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Optical sulfur dioxide sensor based on broadband absorption spectroscopy in the wavelength range of 198–222 nm
Sensors and Actuators B 241 (2017) 146–150
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Optical sulfur dioxide sensor based on broadband absorption spectroscopy in the wavelength range of 198–222 nm Lin Wang a , Yungang Zhang b , Xue Zhou a , Feng Qin a , Zhiguo Zhang a,∗ a b
Condensed-Matter Science and Technology Institute, Harbin Institute of Technology, Harbin 150001, China College of Electrical Engineering, Yanshan University, Qinhuangdao 066004, China
a r t i c l e
a b s t r a c t
i n f o
Article history: Received 30 December 2015 Received in revised form 8 October 2016 Accepted 12 October 2016 Available online 15 October 2016 Keywords: Sulfur dioxide Gas sensing Broadband absorption spectroscopy
A highly sensitive sulfur dioxide detection system based on broadband absorption spectroscopy was developed using the 198–222 nm wavelength range. The compact and simple measurement system was constructed utilizing fiber optoelectronic sensing device. A modified approach based on differential optical absorption spectroscopy was used for the computation of sulfur dioxide concentrations. The results of sulfur dioxide concentration were calibrated and displayed using a LabVIEW-based software. A system detection limit of 17 ppb per meter has been reached, and a ten-fold improvement in sensitive and uncertainty has been achieved for measuring sulfur dioxide concentrations. The system is suitable for monitoring sulfur dioxide concentration in air as well as for the fault diagnosis of gas insulated switchgears. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Sulfur dioxide is one of the most common air pollutants [1–3]. In China, the National Ambient Air Quality Standard (GB 3095-2012) stipulates that the amount of sulfur dioxide contained in the ambient air must not exceed 21 ppb, 52.5 ppb, and 175 ppb on annual, daily, and hourly average, respectively. For these reasons, an accurate system of monitoring sulfur dioxide concentration levels is of great importance to control pollution. In addition, sensitive measurements of low sulfur dioxide concentrations in confined spaces can also be used for the fault diagnosis of gas insulated switchgear (GIS) [4]. The fault of GIS can be diagnosed based on the content and production rate of sulfur dioxide in the airtight shielding gas [5–7]. Currently, the main techniques for monitoring sulfur dioxide in air depend on chemical sensors, broad-band absorption spectroscopic techniques based on differential optical absorption spectroscopy (DOAS), non-dispersive infrared spectroscopy (NDIR), laser-induced fluorescence (LIF), correlation spectroscopy (COSPEC), differential absorption lidar (DIAL), and tunable diode laser absorption spectroscopy (TDLAS) [8–13]. Among these numerous techniques, broad-band absorption spectroscopy is widely used for its superior high signal-to-noise ratio (SNR) and
∗ Corresponding author. E-mail address: [email protected] (Z. Zhang). http://dx.doi.org/10.1016/j.snb.2016.10.055 0925-4005/© 2016 Elsevier B.V. All rights reserved.
strong anti-interference performance. It has been approved by the Environmental Protection Agency (EPA 1995) as an equivalent method for the measurement of gaseous pollutants [14]. Currently, the reported detection limit is 0.2 ppm per meter, when using the spectroscopic absorption band around 300 nm [15]. In these conditions, path lengths of several hundreds of meters are required to achieve low sulfur dioxide concentration measurement for environment monitoring [14]. For the same detection limit, path lengths can be shortened via the utilization of a larger sulfur dioxide absorption cross-section. In the case of sulfur dioxide, the absorption cross-section around 200 nm is 10 times larger than that around 300 nm. Fortunately, Ocean Optics develops one kind of fiber optic spectrometer with a spectral range of 160–270 nm, equipped with a spectral resolution of 0.05 nm. This work presents the development of a highly sensitive sulfur dioxide sensor based on broadband absorption spectroscopy around 200 nm, for which a compact concentration measuring system was constructed. The relationship between sulfur dioxide concentration and its optical parameter (OP) has been established by using the Beer-Lambert law. The optical parameter is defined as the sum of signals recorded from all the measuring points. An automatic recording function has been implemented using a LabVIEW-based software. The continuous monitoring of different concentrations was performed to obtain the detection limit and uncertainty of the system. The demonstration of the superiority of the new system using the 200 nm absorption band was achieved
L. Wang et al. / Sensors and Actuators B 241 (2017) 146–150
Fig. 1. (a) Absorption cross section of sulfur dioxide from 284 to 311 nm. (b) Absorption cross section of sulfur dioxide from 198 to 222 nm.
