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Abstract
We report an open-path and anti-pollution multi-pass cell based tunable diode laser absorption spectroscopy (TDLAS) sensor, which was designed for online measurement of atmospheric H2O and CO2 fluxes. It is mainly composed of two plano-convex mirrors coated on a convex surface, which makes it different from traditional multi-pass cells. This design does not allow a direct contact between the coating layer of the lens and air, thereby realizing the anti-pollution effect of the coating layer. Two DFB lasers operating at 1392 nm and 2004 nm were employed to target H2O and CO2 absorption lines, respectively. Allan analysis of variance indicated that detection limits of H2O and CO2 were 5.98 ppm and 0.68 ppm, respectively, at an average time of 0.1 s. The sensor performance was demonstrated by measuring CO2 and H2O flux emissions at Jiangdu Agricultural Monitoring Station in Jiangsu Province. The results were compared with those obtained using the commercial instrument LI-7500, which is based on non-dispersive infrared technology. The developed gas analysis instrument exhibited good consistency with commercial instruments, and its accuracy was comparable; thus, it has strong application prospects for flux measurements in any ecosystem.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
In recent years, climate change and ecological environment problems caused by greenhouse gases (GHGs) have become the focus of the world's attention. Water vapor (H2O) and carbon dioxide (CO2) have been confirmed as two influential GHGs. Currently, CO2 concentrations exceeding 410 ppm are nearly 50% higher than preindustrial concentrations, and its current elevated levels and rapid growth rates are unprecedented in the past 55 million years of geological record [1,2]. Therefore, high-sensitivity online monitoring of CO2 concentration in the ambient atmosphere can be used to monitor CO2 flux emissions, which is of great significance for understanding the local climate and managing the environment [3–5].
Eddy covariance (EC) technique is the most widely used method for determining surface-atmosphere exchange of energy and scalars [6]. This technique obtains ecological flux densities by calculating the covariance of vertical wind speed fluctuations and the concentration fluctuations during turbulent vertical transport. Furthermore, a gas analyzer is one of the most important parts of the EC system due to the following reasons: First, it can capture minor concentration fluctuations with the help of sufficiently accurate sensors. Second, it can capture the motion of most turbulent eddies; however, the sensors must meet the high-frequency sampling characteristics (≥10 Hz) [7,8]. There are two gas path methods for measuring gas fluxes using EC: the open-path method is an in-situ measurement; however, the closed-path method needs a vacuum pump to measure the gas, which increases the power consumption (up to hundreds and thousands of watts) and weight (up to tens and hundreds of kilograms) of the system [9]. Therefore, the closed-path method is not reliable for long-term field experiments. Moreover, since gas passes through the trachea during pumping, high-frequency turbulence is reduced and data delay occurs [10].
The development of laser spectroscopy and optoelectronic technology has allowed EC systems comprising sonic anemometers to measure flux densities. Spectroscopic technology has the characteristics of high precision and fast response time, which make it suitable for gas analysis via EC systems. For example, techniques based on cavity-ring down spectroscopy (CRDS) and off-axis integrated cavity output spectroscopy (OA-ICOS) are increasingly used for continuous measurement of CO2, H2O, and CH4 in the atmosphere [11–14]. Satar et al. used a CRDS analyzer (Picarro, G2401) to monitor greenhouse gas (CO2, CH4, CO) flux emissions for two years in Switzerland [15]. Despite the high sensitivity of sensors based on OA-ICOS and CRDS, they require complex setups with laser frequency locking to longitudinal cavity resonances [2,16]. These two spectroscopic methods are often used for closed cavity measurements, which are not easy to conduct for an open optical path. Non-dispersive infrared (NDIR) based technology is being increasingly used to measure atmospheric CO2 [17,18]. Hu Yao et al. measured the CO2 at the alpine peatland on the eastern Qinghai-Tibetan Plateau in China by using a sonic anemometer and gas analyzers (LI-7500, LI-COR) based on NDIR [19]. However, NDIR applications are limited due to the poor selectivity caused by using a wideband light source. Tunable diode laser absorption spectroscopy (TDLAS) changes the wavelength of the laser by tuning the injection current and temperature of the semiconductor laser, and scans the absorption spectrum of a specific molecule. TDLAS exhibits the characteristics of high sensitivity, high precision, high selectivity, and fast responsivity [20–22]. It has been widely used for various applications such as atmospheric GHG detection, toxic gas detection in chemical industry parks, breath diagnostics, combustion diagnostics, deep-sea dissolved gas detection, and isotope detection [23–26]. Li. et al. developed a laser analyzer with an open optical path based on derivative absorption spectroscopy for measuring fluxes between ocean and atmosphere [27]. Kai. et al. developed a QCL-TDLAS-based device for measuring NH3 fluxes and performed experiments in the suburbs of Ningbo City, Zhejiang Province, China [28]. There are mainly two different approaches to conduct TDLAS, namely: direct absorption (DAS) and wavelength modulation spectroscopy (WMS). DAS noises are always randomly distributed in all frequency bands [20]. In contrast, WMS provides better noise rejection and sensitivity. The CO2 and H2O concentrations in the air are several hundreds of ppm and a single percentage value, respectively, which implies that the detection sensitivity of TDLAS is sufficient for CO2 and H2O measurement; thus, TDLAS-based systems exhibit better cost performance than that of systems based on CRDS or OA-ICOS.
