The number of the signal pulse represents the number of particles in the sample. To determine the effect of the particle size on the magnitude of the signal, the correlation between the magnitude of the signal and the size of particle was experimentally investigated. The results show that the magnitude of the measured signal is linearly proportional to the square of particle’s diameter. Such a correlation can be used to evaluate particle size from the measured signal. The airborne particle detection method presented in this paper can be used for counting and sizing airborne particles at single particle level. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction With the increasing awareness of the negative impacts of particulate matters (PM) on people’s health [1–3], quantitative evaluation of the size distribution and number concentration of airborne particles is critical to environmental health assessment [4,5]. Therefore, it is greatly needed for developing technologies of detecting and sizing airborne particles. Generally, the current PM detection methods and the commercially available instruments can be categorized into two types: mass concentration evaluation and size distribution evaluation. The classic gravimetric filter, based on measuring the weight change before and after sample passing through the filter, is the most widely used method to obtain particle mass concentrations [6]. While it is simple in construction and easy in operation, it cannot perform real-time measurement. Beta gauges, based on the b penetration theory [7], have been used for a long time for measuring the mass concentration of the PM sample. Once the attenuation of b particles is captured, the mass concentration of the sample is evaluated [8,9]. Using an accurate balance, such as the Tapered Element Oscillating Microbalance (TEOM), is another method for measuring the mass of particulate matters collected on a filter
⇑ Corresponding author. E-mail address: [email protected] (D. Li). http://dx.doi.org/10.1016/j.measurement.2016.06.011 0263-2241/Ó 2016 Elsevier Ltd. All rights reserved.
cartridge. While this method can continually measure the sample [10], it is susceptible to humidity of the sample air. It should be further noted that the above methods can only evaluate the mass of PM in air samples. To obtain other information, such as the number and size of PM which are more important parameters [11], other methods such as optical or electrical methods need to be employed. The details about the technologies for particle size measurement can be found in the reference paper [12]. Optical particle counters can determine the number and size of aerosol particles and have gained wide applications due to its easy operation and relatively low cost [13–15]. It was claimed that some commercial optical sensors are able to detect and count PM of size down to 0.5 lm and some manufacturers even down to 0.3 lm (NIDS Sensor Technologies, 2015). The sensitivity of optical particle counters can be improved by using condensing vapors which can magnify particle size [16,17]. For light scattering method, however, the size measurement can be interfered by the color, refractive index and composition of the particle [18–20]. Furthermore, it cannot provide real-time size distribution measurement [21] and the commercial instruments are very expensive and large in size. Normally, electrical methods are capable of detecting smaller particles. Electrical low pressure impactor [10,22–24] was reported to measure the size distribution of PM. This method works by charging the sample with corona discharge and the amount of
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charges is proportional to the surface area of a particle. Knowing the detected electrical current signals, the size distributions can be determined. However, this method involves complicated measurement systems which greatly limit its applications. Electrical mobility method is capable of measuring submicron and even nano-particles [25–29]. In this method, however, particle charging is usually achieved by applying 1–5 kV DC voltage. Also, it can only work well for particles with good mobility and the configuration of the commercial analyzer is very complicate. Sizing submicron particles by aerodynamic focusing and using a Faraday cup was recently reported [30]. The particle sample was first filtered to a certain size by aerodynamic focusing. After charging the particles with corona, the total current generated by the particles entering the Faraday cup was measured. In this way, the total number of the particles with a certain size is evaluated. As in the case for electrical mobility method, corona generation device is needed which will damage some types of particles. As can be concluded from the above literature review, while the current technologies can size particles in a relative high resolution, most these methods are based on mass average or average of a large particle population. It is still challenge in detecting and sizing particles at single particle level. The increasing demand for particle monitoring with high spatial and temporal resolution requires developing new and low cost PM sensing technology to enhance our ability for monitoring air quality. This paper reports a novel method for detecting and sizing airborne particles at single particle level. By using air flow to force a particle hitting a metal electrode which is grounded, electrostatic charges will be transferred between the particle and the electrode. As a result, an electrical current pulse will be generated and detected by a measurement system. The number of the measured electrical pulses represents the number of particles. The effect of the particle size on the magnitude of the signal was experimentally investigated. A correlation between the particle size and the measured current signal was obtained and can be used to evaluate particle size from the measured signal.
