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Abstract
A novel CH4 sensor based on wavelength modulation spectroscopy with a multipass cell was developed for the soil respiration measurement of CH4. A home-made double-enhanced Herriot-type multipass cell with an effective absorption length of 73.926 m and a fiber-coupled distributed feedback diode laser emission at 1653.74 nm were used to design the sensor. The double enhancement of the effective optical pathlength of the multipass cell, absorption line locking, laser intensity normalization, and temperature control of the multipass cell were used to improve cell performance and achieve a minimum detection limit of 10 ppbv and a measurement precision of 6.4 ppbv. Finally, the potential of the developed CH4 sensor for ecological applications was verified by measuring the soil respiration of CH4 and monitoring of CH4 in the atmosphere over a long period.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Methane (CH4) is a critical non-CO2 greenhouse gas that has attracted considerable research attention for rapidly reducing global warming because of its short response time in atmosphere. A decrease in CH4 abundance would accelerate the effects of reducing the emission of CO2, rendering achieving goals of the Paris agreement less challenging [1]. The CH4 levels increased considerably after the industrial revolution, the increment speed of CH4 abundance in atmosphere fell in the 1990s but increased again in 2007, which resulted in the aggravation of greenhouse effects [2,3]. The variation of CH4 abundance in atmosphere reflects the balance between sources and sinks and is related to industrial emission and ecosystem interaction. Soil can either be a source or a sink of CH4 because of the simultaneously existence of methanogenesis and methanotrophy. In nature, the generation and consumption of CH4 by soil have a similar magnitude. However, improper land use may considerably influence the atmospheric CH4 accumulation rate [4]. Therefore, the measurement of CH4 flux of soils is critical in atmosphere and ecology management.
However, CH4 flux from soils is typically weak and difficult to measure in the atmospheric background. Therefore, requirements for measurement instruments are high. Although non-steady-state non-through-flow (i.e., closed static chamber) combined with discrete samples analysis using gas chromatography is the most widely used method for soil flux measurement, flux is typically underestimated. Furthermore, non-steady-state through-flow (i.e., closed dynamic chamber) is a precise method because of limited flux suppression [5]. In the second method, real-time, continuous, and sensitive measurement is performed by laser absorption spectroscopy (LAS). With the maturity of LAS, monitoring CH4 as well as other greenhouse gas using this novel technology has attracted considerable attention. Cavity-enhanced absorption spectroscopy (CEAS), cavity ring-down spectroscopy (CRDS), photoacoustic spectroscopy (PAS), tunable diode laser absorption spectroscopy (TDLAS) have been studied in laboratory or deployed in field measurements [6–15]. LAS provides in-situ and real-time highly sensitive measurements. In soil flux measurements, the chamber deployment time is typically about several hours and could be even more when a relationship of flux versus time is detailed. However, the long-term stability of LAS is a greater concern than sensitivity.
In the present work, a novel CH4 sensor based on the absorption-line-locked wavelength modulation spectroscopy combined with second harmonic detection was proposed. The sensitivity, stability, and precision of the sensor were evaluated. A novel, double-enhanced multipass cell was designed to increase sensitivity. Laser intensity is modulated with a high frequency sinusoidal waveform to suppress 1/f noise for higher sensitivity [16], and the center wavelength is locked at the peak of absorption line rather than scanned by triangular waveforms to improve the time response and precision. The gas sensing ability of absorption lines (including interferometer and atomic or molecular absorption lines), which are widely used as the wavelength locking Ref. [17–19], was investigated for determining an appropriate locking method. The influences of temperature and laser intensity fluctuations on sensor precision were investigated systematically. Combined with double-enhanced pathlength multipass cells, the optimized CH4 sensor can reach a sensitivity far less than the atmospheric background level. The performance was verified by monitoring the atmosphere and measuring soil respiration.
