CN115343233A - A method and device for real-time measurement of trace gas concentration on an open path - Google Patents

Abstract

The invention discloses a real-time measurement method and a real-time measurement device for the concentration of trace gas on an open path, wherein the used devices comprise a signal generator, two tunable diode lasers, two optical isolators, two optical power amplifiers, a wavelength division multiplexer, two optical fiber beam splitters, two collimating mirrors, a parabolic reflector, a pyramid prism, a dichroic mirror, a beam splitter, four photoelectric detectors, three narrow-band filters, a high-pass filter, a low-pass filter, a data acquisition card and the like; the beat frequency signals of the measuring light and the reference light are used for measuring the path distance, two absorption spectral lines which are easy to obtain are used for measuring the path average temperature, and the absorption spectral line of a single gas to be measured is used for measuring the absolute concentration, so that the dependence on the absorption spectral line of the gas to be measured is reduced; the invention can simultaneously measure the gas concentration, the path average temperature and the path distance on the open path, and can be applied to the long-term monitoring of the greenhouse gas content in the farmland, the livestock farm, the city and other areas.

Description

A method and device for real-time measurement of trace gas concentration on an open path

(1) Technical field

The invention proposes a real-time measurement method and device for trace gas concentration on an open path, belonging to three technical fields of tunable diode laser absorption spectrum, temperature measurement and absolute distance measurement.

(2) Background technology

Rising global temperatures not only have a huge impact on the ecological environment, but also have interfered with the normal functioning of human society. With the entry into force of the Paris Agreement, countries around the world are paying close attention to the issue of greenhouse gas emissions and formulating corresponding emission reduction measures according to their own national conditions. At present, the greenhouse gases emitted by human activities mainly include carbon dioxide, methane, nitrous oxide, hydrofluorocarbons, sulfur hexafluoride, chlorofluorocarbons, ozone, etc. Among them, carbon dioxide and methane contribute more than 90% to the greenhouse effect. Greenhouse gas real-time monitoring technology can not only identify the source of greenhouse gas emissions, but also provide accurate data support for formulating energy-saving and emission-reduction policies.

Gas sensors based on spectral technology have the advantages of non-invasiveness, high selectivity, high precision, high sensitivity, simple structure, small size, and low cost, and have been widely studied in industrial and scientific research fields such as environmental monitoring, industrial process control, and medical diagnosis. with application. The most commonly used spectroscopic techniques in these optical sensors are: ring-down cavity spectroscopy (Cavity Ring-down Spectroscopy, CRDS), quartz-enhanced photoacoustic spectroscopy (Quartz-enhanced Photoacoustic Spectroscopy, QEPAS), chirped laser dispersion spectroscopy (Chirped Laser Dispersion Spectroscopy, CLaDS) ) and Tunable Diode Laser Absorption Spectroscopy (TDLAS). Laura E.Mchale et al. in 2019 "Optics Express" Volume 27, Issue 14, 20084-20097, paper "Open-path cavity ring-downmethane sensor for mobile monitoring of natural gas emissions" The ring-down cavity spectroscopy technology is introduced in detail in mobile monitoring of natural gas emissions, Optics Express. A typical ring-down cavity measurement device uses a pair of high-reflectivity concave spherical mirrors to form a high-precision optical cavity, and the measured gas Packaged in an optical cavity, the light is reflected back and forth between two mirrors, and the attenuation rate of the light in the optical cavity is recorded, and then the concentration of the gas to be measured is determined according to Beer Lambert's law. Kaiyuan Zheng et al. "Quartz-enhanced photoacoustic-photothermal spectroscopy based on quartz-enhanced photoacoustic-photothermal spectroscopy" (Quartz-enhanced photoacoustic-photothermal spectroscopy) paper by Kaiyuan Zheng et al. Fortrace gas sensing, Optics Express) introduced in detail the quartz-enhanced photoacoustic spectroscopy technology, which uses the photoacoustic effect to achieve spectral detection, and usually uses a tuning-fork quartz crystal oscillator as a high-Q factor harmonic oscillator to detect trace gases. The weak sound wave, the tuning fork quartz crystal oscillator is an acoustic quadrupole, so it has strong immunity to environmental noise. This technology has relatively high requirements on the quality of the beam, and it is required that the laser beam cannot touch the surface of the tuning fork when it passes through the gap between the vibrating arms of the tuning fork. Because ring-down cavity spectroscopy and quartz-enhanced photoacoustic spectroscopy have high requirements on the measurement device, these two techniques are not suitable for gas concentration measurement on an open path. When measuring gas concentration, the gas absorption signal is proportional to the product of gas concentration and path length. In order to measure the gas concentration more accurately, it is necessary to obtain a more accurate path length. The traditional method for measuring trace gas concentration is to measure the absorption path length in advance with a measuring ruler or an additional laser rangefinder, and then measure the absorption signal. However, during long-term measurements, the path length may change due to structural deformation or mechanical vibration. Therefore, it is necessary to develop devices and methods that can simultaneously measure path length and gas absorption.