2. Experimental 2.1. Detection principle and the system setup Fig. 1 shows the working principle (Fig. 1(a)) and the experimental system (Fig. 1(b)) of the electrical current pulse method for airborne particle detection. The working principle can be understood as bellows: As shown in Fig. 1(a), either the electrode or the particle in air will have electrostatic charges [31]. The polarity and amount of surface charges are determined by the types of the electrode, the particle and the environment. For example, mean charge concentration values of 478 ± 58 ions cm3 and 1664 ± 998 ions cm3 were reported for particles in typical outdoor air and in highly charged air, respectively [31]. When a charged particle hits the electrode which is grounded, electrostatic charges will be transferred between the particle and the electrode. As a result, an electrical current pulse will be generated. The number of the electrical current pulses represents the number of particles hitting the electrode. Because the amount of charge transfer and hence the electrical current are proportional to the amount of surface charge of the particle which in turn is proportional to particle’s size, the magnitude of the electrical current pulse signals is proportional to the particle’s size. In this way, both the number and sizes of the particles can be evaluated. Fig. 1(b) shows the experimental system setup. The detection system consists of a sensing electrode, an electrometer (KEITHLEY 6517B), a customer-made electrostatic shielding cover, an air Ò pump unit, a LabView based data acquisition module (NI USB6259, NI, USA). The sensing electrode is a commercial Pt electrode (Leici 213, Shanghai, China). The cross section of the sensing electrode is 2 5 mm. The electrode was fixed at a plastic plate which was mounted vertically on a glass base. The air pump unit consists of an accurate screw rail (DSXM60-200, Times Brilliant, Beijing, China) actuated by a mini servo motor (57HBP56AL4TF5, Times Brilliant, Beijing, China), and a plastic syringe. It can
(a)
Upon hitting
Electrode
(b)
Electrode
Computer Air Pump Unit
Sample Tube
Electrometer Fig. 1. Schematic diagram of the detection principle and system setup of the electrical current pulse method for detecting and sizing airborne particles at single particle level. The exchange of electrostatic charges between the particle and the electrode (a); the experimental setup (b).
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(a)
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Fig. 2. Typical signals generated by particles of different sizes. 80 lm SiC particles (a), 50 lm SiC particles (b), 20 lm SiC particles (c), 10 lm SiC particles (d) and 10 lm polystyrene particles (e).
produce adjustable, constant flow rate of air via the syringe. The diameter and length of the syringe are 32 mm and 128 mm respectively. The outlet of the syringe is aimed at the sensing electrode with a separation distance of 1 mm. The particle sample is loaded in the syringe. The moving velocity of the particles shooting out from the syringe can be adjusted by tuning the speed of the motor of the air pump unit. An electrostatic shielding cover is used for protecting the sensing electrode and other electrical components from external static interference. 2.2. Particle preparation and experimental procedure Experiments were conducted with 10 lm polystyrene particles (Fluka, Shanghai, China) and SiC particles of different sizes (Fushi, Shanghai, China). There is no commercial SiC particle samples with a single uniform size available. To prepare the SiC particle samples, the sizes of SiC particles were measured under a microscope (Ti-e, Nikon, Japan). Then the particles of the same size were selected carefully and categorized into one group. In this way, SiC particle samples with 11 different sizes (10–120 lm) were prepared.