147
Fig. 2. (a) Absorption spectrum of sulfur dioxide (black) and slow-change absorption spectrum (red). (b) Structured change absorption spectrum. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
by comparing its technical parameters with a former system based on the absorption band around 300 nm. 2. Theory Fig. 1 shows the absorption cross section of sulfur dioxide in the ultraviolet region. Both the absorption cross-section and the differential absorption cross-section are much larger around 200 nm than that around 300 nm. Therefore, the use of the absorption around 200 nm should improve the sulfur dioxide measuring performance. Considering such a strong structured change in the absorption spectrum around 200 nm, the approach described in reference [15] was used in this study. The measurement of sulfur dioxide concentration is derived from the Beer-Lambert law, which can be expressed as I() = I0 () exp[−C()L − ˛()],
(1)
Fig. 3. Experimental setup of the absorption sensing system for sulfur dioxide.
where I() and I0 () are the incident and received radiation intensities at wavelength , respectively, () (cm2 /molecule) is the absorption cross section, ␣() is the scattering and absorption coefficient of other components, C is the concentration of gas under test, and L is the absorption optical path length. As shown in Fig. 2, the sulfur dioxide absorption spectrum consists of a slow-change portion and a structured change portion. The following expression can be obtained using Eq. (1):
The proposed approach provides a simple and straightforward tool for the assessment of the measuring capability of the system. In fact, other methods, such as the least squares method, can provide equivalent results. To demonstrate that large absorption cross section improves the detection limit of system, the following analysis was carried out. The concentration uncertainty C corresponding to the detection limit of the measurement system, by differentiating formula (4) on both sides, can be expressed as
−C()L = ln(I()/S()),
CL =
(2)
where S() is the slow-change absorption spectrum, and () is the differential absorption cross sections. It has been confirmed that the slow-change portion can be replaced by a polynomial fitting curve [15]. In order to reduce the impact of noise signals, the optical parameter (OP) is defined as the sum of signals recorded from all the measuring points expressed as OP =
| ln(I()/S())|.
(3)
The relationship between optical parameters and concentrations can be expressed as CL =
OP |()|
.
(4)
⁄|I()⁄I(),
(5)
where I()⁄I() is the relative uncertainty of received radiation intensities, and C is the uncertainty of gas concentration under test. As I(), I() and L are constant parameters for certain measuring instruments, it can be seen that C is in inverse proportion of |()|. Considering the fact that the differential absorption cross section around 200 nm is 15 times larger than that around 300 nm, the selection of the absorption band around 200 nm seems preferable to that around 300 nm. 3. Experimentation Fig. 3 shows the experimental setup of the sensing system. It consists of two main components: the sulfur dioxide concentration measurement part and the gas preparation part.
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L. Wang et al. / Sensors and Actuators B 241 (2017) 146–150
3.1. Preparation of sulfur dioxide Different concentrations of sulfur dioxide were prepared by mixing standard gas and pure nitrogen in different proportions. Standard gases of sulfur dioxide (Nanjing Tianze Co., Ltd) at the concentrations of 200 ppm, 50 ppm, 20 ppm and 10 ppm have been used. Accurate gas mixing ratios were obtained from the use of mass flow controllers (MFC, Sevenstar, D07-19C) with a precision of 0.01 L/min. The flow rate of the combined gas was stable, at 1 ± 0.01 L/min, throughout the experimentation. No corrections to pressure were made, for the pressure inside the cell was constant and approximately equal to one atmosphere. The entire experiment was proceeded at room temperature. The impact of sulfur dioxide adsorption was ignored due to the use of flowing gas. 3.2. Sulfur dioxide sensing system A high-pressure deuterium lamp (Hamamatsu 5601, Japan) with a broadband emission spectrum in the ultraviolet range was used as a light source. The light was converted into a parallel beam via a quartz lens with low UV absorption and a focal length of 25 mm. The parallel beam was then transmitted through a 40 cm long sample cell equipped with quartz windows, the cell was used for containing sulfur dioxide. The sample cell had an inlet port and an outlet port near each end of it to guarantee good air exchange. Since the deuterium lamp acts as an incoherent light source, the light interference coming from specular reflection was ignored. The transmitted beam collected by another quartz lens with the same focal length of 25 mm was focused into a multi-mode optical fiber (Ocean Optics OLFV-200-1100). Finally, the light was coupled into a high-resolution fiber optic spectrometer (Ocean Optics Maya2000) working in the spectral range of 160–270 nm. The spectrometer temperature was controlled using a proportional-integral-derivative (PID) loop feed back temperature control system at 40 ◦ C with a fluctuation of 0.3 ◦ C, to prevent excessive changes in the electronic noise of the spectrometer. Data analysis and control of spectrometer were performed automatically via a personal computer using a home-made LabVIEW-based software.
Fig. 5. Real-time collection of OP at 43.04 ppm and definition of OP.