Chirped laser dispersive spectroscopy uses the molecular dispersion effect to achieve spectral detection. It measures the gas concentration by detecting the fluctuation of the refractive index near the gas absorption peak. This method does not depend on the absorption effect and does not need to normalize the received optical power, so it is not only suitable for high-concentration gas detection but also for long optical path gas remote sensing. NartS.Dsghestani et al. in the paper "Analysis and demonstration of atmospheric methane monitoring based on mid-infrared open path chirped laser dispersion spectroscopy" (Analysis and demonstration of atmospheric methane) in Volume 22, Issue 7, pp. 1731-1743 of "Optics Express" in 2014 Methane monitoring by mid-infrared open-path chirped laser dispersion spectroscopy, Optics Express) uses chirped laser dispersion spectroscopy technology to measure the concentration of atmospheric methane on the open path, using a mid-infrared laser with a center wavelength of 7.7942 μm on a path of 90 meters Continuously measure ambient gas for 2 hours, and achieve a lower detection limit of 100ppb methane concentration. This solution uses a mid-infrared quantum cascade laser with a relatively small output power of only 5.8mW, which requires a highly sensitive detector when measuring the gas concentration in an open path. In addition, in order to obtain the best measurement accuracy, the modulation frequency of the laser light intensity needs to reach the order of 3.4 GHz, which requires a high bandwidth of the detector, and the current mid-infrared detector cannot achieve a bandwidth above GHz. Michal Nikodem et al. in the paper "Open-path sensor for atmospheric methane based on chirped laser dispersion spectroscopy" (Open-path sensor for atmospheric methane based on chirped laser dispersion spectroscopy, In Applied Physics B), chirped laser dispersive spectroscopy technology is used to measure the atmospheric methane concentration on an open path of 35 meters, and the methane spectral line near 1.653 μm is selected to continuously measure the ambient gas for 2.7 hours. The results show that the system can reach 1.3ppmv The lower detection limit of methane basically meets the requirements for the measurement of atmospheric methane concentration. This solution requires a high-sensitivity detector with a bandwidth of GHz order. The current photodetectors cannot take into account both high bandwidth and high sensitivity at the same time, which will inevitably reduce the detection sensitivity of the measurement system. In addition, the system needs to introduce a reference gas cell to calibrate the gas concentration to be measured in the atmosphere during the measurement process, and the gas concentration value in the reference gas cell will directly affect the accuracy of the gas concentration to be measured. Wuwen Ding et al. in the paper "Dual-sideband heterodyne of dispersion spectroscopy based on phase-sensitive detection" (Dual-sideband heterodyne of dispersion spectroscopy based on phase-sensitive detection) in Applied Optics, Volume 55, Issue 31, pp. 8698-8704 in 2016 , Applied Optics) introduced in detail the principle of chirped laser dispersion spectroscopy to measure gas concentration, and measured methane gas with a concentration of 940.5ppm.m under normal temperature and pressure. Under the integration time of 30s, the detection of methane concentration The lower limit can reach 0.2ppb. Although this method can achieve high-precision gas concentration measurement, it is necessary to calibrate the relationship between the peak-to-peak value of the dispersion spectrum and the gas concentration before measurement. In order to obtain double sided band dispersion spectrum, the modulation frequency of laser light intensity needs to reach GHz level, which requires high bandwidth of detection system and acquisition system. Yifeng Chen et al. "Optical Communications" Volume 46, Issue 13, pp. 3005-3008 in 2021, "Fugitive methane detection using open-path stand -off chirped laser dispersion spectroscopy, Optics Letters) combined phase-sensitive chirped laser dispersion spectroscopy technology with frequency-modulated continuous wave ranging technology to measure the concentration of methane in the atmosphere. The experimentally measured atmospheric methane concentration was 2.9ppm. This solution cannot measure gas concentration and path length at the same time, and the measurement optical path of the two parameters needs to be manually switched. The measurement system is relatively complicated, and a reference gas cell needs to be introduced for real-time calibration during the measurement process. Andreas Hangauer et al. "Chirped laser dispersion spectroscopy for spectroscopic chemical sensing with simultaneous" (Chirped laser dispersion spectroscopy for spectroscopic chemical sensing with simultaneous range detection, Optics Letters) using chirped laser dispersive spectroscopy to simultaneously measure gas concentration and path length. The gas concentration is related to the peak-to-peak value of the second harmonic of the modulation signal, and the relationship between the two needs to be calibrated in advance; the path length is related to the baseline frequency offset of the modulation signal, and the two are in a linear relationship. By extracting the baseline frequency offset of the modulation signal to obtain the path length. In order to obtain the best gas concentration measurement accuracy, the modulation frequency of the laser intensity needs to be controlled at the GHz level, and a detector with a higher bandwidth is required. In addition, due to the influence of the modulation frequency and the rate of change of the modulation frequency, this method can only achieve a ranging accuracy of the order of 0.1 meters. Mingli Zou et al. "Simultaneous measurement of gas absorption based on dispersive spectroscopy double-sided band heterodyne phase-sensitive detection based on simultaneous measurement of gas absorption and path length" in "Optics Express" Volume 29, Issue 8, pp. 11683-11692 in 2021 and path length based on the dual-sidebandheterodyne phase-sensitive detection of dispersion spectroscopy, OpticsExpress) using phase-sensitive chirped laser scattering spectroscopy to achieve simultaneous measurement of gas concentration and path length. The ranging range of this technology is inversely proportional to the modulation frequency, and the modulation frequency of MHz order can only measure the distance within 10 meters. However, the gas concentration measurement requires a modulation frequency above GHz. In order to balance the measurement requirements of the two parameters, this method can only realize the simultaneous measurement of the two parameters within the range of 1 meter. The above schemes all use chirped laser dispersion spectroscopy to measure gas concentration, which has potential application value in open path gas concentration measurement. However, in practical use, this technology requires an electro-optic modulator with a modulation frequency above GHz and a detector with a corresponding bandwidth. Due to the limitations of electro-optic modulators, detectors and other devices, this method is currently mainly used in the 1-2 μm near-infrared band, where there are fewer selectable spectral lines and the spectral line intensity is weaker. In addition, the gas concentration cannot be obtained directly from the dispersion spectrum. It is necessary to calibrate the relationship between the peak value of the dispersion spectrum and the concentration, and then deduce the measured gas concentration through the calibration relationship between the two. The accuracy of gas concentration measurement depends on the calibration results.