Open-path-based TDLAS is widely used for in-situ online measurement of EC flux. OA-ICOS and CRDS are generally used for closed cavity measurements, while TDLAS is more suitable for open-path measurements. However, the traditional open-path method is unsuitable because its optical path is exposed to air, which causes the coating layer of the lens to be corroded in the atmosphere [29]; consequently, this method damages the system and is not conducive to long-term field work. Therefore, we propose the design of a novel anti-pollution OP-MPC, which can improve the long-term stability of the system.
In this work, we developed a compact open-path CO2 and H2O sensor based on TDLAS-WMS. The novelty of this study lies in the anti-pollution OP-MPC, which has been designed to improve the long-term measurement stability of the open-path system. The optical paths of CO2 and H2O have been integrated in this OP-MPC. To the best of our knowledge, there are no reports on anti-pollution OP-MPCs. The sensor was designed to use 2004 nm and 1392 nm distributed feedback (DFB) lasers combined with a three-dimensional (3D) sonic anemometer for monitoring EC flux. It was deployed at the Jiangdu agricultural monitoring station in Yangzhou City, and the results were compared with those of a commercial NDIR-based instrument (LI-7500, LI-COR).
2. Detection principle and absorption line selection
2.1 Theoretical principle of TDLAS-WMS
In WMS, the frequency of the light $v(t )$ is determined using a slow scan ${v_s}(t )$ with fast sinusoidal modulation at amplitude ${a_m}(t )$ and frequency ${f_m}$:
where θ denotes the phase shift. According to the Beer-Lambert law, the laser intensity transmitted through an absorbing medium of temperature T and pressure P can be written as follows:2.2 Method of flux measurements
The EC method was first proposed by Australian micrometeorologist Swinbank in 1951, and has been widely used ever since to measure surface atmosphere exchange [31]. It mainly involves calculating the turbulent flux by determining turbulent pulsation values. The EC flux F can be defined as follows:
2.3 Selection of CO2 and H2O absorption lines
There are several strong absorption bands for CO2 and H2O from 1 µm to 3 µm. CO2 absorption bands near 1.57 µm and 2.0 µm are free of H2O interference and can be used for CO2 measurements. However, the absorption line intensity at 2.0 µm is stronger than that at 1.57 µm by nearly two orders. Figure 1(a) and (b) shows simulation spectra of the selected CO2 (at 4991.25 cm−1) and H2O absorption lines (at 7181.14 cm−1), based on the HITRAN 2016 database, respectively. The simulation was calculated using 500 ppm CO2 and 1% H2O vapor. Meanwhile, potential interferences from 2 ppm CH4 and 300 ppb N2O were taken into account in the simulation. The measurements of the two gases do not affect each other. The results indicate that the target lines are suitable for the detection of CO2 and H2O. Therefore, the absorption lines of CO2 (centered at 4991.25 cm−1) and H2O (centered at 7181.14 cm−1) were selected in this research. Furthermore, DFB lasers with wavelengths of 2004 nm (DFB-2004, Nanoplus) and 1392 nm (NLK1E5EAAA, NEL) were used.
Fig. 1. Simulation spectra of 500 ppm CO2, 1% H2O, 2 ppm CH4 and 300 ppb N2O at 1 atm in the range of (a) 4988-4993.5 cm−1 and (b) 7179-7185 cm−1, respectively.