4. Results and discussion From the different absorption cross-section values reported in the literature [16–19], the high-resolution absorption spectra of sulfur dioxide at 200 ppm with an optical length of 40 cm around 200 nm and 300 nm were measured at atmospheric pressure and room temperature to calculate absorption cross sections. The corresponding absorption cross sections of sulfur dioxide in the wavelength range of 198–222 nm and 284–311 nm evaluated by the Beer-Lambert law are shown in Fig. 1. The calibration of the sulfur dioxide sensing system using the new absorption band around 200 nm is performed through the relationship between OP and sulfur dioxide concentration, which was established in Fig. 4(a). Absorption spectra for a set of sulfur dioxide concentrations were measured, and corresponding OP values were calculated using the method above. In the calculations, the absorption wavelength range of 198–222 nm was selected to maintain high signal-to-noise ratio. The optical path length was converted into a standard length of 1 m to keep comparability with results in other literature [15]. The good linear relationship (R2 = 0.999) between the measured OP and sulfur dioxide concentration observed in Fig. 4(a) then allowed the derivation of the following calibration expression for sulfur dioxide concentration: CL = 8.3857OP(ppm m).
Fig. 4. (a) Measured OPs for different sulfur dioxide concentrations and their respective fitted values. (b) Fluctuations of OPs for different sulfur dioxide concentrations.
(6)
The zero off-set has been ignored, since it is far lower than the usual measured sulfur dioxide concentrations. The fluctuations of OP values are shown in Fig. 4(b). Continuous monitoring of different sulfur dioxide concentrations was implemented. For each concentration, more than 500 data points were measured with a 4 Hz sampling rate and an adjacent averaging window of 99 points was used to smooth the measured optical parameters. As shown in Fig. 5, the fluctuation of OP was evaluated from the difference between the maximum and minimum, in lieu of the standard deviation. Fig. 4(b) shows that the fluctuations of OP remained below 0.002 throughout the whole sulfur dioxide concentration range. In general, the detection limit of the system was defined as the concentration that brought changes of OP to a level equal to the fluctuation of the system. The detection limit of the system was determined to be 43 ppb with an optical length of 40 cm, which is equivalent to 17 ppb per meter. The superiority of the new sulfur dioxide measurement system was demonstrated by comparing the technical parameters of the new system with a previous system based on the absorption range around 300 nm [15]. The detection limit of the previous system was 0.2 ppm m. The measurement uncertainties associated to the sulfur dioxide concentration at 12 ppm m and 24 ppm m were about
L. Wang et al. / Sensors and Actuators B 241 (2017) 146–150 Table 1 Detection limits of different sulfur dioxide sensing systems. Method
Absorption region
Detection limit
Reference
DOAS system long-path DOAS system UV DOAS system DOAS system
275–315 nm 293–312 nm 200–400 nm 198–222 nm
0.2 ppm m 0.3 ppm m 0.2 ppm m 0.02 ppm m
[15] [20] [21] This work
0.5% with the previous system, whereas they are now about 0.3‰ with the new system. Considering the evaluation methods of the systems were different, the use of absorption band around 200 nm improved the performance by at least 10 times for measuring the sulfur dioxide concentrations. It is a comprehensive result, in terms of improvement of absorption cross section and spectrometer performance. The decrease of the uncertainties and the detection limit verified the analysis in the above theoretical part. The detection limits of other measurement systems using relevant approaches are listed in Table 1. During the process of monitoring sulfur dioxide in air, spectral interference from other compounds, such as O3 , O2 , NO, NO2 , NH3 etc., can occur. For O2 , the absorption in UV occurs at the wavelength lower than 198 nm. The absorption spectral interference is avoided by the selection of the 198–222 nm wavelength range. For O3 , the absorption is slowly varying with wavelength, it can be eliminated by analyzing the spectral structured changes. For NO, the narrow band absorption can be eliminated as reported in Refs. [22,23]. The possibly cumbersome absorption spectral interference comes from NO2 and NH3 , since both of them have structured absorption in the wavelength range of sulfur dioxide measurement. Fortunately, the concentration of NO2 in air is lower than that of SO2 , and its structured absorption cross section is about two orders of magnitude smaller than that of SO2 . The absorption spectral interference of NO2 can then be ignored. For NH3 , the structured absorption is in the same order of magnitude with that of SO2 . In the case, methods, such as the least squares method, can be used to extract concentration information from mixed spectrum with structured absorption spectral interference. 5. Conclusions The feasibility of high-precision sulfur dioxide concentration measurement using the new wavelength range of 198–222 nm has been demonstrated. A compact and simple measurement system was constructed using fiber opto-electronic sensing device and automatic measurements were monitored using LabVIEW-based software. A detection limit of 17 ppb per meter and concentration uncertainties of 0.3‰ were achieved. The sensitivity and uncertainty improvements of the new system showed to be at least ten fold in comparison to existing system based on absorption band around 300 nm. References [1] T. Wang, F. Hendrick, P. Wang, G. Tang, K. Clémer, H. Yu, C. Hermans, C. Gielen, J.F. Müller, G. Pinardi, N. Theys, H. Brenot, M. Van Roozendael, Evaluation of tropospheric SO2 retrieved from Max-DOAS measurements in Xianghe, China, Atmos. Chem. Phys. 14 (2014) 11149–11164. [2] L. Vogel, B. Galle, C. Kern, H. Delgado Granados, V. Conde, P. Norman, S. Arellano, O. Landgren, P. Lübcke, J.M. Alvarez Nieves, L. Cárdenas Gonzáles, U. Platt, Early in-flight detection of SO2 via differential optical absorption spectroscopy: a feasible aviation safety measurement to prevent potential encounters with volcanic plumes, Atmos. Meas. Tech. 4 (2011) 1785–1804. [3] C. Rivera, G. Sosa, H. Wöhrnschimmel, B. de Foy, M. Johansson, B. Galle, Tula industrial complex (Mexico) emissions of SO2 and NO2 during the MCMA 2006 field campaign using a moble mini-DOAS system, Atmos. Chem. Phys. 9 (2009) 6351–6361.