2. Sensor details
The schematic of the CH4 sensor and software procedures are shown in Fig. 1. A distributed feedback diode laser (NEL, NLK1U5FAAA) with a nominal wavelength of 1653.74 nm was used as the light source to detect CH4 absorption in R3 transition near 6046.95 cm−1. A laser controller (Wavelength Electronics, LDTC0520) was used to control the temperature and injection current of laser diodes and the target absorption line corresponding to 18.5 °C and 65 mA. A 95/5 fiber splitter (FOPTO, SMABC-1650-12-05-SM-L-10-FA) was used to split the emitted laser beam into two parts such that 95% traversed the home-made Herriot multipass cell to probe CH4 absorption and 5% passed through the reference cell for locking the laser wavelength to the target absorption line.
Fig. 1. (a) Schematic of the CH4 sensor, (b) distribution of laser spots, and (c) software procedures including: data acquisition, data processing, signal processing, and laser control.
The Herriot multipass cell consisted of two silver coated concave spherical mirrors with a diameter of 75 mm and separated by 33.3 cm. The curvature R of the used spherical mirrors was 1000 mm. The design of the Herriot multipass cell was followed in the theory described in [20,21]. On the mirrors two holes were set of 3 mm diameter for laser entrance / exit and separated with angle of 90°.The distance of the entrance / exit for laser beam of outer and inner circle to the center (O) were 32 mm and 28 mm, respectively. In the conventional Herriot cell arrangements, the incident light is reflected 111times (110 + 1, where 1 refers to outgoing beam in the cell which was not reflected again) between the two concave spherical mirrors, providing an effective optical pathlength of 36.963 m. The simulated light spots on concave spherical mirrors are shown in Fig. 2(a). The effective optical pathlength was enhanced by using the re-entrance method for improving detection performance. Namely, the outgoing light (passing through “Exit”) is then reflected back into the cell (passing through “Entrance”) by two plane mirrors (M1 and M2) as shown in Fig. 2(b) and again reflected 111 times between the two concave spherical mirrors, finally, providing an effective optical pathlength of 73.926 m. The simulated and photographed light spots on the concave spherical mirrors are shown in Fig. 2(b) and Fig. 1(b), respectively. Wedged windows are used in the cell to decrease fringes caused by window.
Fig. 2. Simulated distributions of light spots on the mirrors, (a) conventional Herriot multipass cell and (b) double-enhanced Herriot multipass cell.
The reference cell with an optical pathlength of 5 cm was filled with 6.58% CH4 (in N2) at pressure of ∼100 Torr and is sufficient to generate a strong absorption signal for absorption line locking under a single pass. Next, the transmitted laser intensity was detected by two photodiodes (PD1 and PD2, including home-made pre-amplifier). Two data acquisition cards (DAQ1 and DAQ2) were used because of the limitation of sample rates (250 kS/s). The reference signal from PD1 was acquired by DAQ1 and demodulated by the software-based lock-in amplifier [22]. The reference signal was multiplied by a frequency-tripled orthogonal reference signal with an adjustable phase and subsequently filtered by a lowpass filter to isolate the third harmonic signal. Only the X component of the harmonic signal was used. The third harmonic signal (3f) was fed to the proportion-integration-differentiation (PID) algorithm as the error signal. A software-based signal generator was used to generate 7 kHz, 70 mVpp sinusoidal waveform for modulation. The ultimate modulation signal output through DAQ1 was the sum of the sinusoidal waveform and output of the PID algorithm. The absorption signal acquired by DAQ2 was the second harmonic signal demodulated by the lock-in amplifier (FEMTO, LIA-BVD-150-H) and used for the derivation of the CH4 concentration through linear regression.