Tunable Diode Laser Absorption Spectroscopy (TDLAS) technology currently has two main implementation methods: direct absorption spectroscopy (direct absorption spectroscopy, DAS) and wavelength modulation spectroscopy (wavelength modulation spectroscopy, WMS). Wavelength modulation spectroscopy technology loads a high-frequency modulation signal to the laser, modulates the wavelength, and measures gas parameters by detecting harmonic signals. This technology can effectively suppress the background noise of the system, thereby improving the detection sensitivity, and has been widely used in the measurement of trace gas concentration. Gregory B.Rieker et al. in the 2009 "Applied Optics" volume 48, the 29th issue 5546-5560 paper "Calibration-free wavelength modulation spectrometry for gas temperature and concentration measurement in harsh environments" (Calibration-free wavelength -modulation spectroscopy for measurements of gas temperature and concentration in harsh environments, Applied Optics) introduced in detail the calibration-free wavelength modulation spectroscopy technology, which requires two or more absorption lines of the gas to be measured, and uses colorimetry to measure the gas temperature , and then get the gas concentration. When measuring gas concentration, the path length needs to be determined in advance, and the path length may change during the measurement process, so the path length measured in advance cannot truly represent the path traveled by the laser. Liang Mei et al. in 2011 "Optics Express" Volume 36, Issue 16 3036-3038 paper "Gas Spectroscopy and Optical Path Length Measurement in Scattering Medium Based on Wavelength Modulated Continuous Wave Diode Laser" (Gas spectroscopy and optical path- lengthassessment in scattering media using a frequency-modulated continuous-wavediode laser, Optics Express) combines wavelength modulation spectroscopy technology and frequency modulation continuous wave ranging technology to realize the measurement of gas concentration in porous media. Although this scheme can realize gas concentration and High-precision measurement of path length, but these two parameters cannot be measured at the same time. It is necessary to manually select the parameters for each measurement, and the operation is relatively complicated. Jinbao Xia et al. in 2019, "Optics and Laser Engineering", Volume 117, Issue 4, pages 21-28, "Probing greenhouse gases in turbulent atmosphere based on long-range open path wavelength modulation spectroscopy" (Probing greenhouse gases in turbulent atmosphere) by long-range open-path wavelength modulation spectroscopy, Optics and Laser in Engineering) uses wavelength modulation spectroscopy to measure the concentration of methane and carbon dioxide in the turbulent atmosphere, and continuously measures 10 hours on a path of 2.6 kilometers. The concentration of methane and carbon dioxide The detection limit can reach 2ppb and 20ppm respectively. This method needs to introduce a reference gas cell during the measurement process. The measurement accuracy of the gas concentration to be measured is related to the concentration of the reference gas, the length of the reference optical path, and the distance between the light source and the reflector. The measurement accuracy of the above three parameters will directly affect The accuracy of the gas concentration to be measured. In 2019, "Optics and Laser Engineering", Volume 115, Issue 23, pages 243-248, paper "A portable sensor for in-situ measurement of ammonia based on near-infrared laser absorption spectroscopy" by Xinqian Guo et al. In situ measurement of ammonia based on near-infrared laser absorption spectroscopy, Optics and Laser in Engineering), a set of portable ammonia gas in-situ measurement sensor was built by using multiple reflection gas cell, and the sensor was used to measure 30ppm ammonia gas continuously for 1 hour Measurement, the detection limit of ammonia concentration can reach 0.16ppm. In this solution, before measuring the concentration of ammonia gas, ammonia gas needs to be filled into the gas cell, which is not conducive to reflecting the actual state of the gas to be measured. When the temperature is lower than 415K, ammonia gas will be adsorbed on the glass on the inner wall of the gas cell, resulting in inaccurate measured gas concentration. Guishi Wang et al. "Laser Frequency Locking and Intensity Normalization in Wavelength Modulation Spectroscopy for Sensitive Gas Sensing" (Laser frequency locking and intensity normalization in wavelength modulation spectroscopy for sensitive gas sensing, Optics Express) uses wavelength modulation spectroscopy to measure 50ppm methane at normal temperature and pressure, and the effective optical path of the multiple reflection gas cell used is 26.4 meters. The lower limit of detection for methane concentration is 2.5 ppbv. In actual use, this solution needs to sample the ambient gas and fill it into the gas cell, which will change the actual state of the gas to be measured. Mingli Zou et al. in the 2020 "Optics Express" Volume 28, Issue 8 11573-11582 paper "Acetylene sensing system based on wavelength modulation spectrum based on three rows of circular multiple reflection gas cells" Modulation spectroscopy using a triple-row circular multi-pass cell, Optics Express) built a set of acetylene sensors based on three rows of circular multiple-reflection gas cells based on wavelength modulation spectrum, the effective optical path can reach 21.9 meters, using this sensor for 100.6ppm The detection sensitivity test of acetylene gas shows that the lower detection limit of the sensor is 76.75ppb under the integration time of 340s. When measuring ambient gas, this method also needs to sample the gas to be measured, which changes the real state of the gas to be measured. Chenguang Yang et al. "Simultaneous measurement of gas absorption and optical path length based on the first harmonic phase angle method in wavelength modulation spectroscopy" (Simultaneous measurement of gas absorption and path length by employing the first harmonic phase angle method in wavelength modulation spectroscopy, Optics Express) uses the first harmonic phase angle of wavelength modulation spectroscopy to simultaneously obtain trace gas concentration and path length. The maximum measurement distance of this method is inversely proportional to the modulation frequency of the sine wave in the wavelength modulation spectroscopy technique, and the modulation frequency of the laser needs to be reduced for long-distance measurement. When the modulation frequency of the sine wave is reduced to the kHz level, the phase angle of the first harmonic does not change significantly with distance, resulting in poor spatial resolution. Hongbin Lu et al. in the paper "A RemoteSensor System Based on TDLAS Technique for Ammonia Leakage Monitoring, Sensors" (A RemoteSensor System Based on TDLAS Technique for Ammonia Leakage Monitoring, Sensors) in "Sensors" Volume 21, Issue 7, 2448-2462, 2021 A set of open-path ammonia monitoring sensors based on wavelength modulation spectroscopy technology was built. The sensor was used to measure a simulated ammonia leakage source. The system stability test results showed that the sensor can achieve a lower limit of ammonia concentration detection of 16.6ppm. It can be used to monitor the source of ammonia leakage. Although this scheme can measure the ammonia gas concentration on an open path, the lower limit of detection of the ammonia gas concentration is relatively high, and the ammonia gas concentration in the atmosphere cannot be detected yet. The above schemes all use wavelength modulation spectroscopy to measure the concentration of trace gases. In actual use, this technology usually can only extract the harmonic components after demodulation, and cannot directly obtain the gas concentration from the harmonic components. In order to relate the gas concentration to the harmonic component, it is not only necessary to determine the amplitude of the linear and nonlinear optical intensity modulation of the laser and the phase difference between its frequency modulation, but also to calibrate the harmonic signal using a reference gas. The relationship between the signal and the gas concentration is used to deduce the measured gas concentration, and the accuracy of the gas concentration measurement depends on the accuracy of the calibration results. In addition, the trace gas detection scheme based on wavelength modulation spectroscopy technology does not have high-precision ranging capabilities, and cannot accurately obtain the true path length traveled by the laser, which will affect the accuracy of gas concentration measurement results.