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[4] Yu Zhao, Xianpei Wang, Dangdang Dai, Zhengcheng Dong, Partial discharge early-warning through ultraviolet spectroscopic detection of SO2 , Meas. Sci. Technol. 25 (2014) 1–9. [5] Tao Lin, Dong Han, Guoqiang Zhang, Yilu Liu, Formation characteristics of SF6 decomposition under partial discharge induced by metal protrusions with varing degrees of severity, Electric Power Compon. Syst. 42 (16) (2014) 1839–1848. [6] Shudi Peng, Gaolin Wu, Wei Song, Qian Wang, Application of flower-like ZnO nanorods gas sensor detecting SF6 decomposition products, J. Nanomater. 135147 (2013). [7] Xiaoxing Zhang, Jinbin Zhang, Yichao Jia, Peng Xiao, Ju Tang, TiO2 nanotube array sensor for detecting the SF6 decomposition product SO2 , Sensors-Basel 12 (2012) 3302–3313. [8] Soo Chool Lee, Byung Wook Hwang, Soo Jae Lee, Ho Yun Choi, Seong Yeol Kim, Suk Yong Jung, Dhanusuraman Ragupathy, Duk Dong Lee, Jae Chang Kim, A novel tin oxide-based recoverable thick film SO2 gas sensor promoted with magnesium and vanadium oxides, Sens. Actuators B-Chem. 160 (2011) 1328–1334. [9] D. Ravi Shankaran, N. Uehera, T. Kato, Determination of sulfur dioxide based on a silver dispersed functional self-assembled electrochemical sensor, Sens. Actuators B-Chem. 87 (2002) 442–447. [10] X.T. Lou, G. Somesfalean, Z.G. Zhang, S. Svanberg, Sulfur dioxide measurements using an ultraviolet light-emitting diode in combination with gas correlation techniques, Appl. Phys. B 94 (2009) 699–704. [11] G. Somesfalean, Z.G. Zhang, M. Sjöholm, S. Svanberg, All-diode-laser ultraviolet absorption spectroscopy for sulfur dioxide detection, Appl. Phys. B 80 (2005) 1021–1025. [12] Josef B. Simeonsson, Anand Matta, Ravi Boddeti, Direct measurement of SO2 in air by laser induced fluorescence spectrometry using a nontunable laser source, Anal. Lett. 45 (8) (2012) 894–906. [13] P. Weibring, H. Edner, S. Svanberg, G. Cecchi, L. Pantani, R. Ferrara, T. Caltabiano, Monitoring of volcanic sulphur dioxide emissions using differential absorption lidar (DIAL), differential optical absorption spectroscopy (DOAS), and correlation spectroscopy (COSPEC), Appl. Phys. B 67 (1998) 419–426. [14] Jeong Soon Lee, Young J. Kim, Bongjae Kuk, Andreas Geyer, Ulrich Platt, Simultaneous measurement of atmospheric pollutants and visibility with a long-path DOAS system in urban areas, Environ. Monit. Assess. 104 (2005) 281–293. [15] H.S. Wang, Y.G. Zhang, S.H. Wu, X.T. Lou, Z.G. Zhang, Y.K. Qin, Using broadband absorption spectroscopy to measure concentration of sulfur dioxide, Appl. Phys. B 100 (2010) 637–641. [16] G. Stark, Peter L. Smith, J. Rufus, A.P. Thorne, J.C. Pickering, G. Cox, High-resolution photoabsorption cross-section measurements for SO2 at 295 K between 198 and 220 nm, J. Geophys. Res. 104 (1999) 16585–16590. [17] V. Prahlad, Vijay Kumar, Temperature dependence of photoabsorption cross-sections of sulfur dioxide at 188–220 nm, J. Quant. Spectrosc. Radiat. Transf. 57 (1997) 719–723. [18] J. Rufus, G. Stark, Peter L. Smith, J.C. Pickering, A.P. Thorne, High-resolution photoabsorption cross section measurement of SO2 , 2:220 to 325 nm at 295 K, J. Geophys. Res. 108 (E2) (2003) 5011. [19] D. Blackie, R. Blackwell-Whitehead, G. Stark, J.C. Pickering, P.L. Smith, J. Rufus, A.P. Thorne, High-resolution photoabsorption cross-section measurements of SO2 at 198 K from 213 to 325 nm, J. Geophys. Res. 116 (2011) E03006. [20] Jeongsoon Lee, Ki-Hyun Kim, Young J. Kim, Jaihoon Lee, Application of a long-path differential optical absorption spectrometer (LP-DOAS) on the measurements of NO2 , SO2 , O3 and HNO2 in Gwangju, Korea, J. Environ. Manage. 86 (2008) 750–759. [21] Kong Hwa Chiu, Usha Sree, Sen Hong Tseng, Chien-Hou Wu, Jiunn-Guang Lo, Differential optical absorption spectrometer measurement of NO2 , SO2 , O3 , HCHO and aromatic volatile organics in ambient air of Kaohsiung Petroleum Refinery in Taiwan, Atmos. Environ. 39 (2005) 941–955. [22] Y.G. Zhang, G. Somesfalean, W. Guo, H.S. Wang, S.H. Wu, Y.K. Qin, Z.G. Zhang, An optical system for measuring nitric oxide using spectral separation techniques, Appl. Phys. B 107 (2012) 435–440. [23] Y.G. Zhang, H.S. Wang, G. Somesfalean, Z.Y. Wang, X.T. Lou, S.H. Wu, Z.G. Zhang, Y.K. Qin, Broadband UV spectroscopy system used for monitoring of SO2 and NO emissions from thermal power plants, Atmos. Environ. 44 (2010) 4266–4271.