3. Experimental details
3.1 Design of the anti-pollution multi-pass cell with an open-path plano-convex Mirror
When the traditional OP-MPC is applied in an actual field, film layer of the lens is easily contaminated. Therefore, we designed a plano-convex MPC system to prevent corrosion and contamination of the film layer. The proposed system consists of two opposing plano-convex lenses. The reflective film is coated on the plano-convex surface and placed outside the OP-MPC to form a stable resonant cavity structure. The reflective film material of our OP-MPC is silver, which not only has a low cost but also has a wide spectral response range from visible to middle infrared. The light beam is reflected several times in the OP-MPC cavity and forms a circular light spot on the mirror. This is similar to the ray transfer matrix method utilized in the Herriott cell [32], which uses the line-sphere intersection and ABCD matrix to calculate the spot distribution on the lens. Figure 2 shows the structures of a traditional OP-MPC and the anti-pollution OP-MPC; the yellow lines represent the coating layer and the blue dotted lines represent the sealing structure. The proposed design increases the absorption light path, avoids a direct contact between the lens film layer and polluted gas that needs to be detected, and prevents the corrosion of the lens film layer by atmospheric pollution gases and dust. To facilitate the installation of the fiber collimator and detector, we designed the light entrance and exit holes to be in different positions. The four uncoated holes at the edge of the coated surface ensure that the two beams enter and exit the OP-MPC. The two lenses comprised K9 glass with a diameter of 70 mm, thickness of 8 mm, and convex curvature radius of R = 550 mm; the distance between the two lenses was 42.8 cm. The CO2 and H2O paths lead to 16 and 2 reflections, respectively, thus forming an effective optical path of 6.8 m and 0.85 m, respectively. The optical path diagram of the designed anti-pollution OP-MPC and the light spot distribution on the lens are shown in Fig. 3.
Fig. 3. Optical path diagram of the anti-pollution OP-MPC: (a) optical path structure; (b) light spot distribution.
3.2 Instrument design
The developed detection system is shown in Fig. 4; it comprises a gas analyzer to detect CO2 and H2O concentrations at a speed of 10 Hz, a 3D sonic anemometer to determine vertical wind speed, and a data collector (Campbell, CR6) to collect gas analyzer and wind speed data. The flux data are calculated every half an hour and stored in the SD card.
Fig. 4. Photograph of the TDLAS sensor and schematic of the open-path TDLAS system used for measuring atmospheric H2O and CO2 concentrations.
The gas analyzer comprises an OP-MPC, two lasers, two detectors, two laser control circuit boards, a data acquisition card (NI-USB-6366), and an industrial personal computer (IPC). First, the NI card generates a low-frequency sawtooth wave and superimposes a high-frequency sine wave voltage signal using analog output (AO). This voltage signal passes through the voltage-to-current circuit on the laser control board to achieve current control of the laser. The current control circuit can achieve a center current of 70 mA and scanning current of ±50 mA. The temperature control of the laser was adjusted using an analog proportion integration differentiation (PID) through the driver chip (Maxim, MAX8521) on the circuit board, and the control accuracy reached up to 0.001°C. To ensure that the laser works stably and to avoid wavelength drift in the field environment, we have provided secondary temperature control to the laser in the system. This has improved the environmental adaptability of the system. Meanwhile, the microcontroller (ST, STM32F4) is used to transfer different voltages to the ADC for achieving different temperatures. Consequently, by adjusting the temperature and current of the laser, an optical signal is generated at a wavelength corresponding to the gas absorption center; this allows us to achieve complete control of the laser. The modulated optical signal passes through the collimator and undergoes multiple reflections in the anti-pollution OP-MPC, which has been independently designed in our laboratory. The optical path length of H2O and CO2 is 0.85 m and 6.8 m, respectively. The two light signals after multiple reflections pass through the focusing lens and are converted into current signals by the photodetectors (Hamamatsu), after which they are converted into a voltage signal by the transimpedance amplifier in the preamplifier circuit. The photodetector module has been developed in our laboratory and exhibits low noise and high gain performance. The NI card collects the signal of the detector at 2 MHz/s, transmits it to the IPC via USB in real time, and obtains the 2f signal after digital phase-locked amplification (DLIA) and Butterworth low-pass filtering (B-LPF); this leads to demodulation of the signal. Subsequently, the normalized 2f peak-to-peak value (that is, the concentration value of the signal to be measured) is sent to CR6 through RS232. The sensor has a total weight of 9.2 kg and total power consumption of less than 40 W under normal operation.
4. Results and discussion
4.1 Calibration and measurement precision
In the gas laboratory of the Chinese Academy of Metrology, the WDF-03 dynamic gas distribution device was used to mix 99.9999% nitrogen and 15996 ppm standard CO2 gas in different proportions to produce different CO2 concentrations (3007, 2016, 1024, 768, 512, and 0 ppm). These CO2 concentrations were passed into the analyzer with a cylindrical polymethyl methacrylate airtight cover at a flow rate of 500 ml/min. Consequently, the data are recorded when the concentration is stable. Figure 5(a) and (c) show the measured normalized 2f signals for different concentrations. Figure 5(b) and (d) show the measured response value and concentration, which are used to calibrate the sensor system for more accurate monitoring. Linear relationships are observed between the measured values and concentrations within the measurement range, and all the linear regression R-squared values exceed 0.999. We insert the system in a box with constant temperature and humidity, and then use a dew point meter (Dew star, S-2). Subsequently, different concentrations of water vapor are set. The results are shown in Fig. 4(b) and linearity exceeds 0.999.