Compared with wavelength modulation spectroscopy technology, direct absorption spectroscopy technology can directly extract absorption spectrum from transmitted light intensity, and can realize absolute measurement of gas parameters without standard gas calibration, which makes direct absorption spectroscopy technology irreplaceable in gas concentration measurement. status. Chuantao Zheng et al. in 2016 "IEEE Photonics Technology Communications" Volume 28, Issue 21 3036-3038 paper "Infrared Dual-Gas CH4/C2H6 Sensor Using Two Continuous-Wave Interband Cascaded Lasers" (Infrared Dual- Gas CH4/C2H6 Sensor Using Two Continuous-Wave InterbandCascade Lasers, IEEE Photonics Technology Letters) uses direct absorption spectroscopy to measure the methane concentration in the air with a pressure of 700 Torr at room temperature, and the effective optical path of the multiple reflection gas cell is 54.6 meters , The average concentration of methane in the air was measured to be 2.7ppm. The lower detection limit of methane concentration was analyzed by Allen variance. When the average time was 1s, the lower detection limit of methane concentration was 2.7ppbv. Although this method can achieve high measurement accuracy, it needs to sample the gas to be measured, and the pressure of the sampled gas is slightly less than 1 standard atmospheric pressure, which will affect the accuracy of gas parameter measurement. In 2016, "Applied Physics Letters", Volume 108, Issue 1, pages 1106-1110, "Compact CH4 sensor system based on continuous wave, low power consumption, room temperature interband cascaded lasers" (Compact CH4 sensorsystem based on a continuous-wave, low power consumption, room temperature interband cascade laser, Applied Physics Letters) designed a compact, low power consumption trace gas concentration measurement device, the device uses multiple reflection gas cell as the core component , the system size is limited to a volume of 32*20*17cm ³ , which is convenient for use on mobile devices. The device has an effective optical path of 54.6 meters, and can achieve a detection limit of 1.4 ppbv methane concentration at normal temperature and pressure. This method also needs to sample the gas to be measured, and the output power of the laser is only 1.5mW, so the signal-to-noise ratio of the measurement signal is low. Jingsong Li et al. "Single quantum cascade laser sensor based on dual-spectrum technology simultaneously detects CO, N ₂ O and H in the atmosphere" in "Sensors and Actuators B: Chemistry" in 2016, Volume 231, pages 723-732 ₂ O" (Simultaneous atmospheric CO, N2O and H2O detection using asingle quantum cascade laser sensor based on dual-spectroscopy techniques, Sensors and Actuators B: Chemical) uses direct absorption spectroscopy to detect the ambient gas with a pressure of 100mbar and a temperature of 300K. The measurement shows that the effective optical path of the multiple reflection gas cell is 76 meters, and the central wavelength of the laser is 4566nm. The results show that the sensor can achieve a CO concentration detection lower limit of 1.64ppb and a N ₂ O concentration of 1.15ppb within an integration time of 1s. Lower limit of detection. The method not only needs to sample the gas to be measured, but also the pressure of the sampled gas should be less than 1 standard atmospheric pressure. In 2017, "Optics Express", Volume 25, Issue 25, pages 31876-31888, paper "Interband cascadelaser based mid-infrared methane sensor based on interband cascade laser and adaptive filtering direct absorption spectroscopy" by Fang Song et al. -infrared methane sensor system using a novel electrical-domain self-adaptive direct laser absorption spectroscopy (SA-DLAS), OpticsExpress) using a multi-reflection gas cell with an effective optical path of 16 meters to measure the methane concentration in the air at normal temperature and pressure The measurement was carried out, and the noise of the sensor was suppressed with the adaptive filtering technology, and the filtered direct absorption spectrum data was calculated, and the measured methane concentration in the air was 1.876ppm. When the average time is 6s, this method can achieve the lower detection limit of methane concentration of 43.9ppbv, but the system cannot stabilize the temperature in the gas cell, and the temperature fluctuation will directly affect the measurement accuracy of methane concentration. Nicolas Sobanski et al. "Advances in High-Precision NO2 by Quantum Cascade Laser Absorption Spectroscopy in High-Precision Nitrogen Dioxide Measurement" in Applied Science, Volume 11, Issue 3, pp. 1222-1243 in 2021 Measurement by Quantum Cascade Laser Absorption Spectroscopy, appliedsciences) adopts direct absorption spectroscopy to measure nitrogen dioxide with a pressure of 80mbar, a temperature of 300K, and a concentration of 1ppbv. The effective optical path of the multiple reflection gas cell is 110 meters. The results show that , the sensor can achieve a concentration detection lower limit of 0.8ppbv within an integration time of 150s. This solution also needs to sample the gas to destroy the original state of the gas. The above technologies all use direct absorption spectroscopy to measure the concentration of trace gases. Although this technology has the advantages of wide applicability, simple data interpretation, extraction of absorption spectra, and no need for standard gas calibration, but due to the interference of system background noise and laser power fluctuations, Its detection sensitivity is slightly lower than that of wavelength modulation spectroscopy. For the direct absorption spectroscopy technique, the signal-to-noise ratio of the absorption spectroscopy signal can be improved by increasing the path length, thereby improving the detection sensitivity. In addition, the above method uses the multiple reflection gas cell as the core component of the measurement system to detect the concentration of trace gases, and there is no literature report on the use of direct absorption spectroscopy to measure the concentration of ambient gases in an open path.

Temperature is one of the core parameters in gas concentration measurement. The current gas concentration measurement is to obtain the temperature value near the sensor through temperature measuring devices such as thermometers or thermocouples. The temperature only reflects the temperature at a certain fixed point, and cannot reflect the temperature information on the open path. In order to provide more accurate temperature values, the average temperature of the path is used instead of the temperature of the fixed point to better reflect the temperature change on the open path. According to Beer-Lambert's law, the ratio of the absorption intensities of two spectral lines is a single-valued function of temperature. Therefore, the average temperature on the absorption path can be calculated by using the ratio of the absorption intensities of two different spectral lines of the same substance. The method is called colorimetry. When using the colorimetric method to measure temperature, it is first necessary to select two isolated absorption lines of a certain gas without interference from other gases, and then judge whether the temperature measurement sensitivity of these two absorption lines meets the requirements. In general, it is difficult to select two absorption lines suitable for temperature measurement. Considering that the commonly used photodetector response band is 1100nm-1700nm, taking the absorption line of methane as an example, assuming that the detection range of the sensor is 100-200 meters, under normal temperature and pressure, the spectral line intensity is appropriate and the absorption line is isolated There are: 1.6481μm, 1.651μm and 1.653μm. In addition, considering the cross-interference of water molecules and the sensitivity of temperature measurement by colorimetry, there are no doublet lines that meet the temperature measurement requirements. According to Beer Lambert's law and the principle of colorimetric temperature measurement, the average temperature of the path cannot be measured by a single spectral line, and the prerequisite for gas concentration measurement is to obtain the gas pressure, average path temperature and path length, so only rely on the absorption of methane It is impossible to measure the absolute concentration of the spectral line. In contrast, water molecules have a wide distribution of absorption lines in the range of 1100nm-1700nm, and there are many isolated lines, so it is easy to select two absorption lines suitable for open path average temperature measurement. In order to solve the problem that a single spectral line cannot realize temperature measurement, two absorption spectral lines with appropriate spectral line intensity can be selected in the absorption spectral lines of water molecules, and the average temperature of the same path as that of the gas to be measured can be obtained through a common optical path, so that Complete the gas concentration measurement.