Biographies Lin Wang is currently pursuing her PhD under the supervision of Prof. Zhang. Her research is focused on the development of spectroscopic techniques for pollutant gas concentration monitoring. Yungang Zhang received his M. Sci. Degree and Ph.D. Degree in applied physics at Harbin Institute of Technology in 2004 and 2012, respectively. The focus of his research is to develop spectroscopic techniques for pollutant gas concentration monitoring. Xue Zhou is currently pursuing his PhD under the supervision of Prof. Zhang. Her research is focused on the study of TDLAS technique using different shaped cavities to enhance the equivalent path length of gas absorption.
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Feng Qin is currently a lecturer of physics. His research is focused on the development of luminescent materials based on rare earth ions. Zhiguo Zhang is a professor of Harbin Institute of Technology and director of the Laser Spectroscopy Lab since 2002. He has contributed more than 110 papers in
international journals and owned 9 Chinese patents. His current research interest is mainly focused on upconverting luminescent materials, fluorescent biosensing and spectroscopic techniques for environmental application.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Optical sulfur dioxide sensor based on broadband absorption spectroscopy in the wavelength range of 198–222 nm Lin Wang a , Yungang Zhang b , Xue Zhou a , Feng Qin a , Zhiguo Zhang a,∗ a b
Condensed-Matter Science and Technology Institute, Harbin Institute of Technology, Harbin 150001, China College of Electrical Engineering, Yanshan University, Qinhuangdao 066004, China
a r t i c l e
a b s t r a c t
i n f o
Article history: Received 30 December 2015 Received in revised form 8 October 2016 Accepted 12 October 2016 Available online 15 October 2016 Keywords: Sulfur dioxide Gas sensing Broadband absorption spectroscopy
A highly sensitive sulfur dioxide detection system based on broadband absorption spectroscopy was developed using the 198–222 nm wavelength range. The compact and simple measurement system was constructed utilizing fiber optoelectronic sensing device. A modified approach based on differential optical absorption spectroscopy was used for the computation of sulfur dioxide concentrations. The results of sulfur dioxide concentration were calibrated and displayed using a LabVIEW-based software. A system detection limit of 17 ppb per meter has been reached, and a ten-fold improvement in sensitive and uncertainty has been achieved for measuring sulfur dioxide concentrations. The system is suitable for monitoring sulfur dioxide concentration in air as well as for the fault diagnosis of gas insulated switchgears. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Sulfur dioxide is one of the most common air pollutants [1–3]. In China, the National Ambient Air Quality Standard (GB 3095-2012) stipulates that the amount of sulfur dioxide contained in the ambient air must not exceed 21 ppb, 52.5 ppb, and 175 ppb on annual, daily, and hourly average, respectively. For these reasons, an accurate system of monitoring sulfur dioxide concentration levels is of great importance to control pollution. In addition, sensitive measurements of low sulfur dioxide concentrations in confined spaces can also be used for the fault diagnosis of gas insulated switchgear (GIS) [4]. The fault of GIS can be diagnosed based on the content and production rate of sulfur dioxide in the airtight shielding gas [5–7]. Currently, the main techniques for monitoring sulfur dioxide in air depend on chemical sensors, broad-band absorption spectroscopic techniques based on differential optical absorption spectroscopy (DOAS), non-dispersive infrared spectroscopy (NDIR), laser-induced fluorescence (LIF), correlation spectroscopy (COSPEC), differential absorption lidar (DIAL), and tunable diode laser absorption spectroscopy (TDLAS) [8–13]. Among these numerous techniques, broad-band absorption spectroscopy is widely used for its superior high signal-to-noise ratio (SNR) and
∗ Corresponding author. E-mail address: [email protected] (Z. Zhang). http://dx.doi.org/10.1016/j.snb.2016.10.055 0925-4005/© 2016 Elsevier B.V. All rights reserved.