The polystyrene particles are supplied in a solution. To get dry polystyrene particles, 50 lL spherical polystyrene particles solution and 2.5 mL ethanol (Kermel, Tianjin, China) were mixed together firstly. Afterwards, 50 lL of the mixed solution was added onto a clean glass slide with a pipette (233298A, Eppendorf Research, US). Then the glass slide was put on a hotplate (Torrey Pines, Scientific) with the setting temperature of 70 °C and heated until all of the water were vaporized. Before each measurement, the electrometer was pre-warmed for about 50 min to achieve the rated accuracy. Also, zero correct should be made for each measurement. For each measurement, firstly the electrode was cleaned with ethanol (Kermel, Tianjin, China) to remove the possible impurities on the surface. Then the electrode was flushed with pure water and dried with nitrogen. After flushed with nitrogen, the syringe was loaded with the particles and fixed on the screw rail of the pump unit. The volume flow rate of the air pump unit was set as 0.6 mL/s. Upon turning on the power to the air pump unit, the particles in the syringe will be forced to hit the detection electrode. Under each set of conditions,
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(d)
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Time (s) Fig. 2 (continued)
3. Results and discussions 3.1. Particle detection Fig. 2 displays the typical signals generated by different particles in the measurements. Every downward peak is generated by a particle hitting the electrode. For the SiC particles, the magnitudes of the signals were approximately 36 pA, 17 pA, 256 fA and 32.6 fA for particles with diameters of 80 lm, 50 lm, 20 lm and 10 lm respectively. For the 10 lm polystyrene particle, the average magnitude is about 29.1 fA. It is obvious that such a downward signal is significantly stronger than the noise level which is only about 10 fA. It is also clear that the larger particles will generate signals of larger magnitudes. It should be noted, however, that the types of particle influence the magnitudes of the signals which is demonstrated by the difference in signal magnitudes between the 10 lm polystyrene and the 10 lm SiC particles. The magnitudes of the detected signals are directly related with the surface charges of the particles. Therefore, factors that can influence the surface charges of a particle will eventually influence the magnitude of the detected signal. It has been generally accepted that electrons are transferred in metal–insulator contact [32]. SiC is a semiconductor. Polystyrene can be considered as an insulator. The differences in their surface charge and electric conductivity might be responsible for the different signal magnitudes for these two different particles of the same size. Due to the complexity of contact charge transfer [32] and very limited information on the surface charges of different materials, more systematic investigation on the material effect is needed in the future. Experiments were also conducted for samples carrying mixed particles of different sizes. Fig. 3 shows the typical signals gener-
ated by a sample containing 10 lm, 20 lm and 30 lm SiC particles; the magnitudes of these signals are approximately 50 fA, 226 fA and 510 fA, respectively. These magnitudes are very closer to those shown in Fig. 2(c) and (d). The repeated results clearly demonstrate the reliability of this method. 3.2. Particle size effects on the signal As is clearly demonstrated in Figs. 2 and 3, the particle size directly affects the magnitude of detected signals. Table 1 shows the averaged magnitudes of the signals generated by particles of different diameters. Using the data shown in Table 1, Fig. 4 plots the relationship between the magnitude of the measured signal and the surface area (the square of the diameter) of the particles. By a best-curve-fitting method, the correlation between the measured signal and the square diameter of the SiC particles is given below, with a coefficient R2 = 0.91782, 2
Ioutput ¼ 1:72 d 11:7
ð1Þ
0.2 0 Current (pA)
at least four tests were conducted. Every data point reported in the figures of this paper is the average value of these tests.
10μm
10μm
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20μm
-0.6 30μm
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30μm
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60 50 Time (s)
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Fig. 3. Typical signals of generated by the mixed 10, 20 and 30 lm SiC particles.
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Table 1 Signals generated by particles of different diameters. Particle material
Size (lm)
Polystyrene particle SiC particle
10 10 20 30 40 50 60 70 80 90 100 120
References
Measured signal (fA)
Standard deviation
29.1
4.2
32.6 256 535 3.1 103 1.7 104 2.7 104 3.2 104 3.6 104 4.6 104 5.5 104 5.9 104
11.9 55.3 38.3 5.7 102 3.5 103 2.6 103 3.6 103 5.4 103 1.5 103 2.7 103 4.0 103
1096.63
Measured Signal (pA)
403.43 148.41 54.60 20.09 7.39 2.72 1.00 0.37 0.14 0.05 0.02 0.01 0.15
0.40
1.10
Square Diameter
2.98 3
8.10
μm2
Fig. 4. The relationship between the magnitude of the measured signal and the surface area (the square of the diameter) of the particles.
where Ioutput is the measured current signal amplitude (fA), and d is the diameter of the SiC particle (lm). The size of the SiC particles can be evaluated by using Eq. (1) and the detected current signals. 4. Summary A novel method of detecting single airborne particles is developed. An electrical current will be generated when a particle hitting a metal electrode, because of the electrostatic charge on the surface of the particle. The amplitude of the measured current signal is proportional to the size of the particle. Using the measured current signals, the numbers and sizes of particles can be evaluated when the particles carried by air flow impinging onto the detection electrode. With further improvement, the method presented in this paper offers a simple and reliable tool to detect the airborne particles. Acknowledgements The authors wish to thank the financial support of the Fundamental Research Funds for the Central Universities (3132014336), Liaoning Science Foundation (2014025020) and Liaoning Excellent Talent Supporting Plan (LJQ2014050), Dalian Science and Technology Plan (2014E12SF068) from China to Yongxin Song, the Natural Sciences and Engineering Research Council of Canada through a research grant to D. Li and the financial support from the University 111 Project of China under Grant No. B08046 is greatly appreciated.