3. Analysis and optimizations
3.1 Temperature control
The temperature was controlled for three reasons: (i). distribution changes of the transmitted light spot on the photodetector. After 222 reflections in the Herriot cell, the outgoing light was picked up by the photodetector. However, the supporting material of the cavity mirror typically deform with temperature changes, resulting in misalignment of the optical path and the position changes of the outgoing light spot on the photodetector. Thus, the detected signal fluctuates considerably. (ii) Dependency of the absorption line strength on the temperature. According to the theoretical formula of the line strength [23], the absorption line strength is a function of temperature. The absorbance (i.e., absorption signal) changes with temperature according to Beer-Lambert’s law. (iii). Temperature response of the photodiode. The responsivity of the photodiode varies from the temperature and wavelength of incident light. The output current changes considerably with the temperature if the non-temperature-stabilized photodiode is used. Therefore, the temperature control of multipass cells is essential for improving the stability and sensitivity especially in field campaigns in which large temperature variations exist.
The multipass cell is enclosed in a thermostatic container that is thermostated by a temperature controller (Wavelength Electronics, PTC 5 KCH), two Peltier elements, and a feedback thermistor. The mechanical design of the thermostatic container is shown in Fig. 3. To decrease the time required to reach the equilibrium temperature, two fans are mounted in front of Peltier elements to generate the vortex flow for increasing the replacement rate. The temperature setpoint was set to 0.85 V, which corresponds to approximately 29.2 °C. Figure 4 shows the results of temperature variation and measured signal of indoor air in the conditions of the temperature controller at the ON and OFF state. The coefficient of variation (CV) indicated that the temperature control improved the measurement precision by a factor of approximately 2.5 on the condition of temperature fluctuation of only 1 °C. It should be noted that when investigating the influence of any one of temperature changes, wavelength drifts, and laser intensity fluctuations on sensor performance, the influence of the other two are already minimized, for instance, when investigating the influence of temperature changes, the laser has been locked onto the absorption line center and the signal has been normalized by laser intensity (i.e., the influence of wavelength drifts and laser intensity fluctuations on sensor performance has been minimized). The concentration in brackets is derived with calibration parameters obtained after all optimizations have been performed. Therefore, the concentration derived with these calibration parameters at the temperature controller in the OFF state is untrue but can be used for comparison. These two notes are same for part 3.
Fig. 4. Comparison of temperature control and without, upper: temperature (control off and on), lower: CH4 signals.
3.2 Absorption line locking
The emitted wavelength of the semiconductor laser diode is typically tuned by temperature and injection current, and the internal temperature of laser diode fluctuates with the rapid tuning of the current because the thermal equilibrium state is broken because of lags between the temperature control and tuning current. Therefore, the emitted wavelength drifts and affects the amplitude of absorption signal in the fixed-wavelength WMS. Reducing the frequency of the tuning current can effectively suppress these affections [24]. However, noise immunity also decreases. Thus, absorption line locking is critical for fixed-wavelength WMS, which allows a large integrating time, resulting in a higher signal-to-noise ratio and improves precision. The absorption-line-locked fixed-wavelength WMS is used to introduce an adjustable quantity superimposed on the tuning current to ensure that the emitted wavelength is locked at the center of absorption line. According to WMS theory, the absorption line center is located at the zero-crossing of WMS-3f or baseline-removed WMS-1f signal according to the theoretical simulation in Fig. 5.
Fig. 5. Theoretical simulation of the WMS-nf signal of CH4 at 300 K, 1 atm, 5 cm, and 0.0658 mole fraction.
Considering 1f signal contains a baseline correlated to laser intensity, the 3f locking approach was adopted. As shown in Fig. 1(a), the laser beam was divided into two parts by a fiber splitter with 5% traversing the reference cell. The transmitted light was picked by a photodetector and sent to a software-based lock-in amplifier through DAQ1 to demodulate the 3f signal. An adjustable quantity (direct current, DC) for real-time correction of tuning current was produced by using the PID algorithm. The ultimate injection current to laser diode was the sum of DC and sinusoidal waveform generated by software-based signal generators. The drifts of the output wavelength were monitored by observing the demodulated 3f signal. A comparison of the free running state (Lock OFF) and locking state (Lock ON) is shown in Fig. 6. The standard deviation (SD) of free running state is approximately 20% higher than the SD of the locking state.