Based on the above background, the present invention proposes a real-time measurement method and device for trace gas concentration on an open path, which can simultaneously measure the trace gas concentration, path average temperature and path length on an open path. The measuring device includes two measuring arms, one of which is used to detect the gas to be measured, and the other is used to provide a reference signal. After the laser beams of the two arms are combined, the output contains trace gas concentration, path average temperature and path length. Beat frequency signal of other information. The invention combines direct absorption spectrum technology, frequency modulation continuous wave distance measurement technology, colorimetry temperature measurement and other technologies, and develops a device and method for measuring trace gas concentration on an open path. The method uses frequency modulation continuous wave ranging technology to obtain high-precision distance information, uses colorimetric temperature measurement technology to obtain the path average temperature information of the gas to be measured, and uses direct absorption spectroscopy technology to obtain the absolute concentration information of the gas to be measured. In order to reduce the dependence on the absorption spectrum of the gas to be measured, the present invention will use two easy-to-acquire gas molecule absorption lines to measure the path average temperature, and use a single absorption line to measure the trace gas concentration; The path average temperature of the gas measurement is the same. The device of the invention is simple, portable, and fast in data processing, and is not only suitable for monitoring greenhouse gas emissions of processing enterprises, but also suitable for long-term monitoring of greenhouse gas content in areas such as farmland, livestock farms, and cities.

(3) Contents of the invention

The purpose of the present invention is to solve the deficiencies in the existing trace gas concentration detection technology on the open path, and to provide a real-time measurement device and method for trace gas concentration, path average temperature and path length on the open path. Three technical areas of tuned diode laser absorption spectroscopy, temperature measurement and absolute distance measurement. The components used include a signal generator, two tunable diode lasers, two optical isolators, two optical power amplifiers, a wavelength division multiplexer, two fiber beam splitters, two collimating mirrors, a parabolic reflector mirror, a corner cube, a dichroic mirror, a beam splitter, four photodetectors, three narrow-band filters, a high-pass filter, a low-pass filter and a data acquisition card, etc.

The technical solution adopted in the present invention is: the signal generator is connected with two tunable diode lasers, and by adjusting the injection current of the lasers, the laser frequency of the two lasers changes linearly, and the wavelength output by one of the lasers covers the wavelength of the gas to be measured. One absorption line, the other laser output wavelength covers the absorption lines of two easily accessible gas molecules. The two lasers are respectively connected to the optical isolator. The two laser beams emitted by the optical isolator are amplified by the optical power amplifier respectively, and then a wavelength division multiplexer is used to couple the lasers of the two bands together to realize the common optical path of the two lasers. . The laser light in the common optical path of the wavelength division multiplexer is incident on two fiber optic beam splitters in series for beam splitting, and then three beams of light are obtained, one beam is used as measurement beam (90%), and the other beam is used as reference for FM continuous wave distance measurement Light (5%), and another monitor light (5%) for measuring the laser power of the system. After the measuring light is collimated by the collimating mirror, it passes through the parabolic reflector with a small hole in the center and the gas to be measured in turn, is reflected by the corner cube prism, and then received by the parabolic reflector. A dichroic mirror is used to separate the measuring light by wavelength, the frequency-sweeping laser including the absorption line of the temperature measurement is detected by a photodetector, and the frequency-sweeping laser including the absorption line of the gas to be measured is separated from the reference light by a beam splitter The beams are combined, and the combined laser light is detected by two photodetectors. A narrow-band filter is installed in front of each photodetector to filter out the interference of other light. After the measurement light and the reference light are combined, a beat frequency occurs on the photodetector. The two photodetectors are respectively connected with a high-pass filter and a low-pass filter. The beat frequency signal after the high-pass filter is used for the measurement of the path length. The filtered DC signal is used to measure the concentration of the gas to be measured. The signals of all photodetectors are collected by the acquisition card for subsequent signal processing. The specific implementation process is as follows:

Step 1: The signal generator generates a sawtooth wave signal to modulate the output wavelength of the laser, and the laser controller adjusts the laser frequency output by the tunable diode laser by changing the injection current; in order to make the laser frequency change linearly, according to the injection current and the laser frequency. Relationship, continuously modify the sawtooth waveform until the laser frequency output by the laser changes linearly, and the laser frequency generated by each laser satisfies:

v(t)=v ₀ +at (1) Among them, v ₀ represents the starting frequency of laser frequency modulation, a=Ω/T represents the modulation rate of laser frequency, T represents the period of laser modulation, and Ω represents the bandwidth of laser frequency modulation .

Step 2: Adjust the working temperature of the laser so that the laser wavelength generated by one of the lasers covers one absorption line of the gas to be measured, and the laser frequency generated by the other laser covers two absorption lines of the temperature measurement, in a time-division multiplexed manner Drive two lasers; install an optical isolator between the laser and the wavelength division multiplexer to protect the laser; the two laser beams emitted by the optical isolator are respectively amplified by the optical power amplifier; the wavelength division multiplexer generates two lasers The lasers are coupled together to realize the common optical path of two lasers; two optical fiber beam splitters are connected in series, and the laser is divided into three outputs according to the ratio of 90:5:5, and the ratio of 90% is used as the measurement light, and the light intensity is recorded is I _m , the ratio is 5%, and the two channels are respectively used as the reference light for frequency modulation continuous wave distance measurement and the monitoring light for measuring the laser power of the system, and the light intensities are recorded as I _r and I _monitor respectively.

Step 3: After the measuring light is collimated by the collimating mirror, it passes through the parabolic reflector with a small hole in the center and the gas to be measured in turn, is reflected by the corner cube prism, and then received by the parabolic reflector; the dichroic mirror will To measure light separation, the frequency-sweeping laser including the temperature measurement absorption spectrum will be reflected by a dichroic mirror, and then detected by a photodetector after passing through a narrow-band filter; Transmission, the beam splitter combines the transmitted light and the reference light that does not pass through the gas to be measured. After the combined beam passes through the narrow-band filter, it is detected by two photodetectors, and the detection signals of the two photodetectors pass through a A high-pass filter and a low-pass filter are uploaded to the acquisition card; the change of the laser power of the measurement system is recorded by the fourth photodetector; the signals uploaded to the acquisition card by the four photodetectors are expressed as:

I ₁ (t)=I _m exp(-α _x (v(t))) (2)

I ₃ (t)＝I _r +I _m exp(-α _target (v(t))) (4)

I ₄ (t)＝I _monitor (5)

Among them, I ₁ (t), I ₂ (t), I ₃ (t), and I ₄ (t) are the signals of the four photodetectors collected by the acquisition card; τ ₁ and τ ₂ are the measured τ _m is the time difference between the measurement light and the reference light reaching the photodetector; α _x (v(t)), α _target (v(t)) are the temperature measurement absorption line and the absorption rate at the absorption line of the gas to be measured, α _i (v), i=x, target can be expressed as:

是吸收光谱的线型函数，满足归一化条件，l是表示路径上的位置。Among them, P is the gas pressure in the measurement area, L is the path length, X(l) is the mole fraction of the gas to be measured, T(l) is the temperature of the gas to be measured, and S[T(l)] is the temperature-related spectrum line strength,

is the linear function of the absorption spectrum, which satisfies the normalization condition, and l is the position on the path.