[1] D.W. Dockery, C.A. Pope, X. Xu, J.D. Spengler, J.H. Ware, M.E. Fay, F.E. Speizer, An association between air pollution and mortality in six US cities, N. Engl. J. Med. 329 (1993) 1753–1759. [2] D.W. Dockery, J. Cunningham, A.I. Damokosh, L.M. Neas, J.D. Spengler, P. Koutrakis, F.E. Speizer, Health effects of acid aerosols on North American children: respiratory symptoms, Environ. Health Perspec. 104 (1996) 500. [3] J. Schwartz, D.W. Dockery, Particulate air pollution and daily mortality in Steubenville, Ohio, Am. J. Epidemiol. 135 (1992) 12–19. [4] G. Oberdörster, Pulmonary effects of inhaled ultrafine particles, Int. Arch. Occup. Environ. Health 74 (2000) 1–8. [5] R.M. Harrison, J. Yin, Particulate matter in the atmosphere: which particle properties are important for its effects on health?, Sci Total Environ. 249 (2000) 85–101. [6] A. Charron, R.M. Harrison, S. Moorcroft, J. Booker, Quantitative interpretation of divergence between PM 10 and PM 2.5 mass measurement by TEOM and gravimetric (Partisol) instruments, Atmos. Environ. 38 (2004) 415–423. [7] G. Friedlander, E.S. Macias, J.W. Kennedy, Nuclear and Radiochemistry, third ed., John Wiley & Sons, New York, 1981, p. 222. [8] W. Chueinta, P.K. Hopke, Beta gauge for aerosol mass measurement, Aerosol Sci. Technol. 35 (2001) 840–843. [9] C.T. Chang, C.J. Tsai, C.T. Lee, S.Y. Chang, M.T. Cheng, H.M. Chein, Differences in PM 10 concentrations measured by b-gauge monitor and hi-vol sampler, Atmos. Environ. 35 (2001) 5741–5748. [10] M. Mohr, U. Lehmann, Comparison study of particle measurement systems for future type approval application, EMPA Rep. 2003, 202779. [11] J.P. Shi, R.M. Harrison, D. Evans, Comparison of ambient particle surface area measurement by epiphaniometer and SMPS/APS, Atmos. Environ. 35 (2001) 6193–6200. [12] N. Sabbagh-Kupelwieser, A. Maisser, W.W. Szymanski, From micro-to nanosized particles: selected characterization methods and measurable parameters, Particuology 9 (2011) 193–203. [13] Y. Chun, J. Kim, J.C. Choi, K.O. Boo, S.N. Oh, M. Lee, Characteristic number size distribution of aerosol during Asian dust period in Korea, Atmos. Environ. 35 (2001) 2715–2721. [14] Y. Iwasaka, G.Y. Shi, Z. Shen, Y.S. Kim, D. Trochkine, A. Matsuki, H. Nakata, Nature of atmospheric aerosols over the desert areas in the Asian continent: chemical state and number concentration of particles measured at Dunhuang, China, Water, Air, Soil Pollut. Focus 3 (2003) 129–145. [15] Y.S. Kim, Y. Iwasaka, G.Y. Shi, T. Nagatani, T. Shibata, D. Trochkin, M. Nagatani, Dust particles in the free atmosphere over desert areas on the Asian continent: measurements from summer 2001 to summer 2002 with balloon-borne optical particle counter and lidar, Dunhuang, China, J. Geophys. Res.: Atmos. 109 (2004). D19. [16] N. Collings, K. Rongchai, J.P.R. Symonds, A condensation particle counter insensitive to volatile particles, J. Aerosol Sci. 73 (2014) 27–38. [17] S. Baltzer, S. Onel, M. Weiss, M. Seipenbusch, Counting efficiency measurements for a new condensation particle counter, J. Aerosol Sci. 70 (2014) 11–14. [18] Henryk Eisenberg, Light scattering, Nature 263 (1976) 357. [19] J. Tonnelat, S. Guinand, Light-scattering, Nature 244 (1973) 528. [20] W.H. White, E.S. Macias, R.C. Nininger, D. Schorran, Size-resolved measurements of light scattering by ambient particles in the southwestern USA, Atmos. Environ. 28 (1994) 909–921. [21] L. Wang-Li, Z. Cao, M. Buser, D. Whitelock, C.B. Parnell, Y. Zhang, Techniques for measuring particle size distribution of particulate matter emitted from animal feeding operations, Atmos. Environ. 