strong anti-interference performance. It has been approved by the Environmental Protection Agency (EPA 1995) as an equivalent method for the measurement of gaseous pollutants [14]. Currently, the reported detection limit is 0.2 ppm per meter, when using the spectroscopic absorption band around 300 nm [15]. In these conditions, path lengths of several hundreds of meters are required to achieve low sulfur dioxide concentration measurement for environment monitoring [14]. For the same detection limit, path lengths can be shortened via the utilization of a larger sulfur dioxide absorption cross-section. In the case of sulfur dioxide, the absorption cross-section around 200 nm is 10 times larger than that around 300 nm. Fortunately, Ocean Optics develops one kind of fiber optic spectrometer with a spectral range of 160–270 nm, equipped with a spectral resolution of 0.05 nm. This work presents the development of a highly sensitive sulfur dioxide sensor based on broadband absorption spectroscopy around 200 nm, for which a compact concentration measuring system was constructed. The relationship between sulfur dioxide concentration and its optical parameter (OP) has been established by using the Beer-Lambert law. The optical parameter is defined as the sum of signals recorded from all the measuring points. An automatic recording function has been implemented using a LabVIEW-based software. The continuous monitoring of different concentrations was performed to obtain the detection limit and uncertainty of the system. The demonstration of the superiority of the new system using the 200 nm absorption band was achieved
L. Wang et al. / Sensors and Actuators B 241 (2017) 146–150
Fig. 1. (a) Absorption cross section of sulfur dioxide from 284 to 311 nm. (b) Absorption cross section of sulfur dioxide from 198 to 222 nm.
147
Fig. 2. (a) Absorption spectrum of sulfur dioxide (black) and slow-change absorption spectrum (red). (b) Structured change absorption spectrum. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
by comparing its technical parameters with a former system based on the absorption band around 300 nm. 2. Theory Fig. 1 shows the absorption cross section of sulfur dioxide in the ultraviolet region. Both the absorption cross-section and the differential absorption cross-section are much larger around 200 nm than that around 300 nm. Therefore, the use of the absorption around 200 nm should improve the sulfur dioxide measuring performance. Considering such a strong structured change in the absorption spectrum around 200 nm, the approach described in reference [15] was used in this study. The measurement of sulfur dioxide concentration is derived from the Beer-Lambert law, which can be expressed as I() = I0 () exp[−C()L − ˛()],
(1)
Fig. 3. Experimental setup of the absorption sensing system for sulfur dioxide.
where I() and I0 () are the incident and received radiation intensities at wavelength , respectively, () (cm2 /molecule) is the absorption cross section, ␣() is the scattering and absorption coefficient of other components, C is the concentration of gas under test, and L is the absorption optical path length. As shown in Fig. 2, the sulfur dioxide absorption spectrum consists of a slow-change portion and a structured change portion. The following expression can be obtained using Eq. (1):
The proposed approach provides a simple and straightforward tool for the assessment of the measuring capability of the system. In fact, other methods, such as the least squares method, can provide equivalent results. To demonstrate that large absorption cross section improves the detection limit of system, the following analysis was carried out. The concentration uncertainty C corresponding to the detection limit of the measurement system, by differentiating formula (4) on both sides, can be expressed as
−C()L = ln(I()/S()),
CL =
(2)
where S() is the slow-change absorption spectrum, and () is the differential absorption cross sections. It has been confirmed that the slow-change portion can be replaced by a polynomial fitting curve [15]. In order to reduce the impact of noise signals, the optical parameter (OP) is defined as the sum of signals recorded from all the measuring points expressed as OP =
| ln(I()/S())|.
(3)
The relationship between optical parameters and concentrations can be expressed as CL =
OP |()|
.