Step 4: Extract the beat frequency from the beat signal I ₂ (t), the relationship between the path length and the beat frequency can be expressed as:

Among them, c represents the speed of light in the air, f _IF =aτ _m is the frequency of the beat frequency signal, and the frequency spectrum is obtained by Fourier transforming the signal I ₂ (t) collected by the photodetector, and the maximum amplitude point in the frequency spectrum is The corresponding frequency is the desired frequency f _IF .

Step 5: From formulas (2) and (4) in step 3, it can be seen that the absorptivity at the temperature measurement absorption line and the absorptivity at the absorption line of the gas to be measured are α _i (ν), i=x, target respectively Expressed as:

Among them, the reference light I _r is obtained according to a certain light splitting ratio, which can be calculated proportionally from the monitoring light; the measurement light I _m is obtained by baseline fitting, and I _m exp(-α _i (v)) is selected as For the sections [t ₁ ,t ₂ ] and [t ₃ ,t ₄ ] with minimal absorption on both sides of the absorption section, by means of data fitting, the amplitude of _Im of these two sections is approached to _Im exp(- α _i (v)), get I _m excluding exp(-α _i (v)); the absorption spectrum can be calculated from the absorbance curve α _i (v).

Step 6: Extract the integrated absorption area A _i of the two absorption lines from the absorption rate curve α _x (ν(t)) of the temperature measurement absorption line, i=1 or 2, and calculate the path average temperature by colorimetry, The integral absorption area A _i satisfies:

It can be seen from formula (10) that the ratio of the integrated absorption area at the two absorption lines is only related to the gas temperature:

The average temperature T of the path can be obtained by calculating the ratio of the integrated absorption area; then according to Beer Lambert's law, the concentration of the gas to be measured can be calculated by the following formula:

The present invention has the advantages of combining the easily acquired temperature measurement absorption spectrum with the gas absorption spectrum to be measured, reducing the demand for the gas absorption spectrum line to be measured, and avoiding the failure of two suitable absorption spectrum lines of the gas to be measured. The measurement of the concentration of the gas to be measured cannot be realized. The solution can simultaneously realize the measurement of the gas concentration to be measured, the average temperature of the path and the length of the path on the open path. It is not only suitable for monitoring the greenhouse gas emissions of processing enterprises, but also suitable for the long-term monitoring of greenhouse gas content in farmland, livestock farms, cities and other areas.

(4) Description of drawings

Figure 1 is a structural diagram of the measuring device.

Figure 2 shows the modulation signal of the laser injection current.

(5) Specific implementation methods

Taking the double spectrum line of water molecules to measure the average temperature of the path and the single spectrum line of methane to measure the concentration as examples, and in conjunction with the accompanying drawings, the technical solution of the present invention will be further described. The present invention provides a trace gas on an open path. The real-time measurement method and device of the concentration, the measuring device is as shown in Figure 1, and the measuring device comprises a signal generator 1, a tunable diode laser 2, a tunable diode laser 3, an optical isolator 4, an optical isolator 5, an optical power Amplifier 6, optical power amplifier 7, wavelength division multiplexer 8, optical fiber beam splitter 9, optical fiber beam splitter 10, collimator mirror 11, collimator mirror 12, parabolic mirror 13, dichroic mirror 14, beam splitter Mirror 15, narrowband filter 16, narrowband filter 17, narrowband filter 18, photodetector 19, photodetector 20, photodetector 21, photodetector 22, corner cube prism 23, data acquisition card 24 ,in:

The signal generator 1 is connected to the tunable diode laser 2 and the tunable diode laser 3, and uses a sawtooth waveform to adjust the injection current. By modifying the shape of the sawtooth wave, the tunable diode laser 2 and the tunable diode laser 3 generate The laser frequency changes linearly, the sawtooth wave waveform is shown in Figure 2, and the wavelength output by the tunable diode laser 2 covers one absorption line of methane, and the wavelength output by the tunable diode laser 3 covers two absorption lines of water molecules Wire.

The tunable diode laser 2 is connected to the optical isolator 4 and the optical power amplifier 6 in sequence, and the tunable diode laser 3 is connected to the optical isolator 5 and the optical power amplifier 7 in sequence, and the power is amplified by the optical power amplifier 6 and the optical power amplifier 7 The frequency-sweeping laser is coupled together with a wavelength division multiplexer 8, and realizes the common optical path of the two lasers, and the laser of the common optical path is incident on the optical fiber beam splitter 9 with a splitting ratio of 90:10 for beam splitting, and the splitting ratio is 90%. One beam is used as measuring light, and a beam splitting ratio of 10% is split again by an optical fiber beam splitter 10 with a beam splitting ratio of 50:50, wherein one beam (5%) is used as reference light for frequency modulation continuous wave distance measurement, The other beam (5%) is used to monitor the laser power of the measuring system.

The measuring light is collimated by the collimating mirror 11 , passes through the parabolic reflector 13 with a small hole in the center and the gas to be measured in turn, is reflected by the corner cube prism 23 , and is received by the parabolic reflector 13 .

The dichroic mirror 14 separates the frequency-sweeping lasers output by the tunable diode laser 2 and the tunable diode laser 3 according to the wavelength, and the frequency-sweeping laser for measuring the absorption spectrum of water molecules will be reflected by the dichroic mirror 14 and passed through a narrow-band filter. After 16, it is detected by the photodetector 20, and the detection signal is directly collected by the data acquisition card 24; the frequency-sweeping laser for measuring the methane absorption spectrum will be transmitted by the dichroic mirror 14, and the beam splitter 15 will transmit light and not pass through the gas to be measured The reference light beams are combined, and the combined beams pass through the narrow-band filter 17 and the narrow-band filter 18 respectively, and are detected by the photodetector 19 and the photodetector 21, and the signal measured by the photodetector 19 is filtered by the low-pass filter The data acquisition card 24 collects, and the signal measured by the photoelectric detector 21 is collected by the data acquisition card 24 after being filtered by a high-pass filter; the signal measured by the photoelectric detector 22 is directly collected by the data acquisition card 24 without processing.