66 (2013) 25–32. [22] J. Keskinen, K. Pietarinen, M. Lehtimäki, Electrical low pressure impactor, J. Aerosol Sci. 23 (1992) 353–360. [23] A. Järvinen, M. Aitomaa, A. Rostedt, J. Keskinen, J. Yli-Ojanperä, Calibration of the new electrical low pressure impactor (ELPI+), J. Aerosol Sci. 69 (2014) 150– 159. [24] E. Zervas, P. Dorlhene, L. Forti, C. Perrin, J.C. Momique, R. Monier, B. Lopez, Exhaust gas particle mass estimation using an electrical low pressure impactor, Energy fuels 20 (2006) 498–503. [25] H. Rohmann, Methode zur messung der größe von schwebeteilchen, Z. Phys. 17 (1923) 253–265. [26] K.T. Whitby, W.E. Clark, Electric aerosol particle counting and size distribution measuring system for the 0.015 to 1l size range1, Tellus 18 (1966) 573–586. [27] K.F. Tammet, in: B. Benny (Ed.), The Aspiration Method for the Determination of Atmospheric-ion Spectra, Israel Program for Scientific Translations, Jerusalem, 1970. [28] E.O. Knutson, K.T. Whitby, Aerosol classification by electric mobility: apparatus, theory, and applications, J. Aerosol Sci. 6 (1975) 443–451. [29] B.Y.H. Liu, D.Y.H. Pui, On the performance of the electrical aerosol analyzer, J. Aerosol Sci. 6 (1975) 249–264. [30] Z. Tan, R. Givehchi, A. Saprykina, Submicron particle sizing by aerodynamic dynamic focusing and electrical charge measurement, Particuology 18 (2015) 105–111. [31] F.O. J-Fatokun, L. Morawska, M. Jamriska, E.R. Jayaratne, Application of aerosol electrometer for ambient particle charge measurements, Atmos. Environ. 42 (2008) 8827–8830. [32] A.R. Akande, J. Lowell, Charge transfer in metal/polymer contacts, J. Phys. D: Appl. Phys. 20 (1987) 565–578.
Contents lists available at ScienceDirect
Measurement journal homepage: www.elsevier.com/locate/measurement
Airborne particles detection and sizing at single particle level by a novel electrical current pulse sensor Hang Zhou a, Yongxin Song a, Xinxiang Pan a, Dongqing Li b,⇑ a b
Department of Marine Engineering, Dalian Maritime University, Dalian 116026, China Department of Mechanical and Mechatronics Engineering, University of Waterloo, Waterloo, ON N2L 3G1, Canada
a r t i c l e
i n f o
Article history: Received 10 March 2016 Received in revised form 30 May 2016 Accepted 7 June 2016 Available online 8 June 2016 Keywords: Airborne particle Detecting and sizing Current pulse
a b s t r a c t A novel method for detecting and sizing airborne particles at single particle level is presented in this paper. When a particle hits a metal electrode which is grounded, electrostatic charges will be transferred between the particle and the electrode. As a result, an electrical current pulse will be generated in the measurement system. The number of the signal pulse represents the number of particles in the sample. To determine the effect of the particle size on the magnitude of the signal, the correlation between the magnitude of the signal and the size of particle was experimentally investigated. The results show that the magnitude of the measured signal is linearly proportional to the square of particle’s diameter. Such a correlation can be used to evaluate particle size from the measured signal. The airborne particle detection method presented in this paper can be used for counting and sizing airborne particles at single particle level. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction With the increasing awareness of the negative impacts of particulate matters (PM) on people’s health [1–3], quantitative evaluation of the size distribution and number concentration of airborne particles is critical to environmental health assessment [4,5]. Therefore, it is greatly needed for developing technologies of detecting and sizing airborne particles. Generally, the current PM detection methods and the commercially available instruments can be categorized into two types: mass concentration evaluation and size distribution evaluation. The classic gravimetric filter, based on measuring the weight change before and after sample passing through the filter, is the most widely used method to obtain particle mass concentrations [6]. While it is simple in construction and easy in operation, it cannot perform real-time measurement. Beta gauges, based on the b penetration theory [7], have been used for a long time for measuring the mass concentration of the PM sample. Once the attenuation of b particles is captured, the mass concentration of the sample is evaluated [8,9]. Using an accurate balance, such as the Tapered Element Oscillating Microbalance (TEOM), is another method for measuring the mass of particulate matters collected on a filter
⇑ Corresponding author. E-mail address: [email protected] (D. Li). http://dx.doi.org/10.1016/j.measurement.2016.06.011 0263-2241/Ó 2016 Elsevier Ltd. All rights reserved.