3.3 Minimization of laser intensity fluctuations
Beam steering, window fouling, scattering, and mechanical vibration induce unpredictable fluctuations of received laser intensity. Many methods have been proposed to eliminate fluctuations induced by laser intensity. Cassidy and Reid proposed a novel method of normalization of 2f signals using the 1f signal as the reference for mitigating atmospheric turbulence interference along the absorption path. This method has been widely adopted in many applications [25,26]. Methods, such as using balanced amplified photodetector, to cancel the difference between signal beam and reference beam are highly effective [27]. A method similar to that proposed in a previous study [28] with the raw detector signal (DS) as the reference was used. However, the mean (DS-mean) rather than amplitude of DS was adopted. In the optical thin condition (α(ν) < 0.05), the 1f signal of WMS is independent to absorption [29]. In the atmospheric background, the weak direct absorption signal of CH4 in the raw detector signal can be neglected. Thus, DS-mean can be used as reference to normalize the laser intensity fluctuations and its feasibility was compared to the traditional 1f normalization.
The comparison is performed by measuring indoor air sealed in the absorption cell. The results shown in Fig. 7 revealed that both two normalized signals were smoother than the original 2f signal. The coefficient of variation (CV) between these two methods is close. The normalized signal should be a straight line; however, a slight drift was observed and the possible reasons could be: (1) the changes of the weak interference fringes; (2) the temperature control errors; (3) the changes of pressure in the absorption cell induced by temperature control errors which result in a variation of the modulation index [30].
Fig. 7. Minimization of laser intensity fluctuations and comparison between 2f/DS-mean and 2f/1f: (a) 2f, the demodulated 2f signal, (b) DS-mean, the mean of photodetector signal, (c) 1f, the demodulated 1f signal, (d) 2f/DS-mean, DS-mean normalized 2f signal, and (e) 2f/1f, 1f normalized 2f signal.
4. Sensor performance
The developed sensor was first calibrated after optimizations. CH4 standards were produced with the gas-mixing instrument (Environics, N-4000) by mixing two standard gases: 10.2 ppmv CH4 and pure N2. Pure N2 was used for the measurement of the sensor background (0 ppmv). The best linear fit and 95% confidence interval of measurement results are shown in Fig. 8. The R-square of 0.99952 revealed an excellent linearity of the developed CH4 sensor. The relationship between CH4 concentration and signal is expressed as follows:
where C is the concentration and S is the DS normalized 2f signal. An intercept representing background noise was not removed.To test sensor performance, 2 ppmv CH4 diluted from 10.2 ppmv standard gas was continuously tested for 2 h (Fig. 9(a)). The mean concentration was 2016 ppbv and halfwidth at half maximum is 6.4 ppbv. To determine the detection limit of the developed sensor, Allan deviation analysis was performed as shown in Fig. 9(b). The detection limit was 10 ppbv at the averaging time of 1 s and reduced with increasing averaging time to a minimum value of 1.5 ppbv when the averaging time is equal to 1000 s.
Fig. 9. (a) Continuous measurement of 2 ppmv CH4 and its distribution, (b) Allan deviation analysis.
