Development Of CO₂,H₂O Analyzer Based On TDLAS Technology And Its Application In Flux Measurement

With the increase of greenhouse gas emissions（GHG）,global environmental problems are becoming more and more serious.Exploring the mechanism of GHG emission and absorption is essential to accurately predict and assess the trends and impacts of climate change,and to provide basic data and technical support for the goal of "carbon neutrality and carbon peaking".The chamber method and the eddy covariance method are widely used for GHG flux monitoring.The chamber method is the main method used to observe the exchange of substances and fluxes between soil/vegetation/water bodies and the atmosphere;the eddy covariance method is a method to directly observe the exchange of fluxes between the atmosphere and the surface without making any assumptions.Compared with other observation methods,the eddy covariance method is more extensive in observing spatial scales.Laser absorption spectroscopy technique has the advantages of high resolution,high sensitivity,high accuracy,fast response and high selectivity,which is very suitable for flux gas analysis.Therefore,in this paper,we conducted a study on portable gas analyzer based on tunable diode laser absorption spectroscopy（TDLAS）technology for chamber method flux measurement,and an open-path flux gas analyzer for eddy covariance flux monitoring,and an application study on eddy covariance flux monitoring in combination with 3D ultrasonic anemometer.First,a portable gas analyzer based on TDLAS was designed and developed for online monitoring of CO₂ and H₂O concentrations.The system consists of a selfdesigned small multi-pass cell,a home-made laser driver circuit and a data acquisition circuit,and the optical,electronic and gas circuit parts are integrated into a portable suitcase（Length=388 mm,Width=290 mm,Height=134 mm）,with a total system weight of about 5 kg and a power of about 10 W.The self-designed miniature TDLAS driver module（90 mm×45 mm）,which integrates the laser current and temperature control as well as signal acquisition and demodulation functions,and the test results show that the temperature control accuracy is ± 1.4 mK and the current control accuracy is ±0.5 μA.Simultaneous measurements of CO₂ and H₂O were achieved with a distributed feedback（DFB）laser of 2004 nm and 1392 nm,combined with a home-made small multi-pass cell（34 mm×34 mm×125 mm）.According to the Allan analysis of variance,the detection limits of CO₂ and H₂O were 0.1 ppm and 4 ppm at an average time of 18 s and 35 s,respectively.In addition,the system response time was investigated,and the CO₂ response time from 0 ppm to 450 ppm CO₂ at a flow rate of 500 mL/min was about 10 s.By measuring the atmospheric CO₂ and H₂O concentrations,the performance of the sensor was verified.The results measured by the portable analyzer were compared with those obtained by a commercial instrument based on non-dispersive infrared technology,the LI-7500,and the experimental results showed that the developed portable analyzer measurements were consistent with those of the commercial instrument and their accuracy was comparable.Therefore,the portable TDLAS sensor has promising applications in atmospheric CO₂ and H₂O concentration detection and ecological soil flux monitoring.Then,in this paper,a high-speed TDLAS sensor based on an open-path anti-lens contamination multi-pass cell is designed to meet the demand for online measurement of CO₂ and H₂O fluxes in the external field.Unlike the conventional multi-pass cell,this design mainly consists of a flat convex mirror with the coating layer on the convex side,which makes the coating layer of the lens isolated from air,thus achieving the anti-pollution effect of the coating layer.Based on this,two DFB lasers operating at 2004 nm and 1392 nm,combined with a dual-optical anti-pollution multipass cell,were used to achieve high-precision and rapid measurements of CO₂ and H₂O.The Allan analysis of variance indicated that the detection limits for CO₂ and H₂O were 0.7 ppm and 6 ppm,respectively,at a time average of 0.1 s.The performance of the sensor was verified by measuring CO₂ and H₂O flux emissions at the agricultural monitoring station in Maling village,Jiangdu district,Yangzhou city,Jiangsu province,and comparing the measurement results with those obtained using a commercial instrument based on non-dispersive infrared technology,LI-7500,the developed gas analysis instrument showed good agreement with the commercial instrument and its accuracy met the demand for real-time detection.Therefore,the measurement system has good prospects for application in the field of eddy covariance flux monitoring of ecosystems.