cartridge. While this method can continually measure the sample [10], it is susceptible to humidity of the sample air. It should be further noted that the above methods can only evaluate the mass of PM in air samples. To obtain other information, such as the number and size of PM which are more important parameters [11], other methods such as optical or electrical methods need to be employed. The details about the technologies for particle size measurement can be found in the reference paper [12]. Optical particle counters can determine the number and size of aerosol particles and have gained wide applications due to its easy operation and relatively low cost [13–15]. It was claimed that some commercial optical sensors are able to detect and count PM of size down to 0.5 lm and some manufacturers even down to 0.3 lm (NIDS Sensor Technologies, 2015). The sensitivity of optical particle counters can be improved by using condensing vapors which can magnify particle size [16,17]. For light scattering method, however, the size measurement can be interfered by the color, refractive index and composition of the particle [18–20]. Furthermore, it cannot provide real-time size distribution measurement [21] and the commercial instruments are very expensive and large in size. Normally, electrical methods are capable of detecting smaller particles. Electrical low pressure impactor [10,22–24] was reported to measure the size distribution of PM. This method works by charging the sample with corona discharge and the amount of
59
H. Zhou et al. / Measurement 92 (2016) 58–62
charges is proportional to the surface area of a particle. Knowing the detected electrical current signals, the size distributions can be determined. However, this method involves complicated measurement systems which greatly limit its applications. Electrical mobility method is capable of measuring submicron and even nano-particles [25–29]. In this method, however, particle charging is usually achieved by applying 1–5 kV DC voltage. Also, it can only work well for particles with good mobility and the configuration of the commercial analyzer is very complicate. Sizing submicron particles by aerodynamic focusing and using a Faraday cup was recently reported [30]. The particle sample was first filtered to a certain size by aerodynamic focusing. After charging the particles with corona, the total current generated by the particles entering the Faraday cup was measured. In this way, the total number of the particles with a certain size is evaluated. As in the case for electrical mobility method, corona generation device is needed which will damage some types of particles. As can be concluded from the above literature review, while the current technologies can size particles in a relative high resolution, most these methods are based on mass average or average of a large particle population. It is still challenge in detecting and sizing particles at single particle level. The increasing demand for particle monitoring with high spatial and temporal resolution requires developing new and low cost PM sensing technology to enhance our ability for monitoring air quality. This paper reports a novel method for detecting and sizing airborne particles at single particle level. By using air flow to force a particle hitting a metal electrode which is grounded, electrostatic charges will be transferred between the particle and the electrode. As a result, an electrical current pulse will be generated and detected by a measurement system. The number of the measured electrical pulses represents the number of particles. The effect of the particle size on the magnitude of the signal was experimentally investigated. A correlation between the particle size and the measured current signal was obtained and can be used to evaluate particle size from the measured signal.