The tunable diode laser 2 selects two suitable and easy-to-obtain molecular absorption spectrum lines according to the path length, and the tunable diode laser 3 depends on the type of gas molecules to be detected.

A method for real-time measurement of trace gas concentration on an open path, comprising the steps of:

Step 1: Modulate the wavelengths of the tunable diode laser 2 and the tunable diode laser 3 with a sawtooth wave waveform, and use the relationship between the injection current and the laser frequency to continuously correct the waveform of the injection current until the laser frequency changes linearly. After correction The sawtooth wave waveform of is shown in Figure 2; the laser frequencies generated by tunable diode laser 2 and tunable diode laser 3 all satisfy:

v(t)＝ν ₀ +at (1)

Among them, v ₀ represents the starting frequency of laser frequency modulation, a=Ω/T represents the modulation rate of laser frequency, T represents the period of laser modulation, and Ω represents the bandwidth of laser frequency modulation.

Step 2: Adjust the output wavelength range of tunable diode laser 2 and tunable diode laser 3 by changing the operating temperature of the laser, so that the wavelength output by tunable diode laser 2 covers an absorption line of methane, so that tunable diode laser 3 outputs The wavelength covers the two absorption lines of water molecules, and drives the two lasers in a time-division multiplexing manner; after the tunable diode laser 2, connect the optical isolator 4 and the optical power amplifier 6 in sequence; Optical isolator 5 and optical power amplifier 7; the optical isolator is used to protect the safety of the laser, and the optical power amplifier amplifies the power of the laser, which is convenient for long-distance open path measurement; the wavelength division multiplexer 8 amplifies the frequency-sweeping laser of the two-way amplified laser power Coupling together, and realizing the common optical path of two lasers; The laser of common optical path is incident to the optical fiber beam splitter 9 that beam splitting ratio is 90:10 to carry out beam splitting, and the beam splitting ratio is 90% one bundle is used as measuring light, and light intensity is recorded as _Im , a beam splitting ratio of 10% is split by an optical fiber beam splitter 10 with a splitting ratio of 50:50, wherein one beam (5%) is used as the reference light for frequency modulation continuous wave distance measurement, and the other beam (5%) is used as the monitor light for measuring the laser power of the system, and the light intensities are recorded as I _r and I _monitor respectively.

Step 3: After the measuring light is collimated by the collimating mirror 11, it passes through the parabolic reflector 13 with a small hole in the center and the gas to be measured in turn, and is reflected by the corner cube prism 23 in the original way, and then received by the parabolic reflector 13; The chromatic mirror 14 separates the measurement light according to the wavelength, and the frequency-sweeping laser for measuring the absorption spectrum of water molecules will be reflected by the dichroic mirror 14, and will be detected by the photodetector 20 after passing through the narrow-band filter 16; the frequency-sweeping laser for measuring the absorption spectrum of methane It will be transmitted by the dichroic mirror 14, and the beam splitter 15 combines the transmitted light and the reference light that does not pass through the gas to be measured. and photodetector 21 detection; the change of measuring system laser power is recorded by photodetector 22; the detection signals uploaded to the data acquisition card by four photodetectors are respectively expressed as:

I ₄ (t)＝I _monitor (5)

分别是水分子和甲烷在吸收谱线处的吸收率，α_i(v),i＝H₂O,CH₄可以表示成：Wherein, I ₁ (t), I ₂ (t), I ₃ (t), and I ₄ (t) are the laser light measured by the photodetector 20, the photodetector 21, the photodetector 19 and the photodetector 22, respectively. Strong; τ ₁ and τ ₂ are the time delay introduced when the laser passes through the measurement path and the reference path; τ _m is the time difference between the measurement light and the reference light reaching the photodetector;

are the absorption rates of water molecules and methane at the absorption lines, respectively, α _i (v), i=H ₂ O, CH ₄ can be expressed as:

是吸收光谱的线型函数，满足归一化条件，l是表示路径上的位置。Among them, P is the gas pressure in the measurement area, L is the path length, X(l) is the mole fraction of the gas to be measured, T(l) is the temperature of the gas to be measured, and S[T(l)] is the temperature-related spectrum line strength,

is the linear function of the absorption spectrum, which satisfies the normalization condition, and l is the position on the path.

Step 4: Extract the beat frequency from the beat signal I ₂ (t), the relationship between the path length and the beat frequency can be expressed as:

Among them, c represents the speed of light in the air, f _IF =aτ _m is the frequency of the beat frequency signal, and the frequency spectrum is obtained by Fourier transforming the signal I ₂ (t) collected by the photodetector, and the maximum amplitude point in the frequency spectrum is The corresponding frequency is the desired frequency f _IF .

Step 5: From the formulas (2) and (4) in the step 3, it can be seen that the absorption rate curves of water molecules and methane α _i (v), i=h ₂ o, CH ₄ are expressed as:

Among them, the reference light I _r is obtained according to a certain light splitting ratio, which can be calculated proportionally from the monitoring light; the measurement light I _m is obtained by baseline fitting, and I _m exp(-α _i (v)) is selected as For the sections [t ₁ ,t ₂ ] and [t ₃ ,t ₄ ] with minimal absorption on both sides of the absorption section, by means of data fitting, the amplitude of _Im of these two sections is approached to _Im exp(- α _i (v)), get I _m excluding exp(-α _i (v)); the absorption spectrum can be calculated from the absorbance curve α _i (v).

处提取两条吸收谱线的积分吸收面积A_i，i＝1或2，用比色法计算路径平均温度，积分吸收面积A_i满足：Step 6: Absorption Rate Curve from Water Molecules

Extract the integral absorption area A _i of two absorption lines at , i=1 or 2, calculate the average temperature of the path by colorimetry, and the integral absorption area A _i satisfies:

It can be known from formula (10) that the ratio of the integrated absorption area of water molecules at the two absorption lines is only related to the gas temperature:

The average temperature T of the path is obtained by calculating the ratio of the integrated absorption area; then according to Beer Lambert's law, the concentration of methane can be calculated by the following formula:

The above description of the present invention and its embodiments is not limited thereto, and what is shown in the drawings is only one of the embodiments of the present invention. Without departing from the inventive concept of the present invention, any structure or embodiment similar to the technical solution without creative design shall belong to the protection scope of the present invention.