5. Field tests
5.1 Soil respiration measurements
Soil respiration measurements were performed with a closed dynamic chamber in Hefei, China (117.173°E, 31.909°N) during June 18-20, 2020. The experimental plot is a grass field. The measurement schematic and a home-made chamber with consideration for ventilating to equilibrate pressure difference between inside and outside of chamber are shown in Fig. 10(a). The diameter and height of headspace of the chamber is 10 and 15 cm, respectively. The collar of the chamber was inserted approximately 2 cm into soil. Therefore, the volume of headspace was 1021 cm3 (i.e., 1.021 L). The gas in the chamber was circulated with a flow rate of 1.5 L min−1, controlled by the rotameter and circulating pump, and subsequently dried by a potassium permanganate desiccant to eliminate the potential influence of water on detection before entering the CH4 sensor. The CH4 concentration in the chamber increases to a plateau with time because of saturation effects [31,32]. A sample of soil CH4 flux measurement results is shown in Fig. 10(b). When the plateau is observed, measurements are terminated and the chamber is removed for a period before next measurements. Here, CH4 flux is calculated with equation [33]
where F is the CH4 flux in µmol·m−2·h−1 (the flux units also can be converted to µgCH4·m−2·h−1 by multiplying the F with molar mass of CH4), dC/dt is CH4 accumulation rates in µL·L−1·h−1 (i.e., ppmv·h−1), P is the atmospheric pressure in kPa, V is the volume of headspace in L, A is the basal area of chamber in m2, T is the air temperature in K, and R is the ideal gas constant 8.3145 in L·kPa·K−1·mol−1. The hourly averaged air temperature is obtained from the meteorological station (58321, 31.47°N, 117.18°E). The hourly flux and linear regression of the temperature and CH4 flux are shown in Fig. 11. According to the results, the soil of grass field functions as a source of CH4 during the experiment. A positive correlation between temperature and flux was found. Methane production, which is a microbial-mediated reaction. The increasing temperature can increase the metabolic activities of microorganisms, thus leading to the enhancement of CH4 emission [34,35]. It is difficult to generally describe the soil CH4 flux because it is governed by soil physicochemical properties such as water table, soil redox potential. However, analysis for specific measurements is feasible, for instance, a diurnal pattern of high emission at the day and low emissions at the night were observed at the experimental grassland. This diurnal pattern may be mainly induced by the diurnal changes of temperature. A similar pattern was also observed at peat meadow and urban wetland by Hendriks et al. and Morin et al., respectively [36,37].Fig. 10. (a) Schematic of soil CH4 flux measurements, (b) accumulation and saturation of CH4 in the chamber.
Fig. 11. (a) Hourly soil CH4 flux and temperature, (b) linear regression of temperature and soil CH4 flux.
5.2 Atmospheric CH4 monitoring
Atmospheric CH4 monitoring was performed from August 6 to September 3, 2020. The monitoring spot was at the seventh floor (∼30 m in height). Air was pumped into the CH4 sensor with flow rates of 1.5 L min−1 and dried by using a potassium permanganate desiccant. The measurement results of three typical days in the entire measurement period and hourly variation of entire measurement period are shown in Fig. 12. The mean concentration of atmospheric CH4 over measurement periods was 1.933 ± 0.258 ppm. CH4 concentration in atmosphere has an obvious periodic change, the peak was observed at 6 AM to 8 AM and a gradual decrease of CH4 concentration was also found after the peak was observed. The general diurnal pattern of atmospheric CH4 concentration is in agreement with previous reports, with the difference being that the valley was observed at slightly different time which may be induced by the height of the measurement sites [38,39].
Fig. 12. (a) Continuous measurement of atmospheric CH4 at three typical days, (b) hourly variation of atmospheric CH4 in 28 days.
6. Conclusion
A CH4 sensor operating at 1653 nm with high sensitivity and precision was developed. The precision was less than 0.5% (measured at 2 ppm) and Allan analysis revealed that a detection limit of 10 ppbv and 1.5 ppbv can be achieved by using an averaging time of 1 and 1000 s, respectively. The optimizations of 3f absorption line locking, laser intensity normalization, and temperature control reveal that the developed sensor is low cost, exhibits a fast response, and has a high stability. Soil respiration and atmospheric CH4 were monitored to verify the performance of the developed sensor, and the results revealed that the developed sensor for atmospheric and ecological research exhibits considerable potential for use in applications. In the future, first, double re-entrance (even more) can be applied to the multipass cell for increasing the pathlength so that CH4 sensor performance can be improved. Second, pressure control can be applied to suppress the slow fluctuations observed during long-period measurement.
Funding
National Key Research and Development Program of China (2021YFC2800302); National Natural Science Foundation of China (41730103).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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