2. Experimental 2.1. Detection principle and the system setup Fig. 1 shows the working principle (Fig. 1(a)) and the experimental system (Fig. 1(b)) of the electrical current pulse method for airborne particle detection. The working principle can be understood as bellows: As shown in Fig. 1(a), either the electrode or the particle in air will have electrostatic charges [31]. The polarity and amount of surface charges are determined by the types of the electrode, the particle and the environment. For example, mean charge concentration values of 478 ± 58 ions cm3 and 1664 ± 998 ions cm3 were reported for particles in typical outdoor air and in highly charged air, respectively [31]. When a charged particle hits the electrode which is grounded, electrostatic charges will be transferred between the particle and the electrode. As a result, an electrical current pulse will be generated. The number of the electrical current pulses represents the number of particles hitting the electrode. Because the amount of charge transfer and hence the electrical current are proportional to the amount of surface charge of the particle which in turn is proportional to particle’s size, the magnitude of the electrical current pulse signals is proportional to the particle’s size. In this way, both the number and sizes of the particles can be evaluated. Fig. 1(b) shows the experimental system setup. The detection system consists of a sensing electrode, an electrometer (KEITHLEY 6517B), a customer-made electrostatic shielding cover, an air Ò pump unit, a LabView based data acquisition module (NI USB6259, NI, USA). The sensing electrode is a commercial Pt electrode (Leici 213, Shanghai, China). The cross section of the sensing electrode is 2 5 mm. The electrode was fixed at a plastic plate which was mounted vertically on a glass base. The air pump unit consists of an accurate screw rail (DSXM60-200, Times Brilliant, Beijing, China) actuated by a mini servo motor (57HBP56AL4TF5, Times Brilliant, Beijing, China), and a plastic syringe. It can
(a)
Upon hitting
Electrode
(b)
Electrode
Computer Air Pump Unit
Sample Tube
Electrometer Fig. 1. Schematic diagram of the detection principle and system setup of the electrical current pulse method for detecting and sizing airborne particles at single particle level. The exchange of electrostatic charges between the particle and the electrode (a); the experimental setup (b).
60
H. Zhou et al. / Measurement 92 (2016) 58–62
(a)
60
Current ( pA)
40 20 0 -20 -40 -60 -80 0
30
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(b)
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120
Fig. 2. Typical signals generated by particles of different sizes. 80 lm SiC particles (a), 50 lm SiC particles (b), 20 lm SiC particles (c), 10 lm SiC particles (d) and 10 lm polystyrene particles (e).
produce adjustable, constant flow rate of air via the syringe. The diameter and length of the syringe are 32 mm and 128 mm respectively. The outlet of the syringe is aimed at the sensing electrode with a separation distance of 1 mm. The particle sample is loaded in the syringe. The moving velocity of the particles shooting out from the syringe can be adjusted by tuning the speed of the motor of the air pump unit. An electrostatic shielding cover is used for protecting the sensing electrode and other electrical components from external static interference. 2.2. Particle preparation and experimental procedure Experiments were conducted with 10 lm polystyrene particles (Fluka, Shanghai, China) and SiC particles of different sizes (Fushi, Shanghai, China). There is no commercial SiC particle samples with a single uniform size available. To prepare the SiC particle samples, the sizes of SiC particles were measured under a microscope (Ti-e, Nikon, Japan). Then the particles of the same size were selected carefully and categorized into one group. In this way, SiC particle samples with 11 different sizes (10–120 lm) were prepared.
The polystyrene particles are supplied in a solution. To get dry polystyrene particles, 50 lL spherical polystyrene particles solution and 2.5 mL ethanol (Kermel, Tianjin, China) were mixed together firstly. Afterwards, 50 lL of the mixed solution was added onto a clean glass slide with a pipette (233298A, Eppendorf Research, US). Then the glass slide was put on a hotplate (Torrey Pines, Scientific) with the setting temperature of 70 °C and heated until all of the water were vaporized. Before each measurement, the electrometer was pre-warmed for about 50 min to achieve the rated accuracy. Also, zero correct should be made for each measurement. For each measurement, firstly the electrode was cleaned with ethanol (Kermel, Tianjin, China) to remove the possible impurities on the surface. Then the electrode was flushed with pure water and dried with nitrogen. After flushed with nitrogen, the syringe was loaded with the particles and fixed on the screw rail of the pump unit. The volume flow rate of the air pump unit was set as 0.6 mL/s. Upon turning on the power to the air pump unit, the particles in the syringe will be forced to hit the detection electrode. Under each set of conditions,
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(d)
20 0
Current ( fA)
-20 -40 -60 -80 -100 -120 0
40
20
60
80
140
120