(4)
⁄|I()⁄I(),
(5)
where I()⁄I() is the relative uncertainty of received radiation intensities, and C is the uncertainty of gas concentration under test. As I(), I() and L are constant parameters for certain measuring instruments, it can be seen that C is in inverse proportion of |()|. Considering the fact that the differential absorption cross section around 200 nm is 15 times larger than that around 300 nm, the selection of the absorption band around 200 nm seems preferable to that around 300 nm. 3. Experimentation Fig. 3 shows the experimental setup of the sensing system. It consists of two main components: the sulfur dioxide concentration measurement part and the gas preparation part.
148
L. Wang et al. / Sensors and Actuators B 241 (2017) 146–150
3.1. Preparation of sulfur dioxide Different concentrations of sulfur dioxide were prepared by mixing standard gas and pure nitrogen in different proportions. Standard gases of sulfur dioxide (Nanjing Tianze Co., Ltd) at the concentrations of 200 ppm, 50 ppm, 20 ppm and 10 ppm have been used. Accurate gas mixing ratios were obtained from the use of mass flow controllers (MFC, Sevenstar, D07-19C) with a precision of 0.01 L/min. The flow rate of the combined gas was stable, at 1 ± 0.01 L/min, throughout the experimentation. No corrections to pressure were made, for the pressure inside the cell was constant and approximately equal to one atmosphere. The entire experiment was proceeded at room temperature. The impact of sulfur dioxide adsorption was ignored due to the use of flowing gas. 3.2. Sulfur dioxide sensing system A high-pressure deuterium lamp (Hamamatsu 5601, Japan) with a broadband emission spectrum in the ultraviolet range was used as a light source. The light was converted into a parallel beam via a quartz lens with low UV absorption and a focal length of 25 mm. The parallel beam was then transmitted through a 40 cm long sample cell equipped with quartz windows, the cell was used for containing sulfur dioxide. The sample cell had an inlet port and an outlet port near each end of it to guarantee good air exchange. Since the deuterium lamp acts as an incoherent light source, the light interference coming from specular reflection was ignored. The transmitted beam collected by another quartz lens with the same focal length of 25 mm was focused into a multi-mode optical fiber (Ocean Optics OLFV-200-1100). Finally, the light was coupled into a high-resolution fiber optic spectrometer (Ocean Optics Maya2000) working in the spectral range of 160–270 nm. The spectrometer temperature was controlled using a proportional-integral-derivative (PID) loop feed back temperature control system at 40 ◦ C with a fluctuation of 0.3 ◦ C, to prevent excessive changes in the electronic noise of the spectrometer. Data analysis and control of spectrometer were performed automatically via a personal computer using a home-made LabVIEW-based software.
Fig. 5. Real-time collection of OP at 43.04 ppm and definition of OP.
4. Results and discussion From the different absorption cross-section values reported in the literature [16–19], the high-resolution absorption spectra of sulfur dioxide at 200 ppm with an optical length of 40 cm around 200 nm and 300 nm were measured at atmospheric pressure and room temperature to calculate absorption cross sections. The corresponding absorption cross sections of sulfur dioxide in the wavelength range of 198–222 nm and 284–311 nm evaluated by the Beer-Lambert law are shown in Fig. 1. The calibration of the sulfur dioxide sensing system using the new absorption band around 200 nm is performed through the relationship between OP and sulfur dioxide concentration, which was established in Fig. 4(a). Absorption spectra for a set of sulfur dioxide concentrations were measured, and corresponding OP values were calculated using the method above. In the calculations, the absorption wavelength range of 198–222 nm was selected to maintain high signal-to-noise ratio. The optical path length was converted into a standard length of 1 m to keep comparability with results in other literature [15]. The good linear relationship (R2 = 0.999) between the measured OP and sulfur dioxide concentration observed in Fig. 4(a) then allowed the derivation of the following calibration expression for sulfur dioxide concentration: CL = 8.3857OP(ppm m).
Fig. 4. (a) Measured OPs for different sulfur dioxide concentrations and their respective fitted values. (b) Fluctuations of OPs for different sulfur dioxide concentrations.