Fig. 5. Second harmonic signal at different concentrations of (a) CO2 and (c) H2O. Calibration measurement of the TDLAS system for (b) CO2 and (d) H2O.
The detection limits of the developed laser gas analyzer were evaluated using Allan variance plots [33]. The Allan deviation results for CO2 and H2O concentrations for samples containing 512 ppm carbon dioxide and 1% water are plotted on a log-log scale (Fig. 6). The measurement noise at a data rate of 10 Hz (i.e., 0.01 s averaging time) is 0.68 ppm for CO2 and 5.98 ppm for H2O. With an increase in the averaging time, the minimum CO2 and H2O concentrations are 0.45 and 2.7 ppm at an integration time of 0.6 and 1 s, respectively.
Fig. 6. (a) Continuous measurements of CO2 concentrations at 512 ppm. (b) Continuous measurements of H2O concentrations at 10000 ppm. Allan deviation plots for concentrations of (c) CO2 and (d) H2O.
4.2 Real-time measurement and comparison
To test the CO2 flux emission of the farmland in the wheat season and the performance of the developed gas analyzer, the developed TDLAS system, LI-7500 NDIR equipment, and 3D sonic anemometer were set up on the flux observation tower. Data obtained by the TDLAS system and LI-7500 were transmitted to two CR6s at the same time and real-time flux calculation was performed. The half-hour flux data were processed as an arithmetic average to obtain a flux data point. The experimental site is located at the base of the seed field in Maling Village, Yangzhou City, Jiangsu, China (32°35′49.6″N, 119°42′38.2″E). The CO2/H2O flux emission monitoring and comparison experiment was conducted for nearly one month. The experimental site is shown in Fig. 7.
Fig. 7. Photograph of the installation and comparison of the field measurements obtained using TDLAS and LI-7500 for 24 h.
First, we conducted a test to compare the high-frequency concentration data of 24 h. In Fig. 7, the black area represents the LI-7500 data and the red area represents the TDLAS data. We put the two datasets on the same abscissa, which represents the time coordinate. The water vapor concentration and temperature show a negative correlation, while the CO2 and H2O concentrations exhibit very high consistency and the same change trend over time.
Then, to evaluate the gas analyzer's ability to measure turbulent activity within a certain frequency range, we plot the normalized power spectral density and frequency associated with the high-frequency CO2 and H2O concentrations in Fig. 8. Since our sampling frequency is Fs = 10 Hz, the cut off frequency of the power spectrum is Fc = Fs/2 = 5 Hz. The comparison results shown in Fig. 8 exhibit a downward slope of −3/5, which is in line with the power spectrum of the critical atmospheric turbulence. This shows that both TDLAS system and LI-7500 are able to measure high frequency turbulence in the atmosphere within 10 Hz.
Fig. 8. Power density analysis of concentration measurements obtained using TDLAS and LI-7500: (a) CO2 and (b) H2O.
Finally, we used CAST3B to compare the flux test results, as shown in Fig. 9. The gray area represents the periods wherein rainwater was on the lens, which affected the optical path and caused the instrument data to be abnormal. The flux results calculated by our equipment are linearly correlated with the flux results obtained using LI-7500, and the three-day continuous flux measurement results are selected for analysis. In Fig. 10(a), abscissa is the CO2 flux associated with the TDLAS equipment, and ordinate is the CO2 flux associated with LI-7500. Subsequently, linear fitting is performed using the least square method to obtain the linear correlation coefficient R2 = 0.87. Similarly, Fig. 10(b) shows the H2O flux, and the linear correlation coefficient R2 = 0.96. Furthermore, the negative flux emission of CO2 reached its minimum value (approximately −2 mg /m2s) at 12:00 noon, and the photosynthesis of crops was the most prevalent at this time. Meanwhile, the maximum H2O flux was approximately 100 g /m2s.
5. Conclusions
In this study, we report an open-path gas analyzer based on TDLAS to measure atmospheric CO2 and H2O fluxes. First, we designed a new anti-pollution OP-MPC to improve the stability of the system for long-term external field operations. Second, we used 1392 nm and 2004 nm DFB lasers to measure H2O and CO2 concentrations, respectively, and integrated the two optical paths into a single multi-pass cell with an effective optical path of 0.85 m for H2O and 6.8 m for CO2. The gas analyzer exhibited a detection limit of 5.98 ppm for H2O and 0.68 ppm for CO2 at an averaging time of 0.1 s. Finally, we conducted field experiments with the equipment and implemented flux detection using a 3D ultrasonic anemometer. The obtained results were in good agreement with those achieved using the NDIR-based commercial instrument LI-7500. Overall, the prepared analyzer has strong application prospects for flux measurements in different ecosystems.
Funding
National Natural Science Foundation of China (41730103); National Key Research and Development Program of China (2017YFC0209700).
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 maybe obtained from the authors upon reasonable request.
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