100
Time (s)
(e)
40
Current ( fA)
20 0 -20 -40 -60 -80 -100 0
20
40
60
80
120
100
140
Time (s) Fig. 2 (continued)
3. Results and discussions 3.1. Particle detection Fig. 2 displays the typical signals generated by different particles in the measurements. Every downward peak is generated by a particle hitting the electrode. For the SiC particles, the magnitudes of the signals were approximately 36 pA, 17 pA, 256 fA and 32.6 fA for particles with diameters of 80 lm, 50 lm, 20 lm and 10 lm respectively. For the 10 lm polystyrene particle, the average magnitude is about 29.1 fA. It is obvious that such a downward signal is significantly stronger than the noise level which is only about 10 fA. It is also clear that the larger particles will generate signals of larger magnitudes. It should be noted, however, that the types of particle influence the magnitudes of the signals which is demonstrated by the difference in signal magnitudes between the 10 lm polystyrene and the 10 lm SiC particles. The magnitudes of the detected signals are directly related with the surface charges of the particles. Therefore, factors that can influence the surface charges of a particle will eventually influence the magnitude of the detected signal. It has been generally accepted that electrons are transferred in metal–insulator contact [32]. SiC is a semiconductor. Polystyrene can be considered as an insulator. The differences in their surface charge and electric conductivity might be responsible for the different signal magnitudes for these two different particles of the same size. Due to the complexity of contact charge transfer [32] and very limited information on the surface charges of different materials, more systematic investigation on the material effect is needed in the future. Experiments were also conducted for samples carrying mixed particles of different sizes. Fig. 3 shows the typical signals gener-
ated by a sample containing 10 lm, 20 lm and 30 lm SiC particles; the magnitudes of these signals are approximately 50 fA, 226 fA and 510 fA, respectively. These magnitudes are very closer to those shown in Fig. 2(c) and (d). The repeated results clearly demonstrate the reliability of this method. 3.2. Particle size effects on the signal As is clearly demonstrated in Figs. 2 and 3, the particle size directly affects the magnitude of detected signals. Table 1 shows the averaged magnitudes of the signals generated by particles of different diameters. Using the data shown in Table 1, Fig. 4 plots the relationship between the magnitude of the measured signal and the surface area (the square of the diameter) of the particles. By a best-curve-fitting method, the correlation between the measured signal and the square diameter of the SiC particles is given below, with a coefficient R2 = 0.91782, 2
Ioutput ¼ 1:72 d 11:7
ð1Þ
0.2 0 Current (pA)
at least four tests were conducted. Every data point reported in the figures of this paper is the average value of these tests.
10μm
10μm
-0.2
20μm
10μm
10μm
20μm
-0.4
20μm
-0.6 30μm
30μm
30μm
30μm
-0.8 0
10
20
30
40
60 50 Time (s)
70
80
90
Fig. 3. Typical signals of generated by the mixed 10, 20 and 30 lm SiC particles.
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Table 1 Signals generated by particles of different diameters. Particle material
Size (lm)
Polystyrene particle SiC particle
10 10 20 30 40 50 60 70 80 90 100 120
References
Measured signal (fA)
Standard deviation
29.1
4.2
32.6 256 535 3.1 103 1.7 104 2.7 104 3.2 104 3.6 104 4.6 104 5.5 104 5.9 104
11.9 55.3 38.3 5.7 102 3.5 103 2.6 103 3.6 103 5.4 103 1.5 103 2.7 103 4.0 103
1096.63
Measured Signal (pA)
403.43 148.41 54.60 20.09 7.39 2.72 1.00 0.37 0.14 0.05 0.02 0.01 0.15
0.40
1.10
Square Diameter
2.98 3
8.10
μm2
Fig. 4. The relationship between the magnitude of the measured signal and the surface area (the square of the diameter) of the particles.
where Ioutput is the measured current signal amplitude (fA), and d is the diameter of the SiC particle (lm). The size of the SiC particles can be evaluated by using Eq. (1) and the detected current signals. 4. Summary A novel method of detecting single airborne particles is developed. An electrical current will be generated when a particle hitting a metal electrode, because of the electrostatic charge on the surface of the particle. The amplitude of the measured current signal is proportional to the size of the particle. Using the measured current signals, the numbers and sizes of particles can be evaluated when the particles carried by air flow impinging onto the detection electrode. With further improvement, the method presented in this paper offers a simple and reliable tool to detect the airborne particles. Acknowledgements The authors wish to thank the financial support of the Fundamental Research Funds for the Central Universities (3132014336), Liaoning Science Foundation (2014025020) and Liaoning Excellent Talent Supporting Plan (LJQ2014050), Dalian Science and Technology Plan (2014E12SF068) from China to Yongxin Song, the Natural Sciences and Engineering Research Council of Canada through a research grant to D. Li and the financial support from the University 111 Project of China under Grant No. B08046 is greatly appreciated.
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