(6)
The zero off-set has been ignored, since it is far lower than the usual measured sulfur dioxide concentrations. The fluctuations of OP values are shown in Fig. 4(b). Continuous monitoring of different sulfur dioxide concentrations was implemented. For each concentration, more than 500 data points were measured with a 4 Hz sampling rate and an adjacent averaging window of 99 points was used to smooth the measured optical parameters. As shown in Fig. 5, the fluctuation of OP was evaluated from the difference between the maximum and minimum, in lieu of the standard deviation. Fig. 4(b) shows that the fluctuations of OP remained below 0.002 throughout the whole sulfur dioxide concentration range. In general, the detection limit of the system was defined as the concentration that brought changes of OP to a level equal to the fluctuation of the system. The detection limit of the system was determined to be 43 ppb with an optical length of 40 cm, which is equivalent to 17 ppb per meter. The superiority of the new sulfur dioxide measurement system was demonstrated by comparing the technical parameters of the new system with a previous system based on the absorption range around 300 nm [15]. The detection limit of the previous system was 0.2 ppm m. The measurement uncertainties associated to the sulfur dioxide concentration at 12 ppm m and 24 ppm m were about
L. Wang et al. / Sensors and Actuators B 241 (2017) 146–150 Table 1 Detection limits of different sulfur dioxide sensing systems. Method
Absorption region
Detection limit
Reference
DOAS system long-path DOAS system UV DOAS system DOAS system
275–315 nm 293–312 nm 200–400 nm 198–222 nm
0.2 ppm m 0.3 ppm m 0.2 ppm m 0.02 ppm m
[15] [20] [21] This work
0.5% with the previous system, whereas they are now about 0.3‰ with the new system. Considering the evaluation methods of the systems were different, the use of absorption band around 200 nm improved the performance by at least 10 times for measuring the sulfur dioxide concentrations. It is a comprehensive result, in terms of improvement of absorption cross section and spectrometer performance. The decrease of the uncertainties and the detection limit verified the analysis in the above theoretical part. The detection limits of other measurement systems using relevant approaches are listed in Table 1. During the process of monitoring sulfur dioxide in air, spectral interference from other compounds, such as O3 , O2 , NO, NO2 , NH3 etc., can occur. For O2 , the absorption in UV occurs at the wavelength lower than 198 nm. The absorption spectral interference is avoided by the selection of the 198–222 nm wavelength range. For O3 , the absorption is slowly varying with wavelength, it can be eliminated by analyzing the spectral structured changes. For NO, the narrow band absorption can be eliminated as reported in Refs. [22,23]. The possibly cumbersome absorption spectral interference comes from NO2 and NH3 , since both of them have structured absorption in the wavelength range of sulfur dioxide measurement. Fortunately, the concentration of NO2 in air is lower than that of SO2 , and its structured absorption cross section is about two orders of magnitude smaller than that of SO2 . The absorption spectral interference of NO2 can then be ignored. For NH3 , the structured absorption is in the same order of magnitude with that of SO2 . In the case, methods, such as the least squares method, can be used to extract concentration information from mixed spectrum with structured absorption spectral interference. 5. Conclusions The feasibility of high-precision sulfur dioxide concentration measurement using the new wavelength range of 198–222 nm has been demonstrated. A compact and simple measurement system was constructed using fiber opto-electronic sensing device and automatic measurements were monitored using LabVIEW-based software. A detection limit of 17 ppb per meter and concentration uncertainties of 0.3‰ were achieved. The sensitivity and uncertainty improvements of the new system showed to be at least ten fold in comparison to existing system based on absorption band around 300 nm. References [1] T. Wang, F. Hendrick, P. Wang, G. Tang, K. Clémer, H. Yu, C. Hermans, C. Gielen, J.F. Müller, G. Pinardi, N. Theys, H. Brenot, M. Van Roozendael, Evaluation of tropospheric SO2 retrieved from Max-DOAS measurements in Xianghe, China, Atmos. Chem. Phys. 14 (2014) 11149–11164. [2] L. Vogel, B. Galle, C. Kern, H. Delgado Granados, V. Conde, P. Norman, S. Arellano, O. Landgren, P. Lübcke, J.M. Alvarez Nieves, L. Cárdenas Gonzáles, U. Platt, Early in-flight detection of SO2 via differential optical absorption spectroscopy: a feasible aviation safety measurement to prevent potential encounters with volcanic plumes, Atmos. Meas. Tech. 4 (2011) 1785–1804. [3] C. Rivera, G. Sosa, H. Wöhrnschimmel, B. de Foy, M. Johansson, B. Galle, Tula industrial complex (Mexico) emissions of SO2 and NO2 during the MCMA 2006 field campaign using a moble mini-DOAS system, Atmos. Chem. Phys. 9 (2009) 6351–6361.
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Biographies Lin Wang is currently pursuing her PhD under the supervision of Prof. Zhang. Her research is focused on the development of spectroscopic techniques for pollutant gas concentration monitoring. Yungang Zhang received his M. Sci. Degree and Ph.D. Degree in applied physics at Harbin Institute of Technology in 2004 and 2012, respectively. The focus of his research is to develop spectroscopic techniques for pollutant gas concentration monitoring. Xue Zhou is currently pursuing his PhD under the supervision of Prof. Zhang. Her research is focused on the study of TDLAS technique using different shaped cavities to enhance the equivalent path length of gas absorption.
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Feng Qin is currently a lecturer of physics. His research is focused on the development of luminescent materials based on rare earth ions. Zhiguo Zhang is a professor of Harbin Institute of Technology and director of the Laser Spectroscopy Lab since 2002. He has contributed more than 110 papers in
international journals and owned 9 Chinese patents. His current research interest is mainly focused on upconverting luminescent materials, fluorescent biosensing and spectroscopic techniques for environmental application.