| CO2 and CH4 as greenhouse gases emitted by human activities have a high concentration in the atmosphere,which can absorb long-wave radiation from the sun and the earth’s surface,and release the radiation back into the earth’s atmosphere to cause global warming.Long-term monitoring of CO2 and CH4 concentration in the atmosphere is of great significance for formulating national development strategies of energy conservation and emission reduction.With the development of science and technology,Off-Axis Integrated Cavity Output Spectroscopy(OA-ICOS),as a high-resolution and high-sensitivity absorption spectroscopy technology,is very suitable for atmospheric CO2 and CH4 concentration measurement.Thus,the research and development of instruments based on OA-ICOS has gradually become one of the key topics concern to researchers.The research has made in-depth investigation and study of OA-ICOS,and applies OA-ICOS to development of the instruments.In addition,in order to achieve simultaneous measurement of CO2 and CH4,the Dual-channel OA-ICOS system(Dual-OA-ICOS)has been proposed for the first time.Thus,this research will undoubtedly provide important reference value for applied research work in China.For Dual-OA-ICOS system,the "Dual-channel" mechanical structure is first proposed,which can provide±10 mm distance from axis on X and Y.An adjustable home-made collimator that provides the angle of ±5° tip/tilt along the optical axis is placed on the optical cavity.Two high reflection mirrors are embedded in the optical cavity,and two optical cage systems are respectively installed on the left and right outside the optical cavity.In the left optical cage system,the Dual-channel mechanical structure and the home-made collimator are integrated,which allows the installation of 1603.2 nm CO2 laser and 1651 nm CH4 laser.In the right optical cage system,a focusing lens and a photodetector are integrated,which allows two lasers to be received by the same photodetector.In addition,a time-division multiplexing method is proposed to solve the problem of light intensity superposition on the photodetector.After completing the system design,a large number of detailed optimizations are proposed to reduce system noise.Firstly,concentration filter and spectral filter based on exponential moving average algorithm(EMA)are used to suppress noise.A forty-minute CH4 experiment is designed to establish the relationship between system response time and concentration deviation,which can get the best weight value of 10.On the other hand,the lasers are easily affected by the environment during the long-term measurement process,so that the center wavelength of lasers drift.Therefore,the article proposes a wavelength-locking method for Dual-OA-ICOS system.After 24 hours of CH4 calibration experiment,it can be found that a limit of detection(LoD)of the wavelength locked is reached at 3100 s,and its 1σ can reach 0.39 ppb.the integrated Dual-OA-ICOS system as a prototype to measure the concentration of CO2 and CH4 in the atmosphere.Designing a home-made temperature and pressure control board,and a temperature incubator,the prototype can control the temperature and pressure in the optical cavity.The 18-hour temperature and pressure stability experiment are developed,it can be found that the standard deviation 1σ of temperature is 0.37 mK,and the standard deviation 1σ of pressure measurement was 0.005 Torr.After that,the concentration of CO2 and CH4 are also measured in the same conditions,the original concentration of CO2 and CH41σ is 0.278 ppm,1.794 ppb,and the filtered concentration 1σ is 0.107 ppm and 0.733 ppb,respectively.Compared with the original concentration,the filter concentration accuracy of CO2 and CH4 increases by 2.6 times and 2.4 times respectively.After the laboratory measurement,the Dual-OA-ICOS prototype is sent to the Lin’an Meteorological Observatory in Zhejiang Province for a 22-day comparison experiment with a commercial instrument,the measurement results are basically same. |
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