Claims (3)

1. A real-time measurement method and device for trace gas concentration on an open path are characterized in that a system comprises a signal generator, two tunable diode lasers, two optical isolators, two optical power amplifiers, a wavelength division multiplexer, two optical fiber beam splitters, two collimating mirrors, a parabolic reflector, a pyramid prism, a dichroic mirror, a beam splitter, four photoelectric detectors, three narrow-band filters, a high-pass filter, a low-pass filter, a data acquisition card and the like; wherein: the signal generator is connected with the two tunable diode lasers, the laser frequencies of the two lasers are linearly changed by adjusting the injection current of the lasers, the wavelength output by one of the lasers covers one absorption spectral line of the gas to be measured, and the wavelength output by the other laser covers two absorption spectral lines of gas molecules which are easy to obtain; the two lasers are respectively connected with an optical isolator, two beams of laser emitted by the optical isolator are respectively amplified by a light power amplifier, and then the lasers in two wave bands are coupled together by a wavelength division multiplexer, so that two paths of laser sharing light paths are realized; laser light passing through a common light path of the wavelength division multiplexer is incident to two serially connected optical fiber beam splitters for beam splitting, so that three beams of light are obtained, wherein the power proportion of one beam of light is about 90% and is used as measuring light, the power proportion of one beam of light is about 5% and is used as reference light for frequency modulation continuous wave distance measurement, and the power proportion of the other beam of light is about 5% and is used as monitoring light for measuring the laser power of a system; after being collimated by the collimating mirror, the measuring light sequentially passes through the parabolic reflector with the small hole at the center and the gas to be measured, is reflected by the original path of the pyramid prism, and is received by the parabolic reflector; the dichroic mirror separates the measuring light according to wavelength, and sweep-frequency laser containing temperature measurement absorption spectrum lines is reflected by the dichroic mirror, passes through the narrow-band filter and is detected by a photoelectric detector; the sweep frequency laser containing the absorption spectrum line of the gas to be detected can be transmitted by the dichroic mirror, the beam splitter combines the transmitted light with the reference light which does not pass through the gas to be detected, the combined light is detected by two photoelectric detectors after passing through the narrow-band filter, and signals of the two photoelectric detectors are filtered by a high-pass filter and a low-pass filter respectively before being uploaded to the acquisition card; the change of the laser power of the measuring system is recorded by a photoelectric detector; detection signals of the four photoelectric detectors are uploaded to a computer through an acquisition card to be processed in real time, and the average temperature of the path, the path length and the absolute concentration of the gas to be detected are inverted.

2. The method and the device for real-time measurement of the concentration of the trace gas on the open path according to claim 1, wherein the measurement method uses two easily obtained absorption lines to perform path average temperature measurement, and uses the absorption line of a single gas to be measured to perform absolute concentration measurement, so as to reduce the dependence on the absorption line of the gas to be measured, and comprises the following specific steps:

the method comprises the following steps: the acquisition of the signals uploaded to the four photodetectors of the acquisition card can be represented as:

I ₁ (t)＝I _m exp(-α _x (v(t))) (1)

I ₃ (t)＝I _r +I _m exp(-α _target (v(t))) (3)

I ₄ (t)＝I _monitor (4)

wherein, I ₁ (t) is the measurement signal of the readily accessible absorption spectrum, I ₂ (t) is the measured beat signal, I ₃ (t) is the measurement signal of the absorption spectrum of the gas to be measured, I ₄ (t) is a monitor signal for measuring the laser power of the system; tau. ₁ 、τ ₂ Respectively, the time delay introduced when the laser passes through the measuring path and the reference path; tau. _m Is the time difference between the arrival of the measurement light and the reference light at the photodetector; alpha is alpha _x (v(t))、α _target (v (t)) is the absorption rate at the temperature measurement absorption line and the absorption rate at the gas absorption line to be measured, respectively, alpha _i (v) I = x, target can be expressed as:

where P is the measurement area gas pressure, L is the path length, X (L) is the mole fraction of the gas to be measured, T (L) is the temperature of the gas to be measured, S [ T (L)]Is the line intensity as a function of temperature,

is a linear function of the absorption spectrum, satisfying the normalization condition, l is the position on the representation path;

step two: from the beat signal I ₂ (t) extracting beat frequency, wherein the relationship between the path length and the beat frequency can be expressed as follows:

wherein c represents the speed of light in air, f _IF ＝aτ _m Is the frequency of the beat signal, signal I acquired by the photodetector ₂ (t) Fourier transform is carried out to obtain a frequency spectrum, and the frequency corresponding to the maximum amplitude point in the frequency spectrum is the solved frequency f _IF ；

Step three: according to the formulas (1) and (3) in the step one, the measurement is carried outAbsorption rate at the temperature absorption line and absorption rate alpha at the absorption line of the gas to be measured _i (v) I = x, target is expressed as:

the reference light Ir is obtained according to a certain splitting ratio and can be proportionally calculated from the monitoring light; measuring light I _m Is obtained by fitting a base line, selecting I _m exp(-α _i (v) A section [ t ] with minimal absorption on both sides of the middle absorption section ₁ ，t ₂ ]And [ t ₃ ，t ₄ ]Enabling the I of the two segments to be matched in a data fitting mode _m Is approximated by an amplitude value of _m exp(-α _i (v) Obtaining an amplitude value not containing exp (-alpha) _i (v) I) of this term _m (ii) a The absorption spectrum can be represented by the absorption curve a _i (v) Calculating to obtain;

step four: absorption rate curve alpha from thermometric absorption line _x (v (t)) the integrated absorption area A for extracting the two absorption lines _i I =1 or 2, calculating the path average temperature by colorimetry, integrating the absorption area A _i Satisfies the following conditions:

from equation (9), the ratio of the integrated absorption areas at the two absorption lines is related only to the gas temperature:

obtaining a path average temperature T through the calculated ratio of the integral absorption areas; then, according to beer's lambert law, the concentration of the gas to be measured can be calculated by the following formula:

3. the method and the device for measuring the concentration of the trace gas on the open path in real time according to claim 1, wherein the measuring device couples the swept laser beams of two wave bands together by using a wavelength division multiplexer to realize a common path of two laser beams with different wavelengths; separating the sweep-frequency laser containing the temperature measurement absorption spectrum line from the sweep-frequency laser containing the gas absorption spectrum line to be measured by using a dichroic mirror, wherein a signal containing the temperature measurement absorption spectrum is used for calculating the path average temperature; the sweep-frequency laser containing the absorption spectrum line of the gas to be measured and the reference light are combined by a beam splitter, a beat-frequency signal generated by the combined light is used for calculating the path length, and the direct-current component of the combined light is used for calculating the absolute concentration of the gas to be measured.

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